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Ultrasensitive Three-Dimensional Orientation Imaging of Single Molecules on Plasmonic Nanohole Arrays Using Second Harmonic Generation
Abstract
The ability to detect, image and characterize single molecules non-invasively with high resolution in 3D in deep tissue would provide enormous advantages in the study of biological functions and diseases. Recently, fluorescence-based super-resolution techniques such as stimulated emission depletion (STED) and stochastic optical reconstruction microscopy (STORM) have been developed to achieve near molecular scale resolution. However, such a super-resolution technique for nonlinear label-free microscopy based on second harmonic generation (SHG) is lacking. Due to the nonlinearity of multiphoton excitation, SHG is capable of inherent optical sectioning, achieving deeper penetration in cells/tissues, and providing high contrast and specificity during imaging. Since SHG is label-free and does not involve real-energy level transitions, fluorescence-based super-resolution techniques such as STED cannot be applied to improve the resolution. In addition, due to the coherent and non-isotropic emission nature of SHG, single-molecule localization techniques based on isotropic emission of fluorescence molecule such as STORM will not be appropriate. Single molecule second harmonic generation microscopy is largely hindered due to the very weak non-linear optical scattering cross-sections of SHG scattering processes. Thus, enhancing SHG using plasmonic nanostructures and nanoantennas has recently gained much attention owing to the potential of various nanoscale geometries to tightly confine electromagnetic fields into small volumes. This confinement provides substantial enhancement of electromagnetic field in nanoscale regions of interest, which can significantly boost the nonlinear signal produced by molecules located in the plasmonic hotspots. However, to date, plasmon-enhanced SHG has been primarily applied for the measurement of bulk properties of the materials/molecules and single molecule SHG imaging along with its orientation information has not been realized yet. Herein, we achieved simultaneous visualization and three-dimensional orientation imaging of individual Rhodamine 6G (R6G) molecules in the presence of plasmonic silver nanohole arrays. SHG and two-photon fluorescence microscopy experiments together with finite-difference time-domain (FDTD) simulations revealed ~106-fold nonlinear enhancement factor at the hot spots on the plasmonic silver nanohole substrate, enabling detection of single molecules using SHG. The position and 3D orientation of R6G molecules was determined using the template matching algorithm by comparing the experimental data with the calculated dipole emission images. These findings could enable SHG-based single molecule detection and orientation imaging of molecules which could lead to a wide range of applications from nanophotonics to super-resolution SHG microscopy imaging of biological cells and tissues.
... Individual rhodamine 6G (R6G) three-dimensional orientation [44] 2.1. Single-Nucleic Acid Nanopore Detection ...
... A hybrid approach consisting of plasmonic silver nanohole arrays and second harmonic generation (SHG) microscopy imaging was used to visualize three-dimensional (3D) orientation of individual rhodamine 6G (R6G) molecules [44]. To demonstrate singlemolecule detection, SHG and two-photon fluorescence microscopy experiments along with finite-difference time-domain (FDTD) simulations were conducted. ...
The rapid spread of epidemic diseases (i.e., coronavirus disease 2019 (COVID-19)) has contributed to focus global attention on the diagnosis of medical conditions by ultrasensitive detection methods. To overcome this challenge, increasing efforts have been driven towards the development of single-molecule analytical platforms. In this context, recent progress in plasmonic biosensing has enabled the design of novel detection strategies capable of targeting individual molecules while evaluating their binding affinity and biological interactions. This review compiles the latest advances in plasmonic technologies for monitoring clinically relevant biomarkers at the single-molecule level. Functional applications are discussed according to plasmonic sensing modes based on either nanoapertures or nanoparticle approaches. A special focus was devoted to new analytical developments involving a wide variety of analytes (e.g., proteins, living cells, nucleic acids and viruses). The utility of plasmonic-based single-molecule analysis for personalized medicine, considering technological limitations and future prospects, is also overviewed.
... These results processed by the modified SAI show that the reconstructed images beyond the diffraction-limit can be achieved, which is obviously better than conventional SAI methods. Intriguingly, this directly observed self-imaging phenomenon indicates new potentials for emerging deep-learning-assisted imaging, design of physics-based or physics-aware intelligent technologies for computational imaging [48,49], design of customized speckle [50], and will create new opportunities for bio-imaging (e.g., [2,24,25]), super-resolution imaging through thin scattering layers, and other novel imaging methodologies [51]. ...
Imaging through scattering medium is challenging but important for different applications. Most advances rely on computational image reconstruction from scattering signals. In these conventional investigations, speckles were always treated as scrambled grainy patterns. Directly seeing through scattering diffusers has never been realized. Here, we report a new strategy to see through random diffusers directly using self-imaging of speckles. By analyzing the physics, a direct observation strategy through scattering media is reported with improved image quality. Using this method, we experimentally demonstrated reconstruction-free real-time imaging of static and moving objects with their actual orientation information under single-wavelength and white light illumination. We also proposed a modified speckle autocorrelation imaging (SAI) method inspired by the self-imaging results. Importantly, our strategy requires no pre-calibration or acquisition of point-spread-function, no active control of wavefronts or complicated equipment, nor iterations or carefully adjusted parameters, paving the way towards rapid and high-quality imaging through scattering diffusers.
... Two photons of the same frequency coupled in nanostructures result in a single-photon emission with twice the fundamental frequency. System ground state information and pump light polarization are recorded via SHG signals, which have been used extensively in lasers [1,2], holographic images [3], and measurements of ultrashort-width pulses [4,5], as well as medical imaging applications [6]. In most nonlinear materials, the nonlinear optical conversion is limited due to weak optical responses and light-matter interactions. ...
Plasmonic nanostructures have been regarded as potential candidates for boosting the nonlinear up-conversion rate at the nanoscale level due to their strong near-field enhancement and inherent high design freedom. Here, we design a hybrid metasurface to realize the moderate interaction of Fano resonance and create the dual-resonant mode-matching condition to facilitate the nonlinear process of second harmonic generation (SHG). The hybrid metasurface presents dipolar and octupolar plasmonic modes near the fundamental and doubled-frequency wavelengths, respectively, further utilized to enhance the SHG of low-dimensional MoS 2 semiconductors. The maximum intensity of SHG in hybrid metasurface coupled MoS 2 is more than ten thousand times larger than that of other structure-units coupled MoS 2 . The conversion efficiency is reported to be as high as 3.27 × 10 ⁻⁷ . This work paves the way to optimize nonlinear light–matter interactions in low-dimensional structures coupled with semiconductors.
... Two photons of the same frequency coupled in nanostructures result in a single-photon emission with twice the fundamental frequency. System ground state information and pump light polarization are recorded via SHG signals, which have been used extensively in lasers [1,2], holographic images [3], and measurements of ultrashort-width pulses [4,5] as well as medical imaging [6] applications. In most nonlinear materials, the nonlinear optical conversion is limited due to weak optical responses and light-matter interactions. ...
Plasmonic nanostructures have been regarded as potential candidates for boosting the nonlinear up-conversion rate at nanoscale level due to their strong near-field enhancement and inherent high design freedom. Here, we design a hybrid metasurface to realize the moderate interaction of Fano resonance and create the dual-resonant mode-matching condition to facilitate nonlinear process of second harmonic generation (SHG). The hybrid metasurface presents dipolar and octupolar plasmonic modes near the fundamental and doubled-frequency wavelengths, respectively, which is further utilized to enhance the SHG of low-dimensional MoS 2 semiconductor. The maximum intensity of SHG in hybrid metasurface coupled MoS 2 is more than ten thousand times larger than that of other structure-units coupled MoS 2 , and the conversion efficiency is reported to be as high as 3.27×10 ⁻⁷ . This work paves the way for optimizing nonlinear light-matter interactions in low-dimensional structures coupled semiconductors.
... Two photons of the same frequency coupled in nanostructures result in a single-photon emission with twice the fundamental frequency. System ground state information and pump light polarization are recorded via SHG signals, which have been used extensively in lasers [1,2], holographic images [3], and measurements of ultrashort-width pulses [4,5] as well as medical imaging [6] applications. In most nonlinear materials, the nonlinear optical conversion is limited due to weak optical responses and light-matter interactions. ...
Plasmonic nanostructures have been regarded as potential candidates for boosting the nonlinear up-conversion rate at the nanoscale level due to their strong near-field enhancement and inherent high design freedom. Here, we design a hybrid metasurface to realize the moderate interaction of Fano resonance and create the dual-resonant mode-matching condition to facilitate the nonlinear process of second harmonic generation (SHG). The hybrid metasurface presents dipolar and octupolar plasmonic modes near the fundamental and doubled-frequency wavelengths, respectively, further utilized to enhance the SHG of low-dimensional MoS 2 semiconductors. The maximum intensity of SHG in hybrid metasurface coupled MoS 2 is more than ten thousand times larger than that of other structure-units coupled MoS 2 . The conversion efficiency is reported to be as high as 3.27×10 − 7 . This work paves the way to optimize nonlinear light-matter interactions in low-dimensional structures coupled with semiconductors.
