Optical Inhomogeneities Research Articles

Imaging Spatial Variations of Optical Bandgaps in Perovskite Solar Cells

AbstractA fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer‐scale spatial resolution is developed. This imaging technique utilizes well‐defined and relatively symmetrical band‐to‐band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. Degradation and impurities are found to both cause optical bandgap shifts inside the materials. The results are confirmed with micro‐photoluminescence spectroscopy scans. The excellent agreement between the two techniques opens opportunities for this imaging concept to become a quantified, high spatial resolution, large‐area characterization tool of PSCs. This development continues to strengthen the high value of luminescence imaging for the research and development of this photovoltaic technology.

Radiation-induced loss of silica optical fibres with fluorine-doped cladding

The effect of gamma-irradiation of the fluorine-doped silica depressed cladding graded-index fibre (DCGIF) has been researched. New mechanism of the non-bridging oxygen hole centers (NBOHC) defect formation, conditioned by different mobility of diffusion counter flows of fluorine and oxygen at the core-cladding interface, has been proposed. Using silica tubes with 0.02 wt% of OH groups and hydrogen in free state in MCVD-technology of DCGIF fabrication completely eliminates the NBOHC absorption at the wavelength of 630 nm. However, gamma-irradiation of such fibres results in anomalously high attenuation level in the visible range of optical spectrum. It was established that irradiation of DCGIF leads to light scattering which is probably due to the appearance of optical inhomogeneities in the glass matrix.

Digital holographic imaging of refractive index distributions for defect detection

Any change in physical quantities like density and temperature, affect the refractive index distribution of materials. So, any discontinuity in the translucent materials can be imaged or detected by mapping its spatiotemporally evolving refractive index under thermal stressing. With the help of digital holography, both amplitude and phase of the object under investigation can be obtained and so it emerges as one of the most resourceful tools for high contrast imaging of transparent and semi-transparent materials. The reconstructed phase using digital holography, provides information regarding the refractive index distribution existing in such objects. By mapping the refractive index distributions across the object which is subjected to thermal stress, thermal conductivity of objects can be studied. Moreover, the fact that a defect or any form of optical inhomogeneity will show discontinuity in the refractive index distributions on change in temperature, can be used to detect defects. The wavefront passing through such a region of non-uniformity in refractive index will carry the information about it as a spatio-temporally varying phase and thus will lead to defect detection in phase objects. Here we describe our efforts in the development of a single beam lens less Fourier transform digital holographic interferometric technique for the imaging spatio-temporally evolving refractive index distributions and its application in defect detection.

Adaptive optics via self-interference digital holography for non-scanning three-dimensional imaging in biological samples.

Three-dimensional imaging in biological samples usually suffers from performance degradation caused by optical inhomogeneities. Here we proposed an approach to adaptive optics in fluorescence microscopy where the aberrations are measured by self-interference holographic recording and then corrected by a post-processing optimization procedure. In our approach, only one complex-value hologram is sufficient to measure and then correct the aberrations, which results in fast acquisition speed, lower exposure time, and the ability to image in three-dimensions without the need to scan the sample or any other element in the system. We show proof-of-principle experiments on a tissue phantom containing fluorescence particles. Furthermore, we present three-dimensional reconstructions of actin-labeled MCF7 breast cancer cells, showing improved resolution after the correction of aberrations. Both experiments demonstrate the validity of our method and show the great potential of non-scanning adaptive three-dimensional microscopy in imaging biological samples with improved resolution and signal-to-noise ratio.

Birefringence and incipient plastic deformation in elastically overdriven [100] CaF2 under shock compression

[100] CaF2 single crystals are shock-compressed via symmetric planar impact, and the flyer plate–target interface velocity histories are measured with a laser displacement interferometry. The shock loading is slightly above the Hugoniot elastic limit to investigate incipient plasticity and its kinetics, and its effects on optical properties and deformation inhomogeneity. Fringe patterns demonstrate different features in modulation of fringe amplitude, including birefringence and complicated modulations. The birefringence is attributed to local lattice rotation accompanying incipient plasticity. Spatially resolved measurements show inhomogeneity in deformation, birefringence, and fringe pattern evolutions, most likely caused by the inhomogeneity associated with lattice rotation and dislocation slip. Transiently overdriven elastic states are observed, and the incubation time for incipient plasticity decreases inversely with increasing overdrive by the elastic shock.

Optical Properties and Microdefects in CaMoO4 Single Crystals

Optical inhomogeneities of calcium molybdate crystals in the initial state and after isothermal annealings have been investigated. The value of anomalous birefringence of the samples has been estimated by Mallard’s method; it is shown that annealing reduces this value. Scattering centers have been observed by the Tyndall method. Microscopy has revealed inclusions 2–10 μm in size in all samples. They can’t be attributed to the crucible material inclusions. Light-scattering indicatrices are plotted based on scattering spectroscopy data. Sizes of the scattering centers are estimated.

