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(Color online) (a) THz near-field imaging of a small plant leaf that was raster scanned in front of the stationary near-field detector. (b) The approximate position of the scanned area (5 mm × 5 mm) is indicated by the square. Amplitude (c) and phase image (d) plotted at 1:36 THz. Dark colors correspond to low transmission or a large induced phase, respectively. The leaf was scanned with a spatial resolution of 50 m.
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Imaging and sensing applications based on pulsed terahertz radiation have opened new possibilities for scientific and industrial applications. Many exploit the unique features of the terahertz (THz) spectral region, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Because of their diffrac...
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... the measurement the leaf was positioned in a sample holder that was raster scanned in the x and y directions relative to the stationary detector and illuminating THz beam as shown in Fig. 3(a). A 5 mm × 5 mm section of the leave was scanned with a spatial resolution of 50 m. Each spatial pixel of the image corresponds to an individual THz pulse scan and thus contains spectral information over the entire bandwidth covered by our experiment (from ...
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Nano-spectroscopy in the terahertz frequency range remains challenging despite recent technological progress in developing both THz emitter sources and near-field optical microscopy (SNOM). Here, we combine scattering-type SNOM with a free-electron laser light source, to tune into the 1.3–8.5 THz range. A significant portion of this range, namely,...
Citations
... One is to improve the imaging system, and the other is to adopt a super-resolution reconstruction method. At present, some research teams focus on improving the terahertz imaging system to improve the spatial resolution [8][9][10], while other teams propose to obtain a better image 1. ...
Terahertz (THz) waves are widely used in the field of non-destructive testing (NDT). However, terahertz images have issues with limited spatial resolution and fuzzy features because of the constraints of the imaging equipment and imaging algorithms. To solve these problems, we propose a residual generative adversarial network based on enhanced attention (EA), which aims to pay more attention to the reconstruction of textures and details while not influencing the image outlines. Our method successfully recovers detailed texture information from low-resolution images, as demonstrated by experiments on the benchmark datasets Set5 and Set14. To use the network to improve the resolution of terahertz images, we create an image degradation algorithm and a database of terahertz degradation images. Finally, the real reconstruction of terahertz images confirms the effectiveness of our method.
... IEEE Transactions on Terahertz Science and Technology X-ray diffraction technique by measuring and analyzing a simple sample, namely, a NaCl crystal [27]. Many attempts to develop and improve THz imaging method using samples such as plant tissue and cells have been advanced to act as the appropriately preliminary models: in 1995 Hu and Nuss demonstrated the first far-field THz-TDS imaging system by measuring the water distribution within a leaf (a very simple sample system) [28]; in 2010, the Walther group demonstrated the capabilities with subwavelength spatial resolution of the THz near-field microscopy based on photoconductive antennas, by detecting a dielectric (plant leaf) and a metallic structure (microwire) [29]; and in 2020 our group first realized the single-cell near-field THz spectral imaging by investigating a watermelon pulp cell [15]. In order to test and verify THz imaging based in situ cell detection method, the onion epidermal cells were used as model objects in the current work. ...
The photoconductive antenna microprobe-based terahertz (THz) near-field scanning microscope has proven to overcome the diffraction limit and demonstrated the potential in distinguishing cellular types and states. Here, using the onion epidermal cells as a sample model, detection of single epithelial cell in the onion skin has been realized at amplitude signal around 1 THz. The boundaries of the target cells in a monolayer onion epidermis are clearly depicted with the spatial resolution of 9
μ
m (λ/33). Differentiated THz dielectric response shows clearly the distinctive intracellular and intercellular regions, mainly related to the biowater distribution and cellular heterogeneity, which can be used for
in situ
subcellular identification
.
This finding may have significant implications for the advancement of THz near-field high-resolution bioimaging technology.
... Other techniques involve conversion between terahertz and optical waves to exploit the higher spatial resolution offered by shorter wavelengths. Specifically, a sample can be positioned near to a terahertz source where an emerging terahertz wave converted from an optical excitation is deeply subwavelength 10 . Common terahertz sources for this purpose include an electro-optic crystal and a photoconductive antenna. ...
Two plasma filaments crossing above the target create a subwavelength window for terahertz microscopy that excludes any subwavelength probe in vicinity.
... The resolution reached λ/12 (130 µm) at 0.36 THz for a parthenocissus leaf and a Laurus nobilis leaf[111]. Bitzer et al. obtained a 50 µm resolution for a frequency of 1.36 THz[112]. ...
