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Physics Physics Optics

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Wednesday 14 October · still arriving

Optics

Cavity-tunable Pseudospin-Hall Topology and Edge Transport

Minghui Niu, Yan Meng, Chengle Fan et al. (+5 more)

Topological edge states are a hallmark of topological phases, with their existence tied to bulk band topology through bulk-edge correspondence. In photonic pseudospin-Hall systems, these states exhibit pseudospin-momentum locking, with lattice interactions and interface geometry determining each pseudospin's propagation direction. Reversing the pseudospin-dependent propagation direction without altering the lattice configuration or excitation handedness remains a fundamental challenge in controlling topological edge transport. Here, we theoretically and experimentally demonstrate cavity-tunable topological edge transport in a microwave metasurface embedded within a metallic cavity. Varying the cavity height reshapes resonator interactions, driving band inversion and topological phase transition while preserving the lattice geometry. For a fixed domain-wall configuration, this transition reverses the propagation direction associated with each pseudospin while preserving pseudospin-momentum locking. Excitations with the same handedness are observed to launch edge waves propagating in opposite directions at their respective edge-band frequencies determined by the cavity height. These results establish cavity electrodynamics as an external means of controlling the bulk topology and reconfiguring the directional response of topological interfaces, creating new opportunities for topological wave routing in fixed photonic lattice structures.
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All authors Minghui Niu, Yan Meng, Chengle Fan, Xiaoyuan Jiao, Jidong Song, Perry Ping Shum, Zhen Gao, Mengyao Li

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

Making Weak Interlayer Excitons Absorb: Manipulating the Effective Oscillator Strength via Purcell Engineering of Critical Coupling

Saroj Poudyal, Anabel Atash Kahlon, Michael Klein et al. (+7 more)

Interlayer excitons (IXs) in transition metal dichalcogenide bilayers possess unique dipolar properties, providing a path for exploring correlated many-body phases, electrically tunable dipole-dipole interactions, and long-range exciton transport. However, the large electron-hole separation underlying their dipolar character also results in a significantly reduced oscillator strength, limiting their direct optical interaction with light, and thus the direct probing of their dynamics and many-body interactions. Here, we show that by using Purcell engineering of critical coupling, a weak interlayer transition can absorb as strongly as the dominant intralayer transition, despite its much smaller intrinsic oscillator strength. Using this approach, in an open cavity architecture we demonstrate an order of magnitude IX absorption modulation in bilayer and trilayer MoS2, with doubling of the IX to A-exciton interaction strength weight from ~23% to ~53%. Furthermore, we demonstrate that in a tailored closed cavity architecture, the IX enhancement can reach beyond an order of magnitude, from ~5% to ~83% excitonic absorption, enabling IX absorption similar to intralayer excitons. An analytical description of the interaction is obtained via a closed-form transmission-line model which separates the excitonic and photonic contributions to the optical response, revealing how cavity architecture can be designed towards critical coupling. Our findings establish Purcell engineering of critical coupling as a general route to controlling and enhancing weak excitonic transitions.
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All authors Saroj Poudyal, Anabel Atash Kahlon, Michael Klein, Matan Meshulam, Yonatan Gershuni, Thomas Poirier, James H. Edgar, Mukesh Kumar, Seth Ariel Tongay, Itai Epstein

Optics

Photoacoustic-guided reflection-mode wavefront shaping through dynamic biological tissue

Xuan Liu, Sacha Noimark, Wenfeng Xia et al. (+2 more)

Strong optical scattering limits the depth at which light can be focused and used for imaging in biological tissue. Photoacoustic (PA)-guided wavefront shaping enables non-invasive light focusing in scattering media, but most existing implementations require optical and acoustic access from opposite sides of the sample and a rigidly aligned free-space optical path, which have confined these demonstrations to thin or static samples. Here, we combine a real-valued intensity transmission matrix (RVITM) algorithm with a photoacoustic guidestar (PA-RVITM) in a reflection-mode configuration. Modulated light is delivered through a flexible multimode fiber passing through the central aperture of a focused ultrasound transducer, enabling optical illumination and acoustic detection from the same side of the sample without a free-space combiner. Fiber integration preserved the wavefront shaping performance, and the reflection-mode PA-RVITM system achieved sub-second wavefront optimization with a ~14-fold PA signal enhancement through a diffuser. In ex vivo chicken breast tissue, the enhancement decreased with tissue thickness, reaching ~3.5 through a 2-mm chicken breast layer, with a speckle decorrelation time of ~450 ms. By performing an independent RVITM optimization at each scan position, our system enabled photoacoustic microscopy of a black nylon wire through the same layer, reducing the imaged line width from ~141 {\mu}m to ~102 {\mu}m, close to the nominal diameter of ~100 {\mu}m. This work advances PA-guided wavefront shaping toward a compact, same-side implementation for future biomedical therapeutic and imaging applications.
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All authors Xuan Liu, Sacha Noimark, Wenfeng Xia, Sebastien Ourselin, Tianrui Zhao

