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Physics Physics Optics

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Friday 11 September

Optics

Compact, open, and tunable two-dimensional Fabry-Pérot cavity

Bruno Bender, Andrea Bergschneider, Marcel Baer et al. (+2 more)

In order to enhance light-matter interactions, optically active material, like a semiconductor, can be embedded into an optical cavity. For small cavity mode volumes, which are required for reaching the strong coupling regime, this is typically done by fabrication of microcavities. While guaranteeing a mechanically stable cavity, these systems lack tunability and are difficult to implement for small samples like van der Waals heterostructures (vdW). Here we present a design for a fully tunable two-dimensional (2D) Fabry-Pérot (FP) cavity which can host any material. The cavity can be freely positioned between two confocal high-NA aspheric lenses which allows for optical transmission microscopy and spectroscopy in real and momentum space. In order to maintain maximal mechanical stability, the cavity mirrors are held in a monolithic titanium frame. A pre-loaded flexure joint allows for manual coarse tuning of the cavity length and mirror parallelism, while three piezo actuators in a tripod layout can be used for fine tuning. We find cavity length fluctuations smaller than 100 pm, measure a finesse of 360 and reach the lowest accessible $m=3$ cavity mode (one electric field antinode within the air gap) at a minimal physical mirror separation of 380 nm. In order to demonstrate the 2D nature of the cavity mode we perform momentum space imaging in transmission as well as spectroscopy to measure the cavity dispersion.
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All authors Bruno Bender, Andrea Bergschneider, Marcel Baer, Martin Kroner, Wolf Wüster

Optics · Cross-listed from cond-mat.mes-hall

Superfluorescence in CdS/CdSe/CdS Spherical Quantum Wells Modulated by Excitation Geometry

Chunzheng Bai, Qihao Sun, Yiying Zhu et al. (+6 more)

Superfluorescence (SF) originates from the spontaneous buildup of macroscopic coherence among initially incoherent emitters. Its formation is highly sensitive to dephasing and collective coupling in the system. Here, we observe room-temperature SF in CdS/CdSe/CdS spherical quantum wells. With increasing excitation fluence, the peak emission intensity shows a nearly quadratic increase, while the emission delay and pulse width decrease. Burnham-Chiao ringing is also observed, revealing the characteristic collective radiation dynamics of SF. Further experiments with stripe excitation show that shortening the excitation length L changes the dominant fast emission from amplified spontaneous emission to SF. More importantly, the threshold for SF under stripe excitation is only about 1/30 of that under spot excitation. This large reduction indicates that the propagating radiation field provided by spatially extended excitation favors the buildup of cooperative coherence. These findings provide direct experimental evidence for understanding ultrafast many-body coherence dynamics and offer a route to actively control collective emission at room temperature.
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All authors Chunzheng Bai, Qihao Sun, Yiying Zhu, Ruilin Xu, Bin Gu, Jinsong Qi, Fengrui Hu, Xiaoyong Wang, Jiayu Zhang

Optics

Full complex wavefunction reconstruction with direct-interference ptychography

A. Ducharme and L. Clark

We demonstrate the separable reconstruction of the sample wavefunction amplitude and phase using direct-interference ptychography in scanning transmission electron microscopy. We reconstruct the sample wavefunction in simulated and experimental data under the Weak Object Approximation (WOA) with an extension of single-sideband (SSB) ptychography using information in the double- and triple-overlap regions. The contrast transfer function for the amplitude information is equivalent to the ideal contrast transfer function of Center of Mass imaging (COM). Our results support reporting the imaginary part of SSB reconstructions.
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Optics

Interplay between growth conditions and formation of localized light emitters in epitaxial AlN thin films

Meysam Saeedi, Duc V. Dinh, Andrey N. Anisimov et al. (+3 more)

AlN is a promising material for integrated quantum photonics due to its broad transparency window, its compatibility with CMOS technology, and the possibility to host localized light emitters acting as single photon sources. However, little is understood yet about the microscopic nature and formation mechanisms of these light emitters. In this contribution, we demonstrate that their formation efficiency depends significantly on the growth conditions of the AlN thin film. We reveal that localized light emitters with distinct zero-phonon lines in the 600–800~nm range can be efficiently formed in epitaxial AlN thin films grown under N-rich conditions by exposing the AlN to a post-growth treatment consisting in a proton irradiation followed by an annealing process. In contrast, the density of light emitters formed in AlN grown under Al-rich conditions remains low, even after the thin films have been exposed to the post-growth treatment. These results open a pathway toward a better understanding of the microscopic nature of these light emission centers, as well as their engineered integration as quantum light sources in AlN-based optomechanical platforms.
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All authors Meysam Saeedi, Duc V. Dinh, Andrey N. Anisimov, Mingyun Yuan, Georgy V. Astakhov, Alberto Hernández-Mínguez

Optics

Prior-information based super-resolution optical metrology of 2D nanoscale objects

Jin-Kyu So, Eng Aik Chan, Carolina Rendón-Barraza et al. (+2 more)

