Oblique Illumination Research Articles

Photon upconversion-assisted dual-band luminescence solar concentrators coupled with perovskite solar cells for highly efficient semi-transparent photovoltaic systems.

A luminescent solar concentrator-based photovoltaic system (LSC-PVs) is highly transparent because it harvests solar light via the LSC, a transparent panel containing only fluorophores, and is, therefore, promising as a PV window. However, for the practical use of LSC-PV, achieving high efficiency remains a challenge. Here, we demonstrate an LSC-PV, which is based on the combination of an upconversion (UC)-assisted dual band harvesting LSC and perovskite solar cells (PSCs). We prepare a dual LSC panel consisting of a downshift (DS) LSC that absorbs violet light and an LSC that upconverts the red light. We apply a highly efficient mixed halide PSC with an efficiency of 17.22%. We control the thickness of the LSC panel as well as the dye concentration to maximize the emission from dual LSCs. The dual LSCs coupled with a PSC exhibit a high average-visible-transmittance of 82% and achieve a maximum efficiency of 7.53% at 1 sun (AM 1.5G) illumination. The dual LSC-PSC exhibits a constant efficiency even under oblique solar light illumination and a stable operation with an efficiency retention of 80%.

Спектроскопия слоев на плоскопараллельных подложках

Integral expressions for the reflectance and transmittance spectra of the structure consisted of two thin layers deposited on opposite faces of a plane-parallel substrate at oblique illumination of the structure with partially coherent light are obtained. As a result of the asymptotic analysis of the integrals, approximate analytical formulae are established for calculating the indicated spectra, convenient for use in solving inverse spectrophotometry problems. An aluminum doped zinc oxide layer deposited on a glass substrate is studied. The spectra of the refractive index and absorption coefficient of the layer and the substrate, as well as the thickness of the layer, are determined by processing the reflectance and transmission spectra of the structure, measured for s- and p- polarized waves at two angles of light incidence. The found parameters are used in computational experiments to estimate the applicability limits of the formulated approximations.

Plasmonic sensors with an ultra-high figure of merit

We propose and numerically demonstrate a high-quality hybridized resonant platform, composed of a one-dimensional metal grating and a dielectric cavity. Under a moderate oblique illumination, an ultra-high spectral quality (Q) factor of 1375 is achieved, which shows orders of magnitude larger than that of the system under normal excitation. The high-Q mode results from the strong coupling effect between the surface plasmon polariton of the metal grating and the photonic mode of the dielectric cavity. Based on this hybridized plasmonic resonator, a high-performance sensing scheme with the spectral sensitivity (S) up to 800 nm/RIU (refractive index unit, RIU) is further introduced. Moreover, the figure of merit reaches 1337, indicating a new record for both the localized or propagating surface plasmons based sensors. These features could find applications in sensing and detecting devices, plasmonic switches, and light flow modulators.

Computational Oblique Illumination Microscopy With Isotropic High Resolution

Although various advanced super-resolution microscopies have been developed, conventional microscopes are still the most widely used. Therefore, it is still one of the goals of light microscopists to improve the imaging performance of conventional microscopes. Ernst Abbe had proven the resolution of a bright-field microscope can be improved by oblique illumination in 1873, but the approach has not been widely applied in practice due to the intrinsic drawback that the lateral resolution of the image is anisotropic. In this article, computational oblique illumination microscopy (COIM) is proposed to isotropically improve lateral resolution. Benefiting from outstanding performance of light emitting diodes (LEDs), the programmable symmetrical oblique illumination by LEDs can easily be implemented. COIM uses symmetrical oblique illuminations to enhance the absorption contrast and weaken the phase contrast in images; then an iterative algorithm is used to fuse an image with the isotropic high lateral resolution. Unlike synthetic aperture imaging, COIM does not require phase involvement, so there are no troublesome phase detection schemes. The experimental results indicate that COIM can resolve Element 6 in Group 11 (bar or space width 137 nm) of a USAF test target using a high numerical aperture (NA) objective lens (NA = 1.25) and oblique-illumination sources (center wavelength 520 nm). The proposed technique provides a new way to improve the resolution of bright-field microscope.

