arXiv subjects

Johannes E. Fröch

Publications and source records attributed to Johannes E. Fröch.

At least 19 recordsLinked to original sources

arXiv2026

Metalens-transparent ultrasound transducer module for extended depth photoacoustic microscopy

Optical-resolution photoacoustic microscopy (ORPAM) provides label-free optical-absorption contrast at millimeter-scale tissue depths with micrometer-scale lateral resolution. However, its volumetric coverage is limited by the short depth of focus of conventional optical lenses, whereas system miniaturization is constrained by bulky optics and ultrasound transducers. Here, we present an extended-depth ORPAM system that combines an ultra-thin metalens for optical excitation with a planar transparent ultrasound transducer (TUT) for photoacoustic detection. Fabricated from piezoelectric lithium niobate, the chip-scale TUT provides ~80% optical transparency and enables coaxial optical excitation with simultaneous acoustic detection at 14.4 MHz center frequency. This architecture eliminates the need for bulky acousto-optic combiners and large-volume acoustic coupling arrangements between the transducer and imaging target. We evaluate three ultra-thin metalens designs as hyperbolic, quadratic, and extended depth-of-focus (EDOF), which provide effective axial ranges of 0.3, 0.8, and 1.3 mm, respectively, with lateral resolutions of 1.1 micron for hyperbolic, 1.2 micron quadratic lenses, and 1.5 micron for EDOF across the full axial range. Phantom experiments demonstrate improved visualization of multilayered, inclined, volumetrically distributed targets with the EDOF design compared with the two other metalenses. Label-free imaging in awake, head-fixed mice further demonstrates that EDOF provides extended-depth visualization of subcortical microvasculature without being subjected to anesthesia-induced confounds. This chip-scale metalens-TUT architecture provides a foundation for developing compact photoacoustic microscopy systems.

physics.optics↗

arXiv2026

Broadband Content-Adaptive Moiré Meta-spectrometer

Optical spectroscopy underpins material characterization, chemical sensing, and astronomy, but conventional instruments face a rigid trade-off between footprint, spectral range, and resolution. We demonstrate a content-adaptive spectrometer that overcomes this by co-designing dispersive Moiré meta-optics with a recursive sampling algorithm. Instead of using Moiré metalenses solely for varifocal tuning, we harness the strong chromatic aberration arising from phase-wrapping in their subwavelength metasurface architecture. This hyperchromaticity enables a deterministic, one-to-one mapping between the metasurfaces' mutual rotation angle and the sharply focused wavelength, repurposing the pair as a high-resolution spectral scanner. To accelerate data acquisition, we introduce a content-adaptive recursive sampling protocol that exploits the structural sparsity of physical spectra: a fast coarse sweep identifies high-information regions, followed by successively finer angular refinement only where needed. Using a laboratory prototype spanning 405-980 nm, we reconstruct diverse spectra -- from smooth broadband to sparse multi-line laser emissions -- with nearly 3x fewer measurements on average at matched fidelity (up to 7x for sparse line spectra), achieving 30 dB reconstruction 6.7x faster than conventional uniform sampling. This establishes a framework for intelligent, task-adaptive meta-optical sensors that tightly integrate physical dispersion with computational signal processing for real-time spectrometry.

eess.SP↗

arXiv2026

Intrinsic Limitations of Single Layer Polychromatic Metalens for Virtual Reality Visors

Virtual and augmented reality (VR/AR) visors require compact and lightweight optics. Metalenses have been widely proposed as ultrathin replacements for bulky refractive eyepieces, with performance typically assessed using point spread function (PSF) and modulation transfer function (MTF) measurements. Here, we design, fabricate, and characterize a single-layer silicon nitride metalens optimized for the three emission peaks of an RGB OLED display, and benchmark it against refractive and Fresnel eyepieces. Under coherent illumination, the metalens exhibits a tightly confined PSF and strong mid-to-high spatial-frequency MTF, suggesting excellent optical performance. However, when evaluated in a realistic system-level VR testbed incorporating incoherent OLED illumination, a dynamic-pupil eye model, and near-eye-relevant focal lengths, the same device exhibits pronounced ghosting and background haze. We show that these artifacts arise from the intrinsic multifocal nature of polychromatic diffractive focusing and demonstrate that common mitigation strategies such as narrowband filtering and long-focal-length relay optics merely mask, rather than resolve, the issue. Our results establish that meta-optics for AR/VR must be evaluated under realistic system-level conditions to reveal their true imaging performance.

