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Wednesday 9 September

Optics

Optical design for implementing non-collinear circularly polarized high harmonic generation in an enhancement cavity

Pranshu Dave, Thierry Auguste, Olivier Girard et al. (+2 more)

We introduce an original optical cavity design intended to efficiently output-couple extreme ultraviolet (XUV) light produced via cavity-enhanced high harmonic generation (CE-HHG). It supports the amplification of oppositely circularly polarized modes, crossing with a small angle in the high finesse cavity, where circularly polarized HHG scheme (NCP-HHG) will be implemented. We present an analytical model, numerical simulations, and experimental results obtained with a low-power continuous (CW) laser, demonstrating the strong potential of the cavity as an efficient XUV output coupling method. In particular, it is well suited for producing the 7th harmonic (H7) from an Ytterbium frequency comb at 8.4 eV, which is of particular interest for precision nuclear spectroscopy of Thorium.
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All authors Pranshu Dave, Thierry Auguste, Olivier Girard, Thierry Ruchon, David Bresteau

Optics · Cross-listed from quant-ph

Multiparameter quantum bounds for entanglement-assisted aperture synthesis

Kalaga Madhav, Pieter Kok and Nic Scott

Entanglement-assisted optical interferometry promises diffraction-limited imaging over baselines where light cannot be physically combined, but existing theory treats only a single visibility on one two-station baseline. This work formulates the multiparameter estimation problem for an M-station array imaging an extended scene. A reduction lemma shows that the multimode quantum Fisher information (QFI) equals the mean photon number times the QFI of a single delocalized photon, collapsing the problem to a finite-dimensional one. The local photon-number superselection rule (SSR) erases all phase information from the bare state, while shared entanglement restores a fraction f(r)=r/(1+r) of the QFI on a baseline supplied with r pairs – a factor shown to be achievable, since the reduced two-mode state of any pair is exactly the two-station weak-thermal state. The QFI matrix and its mean Uhlmann curvature reveal numerically that a point source is measurement-compatible, whereas for every extended-source model examined it is incompatible, the Holevo bound exceeding the symmetric-logarithmic-derivative (SLD) bound by up to ~74%. An explicit collective receiver – a global mode-sorting (quantum Fourier transform) measurement – then attains a weighted variance within ~7% of the SLD bound and roughly an order of magnitude below an explicit pairwise receiver, and a noisy-resource advantage threshold gives the break-even baseline (~20 km for near-term parameters) beyond which repeater-distributed entanglement beats direct transmission. Finally, the optimal allocation of a finite entanglement budget is a convex program with a closed-form proportional (square-root-law) solution favoring low-visibility baselines. These results recast quantum aperture synthesis as collective multiparameter estimation and give concrete design targets, illustrated on the CHARA array.
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Optics

Laboratory X-ray Microscopy Using Fresnel Zone Plate: Method and Practice

Yongshuai Ge and Yuhang Tan

This book was written to share our experience in developing a Fresnel zone plate (FZP) based X-ray microscope. The research began in September 2020, when we were going to build such a scientific imaging instrument from scratch in our laboratory. At that time, we could not find a textbook that systematically explained the entire development process step by step. We therefore had to make progress through trials and mistakes. After nearly five years of hard effort, we finally managed to build such an X-ray microscope in April 2025. Through this experience, we came to realize how challenging it can be to develop a new FZP-based X-ray microscope without systematic guidance. We therefore believe that it is worthwhile to share the learned lessons with readers who may be interested in developing their own FZP-based X-ray microscopes in the future. I would encourage readers to treat this book as a practical manual rather than a traditional textbook. Readers are not expected to be experts in X-ray imaging. Instead, a basic background in college-level physics should be sufficient to understand most of the material. Inevitably, the current edition has limitations and may contain omissions or inaccuracies due to the authors' limited experience and perspective. I sincerely welcome comments, suggestions and discussions from readers, please feel free to contact me at geyongshuai1987@qq.com. Your feedback will be greatly appreciated and will help make future editions of this book more complete.
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Optics

Expanding RCWA capabilities with advanced S-matrix algorithms

Philippe Lalanne and Jean-Paul Hugonin

Modal methods are particularly well suited to layered photonic structures because electromagnetic propagation within each layer is described analytically. Here, we introduce a simple reformulation of the scattering-matrix (S-matrix) approach in which the S-matrix is defined as an intrinsic property of an individual layer, independently of its neighboring interfaces. This separation between layer propagation and interface coupling preserves the numerical stability of the conventional S-matrix formalism while providing a more modular description of multilayer structures. It also enables direct computation of the scattering coefficients between external plane waves and Bloch modes, as well as between Bloch modes themselves, providing generalized Fresnel coefficients for periodic interfaces. In addition, the reformulation allows the optical response to be evaluated simultaneously for many layer thicknesses or incident wavevectors, with only a modest computational overhead compared with a single calculation. Implemented in the RETICOLO freeware, these capabilities provide a practical framework for modal analysis, parameter sweeps, and the design of layered photonic structures.
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Optics

Transition of Photonic Dissipative Dynamics through the Exceptional Point

Yue Cui, Ruihen Jin, Xiong Jiang et al. (+3 more)