Artificial phototropism for omnidirectional tracking and harvesting of light.

Many living organisms track light sources and halt their movement when alignment is achieved. This phenomenon, known as phototropism, occurs, for example, when plants self-orient to face the sun throughout the day. Although many artificial smart materials exhibit non-directional, nastic behaviour in response to an external stimulus, no synthetic material can intrinsically detect and accurately track the direction of the stimulus, that is, exhibit tropistic behaviour. Here we report an artificial phototropic system based on nanostructured stimuli-responsive polymers that can aim and align to the incident light direction in the three-dimensions over a broad temperature range. Such adaptive reconfiguration is realized through a built-in feedback loop rooted in the photothermal and mechanical properties of the material. This system is termed a sunflower-like biomimetic omnidirectional tracker (SunBOT). We show that an array of SunBOTs can, in principle, be used in solar vapour generation devices, as it achieves up to a 400% solar energy-harvesting enhancement over non-tropistic materials at oblique illumination angles. The principle behind our SunBOTs is universal and can be extended to many responsive materials and a broad range of stimuli.

Solid immersion meniscus lens (SIMlens) for open-top light-sheet microscopy.

Open-top light-sheet (OTLS) microscopy has been developed for rapid volumetric imaging of large pathology specimens. A challenge with OTLS microscopy is the transmission of oblique illumination and detection beams through a horizontal sample plate without introducing aberrations. Previous solutions prevented vertical sample movement, constrained the refractive index of the sample, and/or hindered multi-resolution imaging. Here we describe a solid immersion meniscus lens, a wavefront-matching element that suppresses aberrations when illumination and detection beam transition between air and various high-index immersion media, thereby enabling multi-resolution OTLS microscopy of specimens cleared with diverse protocols.

Oblique-plane single-molecule localization microscopy for tissues and small intact animals.

Single-molecule localization microscopy (SMLM), while well established for cultured cells, is not yet fully compatible with tissue-scale samples. We introduce single-molecule oblique-plane microscopy (obSTORM), which by directly imaging oblique sections of samples with oblique light-sheet illumination offers a deep and volumetric SMLM platform that is convenient for standard tissue samples and small intact animals. We demonstrate super-resolution imaging at depths of up to 66 µm for cells, Caenorhabditis elegans gonads, Drosophila melanogaster larval brain, mouse retina and brain sections, and whole stickleback fish.

Polarization dependence of second harmonic generation from plasmonic nanoprism arrays

The second order nonlinear optical response of gold nanoprisms arrays is investigated by means of second harmonic generation (SHG) experiments and simulations. The polarization dependence of the nonlinear response exhibits a 6-fold symmetry, attributed to the local field enhancement through the excitation of the surface plasmon resonances in bow-tie nanoantennas forming the arrays. Experiments show that for polarization of the input light producing excitation of the plasmonic resonances in the bow-tie nanoantennas, the SHG signal is enhanced; this despite the fact that the linear absorption spectrum is not dependent on polarization. The results are confirmed by electrodynamic simulations which demonstrate that SHG is also determined by the local field distribution in the nanoarrays. Moreover, the maximum of SHG intensity is observed at slightly off-resonance excitation, as implemented in the experiments, showing a close relation between the polarization dependence and the structure of the material, additionally revealing the importance of the presence of non-normal electric field components as under focused beam and oblique illumination.

Surface Lattice Resonances in Self-Assembled Arrays of Monodisperse Ag Cuboctahedra.