Spatial-temporal diagnostics of the system of a plasma stream interacting with a surface of heat resistant material

In an automated measuring complex using optical and spectral methods the spatial and temporal changes in the parameters and composition of nitrogen plasma jet were investigated. The plasma jet was flowing out of the nozzle of the plasma torch with 10–12 kK temperature and acting on the sample of MPG-6 graphite. Due to the heating of the sample to the temperatures of 2.5–3 kK the influence of the sublimating material of the sample on the plasma composition and temperature in the near-surface region of the sample was investigated. An original method based on the analysis of movement of optical inhomogeneities in the plasma flow was used to estimate the plasma jet velocity in the region where it interacts with the sample. The combined analysis of the results of two-positioning video recordings opens up the possibility of determining spatial-temporal distributions of the plasma jet velocities, in medium and high pressure environments, in the ranges from few to thousands of m/s and 3–15 kK temperatures.

The Cross-Calibration of Spectral Radiances and Cross-Validation of CO2 Estimates from GOSAT and OCO-2

The Greenhouse gases Observing SATellite (GOSAT) launched in January 2009 has provided radiance spectra with a Fourier Transform Spectrometer for more than eight years. The Orbiting Carbon Observatory 2 (OCO-2) launched in July 2014, collects radiance spectra using an imaging grating spectrometer. Both sensors observe sunlight reflected from Earth’s surface and retrieve atmospheric carbon dioxide (CO2) concentrations, but use different spectrometer technologies, observing geometries, and ground track repeat cycles. To demonstrate the effectiveness of satellite remote sensing for CO2 monitoring, the GOSAT and OCO-2 teams have worked together pre- and post-launch to cross-calibrate the instruments and cross-validate their retrieval algorithms and products. In this work, we first compare observed radiance spectra within three narrow bands centered at 0.76, 1.60 and 2.06 µm, at temporally coincident and spatially collocated points from September 2014 to March 2017. We reconciled the differences in observation footprints size, viewing geometry and associated differences in surface bidirectional reflectance distribution function (BRDF). We conclude that the spectral radiances measured by the two instruments agree within 5% for all bands. Second, we estimated mean bias and standard deviation of column-averaged CO2 dry air mole fraction (XCO2) retrieved from GOSAT and OCO-2 from September 2014 to May 2016. GOSAT retrievals used Build 7.3 (V7.3) of the Atmospheric CO2 Observations from Space (ACOS) algorithm while OCO-2 retrievals used Version 7 of the OCO-2 retrieval algorithm. The mean biases and standard deviations are −0.57 ± 3.33 ppm over land with high gain, −0.17 ± 1.48 ppm over ocean with high gain and −0.19 ± 2.79 ppm over land with medium gain. Finally, our study is complemented with an analysis of error sources: retrieved surface pressure (Psurf), aerosol optical depth (AOD), BRDF and surface albedo inhomogeneity. We found no change in XCO2 bias or standard deviation with time, demonstrating that both instruments are well calibrated.

Experimental determination of microphase separation conversion during photopolymerization of cross-linking monomers to control the evolution of optical inhomogeneities in the volume of the forming polymer

The method of small-angle scattering of optical radiation has been used to study the dynamics of the concentration inhomogeneities evolution in a photopolymerizable composition based on cross-linking monomer, which is restrictedly compatible with its own polymer. The example of oligocarbonate dimethacrylate OCM-2, shows that after the composition photoinitiation is stopped the inhomogeneities evolve depending essentially on the phase state of the polymerizable medium. In the homophase state, these inhomogeneities relax due to diffusion, and in the heterophase state their amplitude grows due to microphase separation. Basing on this regularity an optical technique is proposed to determine the values of conversion at which the polymerizable medium passes from the homophase state to the heterophase state combining the studies on the kinetics of polymerization by IR spectroscopy. The method to optically control the process of concentration inhomogeneities evolution in the volume of the polymerizing layer by switching the polymerization initiation modes during the homo- and heterophase states of the composition is proposed and experimentally realized.

Investigation of heat and mass transfer processes by the method of structured laser radiation caustics

This paper is devoted to investigation of possibilities of using the caustic method for determination of optical inhomogeneities parameters, which arise by heat and mass transfer processes. Conditions of the caustics appearance are considered for longitudinal probing of the diffusion layer of liquid by plane and cylindrical laser beams. Experimental visualization of dynamics of change in the caustics position is presented for various parameters of inhomogeneous media. New method for determination of surface temperature of a cooled body placed in transparent liquid is described.