Previous work has shown that optical metrology of one-dimensional objects, such as slit width, can achieve improved accuracy by using prior information from similar objects to train the metrology estimator. Here, we demonstrate single-shot optical metrology of nanoscale elliptical particles by analysing their diffraction patterns to retrieve length, width and in-plane orientation using a neural-network estimator trained on prior information from nano-ellipses with varied dimensions and orientations. Fisher-information flow analysis was used to optimise the physical parameters of the metrology apparatus and maximise measurement accuracy. Using a 633 nm laser, we measure the dimensions of elliptical particles with accuracy down to ${\lambda}$/128, corresponding to 4.9 nm, and recover their orientation with 5${\deg}$ accuracy. Our results demonstrate the practicality of optical, deep-super-resolution, single-shot, multiparameter measurements of two-dimensional subwavelength objects, with potential relevance to microbiology and nanotechnology applications.
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All authors Jin-Kyu So, Eng Aik Chan, Carolina Rendón-Barraza, Giorgio Adamo, Nikolay I. Zheludev

Optics

Autonomous multifunctional image processing via programmable multimode lasing

Jiawei Wu, Yue Yin, Jianqi Hu et al. (+3 more)

Optical image processing offers a promising pathway to overcome the latency and energy limitations of conventional electronic image processors. However, existing approaches based on passive photonic devices are often constrained by signal attenuation, lack of nonlinearity, fixed functionality, and heavy training overhead. Here, we introduce a programmable image processor based on a highly multimode degenerate cavity laser (DCL), shifting the computational framework from passive extracavity transformation to active intracavity evolution. By manipulating the intracavity loss distribution and exploiting the nonlinear lasing dynamics, we map computational tasks to the spontaneous mode selection within the DCL, realizing image processing directly at the source. We experimentally demonstrate that by simply altering the encoding scheme of input images, the platform can be flexibly reconfigured for multifunctional tasks, including high-fidelity edge detection and robust image denoising. The computation proceeds autonomously without dataset training, featuring an intrinsically low latency ($\sim$90 $\mu\mathrm{s}$) with $\mathcal{O}(1)$ complexity. Furthermore, the high intensity sustained within the resonator enables intracavity nonlinear upconversion of the processed image. This work extends the frontiers of laser applications, providing a compelling candidate for next-generation optical processors.
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All authors Jiawei Wu, Yue Yin, Jianqi Hu, Hao Wang, Xing Fu, Qiang Liu

Optics · Cross-listed from cond-mat.mtrl-sci

Revealing Hidden Inversion Symmetry Breaking in ZrTe$_5$ via Phonon-Assisted Heterodyne Amplification

S. J. Li, J. H. Huang, H. Y. Wu et al. (+13 more)

ZrTe$_5$ is a sensitive topological material where small perturbations can alter its electronic structure. Its equilibrium crystal structure has been widely regarded as centrosymmetric, while recent experiments have raised the possibility of inversion-symmetry breaking. Here we probe this hidden symmetry lowering using nonlinear optical spectroscopy. Although conventional second-harmonic generation does not resolve an equilibrium symmetry-breaking signal, terahertz-field-induced second-harmonic generation (TFISH) reveals it through phonon-assisted heterodyne amplification. A coherently driven infrared-active phonon acts as a local oscillator for the vanishingly weak second-order susceptibility $\chi^{(2)}$, converting an otherwise undetectable symmetry-breaking response into a phonon-frequency modulation of the TFISH signal. The field-linear scaling of this modulation demonstrates $\chi^{(2)}$ is an equilibrium susceptibility rather than a response induced by the THz field. Polarization- and temperature-dependent measurements identify a bulk polar distortion along the crystallographic $a$ axis that persists to room temperature, while the $c$ axis remains nonpolar. These results provide direct optical evidence for equilibrium inversion-symmetry breaking in bulk ZrTe$_5$ and establish a structural constraint for understanding its electronic and topological properties.
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All authors S. J. Li, J. H. Huang, H. Y. Wu, S. P. Zheng, C. J. Kong, B. Xu, H. Wang, T. Dong, L. Yue, D. Wu, Y. Wan, Z. L. Li, X. B. Wang, S. J. Zhang, N. L. Wang, Y. T. Li

Optics

Enhanced chiral response of hybrid photonic structures empowered by Mie resonances

Xingguang Liu, Wenzhe Hao, Ivan Toftul et al. (+3 more)

Chiroptical response provides a powerful route to control light-matter interactions through light handedness, with crucial importance in polarization control, chiral sensing, and nonlinear photonics. Hybrid structures may offer a novel route to engineer such responses by coupling plasmonic field localization with dielectric Mie resonances within a single nanostructure. However, in chiral metasurfaces, hybridization has largely been treated as a means of resonance enhancement, while its role in actively reshaping and reversing intrinsic plasmonic chirality remains largely unexplored. Here, we demonstrate a hybrid metasurface in which a dielectric Mie resonance mediates enhanced chiral response of plasmonic building blocks. By coupling a Si nanocylinder to trapezoidal Au nanobars, a weak and mode-selective plasmonic chiral seed is reorganised into two opposite-handed hybrid resonances, producing near-unity circular dichroism with spectral sign flipping. Extending this mechanism to nonlinear regime, the same hybrid-mode selectivity produces strongly nonlinear chiral emission and near-unity third-harmonic generation circular dichroism of either sign. We believe these results establish plasmonic-Mie hybridization as a novel mechanism for engineering high-purity, sign-switchable linear and nonlinear chirality in compact metadevices.
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All authors Xingguang Liu, Wenzhe Hao, Ivan Toftul, Junqing Li, Yongkang Dong, Yuri Kivshar