... Surface and interface molecules have less or different neighbors and more or different degrees of freedom than bulk molecules. Surface molecules are also reported to exhibit different molecular orientations compared to bulk molecules Sahu et al., 2019). In addition, the effects of solid-glass and solid-liquid on crystallization and dissolution have attracted great interest. ...
Amorphous pharmaceutical solids (APS) are single- or multi-component systems in which drugs exist in high-energy states with long-range disordered molecular packing. APSs have become one of the most effective and widely used pharmaceutical delivery approaches for poorly water-soluble drugs in the last several decades. Considerable efforts have been made to investigate the physical stability and dissolution behaviors of APSs, however, the underlying mechanisms remain imperfectly understood. Recent studies reveal that surface and interface properties of APSs could strongly affect the physical stability and dissolution behaviors. This paper provides a comprehensive overview of recent studies focusing on the physical stability and dissolution behaviors of APSs from both surface and interface perspectives. We highlight the role of surface or interface properties in nucleation, crystal growth, phase separation, dissolution, and supersaturation. Meanwhile, the challenges and scope of research on surface and interface properties in the future are also briefly discussed. This review contributes to a better understanding of the surface- and interface-facilitated processes, which will provide more efficient and rational guidance for the design of APSs.
... Second harmonic generation (SHG), another nonlinear optical phenomenon like two-photon luminescence, has been applied in optical information, laser, nanophotonic, and biomedical technology [19,20]. Nevertheless, SHG is not readily used for detection, because the intensity of which remains virtually unchanged under most conditions except for changes in crystal structure or unstable excitation power [21]. ...
Facile and accurate temperature measurement is crucial in daily life, industrial applications, and scientific research. Here, we present a new strategy for physiological temperature sensing based on dual-emission from second harmonic generation (SHG) signal and two-photon luminescence in a dye-loaded nonlinear metal-organic framework (MOF) composite FIR-8⊃DMASM (DMASM = 4-[p-(dimethylamino)styryl]-1-methylpyridinium). Powder X-ray diffraction (PXRD), thermal gravimetric analysis (TGA), infrared absorption spectrum, single-photon luminescence, and the quantum yield indicate that encapsulation not only provides a two-photon luminescence center, but also improves the stability and quantum efficiency of dyes. Temperature-dependent spectroscopy shows that FIR-8⊃DMASM has excellent sensory performance in the physiological temperature region (20–60 °C) with high ralative sensitivity of 1.36–2.98% oC⁻¹. With good water stability and excellent biocompatibility, FIR-8⊃DMASM can be potentially applied in biological temperature sensing.
... Some of the widely used nanofabrication techniques for fabrication of Ag nanostructures are; by green synthetic method for antibacterial activity [8], nanoholes by nanoimprint lithography for single-molecule imaging [9], tandom nanodisks by electron beam lithography for colour generation applications [10], armrest nanorods by oblique angle deposition for biomolecular detection [5], nanocubes by the solvothermal method for refractive index sensing applications [11], film-coated monolayer of polystyrene nanospheres for SERS applications [12], tetrahedral nanopyramids by nanosphere lithography for plasmonic applications [13] etc. Even though these methods possess their own merits, nanosphere lithography (NSL) has emerged as a good technique as it is simple, low-cost and possible for large-scale fabrications [14]. ...
In the present work, Silver film over nanosphere surface (AgFON) structures were fabricated on a self-assembled monolayer (SAM) of polystyrene spheres (200 nm) by a simple and cost effective drop-casting followed by thermal evaporation techniques. The thickness of Ag thin film was varied from 20nm to 100nm in a step of 20nm. Field emission scanning electron microscopy (FESEM) revealed that the morphology of AgFON changes from nano island to nanoshells with increasing thickness. Reflection spectra of AgFON of thickness >60 nm exhibited a sharp minimum due to the excitation of cavity mode plasmon. Plasmonic sensing capabilities of AgFON have been investigated with respect its thickness. AgFON of 100nm exhibited a bulk sensitivity of 632.54 nm/RIU while 80 nm showed a sensitivity of 365 nm/RIU towards a thin layer of volatile organic compounds such as ethanol, toluene and isopropyl alcohol. Biological molecules such as urea, creatinine, glucose, melamine and glutathione have been tested with the AgFON. The AgFON displays a good capability of detecting 1mM creatinine in an aqueous solution. A successful attempt has been made to detect the creatinine of >1mM in human urine.
... Our research here intends to exploit the above-reported expertise in low-cost soft lithography to fine-tune the optical properties of metal nanostructure arrays at the nanoscale. Controllable nanometer-scale engineering of plasmonic resonance and EM field localization is a key factor in the realization of new functionalities or for enhancing surface-dependent optical phenomena [45][46][47][48][49]. Proper engineering of localized electric fields on the metal surface [44,50], can be exploited in different technological ambits: optical filters [51,52], the amplification of weak spectroscopy signals such as fluorescence [53,54] and Raman scattering [2,55,56], second-harmonic generation [57], and particularly molecular sensing [58]. ...
Conventional nano-sphere lithography techniques have been extended to the fabrication of highly periodic arrays of sub-wavelength nanoholes in a thin metal film. By combining the dry etching processes of self-assembled monolayers of polystyrene colloids with metal physical deposition, the complete transition from increasing size triangular nanoprism to hexagonally distributed nanoholes array onto thin metal film has been gradually explored. The investigated nano-structured materials exhibit interesting plasmonic properties which can be precisely modulated in a desired optical spectral region. An interesting approach based on optical absorbance measurements has been adopted for rapid and non-invasive inspections of the nano-sphere monolayer after the ion etching process. By enabling an indirect and accurate evaluation of colloid dimensions in a large area, this approach allows the low-cost and reproducible fabrication of plasmonic materials with specifically modulated optical properties suitable for many application in biosensing devices or Raman enhanced effects.
... EM enhancement exhibits a strong distance-dependence feature with analyte, producing the major SERS enhancement at ranges of 10 6 to 10 9 [44]. Narrow gaps (especially 10 nm) between NPs and molecules which are called the "hot spots", are able to cause a great 10 6 -fold enhancement of EM field around the NPs (Fig. 1 D) [42,[45][46][47]. Different from EM, CE is originated from the mutual electronic exchanges when a free molecule adhered to the metal surface, contributing about 10 2 to 10 3 of the overall enhancement [48]. ...
Pseudomonas aeruginosa (P. aeruginosa), a ubiquitous opportunistic pathogen, can frequently cause chronic obstructive pulmonary disease, cystic fibrosis and chronic wounds, and potentially lead to severe morbidity and mortality. Timely and adequate treatment of nosocomial infection in clinic depends on rapid detection and accurate identification of P. aeruginosa and its early-stage antibiotic susceptibility test. Traditional methods like plating culture, polymerase chain reaction, and enzyme-linked immune sorbent assays are time-consuming and require expensive equipment, limiting the rapid diagnostic application. Advanced sensing strategy capable of fast, sensitive and simple detection with low cost has therefore become highly desired in point of care testing (POCT) of nosocomial pathogens. Within this review, advanced detection and sensing strategies for P. aeruginosa cells along with associated quorum sensing (QS) molecules over the last ten years are discussed and summarized. Firstly, the principles of four commonly used sensing strategies including localized surface plasmon resonance (LSPR), surface-enhanced Raman spectroscopy (SERS), electrochemistry, and fluorescence are briefly overviewed. Then, the advancement of the above sensing techniques for P. aeruginosa cells and its QS biomarkers detection are introduced, respectively. In addition, the integration with novel compatible platforms towards clinical application is highlighted in each section. Finally, the current achievements are summarized along with proposed challenges and prospects.
... This approach has achieved nearly singlemolecular-level resolution, such as stimulated emission depletion and photo-activated localization microscopy. [213][214][215][216] However, similar to the fluorescence-activated cell sorting technique, it is seriously limited by the fluorescent labeling process. Furthermore, the photobleaching effect will reduce the signal-to-noise ratio since the number of available photons is limited. ...
Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhancing localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even sub-nanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices which results in a very promising alternative technique for high-throughput single bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turns from a 3D problem to a nearly 2D problem. Meanwhile, the operation zone is limited to a pre-defined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating techniques, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Besides, future developmental perspectives of POT techniques are also discussed.
... 143 The three-dimensional orientation of single molecules can be visualized via plasmonenhanced microscopy that is capable of obtaining spatial resolution of ∼1 nm. 144 Water molecules confined in subnanometer channels can be detected by measuring the changes in the tension of the DNA monolayer in nucleic acid films due to hydration. 145 Gating effects allow for probing of the charging dynamics of a material when the distance between layers is close to the ion diameter. ...