Optics

From Single-Particle Thermal Asymmetry to Collective Control: Janus Plasmonic Colloids for Photothermal Applications

Cédric Rousseau, Javier González-Colsa, Victor Joris et al. (+3 more)

Janus nanoheaters provide a route to break the spatial symmetry of thermoplasmonic heating and generate directionally biased thermal gradients at the nanoscale. However, in realistic photothermal scenarios, nanoheaters rarely act as isolated objects, but rather operate within colloidal assemblies where collective thermal and electromagnetic effects reshape the local temperature landscape. Here, we investigate how the intrinsic thermal asymmetry of a single Janus plasmonic nanoheater is transformed when moving from isolated particles to planar random arrays under continuous-wave illumination. Using coupled electromagnetic and steady-state heat-transfer simulations, we first quantify the finite spatial range over which the single-particle Janus signature remains detectable. We show that neighboring nanoheaters introduce two competing effects: a nearly symmetric collective thermal background that screens and homogenizes the Janus factor, and short-range neighbor-induced asymmetries that partially compensate this screening when interparticle distances become comparable to the spatial extent of the single-particle thermal anisotropy. We derive a closed-form analytical model for the array-averaged Janus factor as a function of nanoparticle number and surface density. The model predicts the screening-dominated regime from single-particle quantities and identifies deviations associated with near-field neighbor-induced contributions at higher densities. These results demonstrate a central trade-off for biomedical photothermal applications in continuous-wave operation: collective heating is required to reach relevant temperature increments, but it progressively erases the directional Janus character by thermal homogenization. Our framework provides a practical route to predict and rationally design the spatial temperature distribution in plasmonic assemblies of Janus particles.
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All authors Cédric Rousseau, Javier González-Colsa, Victor Joris, Pablo Albella, Bjorn Maes, Gilles Rosolen

Optics

Near-field contributions to cavity-mediated radiative heat transfer

Antonio Mastropasqua, Martin Eckstein, Daniele Fausti et al. (+1 more)

Embedding materials in optical cavities has emerged as a powerful strategy for controlling their physical properties. In this context, increasing attention has been devoted to the thermal Purcell effect, which describes how the radiative heat transfer between a material and its photonic environ- ment can be modified by the electromagnetic boundary conditions imposed by the cavity. Here, we investigate near-field effects in cavity-mediated radiative heat transfer in planar cavities composed of realistic, dissipative gold mirrors, extending previous work that considered ideal mirrors. We iden- tify the conditions under which near-field effects lead to a substantial enhancement of heat transfer between the material and a photonic thermal bath relative to free space, owing to the strong modi- fication of the photonic local density of states by evanescent modes within the cavity. In particular, we find that radiative heat flow can be enhanced by several orders of magnitude in cavities ranging from nanometer to micrometer scales, depending on the absorber frequency and dipole orientation. These findings suggest that near-field effects may be relevant to polariton chemistry experiments and other systems operating within this cavity length-scale regime. Our results add to the idea that cavities can modify material properties not only by engineering vacuum fluctuations in the strong light-matter coupling regime, but also by controlling radiative dissipation and heat transfer.
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All authors Antonio Mastropasqua, Martin Eckstein, Daniele Fausti, Francesca Fassioli

Optics

Polaritonic self-hybridized critical coupling by Brillouin zone folding induced quasi-bound states in the continuum in bulk WS2 metasurfaces

Kuidong Wang, Xiongwei Guo, Fu Liu et al. (+2 more)

Maximizing the absorption of exciton-polaritons in transition-metal dichalcogenides (TMDCs) under critical coupling conditions is crucial for developing high-efficiency polariton-based photodetectors, solar cells, and perfect absorbers. Here, we propose a polaritonic critical self-hybridized coupling between Brillouin zone folding driven quasi-bound states in the continuum (BZF-QBIC) mode and A excitons in a bulk WS2 metasurface. This polaritonic critical coupling shows robust behavior with respect to the asymmetric parameters of the involved BZF-QBIC mode, in contrast to the asymmetry-sensitive conventional QBIC-based critical coupling. By changing the height of WS2 metasurfaces, we observe a typical anti-crossing behavior with a Rabi splitting of 246.5 meV. Moreover, the maximum absorption value of 0.5 for exciton-polaritons is achieved at the polaritonic critical coupling condition. The large population of excitons provided by bulk WS2 is essential for achieving polaritonic critical coupling and can further enhance polaritonic absorption. This work enables the realization of robust, high-absorption self-hybridized polaritonic critical coupling in TMDCs, and further expedite their applications in high-absorption low-dimensional polaritonic devices.
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All authors Kuidong Wang, Xiongwei Guo, Fu Liu, Jinhai Si, Xun Hou

