Wave-optics Simulations Research Articles

Optical system characterization in Fourier ptychographic microscopy

Fourier ptychographic microscopy (FPM) is a recent technique to overcome the diffraction limit of a low numerical aperture (NA) objective lens by algorithmic post-processing of several low-resolution images. It can increase the space-bandwidth product of an optical system by computationally combining images captured under different illumination conditions. Vignetting determines the spatial extent of the bright and dark regions in the captured images. State-of-the-art analyses treat vignetting as a nuisance that needs to be reduced or excluded from algorithmic consideration using ad hoc decision rules. In contrast, this work investigates vignetting effects as a tool to infer a range of properties of the optical system. Generally, the goal of the FPM reconstruction algorithm is to achieve results that closely resemble the actual specimen at the highest resolution possible. However, as the optimization process does not necessarily guarantee a unique solution, we identify system properties that support alignment between computational predictions and empirical observations, potentially leading to a more accurate and reliable analysis. To achieve this, we characterize the individual system components of the experimental setup and compare experimental data to both, geometrical and wave optical simulations. We demonstrate that using vignetting as an analytical tool enables the modeling of the geometric and coherence properties of the optical system as evidenced by the good agreement between our simulation and experiment.

Limitations of beam-control compensation

In this paper, we use wave-optics simulations to explore the limitations of beam-control compensation. We evaluate performance in terms of the normalized power in a diffraction-limited bucket for the cases of no beam-control compensation, perfect phase compensation, and perfect full-field compensation. From these results, we are able to arrive at the following conclusions: (1) without any form of beam-control compensation, performance begins to degrade when D/r0 > 1; (2) with perfect phase compensation, performance begins to degrade when D/r0 > 1 and (λ/r0)/θ0 > 1; and (3) with perfect full-field compensation, performance begins to degrade when D/r0 > 1 and (λ/D)/θ0 > 1. Here, D is the aperture diameter, r0 is the Fried parameter, λ is the wavelength, and θ0 is the isoplanatic angle. We show (1)–(3) to be true for varying aperture diameters, uniformly distributed turbulence, and varying turbulence profiles. These findings will inform the development of future laser systems that need to sense and correct for the effects of atmospheric turbulence.

Phase recovery from Fresnel incoherent correlation holography using differential Zernike fitting.

Fresnel incoherent correlation holography (FINCH) was created to improve imaging resolution and 3D imaging capabilities using spatially incoherent illumination. The optical setup of a FINCH-based interferometer is closely related to a radial shearing interferometer, which measures the radial phase difference of an input wavefront. By using phase retrieval methodologies from lateral shearing interferometry, namely, differential Zernike fitting (DZF), we show that FINCH-based and radial shearing interferometry can be used for phase retrieval and adaptive optics (AO). In this paper, we describe the phase retrieval algorithm using least squares-based DZF and demonstrate a simple adaptive optics loop with an aberrated point spread function using wave optics simulation. We find that FINCH-based phase retrieval has the advantages of fast phase retrieval measurements, thanks to well-studied least squares-based phase reconstruction methods, improved resolution compared to the Shack-Hartmann-based wavefront sensing, and the simplified optical setup of radial shearing interferometry.

A Framework for Iterative Phase Retrieval Technique Integration into Atmospheric Adaptive Optics—Part I: Wavefront Sensing in Strong Scintillations

The objective of this study, which is divided into two parts, is twofold: to address long-standing challenges in the sensing of atmospheric turbulence-induced wavefront aberrations under strong scintillation conditions via a comparative analysis of several basic scintillation-resistant wavefront sensing (SR-WFS) architectures and iterative phase retrieval (IPR) techniques (Part I, this paper), and to develop a framework for the potential integration of SR-WFS techniques into practical closed-loop non-astronomical atmospheric adaptive optics (AO) systems (Part II). In this paper, we consider basic SR-WFS mathematical models and phase retrieval algorithms, tradeoffs in sensor design and phase retrieval technique implementation, and methodologies for WFS parameter optimization and performance assessment. The analysis is based on wave-optics numerical simulations imitating realistic turbulence-induced phase aberrations and intensity scintillations, as well as optical field propagation inside the SR-WFSs. Several potential issues important for the practical implementation of SR-WFS and IPR techniques, such as the requirements for phase retrieval computational grid resolution, tolerance with respect to optical element misalignments, and the impact of camera noise and input light non-monochromaticity, are also considered. The results demonstrate that major wavefront sensing requirements desirable for AO operation under strong intensity scintillations can potentially be achieved by transitioning to novel SR-WFS architectures, based on iterative phase retrieval techniques.

Wave Optical Modeling of the SEM Column From Source to Specimen.