Engineered nanoporous materials have been extensively employed in the environmental field to take advantage of increased surface area and tunable size exclusion. Beyond those benefits, recent studies have uncovered that the confinement of traditional environmental processes within several nanometer pores exerts unique nanoconfinement effects, such as enhanced adsorption capacity, reaction kinetics, and ion selectivity, compared to their analogous processes without spatial confinement. In this review, we provide a systematic discussion covering the current understanding of nanoconfinement effects reported across diverse fields using similar materials and structures as those being explored in environmental technologies. We further abstract the underlying fundamental physical and chemical principles including molecular orientation and rearrangement, reactive center creation, noncovalent binding, and partial desolvation. Finally, we establish connections between promising nanoconfinement observations and traditional environmental processes to identify challenges and opportunities for the development of innovative functional platforms for environmental applications.
... Compared to fluorescence imaging (which involves absorption of photons), SHG imaging leads to minimal phototoxicity and photobleaching as SHG process is due to the induction of polarization and not an absorption mode [30]. Since its first demonstration to image collagen structures [31], SHG microscopy has emerged as a powerful tool to image extracellular matrices (ECMs) and cellular architectures of thicker tissue (several hundreds of microns thick) samples in their natural environments [30,[32][33][34][35][36], to image single molecule [37], to perform super-resolution localization [38] and also in nanophotonics research [39]. Moreover, SHG is label-free, non-invasive, non-destructive, non-absorptive process hence it's a non-photobleaching imaging tool with high 3D depth of penetration for imaging thick tissue specimens [32,[34][35][36]. ...
We utilized collagen specific second harmonic generation (SHG) signatures coupled with correlative immunofluorescence imaging techniques to characterize collagen structural isoforms (type I and type III) in a murine model of myocardial infarction (MI). Tissue samples were imaged over a four week period using SHG, transmitted light microscopy and immunofluorescence imaging using fluorescently-labeled collagen antibodies. The post-mortem cardiac tissue imaging using SHG demonstrated a progressive increase in collagen deposition in the left ventricle (LV) post-MI. We were able to monitor structural morphology and LV remodeling parameters in terms of extent of LV dilation, stiffness and fiber dimensions in the infarcted myocardium.
... [78][79][80][81][82][83] Recently, it has been demonstrated that plasmon enhanced SHG imaging is capable of single molecule resolution, providing insights into molecule position and orientation with implications of future application to imaging single molecules within nanosystems. 84 Another recent development involves the use of dark field SHG microscopy to image one dimensional defects in grain boundaries of atomically thin monolayer transition metal dichalcogenides. Dark field SHG microscopy uses a spatial filter to reduce the SHG signal originating from homogeneous regions (bulk noncentrosymmetric regions) while improving the spatial selectivity and contrast at the grain boundaries as shown in Figure 5. 85 This technique shows promise in the advancement of the characterization and imaging of nanoscale material interfaces and their defects. ...
Nonlinear microscopy has enabled additional modalities for chemical contrast, deep penetration into biological tissues, and the ability to collect dynamics on ultrafast timescales across heterogeneous samples. The additional light fields introduced to a sample offer seemingly endless possibilities for variation to optimize and customize experimentation and the extraction of physical insight. This perspective highlights three areas of growth in this diverse field: the collection of information across multiple timescales, the selective imaging of interfacial chemistry, and the exploitation of quantum behavior for future imaging directions. Future innovations will leverage the work of the studies reviewed here as well as address the current challenges presented.
... A preliminary study on engineered nanoantennas demonstrated that SHG-based plasmonic sensing can be at least as sensitive as the linear one despite the extremely low SHG yield . The current flourishing of plasmon-enhanced SHG sensing is testified by its recent application to the detection and orientation assessment of single molecules in nanohole arrays (Sahu et al., 2019) and for the successful detection of picomolar concentrations of mercury in blood (Verma et al., 2020). These first studies report sensitivities that only partially outperform those of the linear counterpart. ...
Nonlinear photon conversion is a fundamental physical process that lies on the basis of many modern disciplines, from bioimaging and theranostics in nanomedicine to material characterization in materials science and nanotechnology. It also holds great promise in laser physics with applications in information technology for optical signal processing and in the development of novel coherent light sources. The capability to efficiently generate harmonics at the nanoscale will have an enormous impact on all these fields, since it would allow one to realize much more compact devices and to interrogate matter in extremely confined volumes. Here, we present a perspective on the most recent advances in the generation of nonlinear optical processes at the nanoscale and their applications, proposing a palette of future perspectives that range from material characterization and the development of novel compact platforms for efficient photon conversion to bioimaging and sensing.
... Metallic nanohole arrays (NHAs) have attracted increasing attention after Ebbesen's observation of the extraordinary optical transmission 1 assigned to surface plasmon-mediated light tunneling through periodically arranged subwavelength pores. Subsequently, NHA structures have been employed in diverse application areas including optical filters, 2,3 amplification of weak spectroscopy signals such as fluorescence 4,5 and Raman scattering, [6][7][8][9] second-harmonic generation, 10 and particularly sensing. To date, NHA-based sensors have been utilized for the direct optical probing of proteins, 11,[12][13][14] exosomes, 15 viruses, 16,17 bacteria, 18 and even cancer cells 19,20 and organelles. ...
We report on new approach to rapidly actuate plasmonic characteristics of thin gold film perforated with nanohole arrays that are coupled with arrays of gold nanoparticles. The near-field interaction between localized and propagating surface plasmon modes supported by the structure is actively modulated by changing the distance between nanoholes and nanoparticles and by variations in refractive index symmetry of the structure. It is utilized by the use of thin responsive hydrogel cushion that is allowed to swell and collapse by a temperature stimulus. A detailed experimental study of changes and interplay of localized and propagating surface plasmons is complemented by numerical simulations. We demonstrate that the interrogation of resonant optical excitation to these modes allows for the label-free SPR observation of binding of biomolecules and that it is applicable for in situ SERS studies of low molecular weight molecules attached in the gap between nanoholes and nanoparticles.
In this study, a novel approach for fabricating silver nanohole arrays (AgNAs) on silicon/hyperbolic metamaterial (Si/HMM) substrates is described. This approach involves the use of fixed-size plastic nanoparticles combined with reactive-ion etching, followed by decoration with silver nanoparticles (AgNPs) around the edges of the Ag nanoholes. The HMM structure is composed of six pairs of alternating layers of silver and silicon dioxide (Ag/SiO2). Herein, the interaction of the localized surface plasmon resonance (LSPR) is investigated by the analysis of photoluminescence (PL) responses of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) molecules over the AgNAs on the Si/HMM substrates. Both configurations, i.e., with and without ring-shaped AgNPs, are investigated extensively by a combination of experimental measurements and numerical simulations. The substantially enhanced PL for the proposed substrates can be attributed to increased light absorption, leading to the fine-tuning of surface-enhanced electromagnetic fields. Notably, the PL intensity of DCJTB is significantly enhanced (16.4 times) in comparison with that of the reference substrate. Concurrently, the PL lifetime is reduced by 49.45% (0.91 ns–0.46 ns). These results underscore the transformative potential of these enhanced substrates in reshaping the landscape of sensing platforms. The enhanced PL response coupled with the precisely tunable LSPR effects demonstrates promise for innovative applications in optical and photonic technologies.
In this review, the basic principles of plasmon-enhanced SHG, different methods to enhance the SHG intensity of metal nanostructures, and related applications of SHG based on metal nanostructures are introduced.
In this work, we developed a theoretical model for second-harmonic generation using Jones vectors and matrix calculus. The model was built taking alpha-quartz as a sample within the framework of polarization-resolved second-harmonic generation experiment. An expression for the second-harmonic intensity was thus obtained. The origin of this intensity was discussed and simulations were performed for different values of the incident laser beam polarization. This theoretical development provides new insights into the nonlinear optical phenomenon of second-harmonic generation in alpha-quartz using Jones matrix formalism, revealing the relationship between incident polarization and resulting second-harmonic intensity. The model and simulations enhance understanding of the role of polarization in second-harmonic generation. This study paves the way for developing more comprehensive theoretical models, accounting for other experimental parameters such as wavelength dependence or influence of temperature. Ultimately, this approach could help optimize devices leveraging second-harmonic generation, with potential applications in nonlinear optics and photonics.
Metasurfaces, composed of artificial meta-atoms of subwavelength size, can support strong light–matter interaction based on multipolar resonances and plasmonics, hence offering the great capability of empowering nonlinear generation. Recently, owing to their ability to manipulate the amplitude and phase of the nonlinear emission in the subwavelength scale, metasurfaces have been recognized as ultra-compact, flat optical components for a vast range of applications, including nonlinear imaging, quantum light sources, and ultrasensitive sensing. This review focuses on the recent progress on nonlinear metasurfaces for those applications. The principles and advances of metasurfaces-based techniques for image generation, including image encoding, holography, and metalens, are investigated and presented. Additionally, the overview and development of spontaneous photon pair generation from metasurfaces are demonstrated and discussed, focusing on the aspects of photon pair generation rate and entanglement of photon pairs. The recent blossoming of the nonlinear metasurfaces field has triggered growing interest to explore its ability to efficiently up-convert infrared images of arbitrary objects to visible images and achieve spontaneous parametric down-conversion. This recently emerged direction holds promising potential for the next-generation technology in night-vision, quantum computing, and biosensing fields.
