Oblique Illumination Research Articles

Decomposition-learning-based thick-mask model for partially coherent lithography system.

The simulation of thick-mask diffraction near-field (DNF) is an indispensable process in aerial image calculation of immersion lithography. In practical lithography tools, the partially coherent illumination (PCI) is applied since it can improve the pattern fidelity. Therefore, it is necessary to precisely simulate the DNFs under PCI. In this paper, a learning-based thick-mask model proposed in our previous work is extended from the coherent illumination condition to PCI condition. The training library of DNF under oblique illumination is established based on the rigorous electromagnetic field (EMF) simulator. The simulation accuracy of the proposed model is also analyzed based on the mask patterns with different critical dimensions (CD). The proposed thick-mask model is shown to obtain high-precise DNF simulation results under PCI, and thus is suitable for 14 nm or larger technology nodes. Meanwhile, the computational efficiency of the proposed model is improved up to two orders of magnitude compared to the EMF simulator.

Recent advances in oblique plane microscopy

Abstract Oblique plane microscopy (OPM) directly captures object information in a plane tilted from the focal plane of the objective lens without the need for slow z-stack acquisition. This unconventional widefield imaging approach is made possible by using a remote focusing principle that eliminates optical aberrations for object points beyond the focal plane. Together with oblique lightsheet illumination, OPM can make conventional lightsheet imaging fully compatible with standard biological specimens prepared on microscope slides. OPM is not only an excellent high-speed volumetric imaging platform by sweeping oblique lightsheet illumination without mechanically moving either the sample or objective lens in sample space, but also provides a solution for direct oblique plane imaging along any orientation of interest on the sample in a single shot. Since its first demonstration in 2008, OPM has continued to evolve into an advanced microscope platform for biological, medical, and materials science applications. In recent years, many technological advances have been made in OPM with the goal of super-resolution, fast volumetric imaging, and a large imaging field of view, etc. This review gives an overview of OPM’s working principle and imaging performance and introduces recent technical developments in OPM methods and applications. OPM has strong potential in a variety of research fields, including cellular and developmental biology, clinical diagnostics in histology and ophthalmology, flow cytometry, microfluidic devices, and soft materials.

Image Contrast Enhancement by Using LED Annular Oblique Illumination in Bright-Field Microscopy

Köhler illumination is the most used method for evenly illuminating a sample in bright-field microscopy. However, Köhler illumination cannot provide optimum contrast and high resolution simultaneously. The image contrast for Köhler illumination can be enhanced at the expense of lateral resolution. In this paper, we discuss the main causes of contrast loss with Köhler illumination by simulating unsymmetrical interference, incoherent superposition, and modulation transfer function. We present a method to increase the image contrast by combining the annular oblique illumination with normalization and blind deconvolution. With the progress of optoelectronic devices, the greater advantages of annular oblique illumination will become apparent. The experimental results achieved by using LED annular oblique illumination prove the feasibility of the proposed method.

Polarimetric imaging of back-scattered terahertz speckle fields using a portable scanner.

Speckle patterns observed in coherent optical imaging reflect important characteristic information of the scattering object. To capture speckle patterns, angular resolved or oblique illumination geometries are usually employed in combination with Rayleigh statistical models. We present a portable and handheld 2-channel polarization-sensitive imaging instrument to directly resolve terahertz (THz) speckle fields in a collocated telecentric back-scattering geometry. The polarization state of the THz light is measured using two orthogonal photoconductive antennas and can be presented in the form of the Stokes vectors of the THz beam upon interaction with the sample. We report on the validation of the method in surface scattering from gold-coated sandpapers, demonstrating a strong dependence of the polarization state on the surface roughness and the frequency of the broadband THz illumination. We also demonstrate non-Rayleigh first-order and second-order statistical parameters, such as degree of polarization uniformity (DOPU) and phase difference, for quantifying the randomness of polarization. This technique provides a fast method for broadband THz polarimetric measurement in the field and has the potential for detecting light depolarization in applications ranging from biomedical imaging to non-destructive testing.