Optics

Bias-Modulated Electronically-Tunable Optoelectronic Chromatic Dispersion in Ge PN Photodiodes

Uttama Kumar Saint, Ziv Glasser, Egor Liokumovitch et al. (+3 more)

Optoelectronic Chromatic Dispersion (OED) is a source of significant wavelength-dependent current in photodiodes, offering a pathway to computational spectroscopy in a photodiode-only platform without the need for additional optical or mechanical components. Previous studies have activated OED through light modulation. Here, we present a theoretical and experimental investigation of bias-modulated OED in the C- and L-bands in a Ge PN photodiode, where the applied bias voltage is modulated sinusoidally, and the optical illumination is held constant. The spectral response is due to wavelength-dependent optical absorption together with bias-dependent competition between carrier drift, displacement current, and mobile-charge screening. With the RF phase-shift technique, the maximum measured OED sensitivity is +4.13 deg/nm in the L-band, significantly larger than that of light-modulated OED in Ge. The 15 mdeg phase-noise floor yields a spectral resolution of 3.6 pm. These results establish bm-OED as an electronically reconfigurable dispersive mechanism for applications in chip-scale computational spectrometers, photonic integrated circuits (PICs), on-chip RF photonics, and wavelength-division multiplexing.
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All authors Uttama Kumar Saint, Ziv Glasser, Egor Liokumovitch, Endalamaw Ewnu Kassa, Asaf Albo, Shmuel Sternklar

Optics

Inferring Transition-Dipoles from Two-Dimensional Infrared Spectroscopy with Deep Learning

Hao-Yue Zhang, Rong-Hang Chen and Qing Ai

Two-dimensional infrared (2D IR) spectroscopy is a powerful nonlinear technique for probing vibrational couplings, anharmonicities, coherence, and molecular dynamics. Among the microscopic parameters encoded in 2D IR spectra, transition dipoles are fundamental quantities that characterize molecular structure and govern light-matter interactions. However, reconstructing these quantities from spectroscopic data remains a challenging inverse problem. Here, we introduce a physics-informed convolutional neural network (CNN), Spec2DipoleNet, trained on 2D IR spectra generated via the non-Hermitian Hamiltonian (NHH) method. This multi-task CNN reconstructs the transition dipole matrices under physically-motivated masking constraints. On the validation set, the model achieves a root-mean-square error (RMSE) of 0.085, an $R^2$ of 0.97, and a normalized cross-correlation of 0.98. It reliably recovers matrix elements associated with allowed transitions, while larger deviations are observed for nominally forbidden transitions. Our results suggest that transition dipole operators can be quantitatively inferred from 2D IR spectra, highlighting the potential of physics-informed deep learning for extracting microscopic quantum parameters from multidimensional spectroscopic data.
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Optics

Preservation of photon statistics upon selective molecular modulation in hollow-core fibers

Arturo Mena, Pau Arcos, María Sánchez-Hernández et al. (+7 more)

Preserving the underlying statistical properties of light during frequency transduction is essential for interfacing spectrally disparate quantum systems. To do this successfully, the process needs to be highly efficient, broadband and noiseless. Despite significant progress in this field, satisfying all these conditions remains challenging. Recently, molecular modulation-based transduction in hollow-core fibers has emerged as a promising approach, where biphoton correlations were proven to be preserved during frequency up-conversion. However, beyond this previous demonstration of correlation-maintaining conversion, whether molecular modulation can preserve more general statistical properties of light remains experimentally unexplored. Here, we demonstrate the conservation of photon-number statistics during selective frequency transduction by Raman molecular modulation in a hydrogen-filled hollow-core fiber. As a proof of concept, weak 585-nm signals are selectively converted to 773 nm in the near-infrared and to 471 nm in the blue spectral region with near-unity efficiencies. Phase matching between the generated coherence waves and the transitions of interest is achieved through the effective dispersion of the system, enabling independent selection of the frequency up- and down-conversion processes. The preservation of the photon-number statistics of the shifted signals, along with the large frequency shifts, broad tunability and compatibility with quantum-optical fields, provides a versatile fiber-based platform for quantum frequency transduction. In particular, the demonstrated interfacing between visible and near-infrared wavelengths using low pump energies and low gas pressures offers a route towards connecting spectrally distinct quantum systems, including quantum emitters, sensors and atomic quantum memories, within hybrid quantum networks.
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All authors Arturo Mena, Pau Arcos, María Sánchez-Hernández, Benoit Debord, Frédéric Gérôme, Fetah Benabid, Nicolas Y. Joly, Oskar Arrizabalaga, Joseba Zubia, David Novoa