Second-harmonic generation (SHG) is a noninvasive imaging technique that enables the exploration of physiological structures without the use of an exogenous label. However, traditional SHG imaging is limited by optical diffraction, which restricts the spatial resolution. To break this limitation, we developed a novel approach called multifocal structured illumination microscopy-SHG (MSIM-SHG). By combination of SHG with MSIM, SHG-based super-resolution imaging of material molecules can be achieved, and this SHG super-resolution imaging has a wide range of applications for biological tissues and cells. MSIM-SHG achieved a lateral full width at half-maximum (fwhm) of 147 ± 13 nm and an axial fwhm of 493 ± 47 nm by imaging zinc oxide (ZnO) particles. Furthermore, MSIM-SHG was utilized to quantify collagen fiber alignment in various tissues such as the ovary, muscle, heart, kidney, and cartilage, demonstrating its feasibility for identifying collagen characteristics. MSIM-SHG has potential as a powerful tool for clinical diagnosis and biological research.
Plasmonic tweezers based on periodic nanostructures have been used to manipulate particles through multiple and uniform local surface plasmon (LSP) fields. However, the coverage area of periodic nanostructures is limited, which restricts the range of trapping and manipulation. In this paper, we present a novel approach to achieve large-scale manipulation and trapping of microspheres by uniformly coupled LSP fields on a short-range disordered self-assembled Ag nanoplates (DSNP) film. The DSNP film is prepared by simple and low-cost methods─chemical growth and self-assembly technique, which overcome the challenges of preparing periodic nanostructures with a large coverage area. The uniform and coupled plasmon fields generated by this film provide enhanced electrodynamic interactions with particles, enabling the non-invasive and repeatable trapping of particles in solution. Utilizing sensitive LSPRs, dynamic manipulating particles was achieved by controlling the laser position. This large-scale platform of stable manipulation enabled by the DSNP film opens up new possibilities for the trapping and manipulation of nanoparticles in a variety of applications.
Single‐emitter luminescence is attractive for various applications, including single‐molecular biology, super‐resolution optical imaging, and single‐photon sources. Optical trapping as a particle positioning method is promising for enhancing luminescence by a precise matching of the nanoemitter to the sub‐wavelength mode volume of the photonic structure. Simultaneously satisfying the requirement of both luminescence and trapping enhancement, however, is challenging. Here, stable optical trapping and the in situ excitation of highly bright photoluminescence of a single up‐conversion nanoparticle (UCNP) are demonstrated. A quad‐nanohole structure with two bright‐field modes for enhanced excitation and emission of the UCNP, as well as a dark‐field mode for enhanced optical trapping is designed. It is experimentally demonstrated that the photoluminescence of the UCNP optically trapped in the quad‐nanohole is enhanced by a factor of 87 (as compared to a UCNP trapped in the single nanohole), and this enhancement factor surpasses that from a bowtie nanohole—one of the “gold standards” for local field enhancement. This work provides a route to assemble super‐bright single‐emitter luminescence by optical trapping.
Artificially engineered all-dielectric configurations have received growing attention in the past decades owing to their unique properties in facilitating low-loss and efficient manipulation of electromagnetic radiations. Using the concept of all-dielectric metamaterials, in this study, we demonstrate that a metasurface consisting of all-dielectric unit cells based on a combination of bar and ring resonators efficiently augments the emission of nonlinear harmonics through the use of the optically driven electromagnetically induced transparency (EIT) mode. Initially, we discuss and present the spectral response of the system in the linear regime to judiciously tailor the geometrics of the proposed meta-atom. By taking advantage of the effective third-order nonlinear susceptibility tensors of the entire dielectric metasurface, as well as the resonant EIT feature in the near-infrared wavelengths, we showed that the designed platform facilitates the emission of visible light. The formation of robust near-field enhancements within the unit cell enabled the emission of the third harmonic beam with a high conversion efficiency of 1.75 × 10
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%. Compared to the analogous studies based on nonlinear dielectric meta-optics (i.e., Mie resonators), the proposed configuration facilitates substantially high conversion efficiency and low loss operation. We envision that this understanding paves the way toward the use of asymmetric lineshapes and all-dielectric metasurfaces in the development of coherent light sources and intense lasers.
Metal nanohole arrays show excellent performance when applied for sensing, optical fibers, and surface-enhanced spectroscopy, but they are not ideal candidates for surface-enhanced coherent anti-Stokes Raman scattering (SECARS) because of their low enhancement factor (EF). Here, the finite element method was used to study the dependence of the period, width, and thickness of nanoslits on the EF of SECARS and optical transmission in Au nanohole-slit arrays. Nanoslits across the nanoholes significantly modulated the SECARS signal, and we observed an ∼106 improvement in the EF of SECARS compared with the nanohole-only structure. Uniform and stable 2D hotspots at the open surface of plasmonic nanohole-slit structures provided a huge SECARS EF as high as 18 orders of magnitude. Directional SECARS emission revealed strong forward and backscattering with high directionality, showing a smaller divergence angle of 14° on the reflective side of the nanohole-slit array. These results provide a fundamental understanding of SECARS in coupled nanohole-slit arrays and are useful for designing a SECARS platform with high sensitivity.
As a fundamental security problem, counterfeits pose a tremendous threat to public health and social economy. Herein, we exploit multi-functional nanoinks made of one-dimensional silicon-based nanohybrids for constructing fluorescent and plasmonic security tags. Of particular significance, the presented security solution exhibits triple-layer authentication model, simultaneously featuring the advantages of physical unclonable functions (PUFs), huge-encoding capacity algorithm and artificial intelligence technique. In macroscale, the multi-color fluorescence security signals are used as the first layer, which can be verified through portable smartphone. In the second security layer, the unclonable surface-enhanced Raman scattering (SERS) security signals at low-level magnification could be visualized using confocal Raman system. Taking advantages of coarse grained and quaternary encrypting of signals from Raman at each pixel, the encoding capacity reaches 6.43 × 10²⁴⁰⁸², which is much higher than the value (i.e., 3 × 10¹⁵⁰⁵¹) ever reported. In the third layer, the aggregated SERS signals at high-level magnification Raman mapping produce unrepeatable patterns with shape-specific information. By further applying specifically artificial intelligence (AI), faint features of different SERS images are extracted and trained, allowing 98–100% of recognition accuracy after 1000 learning cycles. Such triple-layer security solution ensures the PUFs, huge encoding capacity and AI authentication simultaneously, providing newly high-performance platform of unbreakable anti-counterfeiting.
All‐optical control of magnetization vectors is considered as a powerful solution that empowers the development of multifunctional integrated optomagnetic devices. The interaction between vectorial light and magnetism has recently experienced a surge of interest due to its prominent ability to steer the magnetic polarization orientations and spatial textures within the subwavelength domain, which, however, usually requires complex wavefront coding optimizers or arduous material fabrication procedures. Herein, an optimization‐free all‐vectorial‐optical strategy for first realizing spatially twist‐controllable and successively polarization‐tunable magnetization at the subdiffraction scale through the inverse Faraday effect is conceived and demonstrated. This facile method relies on the coherent coupling of double crossed azimuthally polarized doughnut‐Gaussian vortex beams in a single optically configured geometry. It is found that both the magnetic twisting manifolds and polarization states, rotating from perpendicular (longitudinal), via hybrid (3D) to in‐plane (transverse) orientations, can be on‐demand mediated by judiciously tuning the crossing angles. It is further unraveled that the orbital angular momenta associated with topological charges of the vortex beams can be viewed as a vital knob to energetically maneuver magnetic twisting and guide magnetization switching. It is believed that the proposed route and presented findings are not only of great theoretical implication in twistronics and optomagnetic topology but also of considerable technological significance in multidimensional high‐density and low‐energy consumption optomagnetic storage and engineering of magnetic topologic materials.
Second-harmonic generation (SHG) is a kind of nonlinear optical phenomenon which has been widely used in optical devices, and factors influencing its signal are very complex. Here, taking advantage of excellent structural designability and overcoming the limitations of various coordinations of lanthanide metals, for the first time a series of lanthanide metal-organic frameworks (Ln-MOFs) with one particular ligand were synthesized and structurally characterized to study the interference of the SHG signal. The optical performance including single-photon fluorescence and SHG was collected and analyzed. It is found that all 13 kinds of Ln-MOFs can be divided into 2 crystal configurations by their individual space groups and Ln-MOFs with coordinated metal atoms from La to Tb possessing the noncentrosymmetric C2 space group exhibit the SHG property, the intensity of which depends on the type of metal atoms, the pumping wavelength, and the size of the single-crystal particles. This is the first time that the relationship between the nonlinear optical properties and the structure, metal atoms, pumping wavelength, crystal size of the whole series of Ln-MOFs is studied systematically, providing a lot of interesting results and enriching the research scope of nonlinear optics and materials science.