Variations in Spectral Absorption Properties of Phytoplankton, Non-algal Particles and Chromophoric Dissolved Organic Matter in Lake Qiandaohu

Light absorption by phytoplankton, non-algal particles (NAP) and chromophoric dissolved organic matter (CDOM) was investigated at 90 sites of a clear, deep artificial lake (Lake Qiandaohu) to study natural variability of absorption coefficients. Our study shows that CDOM absorption is a major contributor to the total absorption signal in Lake Qiandaohu during all seasons, except autumn when it has an equivalent contribution as total particle absorption. The exponential slope of CDOM absorption varies within a narrow range around a mean value of 0.0164 nm−1 ( s d = 0.00176 nm−1). Our study finds some evidence for thIS autochthonous production of CDOM in winter and spring. Absorption by phytoplankton, and therefore its contribution to total absorption, is generally greatest in spring, suggesting that phytoplankton growth in Lake Qiandaohu occurs predominantly in the spring. Phytoplankton absorption in freshwater lakes generally has a direct relationship with chlorophyll-a concentration, similar to the one established for open ocean waters. The NAP absorption, whose relative contribution to total absorption is highest in summer, has a spectral shape that can be well fitted by an exponential function with an average slope of 0.0065 nm−1 ( s d = 0.00076 nm−1). There is significant spatial variability present in the summer of Lake Qiandaohu, especially in the northwestern and southwestern extremes where the optical properties of the water column are strongly affected by the presence of allochthonous matter. Variations in the properties of the particle absorption spectra with depths provides evidence that the water column was vertically inhomogeneous and can be monitored with an optical measurement program. Moreover, the optical inhomogeneity in winter is less obvious. Our study will support the parameterization of the Bio-optical model for Lake Qiandaohu from in situ or remotely sensing aquatic color signals.

Dynamic laser speckle analyzed considering inhomogeneities in the biological sample.

Dynamic laser speckle phenomenon allows a contactless and nondestructive way to monitor biological changes that are quantified by second-order statistics applied in the images in time using a secondary matrix known as time history of the speckle pattern (THSP). To avoid being time consuming, the traditional way to build the THSP restricts the data to a line or column. Our hypothesis is that the spatial restriction of the information could compromise the results, particularly when undesirable and unexpected optical inhomogeneities occur, such as in cell culture media. It tested a spatial random approach to collect the points to form a THSP. Cells in a culture medium and in drying paint, representing homogeneous samples in different levels, were tested, and a comparison with the traditional method was carried out. An alternative random selection based on a Gaussian distribution around a desired position was also presented. The results showed that the traditional protocol presented higher variation than the outcomes using the random method. The higher the inhomogeneity of the activity map, the higher the efficiency of the proposed method using random points. The Gaussian distribution proved to be useful when there was a well-defined area to monitor.

Coherent optical adaptive technique improves the spatial resolution of STED microscopy in thick samples.

Stimulated emission depletion microscopy (STED) is one of far-field optical microscopy techniques that can provide sub-diffraction spatial resolution. The spatial resolution of the STED microscopy is determined by the specially engineered beam profile of the depletion beam and its power. However, the beam profile of the depletion beam may be distorted due to aberrations of optical systems and inhomogeneity of specimens' optical properties, resulting in a compromised spatial resolution. The situation gets deteriorated when thick samples are imaged. In the worst case, the sever distortion of the depletion beam profile may cause complete loss of the super resolution effect no matter how much depletion power is applied to specimens. Previously several adaptive optics approaches have been explored to compensate aberrations of systems and specimens. However, it is hard to correct the complicated high-order optical aberrations of specimens. In this report, we demonstrate that the complicated distorted wavefront from a thick phantom sample can be measured by using the coherent optical adaptive technique (COAT). The full correction can effectively maintain and improve the spatial resolution in imaging thick samples.

Thermal analysis of laser-irradiated tissue phantoms using a novel separation of the variables-based discrete transfer method

ABSTRACTThermal analysis of tissue phantoms subjected to short pulse laser irradiation has been presented. The transient radiative transfer equation (RTE) has been solved using a novel separation of variables-based discrete transfer method (DTM) recently developed by the present authors (Nirgudkar et al. [21]). As an advancement, the solution of RTE has been coupled with Pennes’ bioheat transfer equation for determining the temperature distribution. Homogenous as well as phantoms embedded with optical inhomogeneity have been considered. The numerical model has been verified against the results available in the literature. This study clearly reveals the influence of the nature of the embedded inhomogeneity and its relative contrast on the resultant temperature distribution inside the body of the tissue phantom.