Optics

Computational compressive imaging reconstruction with a simulated-annealing-based Ising solver

Ze Zheng, Xiaoyan Wu, Hang Xu et al. (+3 more)

Computational compressive imaging, such as single-pixel imaging and lensless imaging, is a promising imaging technique, enabling the reconstruction of objects from under-sampled measurements through compressed sensing algorithms or deep learning methods. However, the demand for electronic computational resources grows rapidly when recovering high-resolution and multi-dimensional images and training neural networks with numerous parameters, which has become a bottleneck for the widespread adoption of this technique. Here, we propose a physics-driven reconstruction framework that formulates imaging inverse problems as Ising models and directly recovers target objects through the self-evolution of non-von Neumann Ising machines toward the ground state of the Hamiltonian. We employ a simulated-annealing-based Ising solver to demonstrate this proof-of-concept by reconstructing objects in both single-pixel imaging and a lensless diffuser camera at sampling rates ranging from 1% to 25%. By further incorporating a foreground-sparsity constraint and a smoothness constraint, the reconstruction quality of our method is comparable to or even better than that of mainstream algorithms. This approach has the potential to substantially reduce the electronic computational burden of inverse reconstruction, paving the way for real-time, low-power computational imaging.
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All authors Ze Zheng, Xiaoyan Wu, Hang Xu, Jingzheng Huang, Tailong Xiao, Guihua Zeng

Optics · Cross-listed from quant-ph

Quantum-enhanced phase interrogation of a fiber-optic strain sensor

Romain Dalidet, Marialuisa Capezzuto, Hugo Degert et al. (+7 more)

Fiber-optic sensors enable high-sensitivity measurements of various physical parameters. Here, we demonstrate a quantum-enhanced interrogation of an all-fiber-optic Fabry-Perot resonator, employing entangled two-photon interference to probe directly strain-induced optical phase shifts. The resonator is interrogated by a narrow-linewidth laser that is frequency-locked to its resonance mode, thereby exploiting the cavity's inherent sensitivity to variations in the fiber length. To access the phase fluctuations transferred to the interrogation field due to external perturbations on the sensor, we implement an in-fiber Mach-Zehnder interferometer readout. This configuration allows the sensor to operate both with classical coherent light and with entangled photon pairs generated via spontaneous parametric down-conversion. Harnessing two-photon entangled states corresponding to an N = 2 NOON state, we experimentally demonstrate a reduction of the measurement noise below the standard quantum limit. These results provide clear evidence of the quantum advantage in enhancing fiber-optic sensors under practical constraints on optical power.
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All authors Romain Dalidet, Marialuisa Capezzuto, Hugo Degert, Davide D'Ambrosio, Saverio Avino, Anthony Martin, Sebastien Tanzilli, Laurent Labonte, Paolo De Natale, Gianluca Gagliardi

Optics

Two-Photon Spatial Multiplexing: Image Encoding in Gaussian Noncircularity of Light

Wanting Hou, Zhiyuan Ye, Hong-Chao Liu et al. (+2 more)

The search for new degrees of freedom (DoF) of light is central to advancing optical information encoding and multiplexing. Gaussian statistics are ubiquitous in nature, and here we unlock an unexpected statistical DoF in the joint noncircularity of Gaussian light. For a pair of Gaussian light fields, the covariance- and pseudocovariance-dominated two types of correlations dictate the propagation rules and the image-encoding responses, mapping optical information into coordinate-difference and coordinate-sum momentum spaces, respectively. A hybrid random-phase grating array enables the experimental preparation of noncircular Gaussian light with programmable coexistence, yielding X-shaped intensity correlations while subtly reducing the degree of second-order coherence. Two independently programmed images coexist in the same beams at identical wavelength and polarization, yet are selectively decoded through two-photon correlation geometry without physical filters. Remarkably, both images remain recoverable even when structured stray light is as intense as the signal. These results unlock a hidden statistical dimension of optical information encoding, transforming Gaussian fluctuations into a dual-channel information carrier and joint noncircularity into a programmable optical DoF.
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All authors Wanting Hou, Zhiyuan Ye, Hong-Chao Liu, Jun Xiong, Kaige Wang

