arXiv subjects

Allard P. Mosk

Publications and source records attributed to Allard P. Mosk.

At least 19 recordsLinked to original sources

arXiv2025

Phasebook: A Survey of Selected Open Problems in Phase Retrieval

Phase retrieval is an inverse problem that, on one hand, is crucial in many applications across imaging and physics, and, on the other hand, leads to deep research questions in theoretical signal processing and applied harmonic analysis. This survey paper is an outcome of the recent workshop Phase Retrieval in Mathematics and Applications (PRiMA) (held on August 5--9 2024 at the Lorentz Center in Leiden, The Netherlands) that brought together experts working on theoretical and practical aspects of the phase retrieval problem with the purpose to formulate and explore essential open problems in the field.

cs.IT↗

arXiv2024

Laser-patterned Thin-film Electrodes: Imaging Ion Accumulation and Trapped Nanoparticles

This study introduces a straightforward electrode design featuring sharp edges with a curvature of a few hundred nanometers in radius, with which both ion accumulation and nanoparticle deposition can be observed under an alternating electrical potential. The electrodes, termed 'shark-teeth electrodes', are fabricated using a laser ablation technique optimized for facile nanostructure creation. This method involves successive, overlapping ablated discs in a thin film of gold, producing sharp tips that generate strong electric fields. When electrically polarized in an electrolyte solution, these sharp tips form a screening layer, facilitating the observation of ion and nanoparticle behavior. A total-internal reflection microscope is employed to monitor ion accumulation on these electrodes, demonstrating their capability in iontronic microscopy. Additionally, the same electrodes are used to track nanoparticle trapping under high-frequency alternating potentials. This dual functionality allows for the investigation of electrochemical and physical interactions between ions and colloidal nanoparticles, contributing valuable insights to the field of soft matter.

cond-mat.soft↗

arXiv2024

Origami Nano-gap Electrodes for Reversible Nanoparticle Trapping

We present a facile desktop fabrication method for origami-based nano-gap indium tin oxide (ITO) electrokinetic particle traps, providing a simplified approach compared to traditional lithographic techniques and effectively trapping of nanoparticles. Our approach involves bending ITO thin films on optically transparent polyethylene terephthalate (PET), creating an array of parallel nano-gaps. By strategically introducing weak points through cut-sharp edges, we successfully controlled the spread of nano-cracks. A single crack spanning the constriction width and splitting the conductive layers forms a nano-gap that can effectively trap small nanoparticles after applying an alternating electric potential across the nanogap. We analyze the conditions for reversible trapping and optimal performance of the nano-gap ITO electrodes with optical microscopy and electrokinetic impedance spectroscopy. Our findings highlight the potential of this facile fabrication method for the use of ITO at active electro-actuated traps in microfluidic systems.

physics.app-ph↗

arXiv2024

Noise-robust latent vector reconstruction in ptychography using deep generative models

Computational imaging is increasingly vital for a broad spectrum of applications, ranging from biological to material sciences. This includes applications where the object is known and sufficiently sparse, allowing it to be described with a reduced number of parameters. When no explicit parameterization is available, a deep generative model can be trained to represent an object in a low-dimensional latent space. In this paper, we harness this dimensionality reduction capability of autoencoders to search for the object solution within the latent space rather than the object space. We demonstrate a novel approach to ptychographic image reconstruction by integrating a deep generative model obtained from a pre-trained autoencoder within an Automatic Differentiation Ptychography (ADP) framework. This approach enables the retrieval of objects from highly ill-posed diffraction patterns, offering an effective method for noise-robust latent vector reconstruction in ptychography. Moreover, the mapping into a low-dimensional latent space allows us to visualize the optimization landscape, which provides insight into the convexity and convergence behavior of the inverse problem. With this work, we aim to facilitate new applications for sparse computational imaging such as when low radiation doses or rapid reconstructions are essential.

eess.IV↗

arXiv2023

Wavelength-multiplexed Multi-mode EUV Reflection Ptychography based on Automatic-Differentiation

Ptychographic extreme ultraviolet (EUV) diffractive imaging has emerged as a promising candidate for the next-generation metrology solutions in the semiconductor industry, as it can image wafer samples in reflection geometry at the nanoscale. This technique has surged attention recently, owing to the significant progress in high-harmonic generation (HHG) EUV sources and advancements in both hardware and software for computation. In this study, a novel algorithm is introduced and tested, which enables wavelength-multiplexed reconstruction that enhances the measurement throughput and introduces data diversity, allowing the accurate characterisation of sample structures. To tackle the inherent instabilities of the HHG source, a modal approach was adopted, which represents the cross-density function of the illumination by a series of mutually incoherent and independent spatial modes. The proposed algorithm was implemented on a mainstream machine learning platform, which leverages automatic differentiation to manage the drastic growth in model complexity and expedites the computation using GPU acceleration. By optimising over 200 million parameters, we demonstrate the algorithm's capacity to accommodate experimental uncertainties and achieve a resolution approaching the diffraction limit in reflection geometry. The reconstruction of wafer samples with 20-nm heigh patterned gold structures on a silicon substrate highlights our ability to handle complex physical interrelations involving a multitude of parameters. These results establish ptychography as an efficient and accurate metrology tool.