Probe formation in scanning electron microscope (SEM) is often reduced to objective lens action modeling based on a point-spread function or Fourier transforms. In this study, we present the first complete wave optical modeling of the whole SEM column based on plane-by-plane propagation of the electron beam wavefunction without simplifying the optical system. We identify the challenges in plane-by-plane beam propagation and show how sampling limitations produce aliased results. Through a careful selection and combination of propagators, we have developed a general wave optical propagation method that is able to overcome the aliasing problem to achieve the appropriate probe widths. Using a two-step propagator, we show that it is possible to model the electron beam distribution throughout the column from the virtual source plane to the specimen plane. We also show that our results from the wave optical simulations are consistent with the geometrical theory of probe formation. Finally, as a direct application of this method, we demonstrated that the combined effect of aberrations in the condenser lens and the probe forming objective lens cannot be accurately represented using only the objective lens. Designing beam shaping experiments and studying the effect of partial coherence can be some novel applications.

Deep learning-based general beam synthesis for atmospheric propagation

Optimizing the transmit light beams unlocks the full potential of free-space optical systems. However, designing application-specific light beams remains a challenge, especially for those traversing random media. In this study, we address this gap by proposing a deep learning-based method to generate optimal beams for propagation through atmospheric turbulence. The key mechanism is approximating the receiver statistics through batch-wise computation during the training of a convolutional neural network (CNN). On that basis, statistical performance metrics including average received power, scintillation index, and mean signal-to-noise ratio (SNR) are considered for optimization. Pseudo-modes of the beam are synthesized by weighted superposition of Hermite-Gaussian eigenmodes, enabling the creation of arbitrary complex amplitude profiles, i.e., general beams. An end-to-end implementation framework is designed to facilitate self-supervised learning and eliminate the need for pre-calculated datasets. Effectiveness of the synthesized beam is validated by wave optics simulation and experiments. In particular, comparison with Gaussian Schell-model beams demonstrates that the synthesized beam can achieve lower scintillation and greater intensity at the same time, leading to markedly enhanced receiver SNR. This advantage persists in a wider range of link configurations, extending the application range of stochastic beams.

Circular dichroism of relativistically–moving chiral molecules

Understanding the impact of the relativistic motion of a chiral molecule on its optical response is a prime challenge for fundamental science, but it also has a direct practical relevance in our search for extraterrestrial life. To contribute to these significant developments, we describe a multi–scale computational framework that combines quantum chemistry calculations and full–wave optical simulations to predict the chiral optical response from molecules moving at relativistic speeds. Specifically, the effect of a relativistic motion on the transmission circular dichroism (TCD) of three life–essential biomolecules, namely, B–DNA, chlorophyll a, and chlorophyll b, is investigated. Inspired by previous experiments to detect interstellar chiral molecules, we assume that the molecules move between a stationary observer and a light source, and we study the rotationally averaged TCD as a function of the speed of the molecule.We find that the TCD spectrum that contains the signatures of the molecules shifts with increasing speed to shorter wavelengths, with the effects already being visible for moderate velocities.

Loss on space-ground laser communication downlinks due to optical turbulence: results from wave-optics simulations

Loss on space-ground laser communication downlinks due to optical turbulence: results from wave-optics simulations

Impact of exposure time on optical-phase measurements in turbulence

In this paper, we explore the impact of exposure time on optical-phase measurements collected on light that has propagated through atmospheric-optical turbulence. We model the exposure time by phase averaging over a convective distance, and we quantify the associated impact of imposing an exposure time using the piston- and tilt-removed phase variance. We accomplish this analysis through the development of an analytic solution and wave-optics simulations. In turn, we show that the analytic solution and simulation results are in good agreement when U c τ/D≲0.25, where U c is the convective velocity, τ is the exposure time, and D is the aperture diameter. When U c τ/D≳0.25, the analytic solution underestimates the piston- and tilt-removed phase variance relative to the simulation results, and we discuss these differences. This work, at large, informs wavefront sensing and adaptive-optics efforts, where either the wind speed is high, the system is on a high-speed platform, the beacon is on a high-speed platform, or the beacon signal is very faint thereby requiring long-exposure data collections.