Optics

Femtosecond all-optical polarization manipulation of harmonic generation in an anisotropic strongly coupled plasmon-epsilon-near-zero metasurface

Yingying Zhang, Jinhai Si, Xun Hou et al. (+1 more)

Ultrafast manipulation of light polarization plays a key role in modern high-speed photonics and optoelectronic operations. However, most current polarization-active modulation schemes are restricted by a limited operating wavelength range (visible to infrared), narrow-band resonant fundamental wavelength working condition, and relatively long modulation time (> 1 picosecond). Here, by taking advantage of the transient anisotropic nonlinearity of optical strong coupling between the epsilon-near-zero mode of indium tin oxide (ITO) film and polarization-selective plasmonic resonance, we achieve femtosecond polarization manipulation of broadband light that spans ultraviolet (UV) to visible region by controlling harmonic generation of the signal light under resonant pumping. We demonstrate polarization elliptic rotation of 23.3{\deg} at 407 nm for third-harmonic generation (THG) and 55.0{\deg} at 610 nm for second-harmonic generation (SHG) simultaneously within ~600 fs. In addition, the strongly coupled system can also serve as a universal femtosecond polarization modulation source, imprinting a 32.9{\deg} polarization rotation in the UV region by converting the modulation information of the fundamental signal light to that of THG light via a uniform ITO film in a cascaded configuration. Our study establishes a new paradigm for UV-based, ultrahigh-speed broadband polarization photonics with direct implications for high-speed optical information processing and programmable nonlinear photonic devices.
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All authors Yingying Zhang, Jinhai Si, Xun Hou, Kuidong Wang

Optics

A compact six-beam 3D magneto-optical trap based on multifunctional metasurface chips

Chen Qing, Huijie Sun, Yize Liu et al. (+5 more)

Magneto-optical traps (MOTs), indispensable to cold-atom-based quantum computing and quantum precision measurement, are increasingly desired in compact and stable form. Particularly, the six-beam 3D-MOT, distinguished by symmetric geometry and superior trapping performance, finds particularly broad applications. Conventional implementations, however, rely on complex and bulky free-space optics, hindering miniaturization, practical assembly, and long-term stability. Here, we demonstrate a compact six-beam 3D-MOT system that employs two multifunctional optical metasurface chips to implement the optical-field functionalities required for cooling and trapping atoms. By combining two metasurface chips and a vacuum cell, a photonic-integrated cell module (PICM) is implemented as the core assembly and the volume is only" 73*73*107 mm^3" . In success, 1.3 * 10^3 Rb atoms are trapped and cooled to the temperature of ~183 {\mu}K. It should be noticed that the laser beam is guided to the metasurface through optical fiber, thus such fully fiber-coupled architecture enables straightforward and stable operations. We believe that this work would pave a practical route toward compact and portable 3D-MOT.
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All authors Chen Qing, Huijie Sun, Yize Liu, Yuxuan Liao, Yongzhuo Li, Dengke Zhang, Xue Feng, Yidong Huang

Optics

Strong field confinement in air and enhanced optical Kerr nonlinearity of high-index star-shaped pore membrane metasurfaces

Andrey V. Panov

The optical Kerr effect in high-refractive-index membrane metasurfaces patterned with a square lattice of five-pointed star-shaped pores is investigated numerically using gallium phosphide as a model material. The star geometry supports resonances with pronounced electromagnetic energy confinement, including an anapole state. The sharp tips of the star-shaped pores concentrate the electric energy density in air, exceeding that of Mie voids by a factor of seven. The effective second-order refractive index is enhanced by up to three orders of magnitude relative to bulk GaP, with the largest values obtained near the anapole resonances. The nonlinear response remains stable under polarization rotation and random misalignment of the nanoholes, indicating tolerance to fabrication imperfections. These results establish star-shaped pore lattices in freestanding membranes as a promising platform for nonlinear optics exploiting strong field confinement.
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Tuesday 13 October · still arriving

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

Chern insulator optics

Jason G. Kattan and J. E. Sipe

Chern insulators have optical properties that are strikingly different than ordinary chiral media. Unlike the natural optical activity of the latter, which is a finite-wavelength effect arising from spatial dispersion, the optical activity of a Chern insulator is already present in the long-wavelength limit, and exhibits a striking directional asymmetry in the propagation of circularly polarized light. We study this unnatural optical activity using a recently developed field theory for Chern insulators. For bulk Chern insulators, we derive the normal-incidence transmission and reflection coefficients of multilayer structures, identify the associated Faraday and Kerr angles, and show that these optical rotations provide a direct optical signature of topology at low frequencies. We also show that an interface between bulk Chern insulators can act as a kind of mirror, completely reflecting light at normal incidence when the Chern invariants have opposite signs. Moving to a more realistic model of a two-dimensional current sheet, we study the Faraday and Kerr effects in a thin film of $\mathrm{Mn}\mathrm{Bi}_2\mathrm{Te}_4$ in its Chern insulator phase. The predicted rotations are many orders of magnitude larger than those of optically active media of comparable thickness that are topologically trivial.
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Optics

