Propagation Of Coherent Light Research Articles

Random phase plate designed by the genetic algorithm for decoherence in energy homogenization units of a photolithography illumination system

The microlens array (MLA) is used in the flexible pupil shaping module of lithography illumination system for energy balance. However, the coherence of the light source can lead to interference patterns. Most of the decoherence methods are restricted for the pulse laser and the scanning exposure application. To solve this issue, a specific random phase plate (RPP) is designed to eliminate the interference patterns caused by the coherence of the light source. The model of partially coherent light propagation in the MLAs homogeneous system is established based on the mutual intensity theory, and the genetic algorithm is introduced to design a high-performance RPP. The corresponding measurement device is constructed, and the intensity of the image plane light field is measured. Experiment results indicate that the intensity integral non-uniformity is 32.14% in the x-direction and 66.04% in the y-direction without the RPP, and the intensity integral non-uniformity is 22.95% in the x-direction and 19.18% in the y-direction with the RPP, which verifies the feasibility of the proposed method.

Technique for enhancing the accuracy of the Rayleigh-Sommerfeld convolutional diffraction through the utilization of independent spatial sampling.

The Rayleigh-Sommerfeld diffraction integral (RSD) is a rigorous solution that precisely satisfies both Maxwell's equations and Helmholtz's equations. It seamlessly integrates Huygens' principle, providing an accurate description of the coherent light propagation within the entire diffraction field. Therefore, the rapid and precise computation of the RSD is crucial for light transport simulation and optical technology applications based on it. However, the current FFT-based Rayleigh-Sommerfeld integral convolution algorithm (CRSD) exhibits poor performance in the near field, thereby limiting its applicability and impeding further development across various fields. The present study proposes, to our knowledge, a novel approach to enhance the accuracy of the Rayleigh-Sommerfeld convolution algorithm by employing independent sampling techniques in both spatial and frequency domains. The crux of this methodology involves segregating the spatial and frequency domains, followed by autonomous sampling within each domain. The proposed method significantly enhances the accuracy of RSD during the short distance while ensuring computational efficiency.

Waveguide holography for 3D augmented reality glasses

Near-eye displays are fundamental technology in the next generation computing platforms for augmented reality and virtual reality. However, there are remaining challenges to deliver immersive and comfortable visual experiences to users, such as compact form factor, solving vergence-accommodation conflict, and achieving a high resolution with a large eyebox. Here we show a compact holographic near-eye display concept that combines the advantages of waveguide displays and holographic displays to overcome the challenges towards true 3D holographic augmented reality glasses. By modeling the coherent light interactions and propagation via the waveguide combiner, we demonstrate controlling the output wavefront using a spatial light modulator located at the input coupler side. The proposed method enables 3D holographic displays via exit-pupil expanding waveguide combiners, providing a large software-steerable eyebox. It also offers additional advantages such as resolution enhancement capability by suppressing phase discontinuities caused by pupil replication process. We build prototypes to verify the concept with experimental results and conclude the paper with discussion.

On the exploration of structured light transmission through a multimode fiber in a reference-less system

Recent advancements in optical wavefront shaping have brought multimode fibers (MMFs) into the spotlight as potential contenders for long-haul communication, positioning them as promising substitutes to single-mode fibers. MMFs offer greater data rates, countering the impending congestion of fiber-based networks. Additionally, their suitability for single fiber endoscope procedures presents them as compelling alternatives for minimally invasive endoscopy, providing information comparable to, if not surpassing, current cutting-edge technology. However, the complex modal behavior of light in MMFs hinders the implementation of these promising applications. Hence, precise modal excitation and control are crucial for improving the transmission of structured light in MMFs. This study introduces a groundbreaking approach that achieves the retrieval of the transmission matrix in a single step, thereby facilitating coherent light propagation through highly dispersive MMFs. By combining iterative phase retrieval algorithms with the measurement of phase shifts between experimentally established focal points, potential arbitrary interference control is enabled, leading to effective phase correction. The efficacy of our method is validated through the successful transmission of diverse structured light beams, including Laguerre–Gauss and Hermite–Gaussian types, as well as handwritten characters via MMF. The examination of structured light is simplified using an off-axis holographic technique that accurately captures both intensity and phase information. These results hold significant potential, paving the way for major advancements in long-distance communication and minimally invasive medical procedures, thereby transforming the telecommunications and healthcare sectors.