Optics

Resonance-Enhanced Time Reflection at Photonic Temporal Interfaces

Zeyuan Li, Hammam Bahurmuz, Mohamed H. Mostafa et al. (+5 more)

Photonic temporal interfaces enable dynamic control of light fields, yet strong time reflection at optical frequencies remains challenging because conventional approaches demand large material refractive-index changes on ultrafast timescales. Here, we introduce a resonance-assisted mechanism that harnesses both polarization energy accumulated in a dispersive medium prior to the temporal interface and strongly increased energy supplied by the modulation system. We show that, under a specific critical condition, rapidly increasing the material resonance frequency yields orders-of-magnitude stronger time-reflected power flux density than what conventional plasma-frequency modulation provides. To implement this mechanism in optical systems, we identify two routes based on dielectric and plasmonic structural resonances. For the plasmonic route, we develop an analytical effective-medium model of conducting-oxide cylinder arrays, in which localized surface-plasmon resonances transform the constituent Drude response into a geometrically tunable effective Lorentz response. Using cadmium oxide as a representative material, we predict an enhancement exceeding three orders of magnitude in the summed reflected-mode power coefficient relative to the same homogeneous material under the same modest plasma-frequency modulation, even in the presence of realistic losses. These findings establish spatial resonance engineering as an effective route to strong temporal scattering with reduced demands on intrinsic material tunability.
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All authors Zeyuan Li, Hammam Bahurmuz, Mohamed H. Mostafa, Mohammad S. Mirmoosa, Puneet Garg, Carsten Rockstuhl, Xuchen Wang, Viktar Asadchy

Thursday 10 September

Optics

Inverse reconstruction of dissipative Kerr soliton interactions

Carlo Silvestri, Panayotis G. Kevrekidis, Stephane Coen et al. (+2 more)

We reconstruct the pairwise interaction potential between dissipative Kerr solitons from the linear stability spectrum of a perfect soliton crystal solution of the Lugiato–Lefever equation. The solitons form an overdamped lattice whose positional eigenvalues encode the pairwise-force derivative, mirroring the extraction of microscopic interactions from phonon or relaxation spectra in condensed-matter systems. The method extends beyond quadratic dispersion and assumes no functional form for the interaction. Our theory predicts a novel state -the soft soliton crystal- which has vanishing stiffness and a diverging positional relaxation time, and also explains experimentally observed soliton steps as arising from a sign reversal of the pairwise-force derivative.
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All authors Carlo Silvestri, Panayotis G. Kevrekidis, Stephane Coen, C. Martijn de Sterke, Antoine F. J. Runge

Optics

Frequency-Multiplexed Photonic Reservoir Computing with a Synchronously Pulse-Driven Optical Cavity

Amir Arsalan Arabieh, Simon-Pierre Gorza and Serge Massar

In this work, we introduce a new platform for frequency-multiplexed reservoir computing based on an optical cavity driven by pulses whose repetition period is matched to the cavity roundtrip time. The input symbols are encoded by modulating either the amplitude or the phase of the driving pulses. Our numerical results show that the proposed synchronously pulse-driven cavity operates in a weakly nonlinear regime under anomalous dispersion, while under normal dispersion it exhibits optical bistability with a high-peak-power upper-branch state. By isolating the key physical parameters, we quantify their individual contributions to information-processing performance. We further show that spectral symmetry breaking induced by third-order dispersion and Raman scattering almost doubles the information-processing capacity. Finally, we demonstrate that modulation-induced branch switching within the bistable regime limits stable reservoir operation.
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Optics · Cross-listed from quant-ph

Path-Degenerate Quantum Interferometry for Decoherence Mitigation in Gravitational-Wave Detectors

Jonas Rittmeyer, Niels Boettner, Farid Khalili et al. (+2 more)

Optical decoherence degrades quantum correlations in squeezed states of light, severely limiting the quantum-nondemolition (QND) sensitivity of gravitational-wave detectors. Here, we propose the path-degenerate quantum interferometry scheme that obviates the need for entire optical subsystems—including additional filter cavities, auxiliary parametric amplifiers, and Faraday isolators—thereby drastically reducing spatial mode mismatches while inherently integrating variational output, a long-standing theoretical proposal to further deepen the QND regime. We experimentally demonstrate a core aspect of this scheme, achieving a shot-noise-preserving signal enhancement that directly counteracts the detrimental effects of readout loss. By delivering an improved signal-to-quantum-noise ratio solely through the consolidation and reduction of currently considered optical subsystems, our approach offers a highly optimized route toward enhanced quantum-noise reduction in upcoming LIGO upgrades and next-generation gravitational-wave observatories.
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All authors Jonas Rittmeyer, Niels Boettner, Farid Khalili, Mikhail Korobko, Roman Schnabel