High-resolution retinal matrix imaging via computational motion and aberration correction

Aymeric Melt, Léo Puyo, Victor Barolle et al. (+3 more)

High-resolution optical imaging of the human retina is essential for diagnosing ophthalmic and neurodegenerative diseases. While adaptive optics combined with coherence gating can resolve retinal structures at the cellular scale, it requires complex, costly, and cumbersome hardware to physically correct for ocular aberrations and motion. Matrix imaging offers a powerful digital alternative by leveraging wave linearity to undo aberrations and multiple scattering; however, its in vivo deployment has been hindered by continuous, involuntary eye movements that destroy phase coherence during acquisition. Here, we overcome this fundamental barrier and demonstrate the first in vivo reflection matrix imaging of the human retina using an ultra-simplified optical setup without physical correctors. We introduce a fully computational framework that tracks and compensates for multi-axial ocular motion directly from raw interferometric data, thereby restoring the phase coherence of a multi-spectral distortion matrix. This enables local, high-order aberration correction to achieve diffraction-limited, volumetric imaging of retinal tissues. By shifting the hardware burden to physics-guided numerical computation, this paradigm opens the way for accessible, non-invasive, and high-resolution digital ophthalmology.
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All authors Aymeric Melt, Léo Puyo, Victor Barolle, Claude Boccara, Gereon Hüttmann, Alexandre Aubry

Optics · Cross-listed from physics.chem-ph

Dual-Resonator Organic Microcavities for Triplet-Triplet Annihilation Upconversion

Robert H. Gordon, Rachel C. Kilbride, Michael Hutton et al. (+2 more)

Solid-state triplet-triplet annihilation upconversion offers a compelling route to harvest sub-bandgap photons to increase the efficiency of photovoltaics, but low sensitizer doping required to suppress non-radiative aggregation quenching severely restricts light absorption. Here, we present a dual-resonator planar silver microcavity housing a thermally co-evaporated rubrene:palladium phthalocyanine (PdPC) active layer designed to simultaneously enhance triplet-sensitizer absorption and out-couple upconverted emission. By aligning a dispersive cavity mode with the PdPC absorption peak at $32^\circ$, we achieve a 12-fold enhancement in absorptance at 665 nm. This absorption boost, combined with directional outcoupling through a second cavity mode, outweighs cavity-induced reabsorption losses to yield a 22-fold peak and 4.5-fold integrated enhancement in detected upconversion emission. Overall, this work highlights the potential of microcavity integration to substantially enhance solid-state upconversion performance across low-intensity optoelectronic applications.
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All authors Robert H. Gordon, Rachel C. Kilbride, Michael Hutton, Jenny Clark, Robert D. J. Oliver

Optics

Engineering Color and Angular Stability in Building-Integrated Photovoltaics Using Metal-Oxide Photonic Glasses

Sinan Genc, Talat Ozden and Alpan Bek

Coloring building-integrated photovoltaics poses a dilemma: absorptive pigments cost photocurrent, while ordered photonic structures are iridescent. Amorphous photonic glasses, disordered packings of high-index nanospheres, are proposed to avoid both, yet their angle-independent color has rarely been quantified on a realistic module stack. A non-absorbing scatterer needs a high index with visible transparency, which the Moss rule caps near 2.35. Coupling Mie scattering, the Percus-Yevick structure factor, the exact phase function, and two-flux transfer with absorption and refraction at the front glass, we fix four target colors that a crowd-sourced naming survey calls unambiguously blue, green, gold, and terracotta, and map the recipes reaching them across four metal oxides, four encapsulants, two cell technologies, and wide ranges of particle size, packing fraction, and layer number. All four are reached in a laminated three-layer coating, to within DeltaE00 of 0.8 to 2.5, but not from one oxide: transparent TiO2 renders the blue and only pale warm tones; the green, gold, and terracotta require the blue-band absorption of V2O5, and the low-contrast oxides turn grey at the warm end. The encapsulant governs the color, and the cell technology does not. Disorder does not confer angle independence: the colors hold for an observer moving under a fixed sun but drift by DeltaE00 of 5 to 18 over the day, terracotta least, and a facade lit from above and viewed from below shifts every color towards the blue. Oxide, layer number, and facade geometry must therefore be designed together.
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Optics