High-efficiency broadband polarisation converter via patterned multilayer twisted liquid crystal polymer

ABSTRACT We propose an ultra-compact polarisation converter based on liquid crystal (LC) material for effectively converting the obliquely incident unpolarised light into horizontal linearly polarised beam within the visible frequency range. Multilayer twisted configuration is introduced here to tackle the inherent dispersion issue from the LC itself and extended Jones matrix is employed to analyse the propagation of the electric field inside the LC medium under possible oblique illumination. Spatial angular separation of orthogonal polarised lights and polarisation conversion into the same polarisation direction are realised through specific patterned structure design, which effectively reduce the thickness of the device without sacrificing performance. Experimental results demonstrated that our fabricated LC-based device can deliver >80% polarisation conversion efficiency at normal illumination and 59%–80.5% efficiency for ±10° oblique incidence over the visible spectrum. Therefore, we envision that our presented device might find numerous potential applications associated with polarisation regulation such as vortex beam generator, augmented reality and optical computing.

Preliminary assessment of Onchocerca-induced visual impairment using clinical fundus camera in Gashaka local government area of Taraba state, north eastern Nigeria

IntroductionOnchocerciasis is the world's second leading cause of infectious blindness and remains a major problem in parts of Africa. In light of the efforts targeted towards improving ongoing elimination program, this study assessed onchocerca-induced visual impairments in Gashaka local government areas (LGA) in Taraba State, north-eastern Nigeria. MethodsIn 2019, we recruited 158 consenting visually impaired persons across three communities in Garbabi ward of Gashaka LGA. To avoid confusion with co-endemic trachoma, the integrity of the tarsal conjunctiva, eyelashes were assessed using direct light. The anterior segment of the eye was also examined using a torchlight with oblique illumination. However, the posterior segment of the eye was assessed using a fundus camera. Two photographic images for the left and right eye of each participant were captured using the clinical fundus camera. The photographic eye images that were too dark were discarded, and only clear images were analyzed by two ophthalmologists. An ocular manifestation report was recorded for each participant following consensus between the ophthalmologists. ResultsOf the 316 photographic eye images, almost half 146 (46.2%) from 73 participants were just too destroyed for light to penetrate and was not included in the analysis. Only 170 from 85 participants were clear and examinable. A total of 33 (39%) participants had chorioretinitis suggestive of onchocerciasis, including 22(25.9%) with chorio-retinal atrophy, 7(8.2%) and 4(4.7%) had chorioretinal atrophy in combination with early cataract and signs of trachoma respectively. In addition, 3(3.5%) of the participant had eye images which showed lens opacities, 1(1.2%) showed signs of keratoconus and 1(1.2%) showed a scared and pigmented cornea, possibly due to onchocerciasis. Furthermore, 28 (32.9%) had some ill-defined changes and 19 (22.4%) showed poorly defined chorio-retinal atrophy. ConclusionIn a bid to sustain MDA gains towards elimination of onchocerciasis, this work highlights the need for continuous assessment of onchocerciasis induced visual impairment, strengthening of ivermectin delivery and optimizing compliance and patient care among affected populations. These would require resource acquisition and local capacity building. Our preliminary findings call for further operational research on ocular morbidity as well as future stakeholders' consultations in this important and understudied area.

Self-adaptive retro-reflective Doppler cloak based on planar space-time modulated metasurfaces

As engineered electromagnetic covers based on (space-) time modulated metamaterials and metasurfaces, Doppler cloaks are able to compensate the Doppler effect induced by the motion of a scatterer, making it appear as if it were at rest to a detecting radar system. Perfect Doppler compensation can be theoretically always achieved for any relative velocity and motion direction of the cloaked scatterer with respect to the detecting system. However, the motion can be still detected from the cross section variation of the Doppler cloaked scatterer, especially under oblique incidence illumination. The challenge is, therefore, to have a proper Doppler compensation and maintain the amount of scattered energy toward the detection system as much constant as possible with respect to the illumination angle. In this Letter, we propose the design of self-adaptive retro-reflective planar Doppler cloak composed of a pair of space-time modulated metasurfaces: the first metasurface focuses the incident field in a specific location on the second metasurface, which is designed for enabling retro-reflection and Doppler frequency shift compensation. Here, the self-adaptive Doppler cloak is applied to a metallic planar reflector, moving toward its normal direction, and illuminated by an oblique plane wave. We demonstrate that the proposed Doppler cloak can perform frequency conversion and simultaneously maintain the radar cross section of the reflector as much stable as possible within an angular range of about 60° centered at the normal direction. The self-adaptive Doppler cloak may enhance the undetectability of cloaked moving objects.

Microfluidic Device for High-Resolution Cytoskeleton Imaging and Washout Assays in Physcomitrium (Physcomitrella) patens.