Plasmonic metal nanoparticles arranged in periodic arrays can generate surface lattice plasmon resonances (SLRs) with high Q-factors. These collective resonances are interesting because the associated electromagnetic field is delocalized throughout the plane of the array, enabling applications such as biosensing and nanolasing. In most cases such periodic nanostructures are created via top-down nanofabrication processes. Here we describe a capillary-force-assisted particle assembly method (CAPA) to assemble monodisperse single-crystal colloidal Ag cuboctahedra into nearly defect-free >1 cm2 hexagonal lattices. These arrays are large enough to be measured with conventional ultraviolet-visible spectroscopy, which revealed an extinction peak with a Q-factor of 30 at orthogonal illumination and up to 80 at oblique illumination angles. We explain how the experimental extinction changes with different light polarizations and angles of incidence, and compare the evolution of the peaks with computational models based on the coupled dipole approximation and the finite element method. These arrays can support high Q-factors even when exposed to air, because of the high aspect ratio of the single-crystal nanoparticles. The observation of SLRs in a self-assembled system demonstrates that a high level of long-range positional control can be achieved at the single-particle level.

Characterizing localized surface plasmon resonances using focused radially polarized beam.

We demonstrate a scheme to characterize the localized surface plasmon resonances (LSPRs) of an individual metallic nanorod by employing a focused radially polarized beam (RPB) illumination under normal incidence. The focused RPB has a unique three-dimensional electric field polarization distribution in the focal plane, which can effectively and selectively excite the dipole and multipole plasmon resonances in a metallic nanorod by just moving the nanorod within the focal plane. This performance can be attributed to the mode matching between the excitation electric field of the incident RPB and the LSPRs in a metallic nanorod. Emphatically, in contrast to the commonly used oblique incidence illumination with the linearly polarized light, our proposed scheme is based on the normally incident light illumination and compatible with conventional optical microscopy, which is more scalable for spectroscopic characterization of individual nanostructures.

Relative error in out-of-plane measurement due to the object illumination.

In this work the sensitivity vector is analyzed for collimated and divergent oblique illumination for an out-of-plane arrangement. In the case of collimated illumination, the variation of the sensitivity vector components was considered and a geometric model was proposed for its evaluation. The geometry of the optical setup used allowed us to find sensitivity mostly along the pulling direction; the other two components of the sensitivity vector were relatively small. We measured the displacement induced only along the pulling direction. Errors in the displacement measurement associated with divergent and collimated illumination can be predicted when the sensitivity vector is considered constant. Experimental and theoretical results are presented for the out-of-plane electronic speckle pattern interferometer. The analyzed object was an aluminum plate.

Assessing light absorption contributions in thin periodically-textured silicon absorbers under oblique illumination

Periodic texturing is one of the main techniques to enhance light absorption in thin-film solar cells. The presence of periodicity, such as grating, allows the excitation of guided modes in the structure, thus enhancing absorption. However, grating efficiency in exciting guided modes is highly dependent on the wavelength and incident angle of light. This is relevant especially in solar cells application, where the light source – the sun – is broadband and largely angle-dependent. Nevertheless, most of literature only focuses on the frequency response of periodic texturing, thus neglecting the effect of angular movement of the sun. In this work we use Fourier expansion to calculate the absorption of each type of mode (guided and non-guided) in an absorptive periodic waveguide. The structure is illuminated with TM and TE polarized light and under three different incident angles. Using this method, we are able to calculate the contribution of a guided resonance to total absorption for different angles of incidence. The work here developed and supported by rigorous numerical calculations can be used to better understand light propagation in a periodic waveguide structure, such as thin-film solar cells.

Tilt-invariant scanned oblique plane illumination microscopy for large-scale volumetric imaging.

This Letter presents the first demonstration of multi-tile stitching for large scale 3D imaging in single objective light-sheet microscopy. We show undistorted 3D imaging spanning complete zebrafish larvae and over 1 mm3 volumes for thick mouse brain sections. We use remote galvo scanning for light-sheet creation and develop a processing pipeline for 3D tiling across different axes. With the improved one photon (1p) tilt-invariant scanned oblique plane illumination (SOPi, /sōpī/) microscope presented here, we demonstrate cellular resolution imaging at depths exceeding 330 μm in optically scattering mouse brain samples and dendritic imaging in more superficial layers.