It has been recently suggested that the nonlinear optical processes in plasmonic nanoantennas allow for a substantial boost in the sensitivity of plasmonic sensing platforms. Here we present a sensing device based on an array of non-centrosymmetric plasmonic nanoantennas featuring enhanced second harmonic generation (SHG) integrated in a microfluidic chip. We evaluate its sensitivity both in the linear and nonlinear regime using a figure of merit (FOM = $(\Delta I/I)/\Delta n$) that accounts for the relative change in the measured intensity, \textit{I}, against the variation of the environmental refractive index \textit{n}. While the signal-to-noise ratio achieved in both regimes allows the detection of a minimum refractive index variation $\Delta n_{min} \approx 10^{-3}$, the platform operation in the nonlinear regime features a sensitivity (i.e. the FOM) that is at least 3 times higher than the linear one. Thanks to the surface sensitivity of plasmon-enhanced SHG, our results show that the development of such SHG sensing platforms with sensitivity performances exceeding those of their linear counterparts is within reach.
We numerically design and experimentally test a SERS-active substrate for enhancing the SERS signal of a single layer of graphene (SLG) in water. The SLG is placed on top of an array of silver-covered nanoholes in a polymer and is covered with water. Here we report a large enhancement of up to 2 × 10⁵ in the SERS signal of the SLG on the patterned plasmonic nanostructure for a 532 nm excitation laser wavelength. We provide a detailed study of the light-graphene interactions by investigating the optical absorption in the SLG, the density of optical states at the location of the SLG, and the extraction efficiency of the SERS signal of the SLG. Our numerical calculations of both the excitation field and the emission rate enhancements support the experimental results. We find that the enhancement is due to the increase in the confinement of electromagnetic fields on the location of the SLG that results in enhanced light absorption in the graphene at the excitation wavelength. We also find that water droplets increase the density of optical radiative states at the location of the SLG, leading to enhanced spontaneous emission rate of graphene at its Raman emission wavelengths.
Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs. Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.
A stable nonlinear optical point light source is investigated, based on field enhancement at individual, pointed gold nanocones with sub-wavelength dimensions. Exciting these cones with near-infrared, focused radially polarized femtosecond beams allows for tip-emission at the second harmonic wavelength (second harmonic generation, SHG) in the visible range. In fact, gold nanocones with ultra-sharp tips possess interesting nonlinear optical (NLO) properties for SHG and two-photon photoluminescence (TPPL) emission, due to the enhanced electric field confinement at the tip apex combined with centrosymmetry breaking. Using two complementary optical setups for bottom or top illumination a sharp tip SHG emission is discriminated from the broad TPPL background continuum. Moreover, comparing the experiments with theoretical calculations manifests that these NLO signatures originate either from the tip apex or the base edge of the nanocones, clearly depending on the cone size, the surrounding medium, and illumination conditions. Finally, it is demonstrated that the tip-emitted signal vanishes when switching from radial to azimuthal polarization.
The ability to convert low-energy quanta into a quantum of higher energy is of great interest for a variety of applications, including bioimaging, drug delivery and photovoltaics. Although high conversion efficiencies can be achieved using macroscopic nonlinear crystals, upconverting light at the nanometre scale remains challenging because the subwavelength scale of materials prevents the exploitation of phase-matching processes. Light-plasmon interactions that occur in nanostructured noble metals have offered alternative opportunities for nonlinear upconversion of infrared light, but conversion efficiency rates remain extremely low due to the weak penetration of the exciting fields into the metal. Here, we show that third-harmonic generation from an individual semiconductor indium tin oxide nanoparticle is significantly enhanced when coupled within a plasmonic gold dimer. The plasmonic dimer acts as a receiving optical antenna, confining the incident far-field radiation into a near field localized at its gap; the indium tin oxide nanoparticle located at the plasmonic dimer gap acts as a localized nonlinear transmitter upconverting three incident photons at frequency ω into a photon at frequency 3ω. This hybrid nanodevice provides third-harmonic-generation enhancements of up to 10(6)-fold compared with an isolated indium tin oxide nanoparticle, with an effective third-order susceptibility up to 3.5 × 10(3) nm V(-2) and conversion efficiency of 0.0007%. We also show that the upconverted third-harmonic emission can be exploited to probe the near-field intensity at the plasmonic dimer gap.
When light interacts with metal nanostructures, it can couple to
free-electron excitations near the metal surface. The electromagnetic
resonances associated with these surface plasmons depend on the details
of the nanostructure, opening up opportunities for controlling light
confinement on the nanoscale. The resulting strong electromagnetic
fields allow weak nonlinear processes, which depend superlinearly on the
local field, to be significantly enhanced. In addition to providing
enhanced nonlinear effects with ultrafast response times, plasmonic
nanostructures allow nonlinear optical components to be scaled down in
size. In this Review, we discuss the principles of nonlinear plasmonic
effects and present an overview of their main applications, including
frequency conversion, switching and modulation of optical signals, and
soliton effects.
Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate - nanoscale - range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.
We report the fluorescence lifetime imaging and quantum yield measurement of five different fluorescence dyes spanning different quantum yield and excitation wavelength ranges in solution as well as on irregular nanoplasmonic substrate surface. Due to a distribution of dye molecules at random distances and orientation to the metal nanoplasmonic structure, the dyes showed multi-component lifetime decays on the surface. We have simulated the distribution of lifetime on the surface based on fractional intensity relative to steady-state value and derived an average lifetime with species fraction. From the quantum yield and fluorescence lifetime measurements we calculated the modified radiative and non-radiative decay rates for the dyes due to energy coupling on the substrate. We measured up to 100 fold fluorescence enhancement on nanoplasmonic substrate, and all molecule fluorescence showed not only considerably higher radiative decay rate but also higher non-radiative decay rate.
We demonstrate a new method of determining the three-dimensional dipole orientations of single molecules by direct imaging of the emission patterns in the back focal plane of a high-numerical-aperture objective lens. We compare the reconstructed emission-dipole orientations with a previously established method of absorption-dipole mapping. We find that, for a given number of emitted photons, emission pattern imaging provides better accuracy (1°–2°) than absorption-dipole mapping of single molecules. Compared with some other methods for emission-dipole mapping, the presented method is (1) less sensitive to optical aberrations and adjustment and (2) data analysis is simplified because radiation patterns can be expressed in a simple analytical form.
Second-harmonic generation (SHG) through a proposed thin gold film with a periodic array of subwavelength nanoholes is numerically investigated. By using a recently developed microscopic classical theory and a full-vectorial three-dimensional finite-difference time-domain method, we demonstrate that the mirror symmetry of nanoholes in one direction restricts the polarization state of second-harmonic emission in the same direction. Numerical results show that the second-order nonlinear susceptibility
χ
(
2
)
y
y
y
dominates in the process of SHG when the nanoholes possess mirror symmetry in the x-axis direction. It is also found that the surface plasmon resonance can result in the enhancement of SHG from metallic nanoholes.
Single-molecule fluorescence techniques are key for a number of applications, including DNA sequencing, molecular and cell biology and early diagnosis. Unfortunately, observation of single molecules by diffraction-limited optics is restricted to detection volumes in the femtolitre range and requires pico- or nanomolar concentrations, far below the micromolar range where most biological reactions occur. This limitation can be overcome using plasmonic nanostructures, which enable the confinement of light down to nanoscale volumes. Although these nanoantennas enhance fluorescence brightness, large background signals and/or unspecific binding to the metallic surface have hampered the detection of individual fluorescent molecules in solution at high concentrations. Here we introduce a novel 'antenna-in-box' platform that is based on a gap-antenna inside a nanoaperture. This design combines fluorescent signal enhancement and background screening, offering high single-molecule sensitivity (fluorescence enhancement up to 1,100-fold and microsecond transit times) at micromolar sample concentrations and zeptolitre-range detection volumes. The antenna-in-box device can be optimized for single-molecule fluorescence studies at physiologically relevant concentrations, as we demonstrate using various biomolecules.
Recently, single molecule-based superresolution fluorescence microscopy has surpassed the diffraction limit to improve resolution to the order of 20 nm or better. These methods typically use image fitting that assumes an isotropic emission pattern from the single emitters as well as control of the emitter concentration. However, anisotropic single-molecule emission patterns arise from the transition dipole when it is rotationally immobile, depending highly on the molecule's 3D orientation and z position. Failure to account for this fact can lead to significant lateral (x, y) mislocalizations (up to ∼50-200 nm). This systematic error can cause distortions in the reconstructed images, which can translate into degraded resolution. Using parameters uniquely inherent in the double-lobed nature of the Double-Helix Point Spread Function, we account for such mislocalizations and simultaneously measure 3D molecular orientation and 3D position. Mislocalizations during an axial scan of a single molecule manifest themselves as an apparent lateral shift in its position, which causes the standard deviation (SD) of its lateral position to appear larger than the SD expected from photon shot noise. By correcting each localization based on an estimated orientation, we are able to improve SDs in lateral localization from ∼2× worse than photon-limited precision (48 vs. 25 nm) to within 5 nm of photon-limited precision. Furthermore, by averaging many estimations of orientation over different depths, we are able to improve from a lateral SD of 116 (∼4× worse than the photon-limited precision; 28 nm) to 34 nm (within 6 nm of the photon limit).