physics.optics↗

arXiv2026

Complex wavefront engineering via decoupled space-time modulation

Solid-state Spatial Light Modulators (SLMs) are fundamentally limited in their ability to achieve high spatial complexity and high temporal bandwidth simultaneously. High-speed, low-energy modulation requires sub-wavelength active mode volumes, and sophisticated spatial wavefront engineering necessitates an ultra-fine pixel pitch. While small pixels can simultaneously solve both, in conventional architectures, the dense 2D electrical routing required for such pixels creates an insurmountable physical bottleneck. This results in a compromise between the SLM refresh rate, number of pixels and the field of view. Here, we demonstrate a hybrid architecture that overcomes this limit by spatially decoupling the electrical modulation plane from the optical output plane. By integrating a metasurface doublet with a photonic integrated circuit (PIC)-based optical phased array (OPA), we achieve independent 2D electrical control over each phase-element while simultaneously realizing a three-fold reduction in effective pixel pitch. This decoupling allows us to maintain the small active volume required for high-speed operation, while circumventing the routing constraints of dense spatial array of emitters. We utilize this platform to demonstrate tunable varifocal lensing, 2D beam steering, and 2D holography. Our work provides a scalable foundation for next-generation solid-state SLMs that simultaneously offer high speed, low power consumption, and large field of view.

physics.optics↗

arXiv2025

Meta-optical Miniscope for Multifunctional Imaging

Miniaturized microscopes (miniscopes) have opened a new frontier in animal behavior studies, enabling real-time imaging of neuron activity while leaving animals largely unconstrained. Canonical designs typically use Gradient-Index (GRIN) lenses or refractive lenses as the objective module for excitation and fluorescence collection, but GRIN lenses suffer from aberrations and refractive lenses are bulky and complex. Meta-optics, composed of subwavelength diffractive elements, offer a promising alternative by combining multiple functionalities with significantly reduced footprint and weight. Here, we present meta-optical miniscopes that integrate functionalities including large field of view (FOV), extended depth of focus (EDOF), and depth sensitivity. These meta-optics replace the traditional refractive lens assembly, reducing the total track length of the objective module from 6.7 mm to 2.5 mm while enhancing imaging performance. Our results demonstrate that meta-optical miniscopes can expand the miniscope toolbox and facilitate the development of more compact and multifunctional imaging systems.

physics.optics↗

arXiv2025

Electrically reconfigurable nonvolatile transmissive metasurface in visible

The synergy between metasurfaces and non-volatile phase change materials (PCMs) has created many reconfigurable photonic devices for applications in optical memory, optical computing and optical communications. But these advances have been limited to the infrared wavelengths due to the high loss of PCMs in the visible regime. Here we demonstrate a nonvolatile visible metasurface that is electrically reconfigurable using wide bandgap PCM Sb2S3. Our device supports a resonant mode at 610 nm, a wavelength largely under-explored for PCM-based metasurfaces. By incorporating only a 20 nm thick layer of Sb2S3, we experimentally demonstrate a resonance tuning range of 16 nm. Reversible switching of the metasurface is accomplished in situ using a carefully engineered, ultrathin doped silicon micro-heater. Our work paves the way for integrating PCMs into visible-frequency systems, particularly for human-centric applications such as augmented and virtual reality displays.