Visualizing cytoskeleton dynamics at high spatiotemporal resolution provides valuable insights into the way the dynamics change as well as its interactions with multiple proteins in order to maintain cellular function. Oblique illumination fluorescent microscopy is a popular technique to image cellular events localized near the plasma membrane. In this chapter, we provide detailed protocols for high-resolution cytoskeleton imaging using protonema and gametophore cells of the moss Physcomitrella (Physcomitrium patens) in the microfluidic device. These include preparation of the polydimethylsiloxane (PDMS) device, culture of moss cells, and both short- and long-term oblique illumination fluorescent microscopy. We also describe how to introduce to, and wash out from, the device chemical compounds, such as microtubule-disrupting drugs, during live-cell imaging.

Crossmodal Texture Perception Is Illumination-Dependent.

Visually perceived roughness of 3D textures varies with illumination direction. Surfaces appear rougher when the illumination angle is lowered resulting in a lack of roughness constancy. Here we aimed to investigate whether the visual system also relies on illumination-dependent features when judging roughness in a crossmodal matching task or whether it can access illumination-invariant surface features that can also be evaluated by the tactile system. Participants ( N = 32) explored an abrasive paper of medium physical roughness either tactually, or visually under two different illumination conditions (top vs oblique angle). Subsequently, they had to judge if a comparison stimulus (varying in physical roughness) matched the previously explored standard. Matching was either performed using the same modality as during exploration (intramodal) or using a different modality (crossmodal). In the intramodal conditions, participants performed equally well independent of the modality or illumination employed. In the crossmodal conditions, participants selected rougher tactile matches after exploring the standard visually under oblique illumination than under top illumination. Conversely, after tactile exploration, they selected smoother visual matches under oblique than under top illumination. These findings confirm that visual roughness perception depends on illumination direction and show, for the first time, that this failure of roughness constancy also transfers to judgements made crossmodally.

Generalizing Dipolar Model for the Characterization of Substrated Periodic Metasurfaces Under Oblique Illumination

Most metasurfaces are fabricated on dielectric substrates and have found various applications in manipulating the characteristics of incident fields, from any direction that is normal or at oblique incidences. As a result, a precise analysis of substrated metasurfaces under oblique illuminations is very attractive and important. In this article, the theory of the analytical modeling of periodic planar metasurfaces, placed on a dielectric substrate for normal incidence, is extended to general oblique cases. This method is based on an independent analysis of the electromagnetic behavior of the constituent particles and the interactions of the particles with each other. Since the interaction coefficients are calculated analytically, this method is time effective. The reflectance and transmittance of different substrated metasurfaces under oblique incidence, which is obtained from our proposed method, are compared to the results of numerical full-wave simulations. Two different cases are considered here—a metasurface consisting of isotropic spherical particles and a more general case of a bianisotropic metasurface composed of SRR inclusions. Good accordance has been observed between the results, confirming the accuracy of the proposed method. Furthermore, the effects of different parameters on the interaction coefficients are also investigated.

Polarized iridescence of the tropical carpenter bee, Xylocopa latipes

The tropical carpenter bee, Xylocopa latipes, has metallic-reflecting, iridescent wings. The wing reflectance spectra for TE- and TM-polarized light depend on the angle of light incidence in a way characteristic for dielectric multilayers. Anatomy indicates the presence of melanin multilayers in the wing’s chitinous matrix. A simple optical model of melanin multilayers explains the angle dependence of the wing reflectance spectra. The wing reflections that occur upon oblique illumination exhibit colourful and strongly polarized light patterns, which may mediate intraspecific signaling and mutual recognition by conspecifics.

Combined Scattering, Interferometric, and Fluorescence Oblique Illumination for Live Cell Nanoscale Imaging