eess.IV↗

arXiv2023

Maximum-likelihood estimation in ptychography in the presence of Poisson-Gaussian noise statistics

Optical measurements often exhibit mixed Poisson-Gaussian noise statistics, which hampers image quality, particularly under low signal-to-noise ratio (SNR) conditions. Computational imaging falls short in such situations when solely Poissonian noise statistics are assumed. In response to this challenge, we define a loss function that explicitly incorporates this mixed noise nature. By using maximum-likelihood estimation, we devise a practical method to account for camera readout noise in gradient-based ptychography optimization. Our results, based on both experimental and numerical data, demonstrate that this approach outperforms the conventional one, enabling enhanced image reconstruction quality under challenging noise conditions through a straightforward methodological adjustment.

eess.IV↗

arXiv2023

Sensor fusion in ptychography

Ptychography is a lensless, computational imaging method that utilises diffraction patterns to determine the amplitude and phase of an object. In transmission ptychography, the diffraction patterns are recorded by a detector positioned along the optical axis downstream of the object. The light scattered at the highest diffraction angle carries information about the finest structures of the object. We present a setup to simultaneously capture a signal near the optical axis and a signal scattered at high diffraction angles. Moreover, we present an algorithm based on a shifted angular spectrum method and automatic differentiation that utilises this recorded signal. By jointly reconstructing the object from the resulting low and high diffraction angle images, the resolution of the reconstructed image is improved remarkably. The effective numerical aperture of the compound sensor is determined by the maximum diffraction angle captured by the off axis sensor.

eess.IV↗

arXiv2022

Sub-nanometer measurement of transient structural changes in dye-doped polystyrene microspheres

We present an interferometric spectral-domain optical coherence tomography microscopy setup to detect structural changes using interference of light reflected from different interfaces of the sample. We induce a reproducible nanometer-scale size change in dye-doped 10$~μ$m polystyrene microspheres by the release of Stokes shift energy of dye molecules inside the microspheres, excited by a modulated 532-nm laser. The resulting optical path length difference was measured with a sensitivity of 0.4$~pm/\sqrt{Hz}$ limited by photodetection noise, and reveals elastic as well as inelastic responses, which opens up possibilities for measuring the response of cell-sized biological objects.

physics.optics↗

arXiv2022

Mode mapping Q > 500 000 photonic crystal nanocavities using free carrier absorption

We demonstrate a nonlinear photomodulation spectroscopy method to image the mode profile of a high-Q photonic crystal resonator (PhCR). This far-field imaging method is suitable for ultrahigh-Q cavities which we demonstrate on a Q = 619000 PhCR. We scan the PhCR surface with a 405 nm pump beam that modulates the refractive index by local thermal tuning, while probing the response of the resonance. We enhance resolution by probing at high power, using the thermo-optical nonlinearity of the PhCR. Spatial resolution of the thermo-optical effect is typically constrained by the broad thermal profile of the optical pump. Here we go beyond the thermal limit and show that we can approach the diffraction limit of the pump light. This is due to free carrier absorption that heats up the PhCR only when there is overlap between the optical pump spot and the optical mode profile. This is supported with a thermo-optical model that reproduces the high-resolution mode mapping. Results reveal that the observed enhanced resolution is reached for surprisingly low carrier density.

physics.optics↗

arXiv2021

Optimal control of coherent light scattering for binary decision problems

Due to quantum noise fluctuations, the rate of error achievable in decision problems involving several possible configurations of a scattering system is subject to a fundamental limit known as the Helstrom bound. Here, we present a general framework to calculate and minimize this bound using coherent probe fields with tailored spatial distributions. As an example, we experimentally study a target located in between two disordered scattering media. We first show that the optimal field distribution can be directly identified using a general approach based on scattering matrix measurements. We then demonstrate that this optimal light field successfully probes the presence of the target with a number of photons that is reduced by more than two orders of magnitude as compared to unoptimized fields.

quant-ph↗

arXiv2021

Roadmap on Wavefront Shaping and deep imaging in complex media

The last decade has seen the development of a wide set of tools, such as wavefront shaping, computational or fundamental methods, that allow to understand and control light propagation in a complex medium, such as biological tissues or multimode fibers. A vibrant and diverse community is now working on this field, that has revolutionized the prospect of diffraction-limited imaging at depth in tissues. This roadmap highlights several key aspects of this fast developing field, and some of the challenges and opportunities ahead.