Optical experiments on second-harmonic generation from split-ring-resonator square arrays show a nonmonotonic dependence of the conversion efficiency on the lattice constant. This finding is interpreted in terms of a competition between dilution effects and linewidth or near-field changes due to interactions among the individual elements in the array.
We present a novel plasmonic antenna geometry – the double resonant antenna (DRA) – that is optimized for second-harmonic generation (SHG). This antenna is based on two gaps coupled to each other so that a resonance at the fundamental and at the doubled frequency is obtained. Furthermore, the proximity of the localized hot spots allows for a coupling and spatial overlap between the two field enhancements at both frequencies. Using such a structure, both the generation of the second-harmonic and its re-emission into the far-field are significantly increased when compared with a standard plasmonic dipole antenna. Such DRA are fabricated in aluminium using electron beam lithography and their linear and nonlinear responses are studied experimentally and theoretically.
While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems.
First published in 2006, this book has become the standard reference on nano-optics. Now in its second edition, the text has been thoroughly updated to take into account new developments and research directions. While the overall structure and pedagogical style of the book remain unchanged, all existing chapters have been expanded and a new chapter has been added. Adopting a broad perspective, the authors provide a detailed overview of the theoretical and experimental concepts that are needed to understand and work in nano-optics, across subfields ranging from quantum optics to biophysics. New topics of discussion include: optical antennas; new imaging techniques; Fano interference and strong coupling; reciprocity; metamaterials; and cavity optomechanics. With numerous end-of-chapter problem sets and illustrative material to expand on ideas discussed in the main text, this is an ideal textbook for graduate students entering the field. It is also a valuable reference for researchers and course teachers.
Stimulated emission depletion (STED) microscopy provides subdiffraction resolution while preserving useful aspects of fluorescence microscopy, such as optical sectioning, and molecular specificity and sensitivity. However, sophisticated microscopy architectures and high illumination intensities have limited STED microscopy's widespread use in the past. Here we summarize the progress that is mitigating these problems and giving substantial momentum to STED microscopy applications. We discuss the future of this method in regard to spatiotemporal limits, live-cell imaging and combination with spectroscopy. Advances in these areas may elevate STED microscopy to a standard method for imaging in the life sciences.
We show enhanced second-harmonic generation (SHG) from a hybrid metal-dielectric nanodimer consisting of an inorganic perovskite nanoparticle of barium titanate (BaTiO3) coupled to a metallic gold (Au) nanoparticle. BaTiO3-Au nanodimers of 100 nm / 80 nm sizes are fabricated by sequential capillarity-assisted particle assembly. The BaTiO3 nanoparticle has a non-centrosymmetric crystalline structure and generates bulk SHG. We use the localized surface plasmon resonance of the gold nanoparticle to enhance the SHG from the BaTiO3 nanoparticle. We experimentally measure the nonlinear signal from assembled nanodimers and demonstrate an up to 15-fold enhancement compared to a single BaTiO3 nanoparticle. We further perform numerical simulations of the linear and SHG spectra of the BaTiO3-Au dimer and show that the gold nanoparticle acts as a nanoantenna at the SHG wavelength.
Fluorescence spectroscopy with strong emitters is a remarkable tool with ultra-high sensitivity for detection and imaging down to the single-molecule level. Plasmon-enhanced fluorescence (PEF) not only offers enhanced emissions and decreased lifetimes, but also allows an expansion of the field of fluorescence by incorporating weak quantum emitters, avoiding photobleaching and providing the opportunity of imaging with resolutions significantly better than the diffraction limit. It also opens the window to a new class of photostable probes by combining metal nanostructures and quantum emitters. In particular, the shell-isolated nanostructure-enhanced fluorescence, an innovative new mode for plasmon-enhanced surface analysis, is included. These new developments are based on the coupling of the fluorophores in their excited states with localized surface plasmons in nanoparticles, where local field enhancement leads to improved brightness of molecular emission and higher detection sensitivity. Here, we review the recent progress in PEF with an emphasis on the mechanism of plasmon enhancement, substrate preparation, and some advanced applications, including an outlook on PEF with high time- and spatially resolved properties.
Herein, we utilize surface-enhanced hyper-Raman scattering (SEHRS) under resonance conditions to probe the adsorbate geometry of rhodamine 6G (R6G) on silver colloids. Our results show resonance SEHRS is highly sensitive to molecular orientation due to non-Condon effects, which do not appear in its linear counterpart surface-enhanced Raman scattering (SERS). Comparisons between simulated and measured SEHRS spectra reveal R6G adsorbs mostly perpendicular to the nanoparticle surface along the ethylamine groups with the xanthene ring oriented edgewise. Our results expand upon previous studies that rely on indirect, qualitative probes of R6G's orientation on plasmonic substrates. More importantly, this work represents the first determination of adsorbate geometry by SEHRS and opens up the possibility to study the orientation of single molecules in complex, plasmonic environments.
Surface second harmonic generation (SHG) is a coherent, nonlinear optical technique that is well suited for investigations of biomolecular interactions at interfaces. SHG is surface specific due to the intrinsic symmetry constraints on the nonlinear process, providing a distinct analytical advantage over linear spectroscopic methods, such as fluorescence and UV-Visible absorbance spectroscopies. SHG has the ability to detect low concentrations of analytes, such as proteins, peptides, and small molecules, due to its high sensitivity, and the second harmonic response can be enhanced through the use of target molecules that are resonant with the incident (ω) and/or second harmonic (2ω) frequencies. This review describes the theoretical background of SHG, and then it discusses its sensitivity, limit of detection, and the implementation of the method. It also encompasses the applications of surface SHG directed at the study of protein-surface, small-molecule-surface, and nanoparticle-membrane interactions, as well as molecular chirality, imaging, and immunoassays. The versatility, high sensitivity, and surface specificity of SHG show great potential for developments in biosensors and bioassays. Expected final online publication date for the Annual Review of Analytical Chemistry Volume 10 is June 12, 2017. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The unique optical properties of arrays of metallic nanoparticles are of great interest for many applications such as in optical data storage, sensing applications, optoelectronic devices or as platforms to increase the detection limit in spectroscopic measurements. Nonlinear optical phenomena can also be altered by metallic nanostructures opening new possible applications. In this work, arrays composed of non-centrosymmetric individual structures with three fold axial symmetry made of gold are designed and fabricated using electron beam lithography. The nonlinear optical properties of these structures are investigated using second-harmonic generation microscopy (SHGM) with a femtosecond excitation source set near the plasmon resonance frequency. Modeling of the electromagnetic field distribution around the metallic structures is performed using the Finite Difference Time Domain (FDTD) method, highlighting the confinement of the SHG signal and its polarization dependence. Polarization-resolved measurements are conducted to correlate the SHG signal with the structure and symmetry of the individual nanostructures. Since both two-photon induced photoluminescence (TPPL) and SHG signals are produced upon excitation of these structures, lifetime measurements are performed to further evaluate the magnitude of these two effects.
The recent progress in laser processing reaches a level where a precise fabrication that overcomes the diffraction limit of the far-field optics can be achieved. Laser processing mediated by enhanced near field is one of the attractive methods to provide highly precise structuring with a simple apparatus. In this review, we describe the fundamentals of the electromagnetic near field in the vicinity of small structures and the application of its specific properties for nanomodification. Theoretical and experimental results on nanoablation based on electromagnetic field enhancement due to plasmon polariton excitation and Mie scattering are discussed. High-throughput nanohole fabrication mediated by arrayed nanospheres is discussed, as the coupling effect of near field is also considered. In addition, recent fabrication techniques and their potential applications in nanopatterning, nanoscale deformation, and biophotonics are discussed.
Plasmonics has emerged as an important research field in nanoscience and nanotechnology. Recently, significant attention has been devoted to the observation and the understanding of nonlinear optical processes in plasmonic nanostructures, giving rise to the new research field called nonlinear plasmonics. This review provides a comprehensive insight into the physical mechanisms of one of these nonlinear optical processes, namely second harmonic generation, with an emphasis on the main differences with the linear response of plasmonic nanostructures. The main applications, ranging from the nonlinear optical characterization of nanostructure shapes to the optimization of laser beams at the nanoscale, are summarized and discussed. Future directions and developments, made possible by the unique combination of SHG surface-sensitivity and field enhancements associated with surface plasmon resonances, are also addressed.