To determine the molecular and/or mechanical basis of cell migration using live cell imaging tools, it is necessary to correlate multiple 3D spatiotemporal events simultaneously. Fluorescence nanoscopy and label-free nanoscale imaging can complement each other by providing both molecular specificity and structural dynamics of sub-cellular structures. A combined imaging system would permit obtaining quantitative 3D spatial temporal details of individual cellular components. In this paper, we empirically determined optimal azimuthal scanning angles of rotating beams to achieve simultaneous and label-free nanoscale and fluorescence imaging. Label-free nanoscale imaging here refers to interferometric, bright-field (BF) and dark-field (DF) rotating coherence scattering (ROCS) microscopy, while fluorescence refers to high-inclined laminated oblique (HiLO) and total internal reflection fluorescence (TIRF) imaging. The combined capabilities of interferometric, scattering, and fluorescence imaging enable (1) the identification of molecular targets (substrate or organelle), (2) quantification of 3D cell morphodynamics, and (3) tracking of intracellular organelles in 3D. This combined imaging tool was then used to characterize migrating platelets and endothelial cells, both critical to the process of infection and wound healing. The combined imaging results of more than ∼1000 platelets suggest that serum albumin (bovine) is necessary for platelets to migrate and scavenge fibrin/fibrinogen. Furthermore, we identified new asynchronous membrane fluctuations between the leading and rear edges of a migrating platelet. We further demonstrated that interferometric imaging permits the quantification of mitochondrial dynamics on human lung microvascular endothelial cells. Our data suggests that axial displacement of mitochondria is minimal when it is closer to the nucleus or the leading edge of a cell membrane. Taken together, this combined nanoscopy platform helps to quantify multiple spatial temporal events of a migrating cell that will undoubtedly open ways to new quantitative correlative nanoscale live cell imaging.

Resolution doubling in light-sheet microscopy via oblique plane structured illumination.

Structured illumination microscopy (SIM) doubles the spatial resolution of a fluorescence microscope without requiring high laser powers or specialized fluorophores. However, the excitation of out-of-focus fluorescence can accelerate photobleaching and phototoxicity. In contrast, light-sheet fluorescence microscopy (LSFM) largely avoids exciting out-of-focus fluorescence, thereby enabling volumetric imaging with low photobleaching and intrinsic optical sectioning. Combining SIM with LSFM would enable gentle three-dimensional (3D) imaging at doubled resolution. However, multiple orientations of the illumination pattern, which are needed for isotropic resolution doubling in SIM, are challenging to implement in a light-sheet format. Here we show that multidirectional structured illumination can be implemented in oblique plane microscopy, an LSFM technique that uses a single objective for excitation and detection, in a straightforward manner. We demonstrate isotropic lateral resolution below 150 nm, combined with lower phototoxicity compared to traditional SIM systems and volumetric acquisition speed exceeding 1 Hz.

Aberration-free synthetic aperture phase microscopy based on alternating direction method

Objective measurements of the morphology and dynamics of label-free cells and tissues can be achieved by quantitative phase with low phototoxicity and no photobleaching. Modern quantitative phase imaging is developing toward high throughput and a huge amount of data. The quantitative nature makes it susceptible to optical aberrations when high spatial-frequency content of the object spectrum is recorded by using synthetic aperture. We present an effective and straightforward global phase aberration-free synthetic-aperture phase microscopy (AF-SAPM). We introduce alternating direction method (ADM) in the variable splitting strategy. The optimization in optical fields is decomposed into two solutions with object terms and aberration terms. The inverse problem of aberration extraction is solved by imposing the sparsity regularization on the object and coefficients of polynomial bases. The background aberration in different angle of oblique illumination can be directly decomposed with the Zernike polynomials. Global phase aberration can be suppressed for faithful synthetic aperture reconstruction, which outperforms the existing solutions. We demonstrate the availability of the proposed method in quantitative monitoring of endocrine tumor with highly variable phases.

Advanced Colloidal Sensors Enabled by an Out‐of‐Plane Lattice Resonance

The rational design and dispersion engineering of plasmonic colloid gratings are mainly responsible for advancing refractive index sensors. Herein, a 1D colloidal plasmonic lattice composed of nanoparticle dimer chains fabricated by a combination of interference lithography, soft molding, and colloidal self‐assembly is studied. The bottom‐up approach affords centimeter‐scale arrays of high‐quality colloidal nanoparticles with resonances in the visible range. Demonstrating a special case of collective plasmonic coupling called out‐of‐plane lattice resonance (OPLR) is focused on. Such OPLRs are excited, solely under oblique illumination with transverse magnetic polarization, and these findings with electromagnetic simulations are supported. In addition, it is experimentally found that such resonances are spectrally narrow with a linewidth of 8.08 ± 0.6 nm and have a quality factor of 78.49 ± 5.7, an enhancement by a factor of 11.62 compared with a single particle. The oblique incidence provides field enhancement outside the substrate plane, which is more accessible and sensitive to a surrounding analyte. A simple glycerol–water mixture as a model system to demonstrate this enhanced sensitivity and discuss the potential for a sensing application in an asymmetric refractive index environment is demonstrated. Beyond these sensing applications, this platform has the potential to enhance other processes, such as hot carrier injection or coherent energy transfer.