Terahertz radiation is located in the electromagnetic range between far infrared and microwaves, corresponding to frequencies between 0.1 and 10 THz. This spectral range is currently largely under-exploited, but its application to the study of biological objects has already shown a strong potential, in the detection of skin cancer, ion flow monitoring or biosensors. In the field of biology, which is of particular interest to us here, the terahertz range makes it possible to quantify and discriminate solutes of biological interest thanks to the interaction with low-frequency modes of liquid water, and thus to study biomolecules, microorganisms and cells in their physiological environment. The first part of this thesis work consisted in studying the dynamics of membrane permeabilization of living cells by attenuated total reflection (ATR) with our device based on a femtosecond laser and the generation of ultrashort terahertz pulses. Monolayers of MDCK epithelial cells were exposed to varying concentrations of saponin, a detergent that digs holes in the cell membrane. The dynamics obtained were then compared to a theoretical model describing the physical behavior of the cell layer, taking into account the diffusion of detergent molecules and the physical characteristics of the membrane. The good agreement between experiment and theory indicates that membrane permeabilization is limited mainly by the diffusion of detergent molecules and their binding to the membrane.In a second part, we developed a completely new system based on a continuous terahertz QCL source at 2.5 THz. This new instrument is based on a very simplified design, with a single ATR prism and a single detector, and on the dual modulation of the terahertz beam using a mechanical chopper. This chopper synchronizes the dual modulation and defines the measurement and reference zones. The long-term stability of this device has been greatly enhanced by the precise control of temperature and humidity inside the device. Performance is excellent in both the short and long term. A signal-to-noise ratio of 30 dB is achieved over 300ms, and remains above 30 dB for several hours. In addition, a theoretical and experimental study has been carried out to calibrate the instrument. Thus, the ATR reflection coefficients of several solutions of biological interest (ions, sugars and proteins) were obtained over a wide range of concentrations. A sensitivity at least 20 times higher than that of the existing literature was thus obtained. Thanks to this new high-performance system, we studied the dynamics of membrane permeabilization following the action of photodynamic therapy (PDT). The first results showed that the encapsulation of photosensitizers by micellar vectors significantly improves the efficiency of PDT.
... The size of the THz image of the cell 12 at the 2 nd hour is smaller than the 1 st hour can be ascribed to water-loss induced 13 contraction of the cell. 14 Compared to the 2 nd hour, the size of the cell at the 3 rd hour further shrank due to 15 further water evaporation. But from the 3 rd hour onwards (the 4 th hour and 5 th hour), 16 no obvious size change was observed. ...
Objectives: Terahertz (THz) -based imaging techniques hold great potential for biological and biomedical applications, which nevertheless are hampered by the low spatial resolution of conventional THz imaging systems. In this work, we report a high performance photoconductive antenna microprobe-based near-field THz time-domain spectroscopy scanning microscope.
Materials and Methods: A single watermelon pulp cell was prepared on a clean quartz slide and covered by a thin polyethylene film. The high performance near-field THz microscope was developed based on a coherent THz time-domain spectroscopy system coupled with a photoconductive antenna microprobe. The sample was imaged in transmission mode.
Results: We demonstrate the direct imaging of the morphology of single watermelon pulp cells in the natural dehydration process with our near-field THz microscope.
Conclusions: Given the label-free and nondestructive nature of THz detection techniques, our near-field microscopy-based single-cell imaging approach sheds new light on studying biological samples with THz.
... 11 The LT-GaAs layer is usually grown on a GaAs substrate with temperature around 250 ○ C by molecular beam epitaxy (MBE) equipment. However, regarding the applications where backside laser illumination (the laser beam transmits through the substrate before it illuminates the active layer) is required, 12,13 LT-GaAs substrates cannot be utilized because GaAs is opaque at the excitation laser wavelength. In order to make use of the advantages of LT-GaAs in such applications, Heiliger et al. has demonstrated the transfer of the LT-GaAs layer to a sapphire substrate, which is optically transparent (around 85% of transmission at 800 nm), using an epitaxial liftoff technique. ...
The terahertz (THz) radiation properties of a photoconductive antenna (PCA) fabricated on a GaAs-on-sapphire (GoS) substrate are reported at sub-THz band. The GaAs layer with a thickness of approximately 1 µm was directly deposited on a sapphire wafer by means of molecular beam epitaxy. A butterfly-shaped antenna structure was then fabricated on the GoS substrate by photolithography, and the device was tested as the emitter of an in-house built THz time-domain spectrometer. The performance of this antenna was compared with a commercial one, which had an identical antenna structure but was fabricated on low-temperature-grown GaAs (LT-GaAs). The results showed that the GoS-based PCA radiated an enhanced THz field, which could be as much as 1.9 times that of the LT-GaAs-based PCA, indicating that GoS could be a promising photoconductive material. In addition, the optical transparency of the sapphire substrate allows the device to be illuminated from the backside, which is crucial for THz near-field imaging applications where the sample is usually in close proximity to the front surface of the PCA device.
... There are two main approaches to improve the spatial resolution of THz images, one is to enhance the imaging system, and the other is to adopt a super-resolution method. Some studies focus on improving the THz imaging system to improve the spatial resolution [5][6][7][8][9]. But applying the super-resolution method is a more economical and effective solution to obtain high-resolution (HR) images using only existing imaging systems. ...