Coupling to metal nanoparticles can increase the fluorescence intensity and photostability of fluorescent probes, and this plasmon-enhanced fluorescence is particularly promising for the dimmer fluorescent proteins common in biological imaging. Here, we measure the intensity distribution of single Cy3.5 dye molecules and mCherry fluorescent proteins one at a time as they adsorb on a conformal surface 4.8-61.0 nm thick over a gold nanorod (NR). The emission intensities for both types of fluorophores depend nonmonotonically on the spacer thickness, and an optimal spacer thickness of ∼10 nm is observed for both fluorophores using two different spacer layer materials. Emission from fluorophores coupled to metal nanoparticles is affected by two competing processes: an enhanced spontaneous decay rate and quenching via nonradiative antenna modes. After averaging over a conformal surface, the product of the simulated enhanced local electric field intensity and the quantum efficiency modification reproduces the experimental 10 nm ideal spacer thickness. Overall, up to a 3.4-fold average enhancement in fluorescence intensity was achieved despite the simple geometry, based on biocompatible, tunable, and economic colloidal gold NRs. This study of the distance dependence of single-molecule plasmon-enhanced fluorescence shows promise for super-resolving cellular membrane proteins naturally positioned above an extracellular substrate.
Attainment of spatial resolutions far below diffraction limits by means of optical methods constitutes a challenging task. Here, we design nonlinear nanorulers that are capable of accomplishing approximately 1 nm resolutions by utilizing the mechanism of plasmon-enhanced second-harmonic generation (PESHG). Through introducing Au@SiO2 (core@shell) shell-isolated nanoparticles, we strive to maneuver electric-field-related gap modes such that a reliable relationship between PESHG responses and gap sizes, represented by "PESHG nanoruler equation", can be obtained. Additionally validated by both experiments and simulations, we have transferred "hot spots" to the film-nanoparticle-gap region, ensuring that retrieved PESHG emissions nearly exclusively originate from this region and are significantly amplified. The PESHG nanoruler can be potentially developed as an ultrasensitive optical method for measuring nanoscale distances with higher spectral accuracies and signal-to-noise ratios.
We study, both experimentally and theoretically, the second-order nonlinear response from resonant metasurfaces composed of metal–dielectric nanodisks. We demonstrate that by exciting the resonant optical modes of the composite nanoparticles we can achieve strong enhancement of the second-harmonic signal from the metasurface. By employing a multipole expansion method for the generated second-harmonic radiation, we show that the observed SHG enhancement is due to the magnetic dipolar and electric quadrupolar second-order nonlinear response of the metasurface.
A bifunctional ultrasensitive nanoplasmonic sensor is demonstrated with combined surface plasmon resonance (SPR) and surface-enhanced Raman spectroscopy (SERS) sensing capabilities. Unlike traditional extraordinary transmission (EOT) devices, nano Lycurgus cup array (nanoLCA) contains a hybrid configuration of periodic quasi-3D nanostructure array and dense sidewall metal nanoparticles within each nanostructure, which enables both refractive index sensing and SERS chemical identification on the same device with high sensitivity. The visible plasmon resonance sensitivity of nanoLCA is measured to be as high as 796 nm/RIU with the figure of merit (FOM) of 12.7 so that the device is applied for colorimetric liquid sensing with an ordinary microscopic system. Moreover, the SERS enhancement of the very same nanoLCA for liquid sample is calculated to be 2.8 × 107, which is the highest among all reported EOT-based SERS devices. The urea concentration detection has been demonstrated to show the complementary rapid colorimetric screening and precise SERS identification functions provided by nanoLCA plasmonic sensor for chemical analysis or biological diagnostics in a resource-limited environment.
Confining photons in the smallest possible volume has long been an objective of the nanophotonics community. In this paper, we propose and demonstrate a three-dimensional (3D) gap-plasmon antenna that enables extreme photon squeezing in a 3D fashion with a modal volume of 1.3 × 10(-7) λ(3) (~ 4 × 10 × 10 nm(3)) and an intensity enhancement of 400,000. A three-dimensionally tapered 4-nm air-gap is formed at the center of a complementary nano-diabolo structure by ion-milling 100-nm-thick gold film along all three dimensions using proximal milling techniques. From a 4-nm-gap antenna, a nonlinear second-harmonic signal more than 27,000-times stronger than that from a 100-nm-gap antenna is observed. In addition, scanning cathode-luminescence images confirm unambiguous photon confinement in a resolution-limited area 20 × 20 nm(2) on top of the nano gap.
Semiconductor nanostructures (e.g. nanowires and nanobelts) hold great promise as sub-wavelength coherent light sources, nonlinear optical frequency converters and all-optical signal processors for optoelectronic applications. However, at such small scales, optical second harmonic generation (SHG) is generally inefficient. Herein, we report on a straight-forward strategy using a thin Au layer to enhance the SHG from a single CdS nanobelt by 3 orders of magnitude. Through detailed experimental and theoretical analysis, we validate that the augmented SHG originates from the mutual intensification of the local fields induced by the plasmonic nanocavity and by the reflections within the CdS Fabry-Pérot resonant cavity in this hybrid semiconductor-metal system. Polarization-dependent SHG measurements can be employed to determine and distinguish the contributions of SH signals from the CdS nanobelt and gold film, respectively. When the thickness of gold film becomes comparable to the skin depth, SHG from the gold film can be clearly observed. Our work demonstrates a facile approach for tuning the nonlinear optical properties of mesoscopic, nanostructured and layered semiconductor materials.
We introduce a plasmonic-semiconductor hybrid nanosystem, consisting of a ZnO nanowire coupled to a gold pentamer oligomer by crossing the hot-spot. It is demonstrated that the hybrid system exhibits a second harmonic (SH) conversion efficiency of ∼3 × 10–5%, which is among the highest values for a nanoscale object at optical frequencies reported so far. The SH intensity was found to be ∼1700 times larger than that from the same nanowire excited outside the hot-spot. Placing high nonlinear susceptibility materials precisely in plasmonic confined-field regions to enhance SH generation opens new perspectives for highly efficient light frequency up-conversion on the nanoscale.
To move nanophotonic devices such as lasers and single-photon sources into the practical realm, a challenging list of requirements must be met, including directional emission(1-5), room-temperature and broadband operation(6-9), high radiative quantum efficiency(1,4) and a large spontaneous emission rate. To achieve these features simultaneously, a platform is needed for which the various decay channels of embedded emitters can be fully understood and controlled. Here, we show that all these device requirements can be satisfied by a film-coupled metal nanocube system with emitters embedded in the dielectric gap region. Fluorescence lifetime measurements on ensembles of emitters reveal spontaneous emission rate enhancements exceeding 1,000 while maintaining high quantum efficiency (>0.5) and directional emission (84% collection efficiency). Using angle-resolved fluorescence measurements, we independently determine the orientations of emission dipoles in the nanoscale gap. Incorporating this information with the three-dimensional spatial distribution of dipoles into full-wave simulations predicts time-resolved emission in excellent agreement with experiments.
In the past decade, the surface plasmon resonance of Ag and Au nanoparticles has been investigated to improve the efficiency of photocatalytic processes. The photocatalytic production of fuels is particularly interesting for its ability to store the sun's energy in chemical bonds that can be released later without producing harmful byproducts. This Feature Article reviews recent work demonstrating plasmon-enhanced photocatalytic water splitting, reduction of CO2 with H2O to form hydrocarbon fuels, and degradation of organic molecules. Focus is placed on several possible mechanisms that have been previously discussed in the literature. A particular emphasis is given to several aspects of these mechanisms that are not fully understood and will require further investigation.
A large-area high-density nanoscale Lycurgus cup array is created with 100 times better sensitivity than any other reported nanoplasmonic device. With this device, nanoplasmonic spectroscopy sensing, for the first time, becomes colorimetric sensing requiring only the naked eye or ordinary visible color photography.
The excited-state dynamics of rhodamine 6G (R6G) has been investigated in aqueous solution using ultrafast transient absorption spectroscopy and at the dodecane/water interface using the femtosecond time-resolved surface second harmonic generation (SSHG) technique. As the R6G concentration exceeds ca. 1 mM in bulk water, both R6G monomers and aggregates are excited to a different extent when using pump pulses at 500 and 530 nm. The excited-state lifetime of the monomers is shortened compared to dilute solutions, because of the occurrence of excitation energy transfer to the aggregates, which themselves decay non-radiatively to the ground state with a ca. 70 ps time constant. At the dodecane/water interface, both monomers and aggregates contribute to the SSHG signal to an extent that depends on the bulk concentration, the pump and probe wavelengths, and the polarization of probe and signal beams. The excited-state lifetime of the monomers at the interface is of the order of a few picoseconds even at bulk concentrations where it is as large as several nanoseconds. This is explained by the relatively high interfacial affinity of R6G that leads to a large interfacial concentration, favoring aggregation and thus rapid excitation energy transfer from monomers to aggregates.