Optics · Cross-listed from physics.class-ph

Plasmonics at radio frequencies

Igor I. Smolyaninov, Quirino Balzano, John Mulholland et al. (+2 more)

We demonstrate that low loss radio frequency plasmon-like surface electromagnetic waves may propagate along rough interfaces between air and highly conductive media, such as water or metal. Unlike the well-known Zenneck wave solutions, these surface waves have mode index larger than 1, so that their propagation properties are generally similar to the properties of surface plasmons in the visible frequency range. The wave impedance of these modes is derived, which demonstrates the need to use special plasmonic inducers or exciters for efficient generation of these novel surface electromagnetic modes. Propagation characteristics of these radio frequency plasmons were explored both experimentally and theoretically in the cases of water-air and water-ice-air interfaces, and the two-dimensional character of this propagation has been revealed. Other important applications of radio frequency plasmons were demonstrated, such as communication along metal infrastructure immersed in water, communication along a lake floor, underwater plasmonic radar, as well as plasmonic communications underground and in the dense jungles.
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All authors Igor I. Smolyaninov, Quirino Balzano, John Mulholland, Viktor A. Podolskiy, Dendy Young

Optics · Cross-listed from cond-mat.str-el

Sub-cycle doublon-holon dynamics in one-dimensional Mott insulators revealed by two-color high-harmonic spectroscopy

Lance Hatch, Aditya Verma, Eric Schultz et al. (+7 more)

Solid-state high-harmonic spectroscopy is becoming an emerging tool for probing nonequilibrium many-body dynamics. Yet, direct measurements of strongly driven, sub-optical-cycle dynamics in correlated materials during high-harmonic emission remain largely unexplored. Here, we measure high-harmonic emission chirp in a prototypical one-dimensional Mott insulator, which encodes strongly driven doublon-holon dynamics at sub-optical-cycle timescales. We observe a positive chirp for above band gap harmonics, indicating that high harmonics are dominated by doublon-holon recombinations. We further show a harmonic order-dependent dephasing, which can be understood through different doublon-holon excursion distances associated with each harmonic. These results reveal coherent doublon-holon dynamics and their ultrafast dephasing in Mott insulators, which is relevant to other nonequilibrium light-induced phenomena, such as Floquet engineering.
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All authors Lance Hatch, Aditya Verma, Eric Schultz, Hanjun Yang, Priscila Rosa, Genda Gu, Igor Zaliznyak, Giulio Vampa, Laimei Nie, Hanzhe Liu

Optics

Designing metallo-dielectric antennas for cryogenic applications

Siwei Luo, Tim Hebenstreit, Alexey Shkarin et al. (+3 more)

We present the design of cryogenic metallo-dielectric antennas tailored to single organic emitters, where the choice of host material imposes specific constraints on the antenna geometry. Using dibenzoterrylene (DBT) in para-dichlorobenzene (p-DCB}) as a model system, we show that photon collection efficiencies exceeding 90% can be achieved for arbitrary dipole orientations of the fluorescent molecule. The antenna design is intrinsically broadband and tolerant to emitter positioning within the structure. We further provide concrete fabrication guidelines for the full antenna architecture, and experimentally demonstrate its operating principle by recording a back-focal-plane image (BFP) of a single molecule inside a fabricated antenna.
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All authors Siwei Luo, Tim Hebenstreit, Alexey Shkarin, Jan Renger, Tobias Utikal, Stephan Götzinger

Optics

Heterogeneously Integrated Efficient and Widely Tunable Lasers at 795 nm for Rubidium-Based Quantum Technologies

Max Kiewiet, Stijn Cuyvers, Tom Reep et al. (+8 more)

Scaling quantum processors and optical atomic clocks fundamentally requires orders-of-magnitude reductions in the size, weight, power, and cost of optical control systems. Photonic integration of lasers is critical to fulfill these requirements. At the near-infrared wavelengths required for atomic clocks and quantum computing through manipulation of rubidium atoms, laser integration is hindered by difficulty in light coupling and poor heat dissipation. Here, we introduce a wafer-scalable method utilizing micro-transfer printing to integrate GaAs-based amplifiers in etched recesses, directly butt-coupled to silicon nitride waveguides. We demonstrate extended-cavity single-mode lasers using this integration approach. Our compact microgear laser achieves a narrow 3 kHz fundamental linewidth at an on-chip output power of >22 mW – a record for a single-mode heterogeneously integrated laser in the 780-800 nm band – with a wall-plug efficiency of 9.4%, showcasing the high-power and efficiency potential of this integration approach. Additionally, we demonstrate a widely tunable laser leveraging Vernier filters to achieve lasing with 9 nm coarse tuning, a quasi-continuous fine-tuning range exceeding 140 GHz, and a mode-hop-free tuning range of 45 GHz. Our scalable integrated laser toolkit shows great promise for replacing macroscopic external-cavity diode lasers in next-generation quantum technologies and optical atomic clocks.
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All authors Max Kiewiet, Stijn Cuyvers, Tom Reep, Konstantinos Akritidis, Gaudhaman Jeevanandam, Manuel Reza, Roelof Jansen, Pol Van Dorpe, Günther Roelkens, Kasper Van Gasse, Bart Kuyken