The decay of light in an optical structure depends not only on the intrinsic properties of the material but also on the surrounding electromagnetic environment. This principle has laid the foundation for the engineering of dissipation in photonic emitters. In the conventional wisdom, dissipation is governed by a fixed set of decay channels, each defined by the eigenstates of the structure, and energy leaks through and interacts with these channels. Here we provide experimental evidence that this paradigm fails in non-Hermitian systems. Specifically, we observe an accelerated transient decay in a pair of coupled microcavities tuned near the exceptional point, revealing that photonic dissipation can be governed not by reconfiguring existing loss channels, but rather by restructuring the underlying state space. The universality of this phenomenon is corroborated through two independent control parameters.The finding provides a new perspective on dissipative dynamics in open optical systems and offers a distinct mechanism for controlling transient decay in ultrafast photonic systems.
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All authors Yue Cui, Ruihen Jin, Xiong Jiang, Kaiyang Wang, Can Huang, Qinghai Song

Optics

Non-Hermitian Light Beams

Andrey Novitsky, Dongliang Gao, Iryna Kaputskaya et al. (+4 more)

Non-Hermitian systems provide remarkable features actively studied in modern photonics. Non-Hermiticity is often related to the properties of open structures and devices, which involve lossy and gain channels. Here, we reveal that electromagnetic fields themselves can be designed in a non-Hermitian way, proposing an additional degree of freedom in non-Hermitian physics. Within classical electromagnetic wave theory, we show that the non-Hermitian behaviors may stem not only from the dispersive properties of the waves, but also from the peculiarities of the light spatial spectrum and local structure of the fields. For the nondiffracting light beams, we identify exceptional points detaching symmetric and asymmetric non-Hermitian phases related to the shape of the beam's intensity profile, the non-Hermiticity being well recognized only for non-paraxial beams. In the local structure of the electromagnetic fields and Poynting vector, we determine exceptional lines differentiating non-Hermitian phases stemming from the behavior of streamlines. We believe that the concept of non-Hermitian light beams will enrich our knowledge of the light-matter interaction.
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All authors Andrey Novitsky, Dongliang Gao, Iryna Kaputskaya, Dawei Li, Andrey Bogdanov, Lei Gao, Mikhail Rybin

Optics

Image recognition based on optical spike processing with exciton-polaritons

Olgierd Jeziorski, Jakub Rogala and Krzysztof Tyszka

Exciton-polariton condensates combine ultrafast dynamics with strong optical nonlinearities, making them promising for optical neuromorphic computing. Here, we numerically demonstrate image recognition using the transient response of an exciton-polariton condensate. Downscaled MNIST images are encoded as optical pulse sequences, and the resulting condensate dynamics are sampled and processed by a linear classifier. For ten-class classification, condensate features achieve 92.2% accuracy, compared with 91.4% for raw-pixel linear classification. Combining both feature sets increases the accuracy to 93.2%. For the challenging binary classification of digits 7 and 9, the combined model reaches 97.3%, exceeding both the linear (95.5%) and feed-forward neural-network (96.6%) baselines. These results demonstrate that polariton dynamics can extract useful nonlinear features for ultrafast optical information processing.
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Optics

Stable vortex soliton arrays in disclination-fractal systems

Shuang Shen, Milivoj R. Belić, Ce Shang et al. (+2 more)

Vortex light fields carry orbital angular momentum and have attracted significant attention because of their wide applications in light field manipulations, optical communications, and quantum information processing. In nonlinear media, the balance between diffraction and self-action enables the formation of vortex solitons. However, achieving the stability of vortex solitons remains challenging due to radial and azimuthal modulation instabilities. Here, we report stable vortex solitons and vortex-soliton arrays in disclination-fractal configurations with different rotational symmetries, constructed by applying disclination operations to fractal lattice structures. The stable vortex-soliton arrays—composed of several vortex solitons—can exist along domain walls, resulting from the disclination operation applied to the topologically trivial phase. Their stability is verified through both linear stability analysis and direct simulations of perturbed propagation. For comparison, vortex-soliton arrays in conventional disclination configurations are found to be completely unstable, demonstrating the significance of the fractal configuration in stabilizing these nonlinear states. These findings reveal a new nonlinear excitation mechanism arising from the interplay between fractal geometry and disclination defects, thereby providing theoretical underpinnings for both the improved understanding of multi-field excitation phenomena in complex geometric systems and opening new avenues for the design of photonic devices based on fractal disclination structures.
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All authors Shuang Shen, Milivoj R. Belić, Ce Shang, Yongdong Li, Yiqi Zhang

Optics

Resonant subspace approximation for photonic crystal slabs

Nikolay A. Gippius, Ilia M. Fradkin, Natalia S. Salakhova et al. (+1 more)

