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Images and line profiles of single 1 µm PS beads recorded at the air/silicon interface with the linear polarization set along the X axis (a) and Y axis (b), with X and Y defined as the horizontal and vertical directions in the images. Same at the water/silicon interface (c,d). For all data the background is normalized to unity. Scale bars 2.0 µm, color scales in arbitrary units and identical for (a) and (b). Same for (c) and (d). The double-tipped white arrows mark the direction of polarization. The inset in (b) shows the axial image where Z is the longitudinal (depth) axis, with the scale bar at 5 µm.
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The spatial resolution in far-field mid-infrared (λ>2.5 µm) microscopy and micro-spectroscopy remains limited with the full-width at half maximum of the point-spread function ca. λ/1.3; a value that is very poor in comparison to that commonly accessible with visible and near-infrared optics. Hereafter, it is demonstrated however that polymer beads...
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... In addition, strong water absorption restricts the application for observing subcellular structures in living biological samples. Although the resolution can be improved by using solid immersion lenses 55,56 , the resolution can only reach around λ/2.6, which is not enough to study the subcellular structures and activities in living systems. The recent development in photothermal IR microscopy fills this gap. ...
Far-field chemical microscopy providing molecular electronic or vibrational fingerprint information opens a new window for the study of three-dimensional biological, material, and chemical systems. Chemical microscopy provides a nondestructive way of chemical identification without exterior labels. However, the diffraction limit of optics hindered it from discovering more details under the resolution limit. Recent development of super-resolution techniques gives enlightenment to open this door behind far-field chemical microscopy. Here, we review recent advances that have pushed the boundary of far-field chemical microscopy in terms of spatial resolution. We further highlight applications in biomedical research, material characterization, environmental study, cultural heritage conservation, and integrated chip inspection.
... In addition, strong water absorption restricts the application for observing subcellular structures in living biological samples. Although the resolution can be improved by using solid immersion lenses 59,60 , the resolution can only reach around λ/2.6, which is not enough to study the subcellular structures and activities in living systems. The recent development in photothermal IR microscopy fills this gap. ...
Far-field chemical microscopy providing molecular electronic or vibrational fingerprint information opens a new window for the study of three-dimensional biological, material, and chemical systems. Chemical microscopy provides a nondestructive way of chemical identification without exterior labels. However, the diffraction limit of optics hindered it from discovering more details under the resolution limit. Recent development of super-resolution techniques gives enlightenment to open this door behind far-field chemical microscopy. Here, we review recent advances that have pushed the boundary of far-field chemical microscopy in terms of spatial resolution. We further highlight applications in biomedical research, material characterization, environmental study, cultural heritage conservation, and integrated chip inspection.
... In terms of mid-infrared wavelengths, normally in 2-20 µm range, in order to circumvent the diffraction limit, several variants have been presented such as reflective objectives, crossed polarizations, and central solid immersion lens. [217,218] However, the resolution enhancements www.advancedsciencenews.com www.lpr-journal.org of those endeavors are still limited. Numerical works have demonstrated the potential of subtraction method for enhancing resolution in the mid-infrared range. ...
... The mid-infrared central solid-immersion lens (c-SIL) microscopy exploits the subtraction method of a Gaussian beam with a c-SIL PSF and a second beam modulated using 0-phase plate with a centrally hollow PSF (refer to Table 1 for the equation). [217] Thus, the solid Gaussian image is filtered using an intensity weighted matrix so that matches the hollow beam profile (Figure 8d2,d3). ...
For decades, the spatial resolution of conventional far‐field optical imaging has been constrained due to the diffraction limit. The emergence of optical super‐resolution imaging has facilitated biological research in the nanoscale regime. However, the existing super‐resolution modalities are not feasible in many biological applications due to weaknesses, like complex implementation and high cost. Recently, various newly proposed techniques are advantageous in the enhancement of the system resolution, background suppression, and improvement of the hardware complexity so that the above‐mentioned issues could be addressed. Most of these techniques entail the modification of factors, like hardware, light path, fluorescent probe, and algorithm, based on conventional imaging systems. Particularly, subtraction technique is an easily implemented, cost‐effective, and flexible imaging tool which has been applied in widespread utilizations. In this review, the principles, characteristics, advances, and biological applications of these techniques are highlighted in optical super‐resolution modalities. Extensive variants are developed to improve the existing optical super‐resolution modalities to be more adaptive to biological community. As such, this reviewing work has summarized those techniques that could further enhance the system resolution with background‐free imaging. Systematic study of subtraction methods is also provided combined with the super‐resolution imaging, including the theories, advances, and applications in biology.
... For instance, a silicon SIL has been used to improve the MIR imaging resolution by twofold. [18] Yet, the resolution of the imaging system is still determined by the IR-diffraction limit. The spatial resolution in the photothermal MIR imaging technique, on the other hand, is not determined by the diffraction-limited focal volume of the MIR excitation alone, but rather by the overlap volume of the MIR pump and the visible probe. ...