Optics

Temporal Localisation of Waves from Imaginary Line-Gap Topology

Carlos Camacho, Tom Sheppard, Joshua Feis et al. (+4 more)

For non-Hermitian Hamiltonians, gain, loss, and non-reciprocity produce complex eigenvalues which, in turn, facilitate different kinds of topological phases. One example is the imaginary line-gap phase, where eigenvalues cannot lie on the real line. This notion was recently shown to explain the robust temporal localisation of waves in photonic quantum walks and time-varying metamaterials. In these systems, waves localise around a time interface between topologically inequivalent mediums. At the core of this phenomenon is a $\mathcal{PT}$-symmetric two-mode model, where the non-trivial topology arises due to the $\mathbb{Z}_2$ classification of the AI symmetry class. In this work, we study two-mode models in all non-Hermitian symmetry classes. We find that robust temporal localisation generically follows as a physical consequence of imaginary line-gap topology according to a simple diagnostic: at least one of time-reversal symmetry ($\mathcal{T}^{\hspace{0.05em}2} = 1$) and particle-hole symmetry ($\mathcal{C}^2 = 1$) must be present. Our results provide a comprehensive symmetry-based guide to the observation of the topologically protected temporal localisation of waves.
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All authors Carlos Camacho, Tom Sheppard, Joshua Feis, Oliver Ashfield, Alexander Szameit, Frank Schindler, Hannah M. Price

Optics

Antimony for broadband nanophotonics across the ultraviolet, visible and infrared

Fernando Chacón-Sánchez, Jacek Wojcik, Marina García Pardo et al. (+5 more)

Semimetal elemental antimony (Sb) nanostructures show great potential for applications where nanophotonic properties play a key role, such as phase-change optical memories, non-linear optical elements, photothermal therapy agents, photodetectors and photocatalysts. However, designing advanced Sb-based photonic devices critically requires an accurate and reliable knowledge of the optical response of bulk and nanoscale Sb. Herein, we report for the first time a fully consistent and accurately measured dielectric function for Sb nanoscale films in a wide spectral range from the ultraviolet to the far infrared (4 - 0.04 eV, i.e. ~ 0.3 - 30 $\mu$m), surpassing previous reports that explored a limited spectral range. It is found that the Sb spectral response is driven exclusively by giant interband transitions in the visible up to mid infrared (4 - 0.4 eV, i.e. ~ 0.3 - 3 $\mu$m), and that their contribution dominates over that of free carriers down to 0.12 eV (i.e. ~ 10 $\mu$m). Such spectral response enables Sb nanostructures to display spectrally selective and tunable nanophotonic resonances. First, we showcase interband plasmonic resonances in the visible-to-near infrared for Sb nanogratings. Second, we report giant refractive index dielectric resonances in the mid infrared for nanostructured Sb/dielectric/metal resonant cavities. These findings open a pathway to optimized planar Sb nanoscale designs enabling a tailored light-matter interaction, which will be useful for integrated data, telecom, medical, optoelectronic and energy conversion devices operating in a broad spectral range.
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All authors Fernando Chacón-Sánchez, Jacek Wojcik, Marina García Pardo, Fátima Cabello, Peter Mascher, Fernando Agulló-Rueda, Rosalía Serna, Johann Toudert

Optics · Cross-listed from quant-ph

The Quantum Plumber's Problem

Nicolas G. Underwood, Holger F. Hofmann and Jonte R. Hance

Recent work quantified the notion of quantum counterfactual gain for an extended Elitzur-Vaidman bomb test style scenario, through a connection to the negativity of the Kirkwood-Dirac quasiprobability distribution. We here extend this work to identifying quantum advantage in a new scenario, which we term the “Quantum Plumber's Problem”. In this scenario, we imagine a “quantum plumber”, who knows that one path of an interferometer is blocked, and wants to find the optimal strategy for identifying with certainty which path this is. We discuss various strategies for a generalised path-encoded interferometer, as well as for the specific case of Hofmann's three-path interferometer, introduced in a recent analysis of the relationship between states in five measurement contexts of a three level system. We support our arguments on the relative merit of competing strategies with data collected over many simulated attempts at locating blockages. We also present results for a variant of the game in which the blockage is replaced by a non-demolition detector.
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Optics