A new version of PyWolf for the propagation of partially coherent light in media other than free space

PyWolf is an open-source software capable of performing numerical simulations of partially coherent light propagation from two-dimensional light sources using parallel computation. In the original version of PyWolf, the graphical user interface (GUI) would only show free-space propagation as the only option for propagating partially coherent light, even though users could add their own source models and other objects such as lenses and apertures, which was recognized by PyWolf's GUI. We introduce here the new version of PyWolf, PyWolf 2.0, which among other improvements, enables users to easily add their own propagation medium, which can be used throughout the simulation in different propagation planes. New version program summaryProgram Title: PyWolfCPC Library link to program files:https://doi.org/10.17632/frjscxypkd.2Developer's repository link:https://github.com/tiagoecmagalhaes/PyWolfLicensing provisions: GPLv3Programming language: PythonSupplementary material: Detailed description of the main changes with an example.Journal reference of previous version: Comput. Phys. Commun. 276 (2022) 108336.Reasons for the new version: In the previous paper [1] describing the original version of PyWolf, the propagation kernel was limited to free space, even though optical elements such as lenses and apertures could be added. For propagation in other media (e.g., turbulent medium), users had to substitute the free space propagation by the desired propagation medium since PyWolf always uses the same propagator, which is present in a single Python script.Summary of revisions: Propagation model selection was added to PyWolf's graphical user interface (GUI), enabling users to select the propagation medium of interest. It also enables input parameters (e.g., refractive index) for each propagation model. PyWolf checks for the Python scripts present in the “propagation models” folder at startup and displays them in the GUI, similar to what is done, for instance, for the source models. Then, during the simulation, instead of using the same free space propagators, PyWolf will use the ones that are defined in the selected propagation model. Four examples of propagation models are present in the current version, namely, free space, constant refractive index (besides 1), Kolmogorov, and Tatarskii models [2,3]. Depending on the model used, some propagation quantities may not be valid. For instance, in the current implementation for free space propagation, both the spectral density and degree of coherence are valid, however, that is not the case for the current implementations of the Kolmogorov and Tatarskii models, where only the spectral density is valid. Besides this main feature added to PyWolf, other small issues have been improved and corrected, namely: (i) PyWolf now initiates without needing PyOpenCl platforms or devices; (ii) some errors associated with the importation of packages have been corrected; and (iii) phase correction in the FFT algorithm for odd arrays which can affect the propagation image.Nature of problem: Numerical simulations of the propagation of partially coherent light from planar sources in the Fresnel or far field approximations. Computation time can be improved by using optimized versions of the fast Fourier transform algorithm, but the computation time of other tasks can also be improved through parallel computation. PyWolf aims at using parallel computation to perform time-consuming calculations in the propagation of the cross-spectral density function [4,5].Solution method: The open-source toolkit PyOpenCL is used to perform parallel computation on time-consuming calculations. The user can easily modify and add more features to the software, such as source and propagation models. A graphical user interface is also built using PyQt5 which enables the user to set input parameters, including the user's custom models, view the simulation results, export data, and save and load sessions.

Beam Properties of a Partially Coherent Beam Propagating Horizontally in Atmospheric Turbulence

This study explored the impact of atmospheric turbulence on partially coherent light propagation. Atmospheric turbulence causes random modulation of the intensity and phase of light, resulting in a speckle pattern in the far field. This study focused on partially coherent Gaussian Schell model beams and derived an analytical expression of the cross-spectral density function for their transmission through atmospheric turbulence, based on the generalized Huygens–Fresnel principle and the Tatarski spectrum model. Numerical simulations were used to investigate the effects of the source parameters and turbulence strength on the intensity distribution, beam width, and coherence length of partially coherent light in horizontal atmospheric transmission. The results demonstrate that diffraction-induced broadening primarily affects the intensity distribution of light in free-space transmission. Short transmission distances in atmospheric turbulence have comparable characteristics to those in a vacuum; however, as the turbulence intensity and transmission distance increase, the beam broadening effect amplifies, and the coherence length is reduced. The findings are relevant to the design of acquisition, pointing, and tracking systems for wireless laser communication systems and offer insights into the optimization of optical systems for atmospheric conditions.