Resonant approximations are indispensable for the interpretation and efficient modeling of photonic crystal slabs, yet most of them describe an eigenmode as a pole in the complex energy plane at a fixed set of the remaining parameters. Tracking such poles and their hybridization across the Brillouin zone or upon variation of structural parameters is labor-intensive and severely limits the use of resonant approximations in band-structure calculations and structural optimization. Here we introduce a resonant subspace approximation that treats the photon energy and all other parameters on an equal footing. Considering a wide class of photonic crystal slabs that can be split into two non-resonant parts, we show that their resonances arise solely from the round-trip propagation of coupled Fourier harmonics between these parts, in direct analogy with Fabry–Pérot and waveguide modes. We generalize the scalar round-trip phase to round-trip and phase matrices, whose smooth dependence on all parameters allows us to project the problem onto a small resonant subspace defined at a single anchor point and to extrapolate it throughout a local region of parameter space of arbitrary dimensionality. As a result, rigorous computations at only a few points suffice to reconstruct the band structure, modal linewidths, complex hybridization, and optical spectra as functions of energy, wavevector, geometric dimensions, or permittivity within seconds. We demonstrate the accuracy and versatility of the approach on a strong hexagonal silicon grating, resolving intricate mode hybridization, symmetry-protected features, and subtle geometry-controlled effects that are hardly accessible to straightforward computations. The method is fast, accurate, and readily extensible, offering a practical route to the exploration, design, and optimization of resonant photonic crystal slabs.
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All authors Nikolay A. Gippius, Ilia M. Fradkin, Natalia S. Salakhova, Sergey A. Dyakov

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

Facile hBN-hBN Interfacial Overlap Engineering for Enhanced Quantum Emitter Formation

Nhat Minh Nguyen, Trung Vuong Doan, Md Shakhawath Hossain et al. (+12 more)

Quantum emitters in two-dimensional materials, particularly hBN, are promising platforms for quantum technologies. However, achieving high-density emitters at predetermined locations while preserving optical quality remains challenging. Here, we introduce a facile, cost-effective double-layer all-dry transfer approach to deterministically create overlap regions between hBN flakes. These pre-defined capped regions exhibit a significantly enhanced emitter density, with up to a 15-fold increase compared to uncapped areas. Importantly, this method does not compromise emitter quality: emitters within overlap regions demonstrate excellent optical performance, including high signal-to-background and signal-to-noise ratios, large Debye-Waller factors, high brightness, and strong spectral stability. Possible defect configurations are also discussed to contextualize the observed emission characteristics. This scalable strategy enables preferential formation of quantum emitters in targeted regions, achieving higher densities than simple treatments such as plasma irradiation while avoiding the complexity of advanced fabrication techniques. The approach provides a practical pathway for integrating high-quality quantum emitters into scalable quantum photonic platforms.
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All authors Nhat Minh Nguyen, Trung Vuong Doan, Md Shakhawath Hossain, Akila Elangasinghe, Duc Anh Ngo, Ha Ngoc Duy Huynh, Thi Ngoc Anh Mai, Yongliang Chen, Kenji Watanabe, Takashi Taniguchi, Michael G. Ruppert, Chaohao Chen, Xiaoxue Xu, Toan Dinh, Toan Trong Tran

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

Heteroepitaxial Growth of PbSe on InP Substrates via Lattice-Matched III-V Buffer Layers

Biridiana Rodriguez, Mark Martino, Brody Yeung et al. (+2 more)

Detector cost remains a barrier to the widespread adoption of mid-wave infrared (3-5 micron) technology. PbSe, an inexpensive narrow band gap IV-VI semiconductor that has been used since the early 1940s, delivers high performance for infrared detection despite the abundance of grain boundaries in polycrystalline films. Epitaxial growth, however, could provide superior crystalline quality and interfaces, but suitable substrates remain limited for PbSe. Recent PbSe heteroepitaxy has focused primarily on III-V, II-VI and group-IV substrates that each offer a comparatively narrow range of lattice-matched alloys for heterostructure engineering. Here we show that InP-based heteroepitaxy provides access to a broader materials platform while limiting the lattice mismatch with PbSe to approximately 4%. We grow 150-nm thick PbSe films by molecular beam epitaxy on 200-nm thick In0.53Ga0.47As and In0.52Al0.48As buffers on (001) InP substrates. Reflection high-energy electron diffraction and X-ray diffraction show (001)-oriented rock-salt PbSe with an out-of-plane lattice constant of 6.12 Angstrom on both buffers. Photoluminescence (PL) is observed from room-temperature down to 12 K and the peak wavelength red-shifts from 3.7 to 5.0 microns. Under identical measurement conditions, the room-temperature peak PL intensities from films on In0.53Ga0.47As and In0.52Al0.48As are approximately 1.9x and 1.3x that of a PbSe on GaAs substrate reference, respectively. These results establish an InP-compatible platform for integrating narrow band-gap PbSe with a broad range of ternary and quaternary III-V alloys, including, for example, structures in which In0.53Ga0.47As serves as both a short-wave infrared absorber and a template for PbSe growth.
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All authors Biridiana Rodriguez, Mark Martino, Brody Yeung, Benjamin Sprenger, Leland Nordin

Tuesday 8 September

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

Non-Hermitian topology in driven-dissipative systems: correspondence with quantum correlations and a resource for entanglement

Niladri Chakraborty and Clara C. Wanjura

Directional amplification, in which signals are amplified selectively depending on their propagation direction, is a key resource for quantum information processing and stands in one-to-one correspondence with non-trivial non-Hermitian topology. So far, this correspondence has concerned the mean fields, and thus classical response. Here we turn to the quantum fluctuations, giving access to correlations and entanglement. For phase-preserving amplifiers, we derive analytic expressions for the normal and anomalous correlations, showing that non-trivial topology produces correlations that grow exponentially with the distance between modes and approach the largest values compatible with the uncertainty relations. The associated correlation length diverges at the topological phase transition. Entanglement nonetheless remains local, set by the competition between normalised anomalous correlations and the asymmetry of the mode occupations. For the bosonic Kitaev chain, a phase-sensitive amplifier, the system instead splits into two halves that are internally fully correlated yet mutually uncorrelated. Our work prepares the ground for exploring the quantum properties of non-Hermitian topological systems with state-of-the-art platforms such as cavity optomechanics and superconducting circuits.
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Optics