A Two-Mirror Faceted Projection System for EUV Lithography

Vasiliy A. Es'kin, Egor V. Ivanov and Olga V. Martynova

We propose an all-reflective two-mirror projection system for extreme ultraviolet (EUV) lithography operating at exposure wavelengths of $13.5$~nm (Mo/Si) and $11.2$~nm (Ru/Be), delivering a fourfold ($4\times$) demagnification of the periodic mask pattern at a numerical aperture approaching unity ($\mathrm{NA}_{\max} \approx 0.993$). In contrast to conventional EUV projection objectives that incorporate 6–10 aspheric mirrors with an overall optical throughput of less than $15\%$, the proposed design redirects each accepted discrete spatial diffraction order scattered by the mask onto the wafer via a dedicated pair of planar mirror facets. The number of reflections is strictly fixed at two for all accepted orders, retaining $50$–$60\%$ of the power leaving the mask in each accepted order. We derive a spatial geometry providing rigorous optical path length equalization across all diffraction orders, thereby removing order-dependent propagation phase shifts. Individually optimized 30-bilayer Bragg multilayer coatings are designed for each facet using the transfer matrix method combined with global evolutionary optimization algorithms. The architecture is generalized to a three-dimensional vector formulation with a two-dimensionally periodic mask. Utilizing inverse lithography technology, Fourier parameterization, and a differentiable electromagnetic modal waveguide solver, we solve the synthesis problem for binary absorber masks (La absorber on a Ru/Be/Sr multilayer mirror). We demonstrate simulated aerial images of sub-10-nm features on the wafer (isolated peaks with a full width at half maximum (FWHM) of approximately $5.4$~nm and line pairs with a critical dimension of $6$~nm) and find that the two peaks remain resolved for the tested wafer defocus values from $0$ to $5$~nm along the $z$-axis.
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Optics

Two-step high-accuracy microwave frequency measurement and time-frequency analysis based on optical frequency combs

Wentao Ma, Taixia Shi, Chi Jiang et al. (+1 more)

Broadband microwave frequency measurement and time-frequency analysis are crucial for applications such as electronic warfare. However, when it comes to ultra wideband signal analysis, traditional electronic methods have high analysis accuracy, but intrinsic electronic bottlenecks limit their real-time analysis. Here, we propose and experimentally demonstrate a two-step microwave frequency measurement and time-frequency analysis method based on optical frequency combs. The system first performs coarse frequency localization over the 0-40 GHz range using stimulated-Brillouin-scattering-assisted frequency-to-time mapping (FTTM) and dual-comb channelized reception. The dual-comb is then reapplied for downconverting the signal under test, followed by digital signal processing to achieve high-accuracy unambiguous frequency extraction. Experimental results show that the system achieves mean single-tone frequency measurement errors of less than 10 kHz over 0-40 GHz. We further experimentally measure multi-tone, linearly frequency-modulated, and V-shaped frequency-modulated signals, demonstrating the proposed method's capability for analyzing complex signals.
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All authors Wentao Ma, Taixia Shi, Chi Jiang, Yang Chen

Optics · Cross-listed from physics.app-ph

Scattering-robust Imaging of Azimuthal Features with Enhanced Resolution

Nilakshi Senapati, Abhinandan Bhattacharjee and Anand K Jha

Imaging through scattering media remains a long-standing challenge in numerous real-world applications, ranging from medical imaging to long-distance sensing. Recently, illumination consisting of a single orbital angular momentum (OAM) mode, which is structured in the azimuthal coordinate, has been shown to provide enhanced resolution for imaging objects with azimuthal features, with the resolution becoming maximum at an optimal OAM value. However, in the presence of scattering, single-mode fields, which are spatially fully coherent, cause the imaging resolution to decrease significantly due to speckle formation. In this work, we employ azimuthally partially coherent fields and experimentally demonstrate imaging of azimuthal features with enhanced resolution in the presence of scattering. We show that lower degree of azimuthal coherence in such illumination leads to increased robustness against scattering while the azimuthal structure of the illumination ensures enhanced resolution. We derive the condition for best imaging resolution, and we report increase of imaging contrast in scattering from about 7% to 50% as the illumination is changed from a single-mode fully coherent field to that of an azimuthal partially coherent field.
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Optics · Cross-listed from quant-ph

On-chip squeezed light in the audio frequency band

Xuezhi Zhu, Yunyun Cao, Rui Liu et al. (+7 more)

Squeezed light in the audio-frequency band is a key resource for quantum metrology and quantum sensing. However, realizing stable audio-frequency squeezed light on integrated photonic platforms remains challenging due to technical noise and the difficulty of scalable phase referencing. Here, we demonstrate on-chip generation of audio-band two-mode squeezed states down to 60 Hz in a silica microcavity. To enable phase-stable operation without directly locking fragile quantum modes, we develop a coherent-comb control method in which a weak electro-optic reference comb co-propagates with the vacuum at the quantum frequency modes in an orthogonal polarization. This scheme provides quadrature measurement and long-timescale phase stability, thereby enabling covariance-matrix reconstruction. We verify the entanglement with the positive partial transposition criterion, which confirms inseparability via a minimum symplectic eigenvalue of 0.395 ($<0.5$). Our results establish an experimentally accessible route toward on-chip phase-stable audio-band squeezing and support the scalable framework for continuous-variable quantum information processing with integrated photonics.
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All authors Xuezhi Zhu, Yunyun Cao, Rui Liu, Yaya He, Fan Zhang, Yaqing Zhang, Shiwei Du, Meihong Wang, Xiaoshun Jiang, Xiaolong Su

Optics · Cross-listed from physics.app-ph

Janus Dipoles: Fundamentals, Realizations, and Emerging Applications

Bo Xue, Qiuyang Li, Yuqiong Cheng et al. (+10 more)