Electrostatic Epitaxy of Orientational Perovskites for Microlasers.

Orientational growth of single-crystalline structures is pivotal in the semiconductor industry, which is achievable by epitaxy for producing thin films, heterostructures, quantum wells, and superlattices. Beyond silicon and III-V semiconductors, solution-processible semiconductors, such as metal-halide perovskites, are emerging for scalable and cost-effective manufacture of optoelectronic devices, whereas the polycrystalline nature of fabricated structures restricts their application toward integrated devices. Here, electrostatic epitaxy, a process sustained by strong electrostatic interactions between self-assembled surfactants (octanoate anions) and Pb2+ , is developed to realize orientational growth of single-crystalline CsPbBr3 microwires. Strong electrostatic interactions localized at the air-liquid interface not only support preferential nucleation for single crystallinity, but also select the crystal facet with the highest Pb2+ areal density for pure crystallographic orientation. Due to the epitaxy at the air-liquid interface, direct growth of oriented single-crystalline microwires onto different substrates without the processes of lift-off and transfer is realized. Photonic lasing emission, waveguide coupling, and on-chip propagation of coherent light are demonstrated based on these single-crystalline microwires. These findings open an avenue for on-chip integration of single-crystalline materials.

Partially coherent light propagation through a kinoform lens.

Combining wave optics propagation and geometric ray tracing, the mutual optical intensity (MOI) model is extended to quantitatively simulate the propagation of partially coherent light through a kinoform lens at high speed. The MOI model can provide both a high accuracy and a high efficiency simulation. The intensity and coherence degree distributions at the focal plane are calculated using the MOI model. It is beneficial to improve the focusing capability of the kinoform lens by reducing the coherence or increasing the number of lens steps. In addition, increasing the number of steps is also beneficial to increase the photon flux and reduce the depth of focus.

Propagation of broadband coherent light through LIPSS-based metasurfaces in diamond

We study the propagation of coherent broadband light through laser induced periodic surface structures (LIPSS) fabricated on diamond surfaces. 3D finite-difference time-domain (FDTD) simulations were carried out for a variety of experimentally produced LIPSS morphologies, which include the specific nanometer-scale mesoscopic irregularities arising from the fabrication technique. We compare their performance with sinusoidal grating-like structures, showing that the specific features present in LIPSS nanoripples produce a considerable scattering and diffraction when compared to the ideal nanostructures. With a view on determining the scope of the potential optical and photonic applications of LIPSS, we evaluate the effect of these irregularities on the transmitted spatial beam quality and the spatial phase characteristics of the optical wavefront in a broad spectral range.

Deterministic Assembly of Colloidal Quantum Dots for Multifunctional Integrated Photonics.

Colloidal quantum dots (CQDs) are promising for photonic applications toward lasers, waveguides, and photodetectors. However, integration of high-quality photonic elements into multifunctional devices is still restricted by optical losses stemming from the accumulation of defects and disorder in the solution process. Herein, a platform with a directional Laplace pressure is created for eliminating undesired pinning of liquid fronts in the solution process and boosting ordered assembly of CQDs into designable micro-/nanostructures. The versatility and robustness of this method are demonstrated by deterministic patterning of CQDs with different components and photoluminescence spectra onto various substrates. On the basis of this platform, microring lasers with tunable emission modes, low-loss waveguides, and their coupled structures have been reached for direct on-chip generation and propagation of coherent light. A proof-of-concept demonstration of integrated circuits is also conducted by combining microcavity lasers with waveguides for encoding photonic outputs into information bits.