Stimulated Brillouin Scattering in InGaP-on-Insulator Waveguides

Yuyang Xue, Lisa-Sophie Haerteis, Ryan L. Russell et al. (+8 more)

Stimulated Brillouin Scattering (SBS) is a nonlinear interaction between optical and acoustic waves in solids. First regarded as a parasitic process in optical fibers, it has gathered significant interest in microwave photonic applications such as optical signal processing and narrow linewidth lasers. While many Brillouin system demonstrations have been done on photonic chips, they struggle to provide simultaneous high Brillouin gain and narrow linewidth in a scalable platform, capable of integration with established photonic integrated circuits. Here, we present a novel integrated InGaP-on-SiO$_2$ platform, where a single crystalline InGaP waveguide layer offers superior material properties and strong nonlinearities to support SBS near 1550 nm wavelength. We demonstrate backward SBS with a measured high Brillouin gain coefficient of $588\, \mathrm{W}^{-1}\,\mathrm{m}^{-1}$ and a record narrow linewidth of 5.2 MHz at 9.346 GHz frequency shift. This work establishes a wafer-scale complementary metal-oxide semiconductor compatible fabrication process, paving a scalable path for high gain and narrow linewidth Brillouin photonics with applications such as precision signal processing.
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All authors Yuyang Xue, Lisa-Sophie Haerteis, Ryan L. Russell, Choon Kong Lai, Benjamin J. Eggleton, Michael J. Steel, Glenn Solomon, Kevin L. Silverman, Moritz Merklein, Andreas Boes, Nima Nader

Optics

Evaluating the predictive power of pump-probe imaging contrast of melanin for metastatic outcome: A 71-patient study

David Grass, Emma Bao, Martin C. Fischer et al. (+3 more)

Significance: Most melanoma deaths arise from metastatic spread, yet current staging imperfectly identifies which primary tumors will progress. Melanin structure is altered during malignant transformation, and pump-probe microscopy (PPM) measures melanin excited-state dynamics in standard biopsy sections, offering molecular contrast that is complementary to morphology-based assessment. Aim: To determine whether PPM-derived melanin excited-state dynamics in primary cutaneous melanoma can predict metastatic outcome. Approach: We imaged unstained sections from 71 primary cutaneous melanomas and 18 melanoma metastases. Transient absorption curves were fit to a biexponential excited-state model, and the fit parameters were used as features for learning classifiers to predict metastatic outcome. Results: Melanin dynamics separated primary from metastatic tissue: both excited-state lifetimes were shorter in primaries, with common-language effect sizes up to 0.26. The same parameters did not separate primary tumors by outcome (effect sizes 0.41-0.61), and univariate logistic regression found no significant association for either the excited-state lifetimes or the ESA/GSB ratio. The best classifier reached 74% patient-level accuracy (AUC 0.73), a plateau reached by several unrelated architectures. Conclusions: PPM robustly distinguishes primary from metastatic melanoma tissue, consistent with progressive melanin disaggregation. While it is clear that pump-probe features of melanin thus correlate with tissue differences, melanin dynamics in single sections of the primary tumor, confounded by heterogeneity, do not give robust predictions of metastatic outcome.
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All authors David Grass, Emma Bao, Martin C. Fischer, M. Angelica Selim, Georgia M. Beasley, Warren S. Warren

Optics · Cross-listed from hep-ph

Backward Compton scattering with three vortex particles

Yi Liao, Zhaolong Teng and Hao-Lin Wang

We investigate backward Compton scattering in which an incident vortex (vx) photon collides head-on with a plane-wave (pw) electron and both final-state particles are projected onto vortex states, $\gamma_{\rm vx}+e^-_{\rm pw}\to\gamma_{\rm vx}+e^-_{\rm vx}$. We derive analytically the scattering amplitude and differential cross sections in the energy and cone angle of either final particle. Rotational symmetry implies a selection rule associated with the conservation of the total angular momentum along the collision axis. A Bessel-Gaussian wave packet is adopted for the incident photon to provide a physical normalization and regularize the boundary singularities of ideal Bessel states. We present numerical results for a $10~{\rm MeV}$ electron colliding with a vortex photon of central energy $1~{\rm MeV}$ or $10~{\rm keV}$. The final-photon distributions exhibit strong energy–angle correlations, topological-charge-dependent interference fringes, and systematic shifts of their dominant peaks, allowing different topological charge sectors to be enhanced through angular or energy post-selection. For a longitudinally polarized incident electron, the dominant channels favor final photons of matching helicity. For the $1~{\rm MeV}$ photon benchmark, the final electron can likewise be produced as a MeV-scale vortex state with a sizable cone angle and a large angular-momentum projection. These results demonstrate that triple-vortex backward Compton scattering offers a potential means of generating and controlling high-energy vortex photons and electrons.
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Optics