A scattered volume emitter micropixel architecture for ultra efficient light extraction from DUV LEDs

Deep ultraviolet light-emitting diodes (DUV LEDs) typically suffer from strong parasitic absorption in the p-epitaxial layer and rear metal contact/mirror. This problem is exacerbated by a substantial portion of the multiple quantum well (MQW) emissions having a strong out-of-plane dipole component, contributing to emission in widely oblique directions outside the exit cone of the front semiconductor emitting surface. To address this, we propose an architecture that leverages such a heavily oblique angular emission profile by utilizing spaced-apart or scattered volume emitter micropixels that are embedded in a low-index dielectric buffer film with a patterned top surface. This approach achieves high light extraction efficiency at the expense of enlarging the effective emission area, however, it does not require a high-index (e.g., sapphire) substrate or a lens or a nanotextured epi for outcoupling purposes. Hybrid wave and ray optical simulations demonstrated a remarkable larger than three to sixfold increase in light extraction efficiency as compared to that of a conventional planar LED design with a sapphire substrate depending on the assumed epi layer absorption, pixel size, and ratio of light emission area to the MQW active area. An extraction efficiency three times greater than that of a recent nanotextured DUV LED design was also demonstrated. This architecture paves the way for DUV LEDs to have a plug efficiency comparable to that of mercury lamps while being significantly smaller.

Investigation on the principle and performance of fiber optical reflectometer based on wave optics simulation

Fiber optical reflectometer (FOR) is a novel technique for dense droplet detection that combines conventional phase detection with non-invasive velocity measurement capabilities. In this study, the mechanism of FOR technique and its sensitivity to droplet trajectory are investigated by wave optics simulation and experiment. This paper first describes the simulation method and analyzes the dynamic response of probe during the droplet piercing process. Next, an exploration is conducted on the effective field of view for the probe and the detectable interface inclination. Afterwards, by precisely controlling the droplet size, eccentricity, and trajectory, the simulation reveals the sensitivity of FOR technique to droplet parameters and quantifies the accuracy of measurement on droplet velocity, which provides specific guidance for the design and application of FOR technique. Finally, in the static experiment, the measurement error of the droplet axial velocity is controlled within 13 %, which verifies the reliability of the simulation results.

Nonlinear propagation of chirped laser pulses through a dispersive and turbulent atmosphere

The evolution of ultrashort laser pulses in dispersive, turbulent, nonlinear, and dissipative media is discussed in connection with nonlinear self-focusing collapse and the onset of laser filamentation. In quiescent air, a laser pulse propagating with a peak power greater than a critical power for self-focusing will undergo a catastrophic, transverse collapse until the intensity is large enough for photoionization. At this point, self-focusing is arrested and balanced by plasma refraction, forming a laser filament. By applying an appropriate chirp, the dispersive properties of the medium can be used to enhance this process and control its onset, and to counter dissipative effects such as molecular absorption and atmospheric scattering. This paper presents an analysis of the effect of atmospheric turbulence on the propagation of nonlinear pulses with dispersion compensation (chirp). The analytical results are compared with wave optics simulations and found to be in reasonable agreement as long as the pulse maintains a near-Gaussian spatiotemporal profile.

Scaled-laboratory demonstrations of deep-turbulence conditions

This paper uses five spatially distributed reflective liquid-crystal phase modulators (LcPMs) to accurately simulate deep-turbulence conditions in a scaled-laboratory environment. In practice, we match the Fresnel numbers for long-range, horizontal-path scenarios using optical trombones and relays placed between the reflective LcPMs. Similar to computational wave-optic simulations, we also command repeatable high-resolution phase screens to the reflective LcPMs with the proper path-integrated spatial and temporal Kolmogorov statistics.

Optical characteristics of thin film-based InGaN micro-LED arrays: a study on size effect and far field behavior

Micro-light emitting diodes (µ-LEDs) are considered the key enabler for various high-resolution micro-display applications such as augmented reality, smartphones or head-up displays. Within this study we fabricated nitride-based µ-LED arrays in a thin film chip architecture with lateral pixel sizes down to 1 µm. A metal mirror on the p-side enhances the light outcoupling via the n-side after removal of the epitaxial growth substrate. Mounted devices with pixel sizes ranging from 1×1 to 8×8 µm2 were electro-optically characterized within an integrating sphere and in a goniometer system. We measure increased external quantum efficiencies on smaller devices due to a higher light extraction efficiency (LEE) as predicted by wave optical simulations. Besides this size dependence of the LEE, also the far field properties show a substantial change with pixel size. In addition, we compared µ-LEDs with 40 nm and 80 nm thick aluminium oxide around the pixel mesa. Considerably different far field patterns were observed which indicate the sensitivity of optical properties to any design changes for tiny µ-LEDs. The experimentally obtained radiation behavior could be reasonably predicted by finite-difference time-domain simulations. This clearly reveals the importance of understanding and modeling wave optical effects inside µ-LED devices and the resulting impact on their optical performance.

Fast vector wave optical simulation methods for application on 3D-printed microoptics

Fast vector wave optical simulation methods for application on 3D-printed microoptics

System-level noise performance of coherent imaging systems.