physics.optics↗

arXiv2025

Scalable Ion Fluorescence Collection Using a Trap-Integrated Metalens

A scaled trapped-ion quantum computer will require efficient fluorescence collection across a large area. Here we propose and demonstrate a compact monolithically integrated system featuring a metalens fabricated on the backside of a surface ion trap. A 40$\times$100 $μ$m aperture enables a simulated point-source collection efficiency of 0.91% and a measured point-source detection efficiency of 0.58%. Increasing the aperture area to 40$\times$600 $μ$m boosts the simulated collection efficiency to 3.17%$-$comparable to that of a conventional objective with a numerical aperture of 0.35. Further improvements are possible by co-optimizing the electrode and aperture geometry. An undercut of the electrode substrate at the aperture ensures a large distance between the ion and dielectric substrate without compromising collection efficiency. The metalens directly collimates the collected fluorescence, eliminating the need for a high numerical aperture objective. An array of such readout zones will offer a compact, scalable solution for high-fidelity parallel readout in next-generation trapped-ion quantum processors.

quant-ph↗

arXiv2025

Singleshot Multispectral Imaging via a Chromatic Metalens Array

Real time, singleshot multispectral imaging systems are crucial for environment monitoring and biomedical imaging. Most singleshot multispectral imagers rely on complex computational backends, which precludes real time operations. In this work, we leverage the spectral selectivity afforded by engineered photonic materials to perform bulk of the multispectral data extraction in the optical domain, thereby circumventing the need for heavy backend computation. We use our imager to extract multispectral data for two real world objects at 8 predefined spectral channels in the 400 to 900 nm wavelength range. For both objects, an RGB image constructed using extracted multispectral data shows good agreement with an image taken using a phone camera, thereby validating our imaging approach. We believe that the proposed system can provide new avenues for the development of highly compact and low latency multispectral imaging technologies.

physics.optics↗

arXiv2024

Computed tomography using meta-optics

Computer vision tasks require processing large amounts of data to perform image classification, segmentation, and feature extraction. Optical preprocessors can potentially reduce the number of floating point operations required by computer vision tasks, enabling low-power and low-latency operation. However, existing optical preprocessors are mostly learned and hence strongly depend on the training data, and thus lack universal applicability. In this paper, we present a metaoptic imager, which implements the Radon transform obviating the need for training the optics. High quality image reconstruction with a large compression ratio of 0.6% is presented through the use of the Simultaneous Algebraic Reconstruction Technique. Image classification with 90% accuracy is presented on an experimentally measured Radon dataset through neural network trained on digitally transformed images.

cs.CV↗

arXiv2024

Meta-optical Imaging at Thermal Wavelengths

The field of meta-optics has grown to include metalenses spanning the ultraviolet to terahertz regimes. Imaging is a key application of meta-optics, with recent works demonstrating meta-optical imaging with advanced functionalities including wide field of view, broadband operation, and polarization sensitivity. In this review, we focus on meta-optical imaging for thermal wavelengths. Thermal meta-optics are less well-studied than those in the visible range but have vast potential applications spanning defense, health, and geological sensing. We first introduce these applications and their specific challenges. With compact form-factor and multi-functional capabilities, we suggest that meta-optics are particularly well-suited to meet the needs of imaging in the mid- and long-wave infrared. Then, we review published experimental demonstrations of thermal imaging via meta-optics. These meta-optics vary in complexity from simple hyperboloid metalenses to complex systems composed of engineered meta-atoms and multiple layers of optics. Finally, we suggest some areas where thermal meta-optics may be useful, and we identify some emerging approaches to solve lingering challenges of meta-optical imaging.