PyWolf: A PyOpenCL implementation for simulating the propagation of partially coherent light

We present PyWolf, an open-source software capable of performing numerical simulations of partially coherent light propagation from two-dimensional light sources. PyWolf computes the evolution of a user-defined cross-spectral density function in the Fresnel and far field approximations, which enables the retrieval of second-order optical quantities of interest such as the spectral degree of coherence and spectral density for a given frequency. The open-source tool kit PyOpenCL is used to increase the computation speed. We present examples of propagation of different source models and optical systems to validate our implementation. Performance results for the computation speed when using parallel computation through PyOpenCL is shown. Source models and propagation systems can be easily added to PyWolf, which has a graphical user interface built with PyQt5. This software can be of great utility for partially coherent light simulation problems that are difficult to treat analytically. Program summaryProgram Title: PyWolfCPC Library link to program files:https://doi.org/10.17632/frjscxypkd.1Developer's repository link:https://github.com/tiagoecmagalhaes/PyWolfLicensing provisions: GPLv3Programming language: PythonExternal routines: NumPy, SciPy, Matplotlib, PyOpenCL, PyQt5Nature of problem: Numerical simulations of the propagation of partially coherent light from planar sources in the Fresnel or far field approximations. Computation time can be improved by using optimized versions of the Fast Fourier Transform (FFT) algorithm, but other calculations can only be increased through parallel computation.Solution method: We use the open-source toolkit PyOpenCL to perform parallel computation on time-consuming calculations. The user can easily modify and add more features to the software, such as source models and propagation methods. A graphical user interface is also built using PyQt5 which enables the user to set input parameters, including the user's custom models, view the simulation results, export data, and save and load sessions.

Elucidating the underlying physics in a two-terminal all-perovskite tandem solar cell: A guideline towards 30% power conversion efficiency

We develop an optoelectronic model for a two-terminal all-perovskite tandem solar cell comprising a top cell, a bottom cell, and a recombination junction in between that connects the two sub-cells in series electrically. In short, the model considers incoherent and coherent light propagation in the glass and thin-film layers respectively, as well as charge carrier transport, generation, and recombination. After calibrating the model to the state of the art two-terminal all-perovskite tandem solar cell with an efficiency of 24.5%, we first study the current matching behavior and S-shaped current–voltage curve of this tandem cell due to the recombination junction. Next, the light interference effect is investigated. Layer thicknesses are adjusted to increase the short circuit current. From a loss analysis, the leading order recombination channels are the surface and Shockley–Read–Hall recombinations. Moreover, we show that the power conversion efficiency of this calibrated tandem model can reach 30.5% by adjusting the layer thicknesses, increasing carrier mobility, and reducing recombination loss. Finally, after switching to bifacial operation, the model predicts a power conversion efficiency of around 35%.

Research on partially coherent light propagation through zone plates

The MOI (mutual optical intensity) model is further developed and extended to analyze the mutual optical intensity propagation through a zone plate. This is the first study to quantitatively analyze the influence of the partial coherence property on the spatial resolution of zone plates. Increasing the source coherence length is beneficial for improving the spatial resolution of the zone plate; however, apparent diffraction peaks are observed at the focal plane. Reducing the sizes of the central stop and the order selecting aperture (OSA) can improve the signal-to-noise (SNR) ratio at the cost of the photon flux. Furthermore, we analyze the coherence property propagation through the STXM beamline at Shanghai Synchrotron Radiation Facility (SSRF), where a zone plate is used to focus the light into ∼30 nm. The coherence property increases and the photon flux decreases with decreasing exit slit size, which is beneficial for improving the spatial resolution of the zone plate. Therefore, the spatial resolution and flux can be balanced by optimizing the exit slit size.

Improved Nanophotonic Front Contact Design for High‐Performance Perovskite Single‐Junction and Perovskite/Perovskite Tandem Solar Cells