Resolution enhancement in quantitative phase microscopy

Quantitative phase microscopy (QPM), a technique combining phase imaging and microscopy, enables visualization of the 3D topography in reflective samples, as well as the inner structure or refractive index distribution of transparent and translucent samples. Similar to other imaging modalities, QPM is constrained by the conflict between numerical aperture (NA) and field of view (FOV): an imaging system with a low NA has to be employed to maintain a large FOV. This fact severely limits the resolution in QPM up to 0.82λ/NA, λ being the illumination wavelength. Consequently, finer structures of samples cannot be resolved by using modest NA objectives in QPM. Aimed to that, many approaches, such as oblique illumination, structured illumination, and speckle illumination (just to cite a few), have been proposed to improve the spatial resolution (or the space–bandwidth product) in phase microscopy by restricting other degrees of freedom (mostly time). This paper aims to provide an up-to-date review on the resolution enhancement approaches in QPM, discussing the pros and cons of each technique as well as the confusion on resolution definition claims on QPM and other coherent microscopy methods. Through this survey, we will review the most appealing and useful techniques for superresolution in coherent microscopy, working with and without lenses and with special attention to QPM. Note that, throughout this review, with the term “superresolution” we denote enhancing the resolution to surpass the limit imposed by diffraction and proportional to λ/NA, rather than the physics limit λ/(2n med ), with n med being the refractive index value of the immersion medium.

Nanohole-based phase gradient metasurfaces for light manipulation

The commonly accepted approach for metasurface design utilizes nanopillars with varying diameters. In this study, contrary to usual design approach, we propose and design highly efficient, broadband and polarization-independent nanohole all-dielectric metasurfaces operating in the visible spectrum. High focusing efficiency above 70% is achieved between 450 and 700 nm wavelength region with a numerical aperture (NA) value of 0.60. Moreover, focusing efficiency is succeeded higher than 47% with NA = 0.85 for a design wavelength of 532 nm. Nanohole metasurfaces exhibit less chromatic aberration (<18%) compared to nanopillar based metasurfaces. The nanohole array metasurfaces is investigated under the oblique illumination condition and its performance is found to be satisfactory in a wide range of incidence angles. Furthermore, nanohole and nanopillar metasurfaces are analyzed and their performances are compared for different incidence angles, NAs and operating wavelengths. It is shown that contrary to dielectric pillars commonly deployed in the design of metasurfaces, nanoholes with varying diameters allow phase changes with better performances.

Label-free 3D imaging of weakly absorbing samples using spatially-incoherent annular illumination microscopy

In this paper, we present a simple but effective label-free three dimensional (3D) microscopy for weakly absorbing samples. The proposed technique employs spatially-incoherent annular illumination to enhance absorption contrast of in-focus images and reduce phase contrast. We also employ mechanical scanning along axial direction to acquire a volume of the sample images. A 3D gradient operation is further adopted to remove the background with defocused shadows caused by oblique illumination. As such, a sequence of background-free sectioning images is acquired. The 3D gradient operation results in that only the structural edges of the weakly absorbing sample are visible in the images. We can therefore reconstruct the 3D skeleton structure of the sample from the sectioning image sequence. A label-free diatom is used to verify our technique experimentally. The 3D skeleton structure of the diatom is reconstructed and presented. The proposed technique would find applications in various fields, such as life science, materials science, etc.

Applications of reflection-contrast microscopy, including the sensitive detection of the results of in situ hybridisation a review.