physics.optics↗

arXiv2024

Ultranarrow-linewidth Wavelength-Vortex Metasurface Holography

Ultrathin metasurface holograms, with thicknesses comparable to the operating wavelength, leverage multiple degrees of freedom of light to address independent image channels, thereby significantly enhancing information capacity. Although the wavelength of light can be used to encode holographic image channels, high-capacity wavelength-multiplexing holography has traditionally been achieved only through 3D volume holograms based on Bragg diffraction. We demonstrate ultranarrow-linewidth wavelength-vortex multiplexing holography in ultrathin metasurface holograms. By applying dispersion engineering to the elementary grating functions of a multiplexing hologram, we develop a sparse k-vector-filtering aperture array in momentum space that achieves sharp wavelength selectivity in conjunction with orbital angular momentum selectivity. Further leveraging transformer neural networks for the design of phase-only multiplexing holograms, we reconstruct up to 118 independent image channels from a single metasurface hologram, achieving an ultranarrow linewidth of 2 nm in the visible range. Finally, we apply the developed wavelength-vortex multiplexing metasurface holograms for holographic visual cryptography, achieving unprecedented security with an information rate more than 2500 times higher than that of traditional visual cryptography schemes. Our results open exciting avenues for the use of metasurface holograms in various applications, including 3D displays, holographic encryption, beam shaping, LiDAR, microscopy, data storage, and optical artificial intelligence.

physics.optics↗

arXiv2024

Meta-Optics Triplet for Zoom Imaging at Mid-Wave Infrared

Lenses with dynamic focal length, also called zoom functionality, enable a variety of applications related to imaging and sensing. The traditional approach of stacking refractive lenses to achieve this functionality results in an expensive, heavy optical system. Especially for applications in the mid-infrared, light weight and compact form factor are required. In this work, we use a meta-optic triplet to demonstrate zoom imaging at mid-wave infrared wavelengths. By varying the axial distances between the optics, the meta-optic triplet achieves high quality imaging over a zoom range of 5x, with 50$^\circ$ full field of view in the widest configuration and an aperture of 8 mm. This triplet system demonstrates the potential for meta-optics to reduce conventional components in complex and multi-functional imaging systems to dramatically thinner and lighter components.

physics.optics↗

arXiv2024

Wide Field of View Large Aperture Meta-Doublet Eyepiece

Wide field of view and light weight optics are critical for advanced eyewear, with applications in augmented/virtual reality and night vision. Conventional refractive lenses are often stacked to correct aberrations at wide field of view, leading to limited performance and increased size and weight. In particular, simultaneously achieving wide field of view and large aperture for light collection is desirable but challenging to realize in a compact form-factor. Here, we demonstrate a wide field of view (greater than 60$^\circ$) meta-optic doublet eyepiece with an entrance aperture of 2.1 cm. At the design wavelength of 633 nm, the meta-optic doublet achieves comparable performance to a refractive lens-based eyepiece system. This meta-doublet eyepiece illustrates the potential for meta-optics to play an important role in the development of high-quality monochrome near-eye display and night vision systems.

physics.optics↗

arXiv2024

Compressed Meta-Optical Encoder for Image Classification

Optical and hybrid convolutional neural networks (CNNs) recently have become of increasing interest to achieve low-latency, low-power image classification and computer vision tasks. However, implementing optical nonlinearity is challenging, and omitting the nonlinear layers in a standard CNN comes at a significant reduction in accuracy. In this work, we use knowledge distillation to compress modified AlexNet to a single linear convolutional layer and an electronic backend (two fully connected layers). We obtain comparable performance to a purely electronic CNN with five convolutional layers and three fully connected layers. We implement the convolution optically via engineering the point spread function of an inverse-designed meta-optic. Using this hybrid approach, we estimate a reduction in multiply-accumulate operations from 17M in a conventional electronic modified AlexNet to only 86K in the hybrid compressed network enabled by the optical frontend. This constitutes over two orders of magnitude reduction in latency and power consumption. Furthermore, we experimentally demonstrate that the classification accuracy of the system exceeds 93% on the MNIST dataset.

cs.CV↗

arXiv2024

Beating bandwidth limits for large aperture broadband nano-optics

Flat optics have been proposed as an attractive approach for the implementation of new imaging and sensing modalities to replace and augment refractive optics. However, chromatic aberrations impose fundamental limitations on diffractive flat optics. As such, true broadband high-quality imaging has thus far been out of reach for low f-number, large aperture, flat optics. In this work, we overcome these intrinsic fundamental limitations, achieving broadband imaging in the visible wavelength range with a flat meta-optic, co-designed with computational reconstruction. We derive the necessary conditions for a broadband, 1 cm aperture, f/2 flat optic, with a diagonal field of view of 30° and an average system MTF contrast of 30% or larger for a spatial frequency of 100 lp/mm in the visible band (> 50 % for 70 lp/mm and below). Finally, we use a coaxial, dual-aperture system to train the broadband imaging meta-optic with a learned reconstruction method operating on pair-wise captured imaging data. Fundamentally, our work challenges the entrenched belief of the inability of capturing high-quality, full-color images using a single large aperture meta-optic.