Dynamic Chirality in Photonic Time Crystals

Jianming Mai, Chenwen Yang and Shubo Wang

Temporal modulation offers a fundamentally distinct degree of freedom for active wave control beyond static spatial structuring. Photonic time crystals (PTCs), based on periodic modulation of electromagnetic parameters in time, have expanded photonic band engineering from space to time by enabling controlled energy exchange between light and the modulation. Yet, the use of PTCs to synthesize rotational dynamics and thereby control chirality and circular dichroism (CD) remains largely unexplored. Here, we propose a spatiotemporal PTC whose central cylindrical element is driven by an azimuthally traveling-wave permittivity modulation. Although the structure is geometrically static, its dielectric profile evolves as an effectively rotating pattern in time. This synthetic rotation lifts a static modal degeneracy and produces two nondegenerate counter-rotating states with opposite orbital angular momenta. These chiral modes selectively couple to left- and right-circularly polarized light, giving rise to tunable CD. In addition, the spatiotemporal modulation induces orbital angular-momentum conversion between the Floquet replica bands. Our work reveals the microscopic origin of dynamic chiral response and establishes a strategy for reconfigurable chiral photonics without mechanical motion.
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Optics

Quantized transport of two-dimensional multifrequency solitons

Fangwei Ye, Yaroslav V. Kartashov and Vladimir V. Konotop

We consider Thouless pumping of two-dimensional quadratic solitons composed from coherently interacting fundamental frequency (FF) and second harmonic (SH) components propagating in a periodic $\chi^{(2)}$ material. The pumping is induced by two mutually sliding two-dimensional lattices defined by shallow transverse and longitudinal periodic refractive index modulation. Focusing on solitons in the semi-infinite gap, we find three distinct pumping scenarios: the absence of transport for small-amplitude solitons, non-quantized transport in a transient regime at intermediate amplitudes, and stable quantized transport for solitons with relatively large amplitudes. Different directions of the sliding velocity were investigated resulting in different trajectories of soliton center. The transition to the regime of quantized transport is found to depend strongly on phase mismatch between FF and SH waves that also influences stability properties of two-dimensional $\chi^{(2)}$ lattice solitons. We also show that pumping with longitudinal periods in the x and y directions, whose ratio approximates an irrational number, induces quantized transport whose direction rapidly converges, with increase of the accuracy of the approximation, to a limiting pumping direction determined by this irrational number and corresponding to truly incommensurate longitudinal periods.
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Optics

Encoding Propagation Invariance into Light

Wenxiang Yan, Tianyue Li, Zhuolin Wu et al. (+6 more)

Diffraction governs the axial evolution of optical fields, whereas conventional holographic synthesis primarily controls their transverse structure. Here we add axial diffraction management as an additional design freedom to the transverse_field programmability of holography, enabling the transverse optical function and its diffraction-driven axial evolution to be co_designed. By incorporating established propagation_invariant dynamics into computer_generated hologram and meta_hologram synthesis, we realize task_selectable axial responses in user_defined monochromatic, full-colour and vectorial fields. On a spatial light modulator, the same scalar user_defined field is configured either for a rapidly evolving 1.2_cm depth of field or for an approximately 75_cm propagation_invariant range. Millimetre_scale metasurfaces further enable metre_scale refocusing_free full_colour projection and vectorial colour fields with polarization textures preserved over more than 30 cm. This transverse_axial co_design framework extends holographic field synthesis beyond transverse programmability, providing a broadly compatible route towards task_configurable optical systems and compact multidimensional photonics.
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All authors Wenxiang Yan, Tianyue Li, Zhuolin Wu, Yiyu Zhao, Zhi-Cheng Ren, Xi-Lin Wang, Hui-Tian Wang, Shuming Wang, Jianping Ding

Optics

Universal Broadband Linear Optical Transformations by an Interlaced Structured Integrated Photonic Processor

Ilya V. Kondratyev, Kseniia N. Urusova, Artem S. Argenchiev et al. (+8 more)

Reconfigurable photonic integrated circuits enable applications in a wide range of fields. They represent a powerful platform for signal processing due to the ability to perform parallel computations. In this work, we demonstrate a photonic processor based on a six-channel universal chip fabricated by femtosecond laser writing with an interlaced architecture comprising multiport beam splitters. We exploit and enhance a previously demonstrated calibration procedure for a single building block, and extend it to a complete six-channel interferometer, reconstructing the corresponding interferometer models over a broad spectral range. We validate them by measuring a set of 100 Haar-random unitary matrices, achieving mean amplitude fidelities of 95.9 %, 98.0 %, and 96.0 % for radiation wavelengths 910, 945, and 980 nm, respectively, without on-device optimization. Furthermore, we experimentally realize spectral demultiplexing and multiplexing at these wavelengths, yielding routing fidelities ranging from 90.1 % to 96.5 % and total crosstalk values ranging from -9.6 to -14.4 dB, confirming the applicability of the fabricated photonic processor to practical wavelength-routing tasks.
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All authors Ilya V. Kondratyev, Kseniia N. Urusova, Artem S. Argenchiev, Sergei A. Zhuravitskii, Nikolay N. Skryabin, Alexander D. Golikov, Vadim V. Kovalyuk, Gregory N. Goltsman, Ivan V. Dyakonov, Stanislav S. Straupe, Sergei P. Kulik