The Janus dipole - featuring orthogonally oriented electric and magnetic dipoles with a 90-degree phase difference - has emerged as a powerful paradigm for wave manipulation. Unlike traditional Huygens dipoles used for directional control, this unique configuration exhibits strongly asymmetric, face-selective near-field behavior while maintaining a quasi-isotropic far-field radiation pattern. These remarkable properties make the Janus dipole an essential platform for directional wave shaping, with wide-ranging applications in on-chip photonics, quantum interactions, and wireless power transfer. This review systematically traces the rapid development of the Janus dipole from its foundational theoretical inception to its diverse implementation platforms across optical, microwave, and acoustic frequencies. In this paper, we explore the governing principles, classify realization strategies into passive Janus dipoles, active Janus dipoles, and advanced near-field coupling control, and highlight emerging frontiers. By bridging foundational electrodynamics with advanced device engineering, this paper serves as an essential reference and roadmap for researchers designing next-generation, highly integrated, and compact wave-manipulation systems.
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All authors Bo Xue, Qiuyang Li, Yuqiong Cheng, Wenbo Ma, Xuhuinan Chen, Wanting Tong, Junho Jung, Runqi Jia, Ke Chen, Yijun Feng, Xiao Lin, Shubo Wang, Alex M. H. Wong

Optics · Cross-listed from cond-mat.mtrl-sci

Measuring chiral phonons

Rahul Rao, Hanyu Zhu, Nicholas Kotov et al. (+7 more)

Chiral phonons are quantized vibrations where the atomic motion in a solid breaks improper rotation symmetries. In many cases, chiral phonons possess angular momenta and are therefore selective to circularly polarized light. Both fundamental and applied research efforts on chiral phonons have been gaining increasing attention owing to their importance in a variety of fields including spintronics, spin-selective chemical reactions, thermal transport, quantum information processing and biosensing, where the bi-directional spin-lattice coupling enabled by chiral phonons can be harnessed in new ways, and potentially lead to new functionalities. Thus far, the studies of chiral phonons across diverse materials platforms have evolved largely independently within these fields, but the experimental techniques are often interrelated. In this perspective, we present a detailed description, as well as advantages and disadvantages of the current approaches for experimentally measuring chiral phonons in chiral and achiral materials. We conclude with a discussion of new methods for measuring chiral phonons. Ultimately, this work seeks to offer an experimental guide for systematically investigating the properties of chiral phonons in various materials systems and applications.
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All authors Rahul Rao, Hanyu Zhu, Nicholas Kotov, Thuc T. Mai, Maria F. Muñoz, Dali Sun, Jun Liu, Wonjin Choi, Renee R. Frontiera, Angela R.H. Hight Walker

Wednesday 9 September

Optics · Cross-listed from q-bio.QM

PyFLI: A Python Library for Simulation, Parameter Estimation, and Benchmarking in Fluorescence Lifetime Imaging

Vikas Pandey, Ismail Erbas, Margarida Barroso et al. (+2 more)

Fluorescence lifetime imaging (FLI) measures the temporal decay of fluorescence after excitation and provides quantitative information about a fluorophore's local environment and molecular interactions. Depending on the fluorophore and experimental design, lifetime can report changes associated with pH, oxygenation, cellular metabolism, and Forster resonance energy transfer. These properties make FLI useful across microscopy, biophysics, biomedical optics, and preclinical imaging, where the same type of molecular contrast can be studied across different biological scales. FLI measurements, however, are acquired with instruments that record fluorescence in different ways. Intensified charge-coupled device (ICCD) cameras, single-photon avalanche diode (SPAD) arrays, and time-correlated single-photon counting (TCSPC) systems differ in temporal sampling, data organization, detector noise, instrument response, and file format. Lifetime-estimation methods also make different assumptions about the recorded decay, while learning-based approaches require realistic training and validation data for which the underlying parameters are known. PyFLI is an open-source framework for FLI processing and standardized data simulation. It imports measurements from acquisition systems, simulates labeled data under configurable acquisition and noise conditions, and provides complementary approaches for parameter estimation. These include nonlinear least-squares fitting (NLSF), maximum-likelihood estimation (MLE), phasor analysis, rapid lifetime determination (RLD), Laguerre-based estimation, and optional Bayesian and deep-learning inference. CPU and GPU processing support image-scale analysis, while reconstruction, visualization, statistical analysis, and cross-software comparison provide tools for evaluating results. PyFLI includes compressed-sensing reconstruction for single-pixel hyperspectral FLI.
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All authors Vikas Pandey, Ismail Erbas, Margarida Barroso, Stefan Radev, Xavier Intes

Optics

Quantum eraser enables single-shot quantum holography with undetected photons

Naru Yoneda, Nao Sakiyama, Luis Ordonez et al. (+3 more)