The front contact of solar cells greatly influences the optoelectronic performance of perovskite solar cells (PSCs) by controlling the coherent light propagation as well as charge transport within the device. Herein, the nanophotonic front contact consisting of multilayer nanodomes and nanoholes for high‐efficiency perovskite single‐junction and perovskite/perovskite tandem solar cells (PVK/PVK TSCs) is investigated. The optical and electrical characteristics of solar cells are investigated by conducting an advanced 3D numerical approach with the combination of finite‐difference time‐domain (FDTD) and finite‐element method (FEM) simulations embedded with the particle swarm optimization (PSO) algorithm. The numerical modeling is validated by fabricating a set of efficient PSCs, optimized to a power conversion efficiency (PCE) of 17.9%, VOC of 1.07 V, JSC of 21.8 mA cm−2 and fill factor (FF) of 77%. The nanophotonic device results in improved JSC by 10−15%, resulting from 10−15% enhanced light incoupling compared with the planar device, while also strengthening the omnidirectional capabilities at angles of illumination as high as 40°. The optimized nanophotonic front contact results in PCEs of >23% and >30% (matched JSC ≈18 mA cm−2) for single‐junction PSCs and PVK/PVK TSCs, respectively. Details of the nanophotonic front contact, device, and fabrication process are provided.

Examining the Effects of Fluid Velocity Gradients on 4D Digital Holographic PIV/PTV Measurements

4D digital holographic PIV/PTV (4D-DHPIV/PTV) methods have demonstrated theoretical viability due to their relative ease of setup and high spatial resolution (Soria (2018)). This study investigates how velocity gradients related to different flow regimes and their magnitudes affect 3-component–3-dimensional (3C-3D) digital holographic PIV measurement uncertainty. The error introduced by velocity gradients within the interrogation volume is studied by simulating particles in a velocity field, with a given constant velocity gradient superimposed on a uniform flow from which a time-series of hologram pairs are generated and the 3C-3D velocity fields and their errors are determined using 4D-DHPIV/PTV Sun et al. (2020). Hologram pairs are simulated by modelling the propagation and particlediffraction of coherent laser light using the angular spectrum method (Goodman (1996)). The hologram reconstruction then involves direct reconstruction, followed by deconvolution, a particle position refinement and a hologram subtraction step (Sun et al. (2020)). The particle positions obtained from 4D-DHPIV/PTV are then used to resolve particle displacement measurements using 3D cross-correlation digital analysis with a 3D Gaussian fit to sub-pixel resolution (Soria (2006)). The effects of velocity gradients on the displacement uncertainty and bias error have been investigated by undertaking Monte Carlo simulations under a range of velocity gradient environments. Specifically, 5 common velocity gradients have been studied, which included pure strain, pure vorticity and x, y and z-directional shear. The results indicate that the novel 4D-DHPIV/PTV has poorer accuracy and precision in the z-propagation axis, resulting in larger minimum uncertainties and bias errors. The errors in the z axis are also significantly less affected by velocity gradients in the z direction when compared to the effects of x and y directional velocity gradients on x and y errors respectively. Furthermore, the rate of cross-correlation maximum and SNR decrease are approximately 1.36 times slower due to velocity gradients in the z axis than other axes.

Analysis of partially coherent light propagation through the soft X-ray interference lithography beamline at SSRF.

The mutual optical intensity (MOI) model is extended to the simulation of the interference pattern produced by extreme ultraviolet lithography with partially coherent light. The partially coherent X-ray propagation through the BL08U1B beamline at Shanghai Synchrotron Radiation Facility is analysed using the MOI model and SRW (Synchrotron Radiation Workshop) method. The fringe intensity at the exposure area is not uniform but has similar envelope lines to Fresnel diffraction, which is explained by the diffraction from the finite grating modelled as a single aperture. By balancing the slit size and photon stop size, the fringe visibility, photon flux and intensity slope can be optimized. Further analysis shows that the effect of pink light on the aerial images is negligible, whereas the third-harmonic light should be considered to obtain a balance between high fringe visibility and high flux. Two grating interference exposure experiments were performed in the BL08U1B beamline. The aerial image depth showed that the polymethyl methacrylate photoresist depth was determined by the X-ray coherence properties.