The observation with RCM of the reflection from reaction products produced by nonisotopic in situ hybridisation and a peroxidase staining, has recently facilitated the identification of single copy genes. RCM also reveals light microscope structures in stained ultrathin (0.1 μm) epon and lowicryl sections. In such preparations no out-of-focus blur can be observed. Confocal optics are thus not needed to obtain high-quality images of ultrathin sections at the highest conventional light microscope optical resolution, and with higher image-contrast than can be obtained by classical absorption microscopy. Such RCM images provide the same type of information as confocal scanning microscope images. A sequential ultrathin section of the same microscopic specimen can be examined with electron microscopy for a simple and accurate (CLEM) procedure. For some applications RCM can replace (CLSM). LAY DESCRIPTION: With reflection contrast microscopy (RCM), a microscope image can be observed if micromirror-like reflecting substances are present in the reaction product of cytochemical stains. Using an antiflex objective for RCM, the reflected incident light from the stained microscope specimen can be observed with only a small amount of unwanted stray-light in the image background. Stray-light is caused by reflection of incident light from the surface of the lenses in the microscope objective and in the microscope tube (using a 50% beam splitter for epi-illumination). Several authors who compared RCM with bright-field and fluorescence microscopy, found RCM to be a sensitive microscope method in many applications. It has been reported that RCM, using in situ hybridisation methods, can detect DNA sequences in single copy genes. In the literature is mentioned that single copy genes have an estimated mass of less than a trillionth of a gram of DNA. From stained ultrathin microscope sections with less than 0.1 μm thickness, multicoloured images with a high image contrast, optimal resolution and no out-of-focus blur can be observed with RCM. To achieve this, RCM uses an antiflex microscope objective, with a quarter-wave plate mounted on the front lens in combination with a polarising filter cube inserted in the epi-illuminator. RCM can also make use of oblique illumination, for a further increase of the image-contrast and the resolution of the microscope image. A central stop has then to be inserted in the aperture plane of an epi-illuminator.

Contrast enhancement by oblique illumination microscopy with an LED array

Oblique illumination microscopy is a contrast-enhancing technique accomplished by changing the illumination angle. Conventionally, oblique illumination is achieved by illuminating the sample with a part of the light coming from the condenser lens. This paper describes a low-cost oblique illumination microscope with an LED array. The LED array can electrically change asymmetric illumination patterns. By oblique illumination, we enhanced the contrast of obtained images compared with that of a bright-field image. The contrast obtained by oblique illumination was approximately 3.5 times higher than those of bright-field images obtained by a standard bright-field microscope. The feasibility of oblique illumination microscopy with an LED array is demonstrated by observing diatom cells and buccal epithelial cells.

Dynamic modulation yields one-way beam splitting

This article demonstrates the realization of an extraordinary beam splitter based on nonreciprocal and synchronized photonic transitions in obliquely illuminated space-time-modulated (STM) slabs which impart the coherent temporal frequency and spatial frequency shifts. As a consequence of such unusual photonic transitions, a one-way beam splitting and amplification is exhibited by the STM slab. Beam splitting is a vital operation for various optical and photonic systems, ranging from quantum computation to fluorescence spectroscopy and microscopy. Despite the beam splitting is conceptually a simple operation, the performance characteristics of beam splitters significantly influence the repeatability and accuracy of the entire optical system. As of today, there has been no approach exhibiting a nonreciprocal beam splitting accompanied with transmission gain and an arbitrary splitting angle. Here, we show that oblique illumination of a periodic and semi-coherent dynamically-modulated slab results in coherent photonic transitions between the incident light beam and its counterpart space-time harmonic (STH). Such photonic transitions introduce a unidirectional synchronization and momentum exchange between two STHs with same temporal frequencies, but opposite spatial frequencies. Such a beam splitting technique offers high isolation, transmission gain and zero beam tilting, and is expected to drastically decrease the resource and isolation requirements in optical and photonic systems. In addition to the analytical solution, we provide a closed-form solution for the electromagnetic fields in STM structures, and accordingly, investigate the properties of the wave isolation and amplification in subluminal, superluminal and luminal ST modulations.

Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers.

Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with "open" structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.