Quantum holography with undetected photons (QHUP) enables the complex amplitude of infrared to be acquired beyond the sensitivity range of silicon based sensors, through induced coherence withoud induced emission. QHUP generally requires multiple phase-shifted holograms to retrieve the complex-amplitude distribution of an object, which limits measurement stability and temporal resolution. Although classical parallel phase-shifting holography provides a favorable balance between temporal and spatial resolution, it has not previously been implemented in QHUP because of the constraints imposed by photon distinguishability. In this study, we propose a quantum eraser-based geometric phase-shifting QHUP. By accurately erasing not only polarization distinguishability but also temporal distinguishability arising from differences in the optical paths, the complex-amplitude distribution of an object can be retrieved in a single shot using a silicon-based image sensor equipped with a polarization-filter array. This single-shot acquisition, combined with real-time processing, enables the observation of dynamic objects in the near-infrared spectral region. To the best of our knowledge, these experimental results constitute the first real-time acquisition of the complex-amplitude distribution of a near-infrared object using QHUP with a silicon-based image sensor equipped with a polarization-filter array. Furthermore, to enhance the reproducibility and applicability of this study, the experimental optical setup, bill of materials, and source code for real-time imaging have been made openly available. The ability to retrieve the complex-amplitude information carried by undetected photons in real time is expected to extend the applicability of QHUP to a wide range of fields, including spectroscopic imaging and semiconductor inspection.
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All authors Naru Yoneda, Nao Sakiyama, Luis Ordonez, Vishal Prajapati, Kanako Yoshimura, Osamu Matoba

Optics

New Method for Path-length Equalization of Long Single-mode Fibers for Interferometry

M.Anderson, J. D. Monnier, K. Ozdowy et al. (+2 more)

The ability to use single mode (SM) fibers for beam transport in optical interferometry offers practical advantages over conventional long vacuum pipes. One challenge facing fiber transport is maintaining constant differential path length in an environment where environmental thermal variations can lead to cm-level variations from day to night. We have fabricated three composite cables of length 470 m, each containing 4 copper wires and 3 SM fibers that operate at the astronomical H band (1500-1800 nm). Multiple fibers allow us to test performance of a circular core fiber (SMF28), a panda-style polarization-maintaining (PM) fiber, and a lastly a specialty dispersion-compensated PM fiber. We will present experimental results using precision electrical resistance measurements of the of a composite cable beam transport system. We find that the application of 1200 W over a 470 m cable causes the optical path difference in air to change by 75 mm (+/- 2 mm) and the resistance to change from 5.36 to 5.50 {\Omega}. Additionally, we show control of the dispersion of 470 m of fiber in a single polarization using white light interference fringes ({\lambda}c=1575 nm, {\Delta}{\lambda}=75 nm) using our method.
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All authors M.Anderson, J. D. Monnier, K. Ozdowy, J. Woillez, G. Perrin

Optics

Quantitative Fabrication-Error Reduction in Optomechanical Crystal using Proximity Error Correction

Pratip Ghosh, Anohita Mallick and Akshay K. Naik

We demonstrate the use of proximity effect correction (PEC) in electron beam lithography (EBL) to improve the fabrication fidelity of dense photonic crystal structures. Monte Carlo simulations were employed to model electron scattering and determine the proximity function of the resist-substrate system. Based on this, a computational dose-modification scheme was implemented to compensate for nonuniform energy deposition during exposure. In addition, SEM-based image analysis was performed to quantitatively assess structural differences among uniformly exposed, manually dose-modified, and PEC-fabricated devices by comparing extracted geometries with the reference GDS design. The analysis revealed reduced dimensional deviation, improved spatial uniformity, and lower edge roughness in the PEC-corrected structures. These improvements resulted in a significantly enhanced optical quality factor in the fabricated photonic crystal cavities.
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Optics

Surrogate-Assisted Inverse Design and Temperature-Dependent Electrothermal Analysis of an All-Oxide Narrowband Thermophotovoltaic Emitter

Bibekananda Nath, Kawshik Nath and Ahmed Zubair

A narrowband emitter aligning with the bandgap of the underlying solar cell is essential for improving the spectral efficiency and thermal stability of thermophotovoltaic (TPV) systems. Emitters based on oxide materials present a promising solution to the optical and mechanical performance degradation of traditional emitters, which employ metal-dielectric structures that experience metal oxidation and structural deterioration at high temperatures. Here, we presented a surrogate-assisted inverse-design framework for a narrowband 1D grating emitter comprising ITO and Al2O3 layers on a sapphire substrate. We performed Bayesian optimization over the trained ExtraTrees surrogates on a penalty-augmented objective containing a peak-emission constraint (E_peak > 0.90) while minimizing the full width at half maximum (FWHM) and maximizing the fraction of emission concentrated within the selected peak-centered spectral band to acquire a high, narrowband peak emission. The surrogate model was trained using a dataset of emission spectra obtained from the finite-difference time-domain (FDTD) by systematically varying the layers' thicknesses and the structure's period as the input features, and the narrowband emission's figure of merit (FOM) ((E_peak), wavelength of peak emission (_peak), FWHM, in-band fraction (f_in) for determining the amount of emission outside the peak band, and concentration of peak emission near the peak) was used as the prediction target. The resulting set of predictions for the optimized structure was further validated using the FDTD method.
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