We propose an effective and robust method for terahertz (THz) image super-resolution based on a deep convolutional neural network (CNN). A deep CNN model is designed. It learns an end-to-end mapping between the low- and high-resolution images. Blur kernels with multiple width and noise with multiple levels are taken into the training set so that the network can handle THz images very well. Quantitative comparison of the proposed method and other super-resolution methods on the synthetic THz images indicates that the proposed method performs better than other methods in accuracy and visual improvements. Experimental results on real THz images show that the proposed method significantly improves the quality of THz images with increased resolution and decreased noise, which proves the practicability and exactitude of the proposed method.
... In Figure 13, we see how the initial work on sub-wavelength imaging was driven by methods such as tapered apertures or waveguides [101,107,110]. LTEM [146] and near-field detection methods [112,[163][164][165][166] moved THz imaging to the microscale, and the further push down to the nanoscale came with the implementation of the tip-based approaches in THz imaging systems [123,128,134,167,168]. For the sake of comparison, we have also included apertureless imaging reports in the mid-IR [141,169,170] and MW [121] as well as the stimulated emission depletion microscopy (STED), which first was developed by Hell and co-workers in 1994 [171], and for which the Nobel Prize in Chemistry was awarded in 2014. ...
... In Figure 13, we see how the initial work on subwavelength imaging was driven by methods such as tapered apertures or waveguides [101,107,110]. LTEM [146] and near-field detection methods [112,[163][164][165][166] moved THz imaging to the microscale, and the further push down to the nanoscale came with the implementation of the tip-based approaches in THz imaging systems [123,128,134,167,168]. For the sake of comparison, we have also included apertureless imaging reports in the mid-IR [141,169,170] and MW [121] as well as the stimulated emission depletion microscopy (STED), which first was developed by Hell and coworkers in 1994 [171], and for which the Nobel Prize in Chemistry was awarded in 2014. ...
This article reviews recent advances in terahertz science and technology that rely on confining the energy of incident terahertz radiation to small, very sub-wavelength sized regions. We focus on two broad areas of application for such field confinement: metamaterial-based nonlinear terahertz devices and terahertz near-field microscopy and spectroscopy techniques. In particular, we focus on field confinement in: terahertz nonlinear absorbers, metamaterial enhanced nonlinear terahertz spectroscopy, and in sub-wavelength terahertz imaging systems.
... For many applications, including spectroscopy 3 , inspection 4 , communication 5 , and coherent control 6 , tunability of the narrow-band THz emission is required. In addition, THz techniques are often limited in terms of spatial resolution 7,8 . Owing to the relatively long wavelength of the THz field, diffraction usually limits the spatial resolution to the order of 10-100 µm, thereby making it difficult to resolve features that are substantially below this scale. ...
A nanojunction structure developed by researchers in the US can generate tunable terahertz radiation over a wide bandwidth. Terahertz radiation, occupying the upper end of microwave and lower end of infrared, causes atoms and molecules to resonate, revealing useful information about the physical structure of matter. However, terahertz radiation is difficult to generate. Lu Chen and co-workers in Jeremy Levy’s group at the University of Pittsburgh used an atomic force microscope to create nano-sized junctions between lanthanum aluminate and strontium titanate. By shining two frequencies of laser light onto a voltage-biased nanojunction, they could produce radiation at the difference of the two frequencies as high as 100 terahertz. Because the nanojunction can both generate and detect terahertz radiation, it could be used to manipulate and observe nanoscale objects such as quantum dots or individual molecules.
... Radiation damage through ion (e.g., Oþ, Arþ) implantation is normally used to drastically reduce the intrinsic-like carrier lifetime to be less than ps [45,52], which is preferred for broadband generation. On the other hand, the sapphire substrate has an advantage of optical transparency around 800 nm, which is useful for potential THz near-field applications [53][54][55]. The GaAs material is demonstrated to have higher THz radiation compared to the ion-damaged SoS substrate as reported in [56], based on a 40-mmlong and 10-mm-wide (6 mm slot) dipole emitter antenna, as shown in Figure 3. 16. ...
... Several works on THz near-field imaging, such as dynamic aperture, tapered metal tip and EO near-field emission microscopy [71]- [73], have been done previously to investigate the system properties. Meanwhile, most of the reported THz near-field imaging systems utilize the detection mode [53,54,74], in which the sample under test is in close proximity to the THz detector. Emission mode using a single PCA for near-field imaging is reported in [75]. ...
... Other 2D image demonstrations have been reported in the literature as well, using the THz near-field spectroscopic technique. For example, in [54], the images of a small portion of leaf are formed using the single frequency transmission magnitude and phase at 1.36 THz as plotted in Figure 3.39(a), where the leaf sample is placed in direct proximity to the dipole-shape THz detector (with 5 mm dipole gap) fabricated on the RD-SoS substrate. The leaf is scanned with a spatial step of $50 mm. ...