Optical Formation of Waveguide Elements in Photorefractive Surface Layer of a Lithium Niobate Sample

Formation of channel optical waveguides due to the sequential point-to-point exposure of local stripe-like regions of Y-cut lithium niobate sample surface is experimentally investigated. The surface layer of the sample is thermally doped with Cu ions to increase its photorefractive sensitivity. The laser radiation with wavelength of 532 nm and optical power of 10 mW is used for the crystal exposure in experiments. The optical inhomogeneities formed during the sample exposure are studied with their probing by laser beams with wavelength of 633 nm.

Absolute measurement for optical inhomogeneity of parallel plate using phase-shifting interferometry

Optical inhomogeneity is an important index to evaluate optical transmission material. We propose an absolute measurement method for optical inhomogeneity of the parallel plate with phase-shifting interferometry (PSI). Compared with the window-flipping method, we introduce another transmission flat and add two cavity measurements between the two transmission flats and the reflective flat with the assistance of a Fizeau interferometer. Simulation and experiment results show that the method can effectively eliminate the disturbances of both surfaces of the parallel plate, the reflective flat, and the system error of the interferometer. It reduces the requirement for surface accuracy of the transmission and reflective flats. It is an absolute measurement method for the optical inhomogeneity of the parallel plate, which can be realized with traditional phase-shifting interferometry.

Controlling optical inhomogeneity of LiNbO3 single crystals

Different ways of inducing changes in the optical inhomogeneity of single crystals of lithium niobate are described. These include the application of a conductive coating in combination with the heating of samples and exposure to the field of a corona discharge, to UV radiation from a Nd:YAG laser, and to the electron beam of an electron microscope. All such effects can be used to record images.

Collaborative effects of wavefront shaping and optical clearing agent in optical coherence tomography

We demonstrate that simultaneous application of optical clearing agents (OCAs) and complex wavefront shaping in optical coherence tomography (OCT) can provide significant enhancement of penetration depth and imaging quality. OCA reduces optical inhomogeneity of a highly scattering sample, and the wavefront shaping of illumination light controls multiple scattering, resulting in an enhancement of the penetration depth and signal-to-noise ratio. A tissue phantom study shows that concurrent applications of OCA and wavefront shaping successfully operate in OCT imaging. The penetration depth enhancement is further demonstrated for <italic<ex vivo</italic< mouse ears, revealing hidden structures inaccessible with conventional OCT imaging.

Reconstruction of an optical inhomogeneity map improves fluorescence diffuse optical tomography

We propose a new reconstruction algorithm for fluorescence diffuse optical tomography, which is designed for highly heterogeneous objects, such as biological tissues. It is a two-step algorithm that exploits continuous-wave measurements acquired at both excitation and fluorescence wavelengths. First, an optical inhomogeneity map, which depends on both absorption and diffusion coefficients, is obtained from excitation measurements. Second, the fluorescence distribution is reconstructed considering the recovered optical inhomogeneity map. The algorithm includes dimensionality reduction techniques, namely measurement compression and structured illumination, which significantly reduce acquisition and reconstruction times. The algorithm has been tested on experimental data acquired from tissue-mimicking phantoms considering sparsity priors. We demonstrate the feasibility and effectiveness of this new approach that provides improved reconstruction quality when compared with the standard normalized Born method.

Numerical investigation of thermal response of laser-irradiated biological tissue phantoms embedded with gold nanoshells

The work presented in this paper focuses on numerically investigating the thermal response of gold nanoshells-embedded biological tissue phantoms with potential applications into photo-thermal therapy wherein the interest is in destroying the cancerous cells with minimum damage to the surrounding healthy cells. The tissue phantom has been irradiated with a pico-second laser. Radiative transfer equation (RTE) has been employed to model the light-tissue interaction using discrete ordinate method (DOM). For determining the temperature distribution inside the tissue phantom, the RTE has been solved in combination with a generalized non-Fourier heat conduction model namely the dual phase lag bio-heat transfer model. The numerical code comprising the coupled RTE-bio-heat transfer equation, developed as a part of the current work, has been benchmarked against the experimental as well as the numerical results available in the literature. It has been demonstrated that the temperature of the optical inhomogeneity inside the biological tissue phantom embedded with gold nanoshells is relatively higher than that of the baseline case (no nanoshells) for the same laser power and operation time. The study clearly underlines the impact of nanoshell concentration and its size on the thermal response of the biological tissue sample. The comparative study concerned with the size and concentration of nanoshells showed that 60nm nanoshells with concentration of 5×1015mm−3 result into the temperature levels that are optimum for the irreversible destruction of cancer infected cells in the context of photo-thermal therapy. To the best of the knowledge of the authors, the present study is one of the first attempts to quantify the influence of gold nanoshells on the temperature distributions inside the biological tissue phantoms upon laser irradiation using the dual phase lag heat conduction model.