We incorporate dielectric indium tin oxide nanocrystals into the hot-spot of gold nanogap-antennas and perform third harmonic spectroscopy on these hybrid nanostructure arrays. The combined system shows a twofold increase of the radiated third harmonic intensity. In order to identify the origin of the enhanced nonlinear response we perform finite element simulations of the nanostructures, which are in excellent agreement with our measurements. We find that the third harmonic signal enhancement is mainly related to changes in the linear optical properties of the plasmonic antenna resonances, when the ITO nanocrystals are incorporated. Furthermore, the dominant source of the third harmonic is found to be located in the gold volume of the plasmonic antennas.
We report the growth mechanisms for the formation of 40 nm star shape gold nanocrystals using real time TEM and in situ two-photon scattering technique. The overall process consists of several intermediate steps and these are: the nucleation process for the formation and accumulation of nanoseeds, Ostwald ripening process for the formation of nanoflower and intraparticle ripening process for the formation of the star sharp nanoparticles via nanocrowns. We demonstrates that the real time shape evolution of intermediate colloidal nanoparticles and their number density change can be fully accessed during the synthesis of star shaped gold nanoparticles using time dependent in situ TPS experiment.
Sensing lane boundaries is a core capability for advanced automotive functions such as collision warning, collision avoidance and automatic vehicle-guidance. Part I of this study described special image-processing algorithms for the detection of lane boundaries and vehicle tracking, using images from a video camera. Part II of this study describes a new algorithm for detecting lane boundaries using template matching. This technique was selected because of its speed and its ability to include additional knowledge--two characteristics which are required for real-time, on-board vehicle applications. The algorithm has been tested successfully on over 3000 frames of videotape from interstate highways I-75 and I-94.
Size-controlled synthesis of metal nanostructures has opened many new possibilities to design ideal building blocks for future nanodevices. By solution-based method, silver triangular nanoprisms of different sizes (edge length) have been synthesized. First hyper-polarizabilities of silver triangular nanomaterials were measured using hyper-Rayleigh scattering technique. We have shown that both linear and nonlinear optical properties are highly dependent on the size of silver triangular nanoprism. We provide experimental evidence for higher multipolar contribution to NLO response of silver nanoprism.
Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, molecular nanotechnology and biotechnology. Molecular vibrations provide a valuable 'fingerprint' for such identification. Vibrational spectroscopy based on tip-enhanced Raman scattering allows us to access the spectral signals of molecular species very efficiently via the strong localized plasmonic fields produced at the tip apex. However, the best spatial resolution of the tip-enhanced Raman scattering imaging is still limited to 3-15 nanometres, which is not adequate for resolving a single molecule chemically. Here we demonstrate Raman spectral imaging with spatial resolution below one nanometre, resolving the inner structure and surface configuration of a single molecule. This is achieved by spectrally matching the resonance of the nanocavity plasmon to the molecular vibronic transitions, particularly the downward transition responsible for the emission of Raman photons. This matching is made possible by the extremely precise tuning capability provided by scanning tunnelling microscopy. Experimental evidence suggests that the highly confined and broadband nature of the nanocavity plasmon field in the tunnelling gap is essential for ultrahigh-resolution imaging through the generation of an efficient double-resonance enhancement for both Raman excitation and Raman emission. Our technique not only allows for chemical imaging at the single-molecule level, but also offers a new way to study the optical processes and photochemistry of a single molecule.
We report optical second harmonic generation studies of the organic dye molecule rhodamine 6G spin cast on fused silica surfaces. The concentration dependence of the second harmonic response demonstrates oscillatory behavior with a period corresponding to the concentration required for monolayer surface coverage. This behavior reflects the formation of ordered molecular adlayers which persist for approximately five periods. Polarized SHG studies confirm orientational anisotropy of the dye molecules and allow the orientation within adjacent layers to be determined. Optical absorbance measurements of the films indicate the onset of rhodamine 6G aggregate formation at surface coverages of approximately one monolayer. However, the onset of dimer or aggregate fluorescence is observed to occur only at much higher surface coverages, consistent with the loss of orientational order within the adlayers. Our results indicate strong adsorbate−substrate interaction which gives rise to orientational anisotropy within the first molecular layer. Well-defined order within subsequent layers is determined by interlayer adsorbate−adsorbate aggregation and decays on a length scale of several molecular diameters. These results provide a direct measure of the extent of interfacial ordering at the solid/air interface.
Plasmon coupling is known to enhance the two-photon excitation photoluminescence of metal nanoparticles significantly. Here, Au and Ag nanospheres of different sizes were prepared to systematically investigate the effects of particle size on plasmon coupling enhanced two-photon excitation photoluminescence. An oppositely charged polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA), was used to induce the coupling of Au and Ag nanospheres. The two-photon excitation photoluminescence enhancement factor was found to first increase and then decrease with the increasing particle size for both Au and Ag nanospheres. Optimum enhancement factors of 25-fold and 14-fold were obtained for coupled 55-nm Au nanospheres and 50-nm Ag nanospheres, respectively. The coupled Au and Ag nanospheres displayed two-photon action cross sections of up to 9 × 10(4) GM per particle (where 1 GM = 10(-50) cm(4) s/photon). Similar to Ag nanoparticles, Au nanoparticles also displayed large coupling induced enhancement of two-photon excitation photoluminescence. Considering their excellent biocompatibility, high inertness, and easy preparation, Au nanoparticles are expected to find many new applications in two-photon biosensing and bioimaging.
Polarization spectroscopy of single fluorescent molecules is used to probe their rotational dynamics. When a molecule is immobilized on a dry surface, its in-plane dipole orientation is precisely determined by utilizing its transition dipole moment. An angular offset between the absorption and the emission dipoles was detected from a single fluorophore revealing its binding geometry to the surface. In an aqueous environment, DNA-tethered fluorophores display dynamics that are well-described by a hindered rotational diffusion model. A detailed description of the model is given, including calculations to estimate depolarization effects resulting from the high numerical aperture objective used to collect fluorescence photons. Protein-conjugated fluorophores display very distinct dynamics with continuous evolution of the rotational profile, possibly reflecting fluctuations in the polypeptide chain. When protein-conjugated fluorophores are allowed to freely diffuse in solution, it is possible to determine the fluorescence polarization anisotropy of each molecule that traverses the laser beam. The anisotropy values could, in principle, be used to identify conformational states of single molecules without the potential artifacts associated with surface immobilization.
The ultimate detection limit in analytic chemistry and biology is the single molecule. Commonly, fluorescent dye labels or enzymatic amplification are employed. This requires additional labeling of the analyte, which modifies the species under investigation and therefore influences biological processes. Here, we utilize single gold nanoparticles to detect single unlabeled proteins with extremely high temporal resolution. This allows for monitoring the dynamic evolution of a single protein binding event on a millisecond time scale. The technique even resolves equilibrium coverage fluctuations, opening a window into Brownian dynamics of unlabeled macromolecules. Therefore, our method enables the study of protein folding dynamics, protein adsorption processes, and kinetics as well as nonequilibrium soft matter dynamics on the single molecule level.
Here we report the experimental observation of circular dichroism in the second-harmonic field (800-400 nm conversion) generated by self-organized gold nanowire arrays with subwavelength periodicity (160 nm). Such circular dichroism, raised by a nonlinear optical extrinsic chirality, is the evident signature of the sample morphology. It arises from the curvature of the self-assembled wires, producing a lack of symmetry at oblique incidence. The results were compared, both in the optical linear and nonlinear regime, with a reference sample composed of straight wires. Despite the weak extrinsic optical chirality of our samples (not observable by our optical linear measurements), high visibility (more than 50%) was obtained in the second-harmonic generated field.
We present efficient parallel algorithms for image template matching on hypercube SIMD machines of sizes N 2 , N 2 ×M 2 , and N 2 ×K 2 processing elements. For an N×N image and M×M window with N 2 PE ' s, we present a simple optimal parallel algorithm with 0(M 2 +logN) time. Then N 2 ×M 2 PE ' s case is solved in 0(log N) time and the N 2 ×K 2 PE ' s case is solved in 0(M 2 /K 2 +logN) time. We also design a parallel algorithm for the N 2 PE ' s case using constant space/PE which runs in 0(M 2 *log * (M)+logN) time. All these algorithms have superior time performance compared to known results.
We present the use of Au bowtie nanoantenna arrays (BNAs) for highly efficient, multipurpose particle manipulation with unprecedented low input power and low-numerical aperture (NA) focusing. Optical trapping efficiencies measured are up to 20× the efficiencies of conventional high-NA optical traps and are among the highest reported to date. Empirically obtained plasmonic optical trapping "phase diagrams" are introduced to detail the trapping response of the BNAs as a function of input power, wavelength, polarization, particle diameter, and BNA array spacing (number density). Using these diagrams, parameters are chosen, employing strictly the degrees-of-freedom of the input light, to engineer specific trapping tasks including (1) dexterous, single-particle trapping and manipulation, (2) trapping and manipulation of two- and three-dimensional particle clusters, and (3) particle sorting. The use of low input power densities (power and NA) suggests that this bowtie nanoantenna trapping system will be particularly attractive for lab-on-a-chip technology or biological applications aimed at reducing specimen photodamage.