physics.optics↗

arXiv2021

Observation of mutual extinction and transparency in light scattering

Interference of scattered waves is fundamental for modern light-scattering techniques, such as optical wavefront shaping. Recently, a new type of wavefront shaping was introduced where the extinction is manipulated instead of the scattered intensity. The underlying idea is that upon changing the phases or the amplitudes of incident beams, the total extinction will change due to interference described by the cross terms between different incident beams. Here, we experimentally demonstrate the mutual extinction and transparency effects in scattering media, in particular, a human hair and a silicon bar. To this end, we send two light beams with a variable mutual angle on the sample. Depending on the relative phase of the incident beams we observe either nearly zero extinction, mutual transparency, or almost twice the single-beam extinction, mutual extinction, in agreement with theory. We use an analytical approximation for the scattering amplitude, starting from a completely opaque object and we discuss the limitations of our approximation. We discuss the applications of the mutual extinction and transparency effects in various fields such as non-line-of-sight communications, microscopy, and biomedical imaging.

physics.optics↗

arXiv2021

Maximum information states for coherent scattering measurements

The use of coherent light for precision measurements has been a key driving force for numerous research directions, ranging from biomedical optics to semiconductor manufacturing. Recent work demonstrates that the precision of such measurements can be significantly improved by tailoring the spatial profile of light fields used for estimating an observable system parameter. These advances naturally raise the intriguing question of which states of light can provide the ultimate measurement precision. Here, we introduce a general approach to determine the optimal coherent states of light for estimating any given parameter, regardless of the complexity of the system. Our analysis reveals that the light fields delivering the ultimate measurement precision are eigenstates of a Hermitian operator which quantifies the Fisher information based on the system's scattering matrix. To illustrate this concept, we experimentally show that these maximum information states can probe the phase or the position of an object that is hidden by a disordered medium with a precision improved by an order of magnitude as compared to unoptimized states. Our results enable optimally precise measurements in arbitrarily complex systems, thus establishing a new benchmark for metrology and imaging applications.

physics.optics↗

arXiv2021

Scattering invariant modes of light in complex media

Random scattering of light in disordered media is an intriguing phenomenon of fundamental relevance to various applications. While techniques such as wavefront shaping and transmission matrix measurements have enabled remarkable progress for advanced imaging concepts, the most successful strategy to obtain clear images through a disordered medium remains the filtering of ballistic light. Ballistic photons with a scattering-free propagation are, however, exponentially rare and no method so far can increase their proportion. To address these limitations, we introduce and experimentally implement here a new set of optical states that we term Scattering Invariant Modes (SIMs), whose transmitted field pattern is the same, irrespective of whether they scatter through a disordered sample or propagate ballistically through a homogeneous medium. We observe SIMs that are only weakly attenuated in dense scattering media, and show in simulations that their correlations with the ballistic light can be used to improve imaging inside scattering materials.

physics.optics↗

arXiv2021

Efficient and flexible approach to ptychography using an optimization framework based on automatic differentiation

Ptychography is a lensless imaging method that allows for wavefront sensing and phase-sensitive microscopy from a set of diffraction patterns. Recently, it has been shown that the optimization task in ptychography can be achieved via automatic differentiation (AD). Here, we propose an open-access AD-based framework implemented with TensorFlow, a popular machine learning library. Using simulations, we show that our AD-based framework performs comparably to a state-of-the-art implementation of the momentum-accelerated ptychographic iterative engine (mPIE) in terms of reconstruction speed and quality. AD-based approaches provide great flexibility, as we demonstrate by setting the reconstruction distance as a trainable parameter. Lastly, we experimentally demonstrate that our framework faithfully reconstructs a biological specimen.

eess.IV↗

arXiv2021

Optimizing illumination for precise multi-parameter estimations in coherent diffractive imaging

Coherent diffractive imaging (CDI) is widely used to characterize structured samples from measurements of diffracting intensity patterns. We introduce a numerical framework to quantify the precision that can be achieved when estimating any given set of parameters characterizing the sample from measured data. The approach, based on the calculation of the Fisher information matrix, provides a clear benchmark to assess the performance of CDI methods. Moreover, by optimizing the Fisher information metric using deep learning optimization libraries, we demonstrate how to identify the optimal illumination scheme that minimizes the estimation error under specified experimental constrains. This work paves the way for an efficient characterization of structured samples at the sub-wavelength scale.

physics.optics↗

arXiv2020

Resampling the transmission matrix in an aberration-corrected Bessel mode basis

The study of the optical transmission matrix (TM) of a sample reveals important statistics of light transport through it. The accuracy of the statistics depends strongly on the orthogonality and completeness of the basis in which the TM is measured. While conventional experimental methods suffer from sampling effects and optical aberrations, we use a basis of Bessel modes of the first kind to faithfully recover the singular values, eigenvalues and eigenmodes of light propagation through a finite thickness of air.

physics.optics↗

arXiv2020

Influence of the local scattering environment on the localization precision of single particles

We study the fundamental limit on the localization precision for a subwavelength scatterer embedded in a strongly scattering environment, using the external degrees of freedom provided by wavefront shaping. For a weakly scattering target, the localization precision improves with the value of the local density of states at the target position. For a strongly scattering target, the localization precision depends on the dressed polarizability that includes the back action of the environment. This numerical study provides new insights for the control of the information content of scattered light by wavefront shaping, with potential applications in sensing, imaging, and nanoscale engineering.

physics.optics↗