Optics

Harmonics of femtosecond and continuous-wave radiation for precision spectroscopy: generation efficiency and spectral line shape

Nikita Zhadnov, Olga Khronusova, Alexey Russkikh et al. (+2 more)

Precision spectroscopy in the UV and VUV ranges, including spectroscopy of the thorium-229 nuclear transition (148.4 nm), requires narrow-band sources obtained by frequency multiplication of stabilized lasers. Upon harmonic generation, the radiation intensity drops and the phase noise grows. In this work, both factors are analyzed for two types of primary sources: a continuous-wave single-frequency laser and a femtosecond optical frequency comb. It is shown that in the weak-conversion regime for equal average input powers, the power of the central mode in the spectrum of the Kth harmonic of femtosecond radiation matches the harmonic power of continuous-wave radiation for K=2 and noticeably exceeds it for K>2. Within the first-order dispersion approximation, group-velocity mismatch reduces the total conversion efficiency without affecting the power of the exactly phase-matched central mode. The transformation of the phase noise of an ultrastable laser upon harmonic generation leads to two effects that depend on K: the harmonic linewidth grows linearly, whereas the power fraction in the carrier decreases as a Gaussian function and is determined by the high-frequency noise of the stabilization loop (the servo bumps). Measurements of the phase-noise spectra of a laser stabilized to a Fabry-Perot cavity show that for a typical rms phase excursion of ~100 mrad the carrier loses about half of its power already at the eighth harmonic, while for a non-optimal stabilization-loop gain carrier collapse occurs at the second-third harmonic.
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All authors Nikita Zhadnov, Olga Khronusova, Alexey Russkikh, Nikolay Kolachevsky, Anatoly Masalov

Optics

Computational analysis and performance optimization of SrScCu3Se4-based solar cells using COMSOL Multiphysics

Khalid Mahmud, Md. Nahid Hasan, Md. Islahur Rahman Ebon et al. (+2 more)

Transition metal-based quaternary semiconductors show strong magneto-optical and thermoelectric properties, yet their photovoltaic performance in realistic three-dimensional device architectures remains underexplored. In this work, we investigate a n-ZnSe/p-SrScCu3Se4/p+-WSe2 quaternary chalcogenide heterostructure using three-dimensional finite-element simulations in COMSOL Multiphysics. The model self-consistently couples wavelength-dependent optical generation, drift-diffusion carrier transport, and thermal loss analysis under AM1.5G, 1-sun illumination within a fully coupled opto-electro-thermal framework. The effect of absorber width, acceptor level, and bulk defects on photovoltaic performance, carrier generation, and recombination profile is investigated in this study. Quaternary chalcogenide device produces an open circuit voltage (VOC) of 1.02 V, short-circuit current density (JSC) of 32.472 mA/cm2, fill factor (FF) of 88.212%, and power conversion efficiency (PCE) of 29.217% under optimized conditions. The quantum efficiency (QE) results indicate that the device effectively transforms incident light into charge carriers within the visible spectrum, however absorption and carrier collecting performance diminish in the near-infrared range. The electrothermal simulations indicate modest, spatially non-uniform temperature increases relevant to Joule heating and nonradiative recombination heating within the active layers. Overall, these findings ensure that SrScCu3Se4, a quaternary chalcogenide, is a promising absorber material and offers a strong experimental design consideration for obtaining high-performance, thermally stable three-dimensional photovoltaic device topologies.
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All authors Khalid Mahmud, Md. Nahid Hasan, Md. Islahur Rahman Ebon, Bipanko Kumar Mondal, Jaker Hossain

Optics

Magic-free coexisting photonic and phononic moiré flat bands

Ziming Chen, Kaiyu Cui, Ning Wu et al. (+8 more)

Moiré flat bands enhance localization and interactions through suppressed group velocity, but existing approaches largely target a single physical field because distinct excitations generally require different, finely tuned magic configurations. Here we introduce a flat-band mechanism based on strong diffractive hybridization among moiré-folded bands. Period-mismatched modulations open distinct coupling channels whose hybridization renormalizes the band dispersion. An effective Hamiltonian shows that increasing the diffractive coupling progressively suppresses the group velocity, driving the system toward a flat-band regime without field-specific magic configurations. This coupling-induced mechanism enables band flattening across distinct physical excitations. We demonstrate this mechanism in a single-layer moiré optomechanical crystal, where photonic and phononic flat bands are simultaneously realized, and their localized modes and optomechanical interaction are experimentally observed. Beyond photonic and phononic systems, this mechanism may extend to other wave and quasiparticle platforms, providing a general route to co-localizing and coupling distinct physical fields in moiré systems.
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All authors Ziming Chen, Kaiyu Cui, Ning Wu, Shuyuan Li, Chenxuan Wang, Xue Feng, Fang Liu, Wei Zhang, Hao Sun, Yongzhuo Li, Yidong Huang

Optics

Fiber-based electro-optic dual-comb light source for fast linear and nonlinear spectroscopy

Debanuj Chatterjee, Fabio Motetta, Simon Boivinet et al. (+2 more)