Probing topological edge states in multilayer dielectric photonic nanostructures

Eva Otero, Ivan Toftul, Michael Scalora et al. (+3 more)

Topological photonics has recently attracted a significant attention, leading to many theoretical studies and predictions. However, direct experimental demonstrations of topology-driven effects, particularly in nanostructures, are rarely proven. In this work, we experimentally investigate multilayer photonic nanostructures composed of alternating subwavelength dielectric layers, arranged according to the topological Su-Schrieffer-Heeger model. By engineering different edge truncations, we probe different supported topological phases. Our theoretical predictions and experimental measurements reveal strong field confinement at the surface exclusively in the configuration supporting topological edge states, providing their localized character and topological origin. First, we demonstrate enhanced absorption in an ultrathin absorbing overlayer deposited on the surface when the topological surface mode is present, highlighting the potential for surface-sensing applications. Second, we observe enhanced second-harmonic generation in reflection from the surface, demonstrating the potential of topological states for surface enhanced nonlinear optics enabled by the strong localization of the electromagnetic field at the interface.
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All authors Eva Otero, Ivan Toftul, Michael Scalora, Jose Trull, Crina Cojocaru, Yuri Kivshar

Optics

Mode locking by phase-sensitive amplification

Francesco De Lucia, Yifan Sun, Georges Semaan et al. (+4 more)

Mode locking is the mechanism by which lasers organize many optical modes into stable trains of ultrashort pulses. It underpins optical frequency combs, precision timing and spectroscopy, and provides the phase-coherent waveforms required for applications ranging from strong-field physics to optical frequency metrology. Conventional pulse-forming mechanisms select pulse amplitude without fixing its phase relative to an external optical reference, leaving pulse formation and optical-phase stabilization as separate functions. Phase-sensitive amplification offers a different possibility: the optical reference can participate directly in pulse formation. Here we demonstrate this principle in a continuously pumped oscillator. Kerr-induced phase evolution and cavity detuning turn pump-referenced parametric gain into a power-dependent discriminator that supports stable single-soliton operation. Roundtrip-resolved interferometry reveals that the soliton optical phase remains referenced to the master laser modulo $\pi$. These results establish phase-sensitive mode locking, in which pulse-amplitude selection and optical-phase referencing arise from the same coherent nonlinear feedback.
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All authors Francesco De Lucia, Yifan Sun, Georges Semaan, Clement Dupont, Nicolas Englebert, Simon-Pierre Gorza, François Leo

Optics

Continuous-wave 148.4 nm Generation in Quasi-phase Matched BaMgF4

Yuanzhuang Bu, Xiangting Wang, Ruiyin Gu et al. (+6 more)

We report continuous-wave (CW) vacuum-ultraviolet (VUV) generation at 148.4 nm by second-harmonic generation of 296.8-nm fundamental light in quasi-phase matched (QPM) BaMgF4 (BMF). At the incident power of 170 mW, a VUV output power of approximately 31 pW can be obtained. The quadratic dependence of the signal on the incident power in the QPM region and its attenuation in the ambient environment support the identification of VUV generation. The polarization dependence shows a sine-function shape with a 15% output attenuation from the maximum to the minimum. These results highlight the potential of BMF as a nonlinear medium for CW VUV generation.
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All authors Yuanzhuang Bu, Xiangting Wang, Ruiyin Gu, Chongtai Liu, Luxin Jiang, Xinqing Ding, Ke Zhang, Yongkai Li, Kemi Xu

Optics

Continuous-wave 148.4-nm generation in strontium tetraborate for nuclear absorption spectroscopy

Hanlin Wang, Zhouzhi Tan, Conghui Huang et al. (+13 more)

Continuous-wave light near 148.4 nm enables direct absorption spectroscopy of the thorium-229 nuclear transition. We demonstrate single-pass frequency doubling in strontium tetraborate, delivering an estimated 8.1 nW at 920 mW pump power. At this power, oxygen admission leaves a background of 1.22 kHz, corresponding to an estimated signal-to-background ratio of 3.8 x 10^4 after current-integration calibration. Several crystals grown by top-seeded solution growth produce polarization-dependent VUV signals. Temperature scans reveal distinct maxima among specimens cut from one boule and among positions within a specimen. The source sustains operation over 33.3 h. In a separate 3-h record acquired with 1-ms gates, local referencing achieves near-Poisson averaging, reaching a fractional Allan deviation of 3.5 x 10^-5 at 1000 s. These results establish a practical VUV source for stable, long-duration spectroscopy of weak nuclear absorption.
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All authors Hanlin Wang, Zhouzhi Tan, Conghui Huang, Jinming Liu, Xuan Fan, Runzhe Yang, Meng Wang, Tung-Hsun Chung, Yong-Heng Huo, Yin Hang, Zhiwei Jiao, Heng Tu, Lei Zhang, Xu-Fei Yin, Pengfei Wang, Zhiqiang Zhang