We studied the use of vibrationally resonant, third-order sum-frequency generation (TSFG) for imaging of biological samples. We found that laser-scanning TSFG provides vibrationally sensitive imaging capabilities of lipid droplets and structures in sectioned tissue samples. Although the contrast is based on the infrared-activity of molecular modes, TSFG images exhibit a high lateral resolution of 0.5 µm or better. We observed that the imaging properties of TSFG resemble the imaging properties of coherent anti-Stokes Raman scattering (CARS) microscopy, offering a nonlinear infrared alternative to coherent Raman methods. TSFG microscopy holds promise as a high-resolution imaging technique in the fingerprint region where coherent Raman techniques often provide insufficient sensitivity.
... In both cases, the lines were not Importantly, the results have demonstrated a significant improvement over previous non-ATR-based high resolution FTIR imaging studies that also published the contrast profiles of the USAF target including using multi-beam synchrotron imaging [30], CaF 2 hemispheres [27] or the high magnification approach in transmission mode [31,32]. A similar improvement in spatial resolution was previous observed in ATR mode [33] and more recently when a Si immersion lens was used in back scattering mode (non-USAF target measurement) [34]. However, the latter work was not demonstrated with live cells but with polystyrene beads. ...
FTIR imaging is a label-free, non-destructive method valuably exploited in the study of the biological process in living cells. However, the long wavelength/low spatial resolution and the strong absorbance of water are still key constrains in the application of IR microscopy ex vivo. In this work, a new retrofit approach based on the use of ZnS hemispheres is introduced to significantly improve the spatial resolution on live cell FTIR imaging. By means of two high refractive index domes sandwiching the sample, a lateral resolution close to 2.2 μm at 6 μm wavelength has been achieved, i.e. below the theoretical diffraction limit in air and more than twice the improvement (to ~λ/2.7) from our previous attempt using CaF2 lenses. The ZnS domes also allowed an extended spectral range to 950 cm⁻¹, in contrast to the cut-off at 1050 cm⁻¹ using CaF2. In combination with synchrotron radiation source, microFTIR provides an improved signal-to-noise ratio through the circa 12 μm thin layer of medium, thus allowing detailed distribution of lipids, protein and nucleic acid in the surround of the nucleus of single living cells. Endoplasmic reticula were clearly shown based on the lipid ν(CH) and ν(C=O) bands, while the DNA was imaged based on the ν(PO2⁻) band highlighting the nucleus region. This work has also included a demonstration of drug (doxorubicin) in cell measurement to highlight the potential of this approach.
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The online version of this article (10.1007/s00216-018-1245-x) contains supplementary material, which is available to authorized users.
... These earlier proposals envisaged the exploitation of reflective objectives typically used for imaging in the mid-IR but for which the numerical apertures (NA) remain practically limited so that the PSF with a Gaussian beam exhibits a full-width at half maximum (FWHM) that is not narrower than ca. λ/1.3 in air [16,17]. With a central solid immersion lens (c-SIL) [18,19], the mid-IR FWHM is reduced to λ/2 along the axis normal to the direction of polarization and the reconstruction of sparse specimens with a resolution of λ/2.6 was demonstrated when combining images recorded with crossed polarizations [17]. ...
... λ/1.3 in air [16,17]. With a central solid immersion lens (c-SIL) [18,19], the mid-IR FWHM is reduced to λ/2 along the axis normal to the direction of polarization and the reconstruction of sparse specimens with a resolution of λ/2.6 was demonstrated when combining images recorded with crossed polarizations [17]. ...
... The mid-IR microscope used in these experiments has been described in [17] with the difference here that one of the two beam paths accommodates a π-step phase plate to generate the half-moon beam [ Fig. 1(a)]. Briefly, the microscope is operated with a mid-IR PPLNbased synchronously-pumped optical parametric oscillator (LASERSPEC) pumped by a fibre laser (41 MHz, 40 ps, 1030 nm, 2.9 W, MULTITEL). ...
We have experimentally investigated the enhancement in spatial resolution by image subtraction in mid-infrared central solid-immersion lens (c-SIL) microscopy. The subtraction exploits a first image measured with the c-SIL point-spread function (PSF) realized with a Gaussian beam and a second image measured with the beam optically patterned by a silicon π-step phase plate, to realize a centrally hollow PSF. The intense sides lobes in both PSFs that are intrinsic to the SIL make the conventional weighted subtraction methods inadequate. A spatial-domain filter with a kernel optimized to match both experimental PSFs in their periphery was thus developed to modify the first image prior to subtraction, and this resulted in greatly improved performance, with polystyrene beads 1.4 ± 0.1 µm apart optically resolved with a mid-IR wavelength of 3.4 µm in water. Spatial-domain filtering is applicable to other PSF pairs, and simulations show that it also outperforms conventional subtraction methods for the Gaussian and doughnut beams widely used in visible and near-IR microscopy.
This position paper provides a topical overview on the translation of infrared (IR)‐based imaging techniques in metrological applications related to advance manufacturing with a particular focus on in situ imaging to obtain real‐time process information. Fast imaging techniques using optical, X‐ray, e‐beam, ultrasound, or scanning probes are widely used in non‐destructive testing (NDT), the translation of which to metrology requires detailed consideration of the technique in question as well as the instrumentation and adaption choices available. We review the use of IR imaging for metrology in advanced manufacturing based on the current use of IR thermography (IRT) and spectro‐microscopy. We then discuss the opportunity of, and barriers against, the use IR techniques as potential metrological tools in advanced manufacturing and discuss technological directions that can be taken to enable rapid, real‐time ambient measurements.