Dual-comb spectroscopy (DCS) enables rapid, broadband and high-resolution optical measurements by mapping optical spectra into the radio frequency (RF) domain. However, conventional DCS systems are fundamentally constrained by a tradeoff between optical bandwidth and interferogram acquisition speed, limiting their overall performance. Here, we demonstrate an all-fiber, polarization-maintaining (PM) frequency-agile electro-optic modulation (EOM) dual-comb source that simultaneously achieves a broad optical bandwidth of 10 THz and a high interferogram acquisition speed of up to 2.5 MHz. The high acquisition rate is enabled through an in-phase/quadrature (IQ) modulator-based architecture to shift the carrier frequency of one of the combs. We illustrate the performance of the source through proof-of-concept linear spectroscopy and nonlinear dual-comb coherent anti-Stokes Raman scattering (CARS) spectroscopy measurements. The combination of large spectral coverage, high refresh rate and an all-PM fiber configuration makes this dual-comb platform attractive for applications such as rapid molecular spectroscopy and high-speed nonlinear spectroscopic imaging.
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All authors Debanuj Chatterjee, Fabio Motetta, Simon Boivinet, Hervé Rigneault, Arnaud Mussot

Optics · Cross-listed from quant-ph

Performance of the subcarrier-wave quantum key distribution in the presence of spontaneous Raman scattering noise generated by classical DWDM channels

Fedor Kiselev, Roman Goncharov, Natalia Veselkova et al. (+3 more)

In this paper we study performance of the subcarrier-wave quantum key distribution system (SCW QKD) in the presence of spontaneous Raman scattering (SpRS) noise generated by classical channels of dense wavelength division multiplexing (DWDM) network within a single-mode optical fiber. We present the mathematical model for evaluation of the quantum bit error rate (QBER) and the secure key generation rate with the SpRS noise taken into account. We consider two regimes of the SCW QKD system: the continuous wave regime that uses continuous wave laser and the pulsed regime. For these regimes, performance of the system is analyzed depending on receiver sensitivity of classical DWDM. It is found that the pulsed regime outperforms the continuous wave regime in both the secure key generation rate and the maximum achievable distance.
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All authors Fedor Kiselev, Roman Goncharov, Natalia Veselkova, Eduard Samsonov, Alexei D. Kiselev, Vladimir Egorov

Optics · Cross-listed from quant-ph

A theoretical study of subcarrier-wave quantum key distribution system integration with an optical transport network utilizing dense wavelength division multiplexing

Fedor Kiselev, Natalia Veselkova, Roman Goncharov et al. (+1 more)

In this paper we study the performance of subcarrier-wave quantum key distribution (SCW QKD) in the presence of classical channels of optical transport network utilizing dense wavelength division multiplexing (DWDM). We consider the impact of spontaneous Raman scattering noise as well as the four-wave mixing and channel isolation efficiency. We calculate quantum bit error rate as well as the secure key generation rate of SCW-QKD protocol for different parameters of DWDM system and quantum channel allocations. Our calculations show, that quantum channel allocation at the wavelength of 1310 nm is preferable to allocation at C-band due to lower nonlinear noise, despite fiber's larger attenuation at this wavelength.
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All authors Fedor Kiselev, Natalia Veselkova, Roman Goncharov, Vladimir Egorov

Optics

BRAHMS: A cross-platform graphical toolkit for (3+1)D simulation of three-wave mixing in $\chi^{(2)}$ nonlinear media, with GPU and CPU backends

Alfredo Daniel Sanchez Rossi and Daniel Alejandro La Valle Huignard

We present BRAHMS, a cross-platform (Linux/Windows) graphical user interface, with GPU (CUDA) and CPU (OpenMP) backends, for the efficient and accurate simulation of three-wave mixing processes involving focused and pulsed Gaussian beams. The package solves the coupled $\chi^{(2)}$ nonlinear Schrödinger equations, including diffraction, dispersion, walk-off, and phase-mismatch effects simultaneously, in full (3+1)D. To our knowledge, this is the first open-source package that solves the complete second-order (three-wave-mixing) nonlinear-optics problem in (3+1)D while fully exploiting the parallel capabilities of modern GPUs, with an equivalent CPU backend available for users without GPU hardware. The GPU implementation is inherently scalable thanks to its Thrust-based design. The package provides a valuable tool for experimental design and for studying three-dimensional field propagation in nonlinear three-wave interactions, supporting applications such as second-harmonic generation (SHG), sum-frequency generation (SFG), and optical parametric generation (OPG). Its GUI requires no programming experience, so that users can readily set up simulations and interpret their results.
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Optics

Thresholdless corner vortex solitons in fractal Sierpiński topological insulators

Yiqi Zhang, Alexander V. Kireev, Victor O. Kompanets et al. (+12 more)