An apparatus for qualitative assessment of the shading ratio of oblique illumination and real-time high-contrast imaging.

Oblique illumination imaging can significantly improve the contrast of transparent thin samples. However, in traditional oblique illumination methods, either the condenser is offset or a block is added to the condenser, which makes it complicated and challenged to build a stable oblique illumination imaging. Herein, we present a method to measure the optimal shading ratio of oblique illumination in an inverted microscope, and develop an apparatus for stable high-speed high-contrast imaging with uniform brightness. At optimal shading ratio, the oblique illumination imaging has better imaging quality than differential interference contrast, which characteristic is independent on sample. In oblique illumination with low magnification objective, the images have uneven brightness. According to target brightness, we have developed a brightness unevenness correction algorithm to form uniform background brightness for oblique illumination. Integrating the algorithm with imaging acquisition, corrected oblique illumination microscopy is appropriate to observe living cells with high contrast.

Computational single-objective scanning light sheet (cSOLS)

Single-objective scanning light sheet (SOLS) imaging has fueled major advances in volumetric bioimaging because it supports low phototoxic, high-resolution imaging over an extended period. The remote imaging unit in the SOLS does not use a conventional epifluorescence image detection scheme (a single tube lens). In this paper, we propose a technique called the computational SOLS (cSOLS) that achieves light sheet imaging without the remote imaging unit. Using a single microlens array after the tube lens (lightfield imaging), the cSOLS is immediately compatible with conventional epifluorescence detection. The core of cSOLS is a Fast Optical Ray (FOR) model. FOR generates 3D imaging volume (40 × 40 × 14 µm3) using 2D lightfield images taken under SOLS illumination within 0.5 s on a standard central processing unit (CPU) without multicore parallel processing. In comparison with traditional lightfield retrieval approaches, FOR reassigns fluorescence photons and removes out-of-focus light to improve optical sectioning by a factor of 2, thereby achieving a spatial resolution of 1.59 × 1.92 × 1.39 µm3. cSOLS with FOR can be tuned over a range of oblique illumination angles and directions and, therefore, paves the way for next-generation SOLS imaging. cSOLS marks an important and exciting development of SOLS imaging with computational imaging capabilities.

Absorption of nonpolarized light by a two-dimensional ensemble of spherical silver particles under oblique illumination

The problem of absorption of nonpolarized (natural) light by a two-dimensional ensemble (monolayer) of spherical particles under oblique illumination is considered. The solution is based on the statistical theory of multiple scatteing of waves. The written equations make it possible to find the optimal conditions for the absorption of directed light to increase the absorption efficiency of monolayers with a periodic and partially ordered spatial arrangement of particles. The results of the calculations are presented for the absorption coefficient of polarized and natural light by ensembles of silver particles in a nonabsorbing medium. The particle size and monolayer filling factor are chosen so as to illustrate the dependence of absorption on the angle of incidence under conditions of resonance effects caused by the spatial organization of particles. It is shown that in the region of the resonant absorption peak a normally illuminated monolayer with a triangular lattice of silver particles can absorb almost an order of magnitude more incident light than a partially ordered layer. The absorption coefficient of a monolayer under directional oblique illumination can be almost an order of magnitude larger than under normal illumination. The results can be used to optimize the design of opto-electronic devices based on particulate layers.

Intradural Small Bowel Herniation.

Online supplemental material is available for this article.

Femtosecond pulsed laser ablation for paint removal at oblique illumination: Effect of the incidence angle

This research studies how the angle of incidence affects the ablation process when a pulsed femtosecond laser with a wavelength of 1040 nm and a high repetition rate is applied to spray paints used for graffiti. Flat test samples were painted using four different spray paints (red, blue, black and silver). All paints had an organic base but different physicochemical properties. For each paint, the ablation process was characterized using confocal microscopy, determining the threshold fluence and the penetration depth of the laser radiation at normal incidence. For different sample tilt angles in the range of 0°–60°, the effect of oblique incidence was analysed in terms of its effect on the ablation rate and the geometric properties (depth and width) of the ablation grooves on each paint. The results show a clear decrease in the ablation rate as the angle of incidence moves away from normal incidence. Moreover, it can also be observed that for a given angle, groove depth is the property most affected by beam tilt. These results represent a necessary step in addressing both cleaning and other laser ablation processes on non-flat surfaces in a controlled and reproducible manner.