physics.optics↗

arXiv2023

Broadband Thermal Imaging using Meta-Optics

Subwavelength diffractive optics known as meta-optics have demonstrated the potential to significantly miniaturize imaging systems. However, despite impressive demonstrations, most meta-optical imaging systems suffer from strong chromatic aberrations, limiting their utilities. Here, we employ inverse-design to create broadband meta-optics operating in the long-wave infrared (LWIR) regime (8 - 12 $μ$m). Via a deep-learning assisted multi-scale differentiable framework that links meta-atoms to the phase, we maximize the wavelength-averaged volume under the modulation transfer function (MTF) of the meta-optics. Our design framework merges local phase-engineering via meta-atoms and global engineering of the scatterer within a single pipeline. We corroborate our design by fabricating and experimentally characterizing all-silicon LWIR meta-optics. Our engineered meta-optic is complemented by a simple computational backend that dramatically improves the quality of the captured image. We experimentally demonstrate a six-fold improvement of the wavelength-averaged Strehl ratio over the traditional hyperboloid metalens for broadband imaging.

physics.optics↗

arXiv2023

Non-volatile Phase-only Transmissive Spatial Light Modulators

Free-space modulation of light is crucial for many applications, from light detection and ranging to virtual or augmented reality. Traditional means of modulating free-space light involves spatial light modulators based on liquid crystals and microelectromechanical systems, which are bulky, have large pixel areas (~10 micron x 10 micron), and require high driving voltage. Recent progress in meta-optics has shown promise to circumvent some of the limitations. By integrating active materials with sub-wavelength pixels in a meta-optic, the power consumption can be dramatically reduced while achieving a faster speed. However, these reconfiguration methods are volatile and hence require constant application of control signals, leading to phase jitter and crosstalk. Additionally, to control a large number of pixels, it is essential to implement a memory within each pixel to have a tractable number of control signals. Here, we develop a device with nonvolatile, electrically programmable, phase-only modulation of free-space infrared radiation in transmission using the low-loss phase-change material (PCM) Sb2Se3. By coupling an ultra-thin PCM layer to a high quality (Q)-factor (Q~406) diatomic metasurface, we demonstrate a phase-only modulation of ~0.25pi (~0.2pi) in simulation (experiment), ten times larger than a bare PCM layer of the same thickness. The device shows excellent endurance over 1,000 switching cycles. We then advance the device geometry, to enable independent control of 17 meta-molecules, achieving ten deterministic resonance levels with a 2pi phase shift. By independently controlling the phase delay of pixels, we further show tunable far-field beam shaping. Our work paves the way to realizing non-volatile transmissive phase-only spatial light modulators.

physics.optics↗

arXiv2023

Spectrally-encoded non-scanning imaging through a fiber

With the advent of neuroimaging and microsurgery, there is a rising need for capturing images through an optical fiber. We present an approach of imaging through a single fiber without mechanical scanning by implementing spatial-spectral encoding. The spectral encoding is achieved through a microfabricated spectral filter array, where light from different spatial pixels is coded with a highly orthogonal spectrum. The image is then computationally recovered via pseudo inverse of the encoding process. We demonstrate imaging of a $4 \times 4$ binary object at the proximity of the spectral filter array using $560-625nm$ wavelength band. The recovered image maintains an error rate of $<11\%$ when measured using a spectrometer with a spectral resolution of $1.5nm$. The image remains unchanged with fiber bending or moving. Thus our approach shows a more robust way to image through a single optical fiber, with potential applications in compact endoscopes and angioscopes.

physics.optics↗