Quantized vortices are ubiquitous in physics, spanning superconductivity, astrophysics, superfluid condensed matter systems, and nonlinear optics. Yet embedding vorticity into topologically protected nonlinear states has remained a major challenge, with all previously observed corner solitons in higher-order topological insulators (HOTIs) exhibiting only trivial phase distributions. Here, we report on the first realization of stable topological corner vortex solitons in a photonic fractal HOTI. Using an array of laser-written waveguides in the shape of Sierpiński gasket with a controllable distortion, we design linear topological vortex modes, from which nonlinear corner vortex solitons bifurcate. Moreover, we demonstrate that these solitons exhibit exceptional robustness across a broad power range and, unlike vortex solitons in topologically trivial lattices, form without a power threshold. Our results introduce the angular momentum degree of freedom into the physics of topological corner modes, opening prospects for topologically protected vortex-based photonics.
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All authors Yiqi Zhang, Alexander V. Kireev, Victor O. Kompanets, Sergey Y. Alyatkin, Nikita S. Kostyuchenko, Sergei A. Zhuravitskii, Nikolay N. Skryabin, Khalil Sabour, Alexander A. Kalinkin, Yongdong Li, Sergei P. Kulik, Pavlos G. Lagoudakis, Sergey V. Chekalin, Yaroslav V. Kartashov, Victor N. Zadkov

Optics

Integrated yoctosecond-precision timing detector

Jie Yang, Tong Wang, Yulin Shen et al. (+12 more)

Precise timing detection is essential for exploring ultrafast phenomena in fields ranging from free-electron lasers to ultra-high-power laser facilities. However, achieving simultaneous high resolution and large dynamic range remains a fundamental challenge, and state of the art systems are often constrained by their physical size, power requirements, and limited scalability. Here we introduce an integrated dual electro optic sub cycle timing detector (DEST) that overcomes these limitations. In a proof of principle measurement, the device resolves timing jitter as small as 11 yoctoseconds (ys, $10^{-24}$ s) at 1 MHz–equivalent to the transit time of light across two protons–while maintaining an unambiguous measurement range of 6.15 ps and a dynamic range exceeding 155 dB. The core detection unit is miniaturized to chip scale dimensions of 18 mm x 2 mm x 1 mm on a thin film lithium niobate platform, ensuring inherent stability and immunity to environmental disturbances. Moreover, the architecture naturally lends itself to massive parallelization through array integration, with the potential to push timing precision to the sub 10 rontosecond (rs, $10^{-27}$ s) level within a 1 $m^2$ footprint. This combination of extreme sensitivity, wide dynamic range, compact size, and scalability opens new avenues for detecting previously inaccessible weak signals, including those from gravitational waves, quantum vacuum fluctuations, and beyond.
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All authors Jie Yang, Tong Wang, Yulin Shen, Dehui Pan, Zhichao Chen, Ming-Yang Zheng, Bin Wang, Shaobo Fang, Yi Zhang, Ke Zhang, Jiahui Yao, Minghua Chen, Guorong Wu, Xueming Yang, Ming Xin

Optics

Topology of Photonic Time Crystals under Complex Refractive Index Modulation

Huili Lin, Zhaohui Dong, Xianfeng Chen et al. (+1 more)

Photonic time crystals, constructed by periodic modulation of electromagnetic parameters in time, hold unique topological properties. Here, we explore permittivity and conductivity co-modulated PTCs (PCM PTCs) by dynamically tuning permittivity and conductivity simultaneously of the spatially-uniform materials, and discover their distinctive non-Hermitian effects. We find that such PCM PTCs hold a global shift of the Floquet spectrum along the imaginary quasifrequency axis, which controls field amplification and decay in both momentum bands and band gaps. By further considering the temporal interface consisting of two different PCM PTCs, we can unveil the relationship between the emergence of topological edge states and momentum band inversion by analyzing the eigenstates at exceptional points. Moreover, the characteristic of the topology of PCM PTCs can also be captured from the complex Dirac mass. Our work studies the topology of PTCs in complex modulation regime, offering new opportunities for manipulating topological edge states in the time domain.
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All authors Huili Lin, Zhaohui Dong, Xianfeng Chen, Luqi Yuan

Optics · Cross-listed from physics.atom-ph

A Portable Dual-Color Two-Photon Rubidium Optical Frequency Standard

Sarah K. Scholten, Emily Ahern, Clayton Locke et al. (+8 more)

Portable atomic clocks are essential in a wide variety of applications, most notably in the operation of global navigation satellite systems. Existing portable atomic clocks utilizing microwave-based interrogation schemes are now routinely eclipsed by the next generation of atomic frequency standards based on optical interrogation. While optical frequency standards demonstrate greatly improved frequency stability, they have only recently reached a level of technical maturity required to demonstrate this improved performance outside of well curated laboratory environments. Here, we demonstrate a fully autonomous and portable optical frequency standard based on an efficient dual-color excitation of the $5S_{1/2}\rightarrow5D_{5/2}$ two-photon transition in $^{87}$Rb. The standard utilizes a combination of robust, highly developed commercial-off-the-shelf telecommunications technologies and a fully integrated portable optical frequency comb, providing the optical and microwave outputs vital for interfacing with existing electronic systems and infrastructure. The system demonstrates a fractional frequency stability of $1.9\times10^{-13}$ at 1s of integration time, reaching $3.5\times10^{-15}$ at 8000s of integration time without the need for drift removal. This portable demonstrator unit marks a significant achievement in the development of Rb optical atomic frequency standards, and for the deployment of optical atomic frequency standards outside of the laboratory.
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All authors Sarah K. Scholten, Emily Ahern, Clayton Locke, Christopher J. Billington, Nicolas Bourbeau Hébert, Montana Nelligan, Ashby P. Hilton, Rachel F. Offer, Elizaveta Klantsataya, Christopher Perrella, Andre N. Luiten