We provide an in-depth analysis of noise considerations in coherent imaging, accounting for speckle and scintillation in addition to "conventional" image noise. Specifically, we formulate closed-form expressions for total effective noise in the presence of speckle only, scintillation only, and speckle combined with scintillation. We find analytically that photon shot noise is uncorrelated with both speckle and weak-to-moderate scintillation, despite their shared dependence on the mean signal. Furthermore, unmitigated speckle and scintillation noise tends to dominate coherent-imaging performance due to a squared mean-signal dependence. Strong coupling occurs between speckle and scintillation when both are present, and we characterize this behavior by fitting a scale factor capable of generating variances in closed form. We verify each of these claims through a series of wave-optics simulations, and we see strong agreement in general between numerical results and theoretical predictions. Our findings allow us to confidently gauge signal-to-noise ratio (SNR) expectations when active illumination produces coherent noise.

Improved active-tracking performance through Hadamard speckle contrast reduction.

We report wave-optics simulation results and benchtop experimental findings that demonstrate reduced centroiding error through a Hadamard speckle contrast reduction (HSCR). The method involves projecting multiple orthogonal phase patterns onto an actively imaged object within a single camera resolution cell and integration time. Ideally, performance improves in proportion to the square root of the number of such phase realizations applied. Using 16 subpixels per camera pixel, our simulated track performance consistently meets this expectation with a decent experimental agreement, particularly at smaller object sizes. This outcome has promising implications for active tracking and wavefront sensing, extending the utility of HSCR beyond its known benefits to coherent image quality.

Measuring laser-speckle statistics inscaled-laboratory experiments.

Generating fully developed speckle in a repeatable way is of interest to ongoing scaled-laboratory experiments. Such experiments often look to validate theoretical and numerical predictions for numerous laser-based applications. Unfortunately, experimental constraints such as camera-pixel sampling, residual-sensor noise, and cover-glass etaloning limit one's ability to match the statistics of fully formed speckle in a straightforward way. In this paper, we develop expressions for the speckle probability density function (PDF) and speckle contrast, which account for the effects of camera-pixel sampling (relative to the size of the speckles), as well as Gaussian-distributed additive noise. We validate these expressions using wave-optics simulations, which also account for the separate effects of cover-glass etaloning. Next, we set up an experiment that limits the effects of the cover-glass etaloning (as much as possible). The results show excellent agreement with the expressions we develop for the speckle PDF and speckle contrast. This agreement will enable future scaled-laboratory experiments to match the statistics of fully developed speckle in a straightforward way.

Synchronous optical intensity and phase measurements to characterize Rayleigh-Bénard convection.

Propagation of a laser beam through the Rayleigh-Bénard (RB) convection is experimentally investigated using synchronous optical wavefront and intensity measurements. Experimental results characterize the turbulence strength and length scales, which are used to inform numerical wave optic simulations employing phase screens. Experimentally found parameters are the refractive index structure constant, mean flow rate, kinetic and thermal dissipation rates, Kolmogorov microscale, outer scale, and shape of the refractive index power spectrum using known models. Synchronization of the wavefront and intensity measurements provide statistics of each metric at the same instance in time, allowing for two methods of comparison with numerical simulations. Numerical simulations prove to be within agreement of experimental and published results. Synchronized measurements provided more insight to develop reliable propagation models. It is determined that the RB test bed is applicable for simulating realistic undersea environments.

Spatiotemporally controlled drug delivery via photothermally driven conformational change of self-integrated plasmonic hybrid nanogels

BackgroundSpatiotemporal regulation is one of the major considerations for developing a controlled and targeted drug delivery system to treat diseases efficiently. Light-responsive plasmonic nanostructures take advantage due to their tunable optical and photothermal properties by changing size, shape, and spatial arrangement.ResultsIn this study, self-integrated plasmonic hybrid nanogels (PHNs) are developed for spatiotemporally controllable drug delivery through light-driven conformational change and photothermally-boosted endosomal escape. PHNs are easily synthesized through the simultaneous integration of gold nanoparticles (GNPs), thermo-responsive poly (N-isopropyl acrylamide), and linker molecules during polymerization. Wave-optic simulations reveal that the size of the PHNs and the density of the integrated GNPs are crucial factors in modulating photothermal conversion. Several linkers with varying molecular weights are inserted for the optimal PHNs, and the alginate-linked PHN (A-PHN) achieves more than twofold enhanced heat conversion compared with others. Since light-mediated conformational changes occur transiently, drug delivery is achieved in a spatiotemporally controlled manner. Furthermore, light-induced heat generation from cellular internalized A-PHNs enables pinpoint cytosolic delivery through the endosomal rupture. Finally, the deeper penetration for the enhanced delivery efficiency by A-PHNs is validated using multicellular spheroid.ConclusionThis study offers a strategy for synthesizing light-responsive nanocarriers and an in-depth understanding of light-modulated site-specific drug delivery.Graphical