Optoelectronic modeling and sensitivity analysis of a four-terminal all-perovskite tandem solar cell – Identifying pathways to improve efficiency

We develop an optoelectronic model for a four-terminal all-perovskite tandem solar cell comprising a top and a bottom cell that are mechanically stacked together. In essence, the model captures conservation of charge carriers, the electric field via the Poisson equation, the coherent and incoherent light propagation and subsequent carrier generation and recombination mechanisms. Experiments for the stand-alone bottom and top cells as well as the tandem cell calibrate and validate the model with overall good agreement. To elucidate the relative importance of the geometrical and material parameters, a stochastic Monte Carlo simulation is carried out with a sample size of ten thousand for thirty-six simultaneously varied parameters. These parameters are ranked according to their influence on the efficiencies of the top, bottom, and tandem cell: The thicknesses for the top- and bottom-cell transparent electrode layers are ranked amongst the highest. Finally, we show how one can fine-tune the layer thicknesses to enhance the tandem efficiency from the base case of 23.1% to 24.4% and carry out further optimizations.

Intensity-dependent speckle contrast of second harmonic light in a nonlinear disordered medium.

Coherent light propagation and scattering in diffuse media realize an interference effect that manifests as a random spatial distribution of bright and dark spots called speckle. Laser speckle contrast is a crucial parameter that quantifies light behavior in diffuse media, often of help in biomedical imaging applications. With several ultra-intense lasers coming onto the scene in recent years, the probability of nonlinearly generating harmonic light has considerably increased. To our knowledge, there are no studies on the simultaneous measurement of local speckle contrast (LSC) at fundamental and harmonic wavelengths. In this work, we experimentally characterize the intensity-dependent global and local speckle contrast at λ=1064nm (IR) and λ=532nm [second-harmonic generation (SHG)] in a diffuse sample made of potassium dihydrogen phosphate. Under pulsed excitation with IR light, second-harmonic light is generated and undergoes scattering, leading to speckle at both wavelengths. The radiation pressure on the sample induces displacement of the particles of the medium, which leads to the temporal averaging and hence reduction in speckle contrast of IR. The decrease in speckle contrast with input power is consistent with the theoretically predicted behavior. Next, the probability distribution of LSC for IR and SHG are observed to follow log-normal distributions with distinctive parameters, measured for co-polarized and counter-polarized channels. Parameter evolution with box-size is fit to the established global model for speckle contrast and also another alternative model. The fit parameters, illustrated as a function of input power, indicate possibilities of widening the global model to second-harmonic light.

Optical and electrical optimization of all-perovskite pin type junction tandem solar cells

A definitive breakthrough of perovskite solar cells towards large scale industrialization is believed to be the demonstration of higher efficiencies than conventional silicon technology, suggesting the exploration of perovskite tandem cell configurations. Since high efficiency tandem solar cells require careful optimization of photoactive as well as contact and additional functional layers, we propose an optical-electrical model to obtain the optimum layer thicknesses and the attainable electrical output parameters of two-terminal perovskite-perovskite tandem solar cells. The optical model takes into account the coherent propagation of light through the layer stack comprising two perovskite semiconductors and the corresponding contact layers, while the electrical model assumes two series-connected analytical current/voltage equations for pin solar cells. This model allows to assess the impact of the main physical parameters on each perovskite layer without requiring the high specificity needed in more rigorous numerical simulations. Efficiencies above 34% are predicted considering available perovskites with non-optimum bandgap and contact materials already proven in efficient laboratory solar cells. The requisite to attain such efficiencies is that recombination at the interfaces between the perovskite and contact materials is kept low in both bottom and top cells. Furthermore, within the assumption of non-optimum bandgaps of currently available perovskites, the simulation results suggest that efficiencies around 37% are possible when adopting contact materials with smaller absorption, more adequate refraction indices, and lower resistivity.

Coherent light propagation through cold atomic clouds beyond the independent scattering approximation

We calculate the relative permittivity of a cold atomic gas under weak probe illumination, up to second order in the density. Within the framework of a diagrammatic representation method, we identify all the second order diagrams that enter into the description of the relative permittivity for coherent light transmission. These diagrams originate from pairwise position correlation and recurrent scattering. Using coupled dipole equations, we numerically simulate the coherent transmission with scalar and vector waves, and find good agreement with the perturbative calculations. We applied this perturbative expansion approach to a classical gas at rest, but the method is extendable to thermal gas with finite atomic motion and to quantum gases where non-trivial pair correlations can be naturally included.