Optics · Cross-listed from math-ph

Spectral Theory of an Exactly Solvable Quantum-Optical Oscillator

Sam Johar and Joseph Kraisler

We study a one-dimensional single-excitation model of a scalar photon field coupled to a continuum of two-level atoms with quadratic density. The resulting matrix Hamiltonian has an unbounded spatially varying coupling and is not semibounded. We prove that it is essentially self-adjoint and determine its spectrum explicitly. The continuous spectrum is the half-line $(-\infty,\Omega]$, where $\Omega$ is the atomic resonance frequency, while the spectrum above $\Omega$ consists of a strictly increasing sequence of simple eigenvalues. These eigenvalues are characterized by the interlaced zeros of the Airy function and its derivative and satisfy \begin{align*} \omega_n = \left(\frac{3\pi g n}{4}\right)^{1/2} + \frac{\Omega}{4} + O(n^{-1/2}). \end{align*} The analysis is based on a nonlinear spectral reduction that yields a confining massless relativistic oscillator above $\Omega$ and an inverted oscillator below $\Omega$.
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Optics

Orbital Angular Momentum Transfer and Beam-Quality Limits in Fork-Grating Electron Diffraction

P.K. Tsiopa and S.S. Baturin

We analyze fork-grating generation of electron vortex beams beyond the ideal single-mode approximation. The incident beam is described as a paraxial Gaussian ensemble with finite transverse momentum spread and a first electromagnetic Hartree phase accumulated before the grating. Within Fresnel propagation we derive an analytical expression for each diffraction order and determine the corresponding orbital-angular-momentum statistics. The mean OAM of a selected order remains fixed by the topological rule, $\langle L_z\rangle_m=mp\hbar$, while its variance is set by the residual transverse momentum spread of the beam prepared at the grating. For the Gaussian mixture, the transverse purity is $\mathcal P_{\perp,m}=[1+2(\Delta L_{z,m}/\hbar)^2]^{-1}$. The observed annular profile is governed by a separate propagation-dependent blurring scale, so a beam can retain well-defined mean OAM while having low transverse purity or a partially filled vortex minimum. Within the radial phase-screen approximation, the Hartree self-field modifies the propagated profile while preserving the full-plane OAM moments and transverse purity.
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Optics · Cross-listed from physics.atom-ph

Theory of Sub-Doppler Spectroscopy of Alkali Metal Vapors in the Field of Counterpropagating Bichromatic Laser Beams: Probe Beam Regime

A. M. Mikhailov and D. V. Brazhnikov

Sub-Doppler resonances observed in miniature cells ($V \ll 1 cm^3$) with Rb or Cs atoms in the field of counterpropagating bichromatic laser beams are of interest for the creation of a miniature quantum frequency standard (QFS) of the optical range. However, a theory of these resonances that takes into full account the real structure of energy levels in an atom has not yet been constructed, which would make it possible to obtain explicit analytical expressions for the absorption coefficient of the medium. This paper addresses this gap using the density matrix formalism and the algebra of irreducible tensor operators. Namely, we obtained an analytical expression for the absorption coefficient of the probe beam in the presence of a more intense counterpropagating beam (pump-probe regime). Moreover, the calculations were carried out for arbitrary parameters of laser beam polarization, arbitrary angular momenta of optical dipole transitions in the atom and various relaxation processes. The constructed theory allows us to correctly take into account the simultaneous action of various nonlinear optical effects, in contrast to the simplified three-level model of the atom, and also to investigate the partial contribution of each of them to the overall observed resonant circuit. The analysis performed, in particular, made it possible to predict an increase in the amplitude of the sub-Doppler resonance when using counterpropagating light beams with opposite circular polarizations in relation to the configuration of counterpropagating beams with orthogonal linear polarizations, which is often used in practice.
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Optics

Optical parametric amplification of spatiotemporal optical vortices

Houan Teng, Chenhao Wan and Qiwen Zhan

Spatiotemporal optical vortices (STOVs) are structured light featuring a phase singularity in the spatiotemporal domain, exhibiting unique topology that couples spatial and temporal dimensions. To date, amplification of STOVs remains unexplored, and their wavelength tunability is limited by available light sources. In this work, we present the theoretical analysis and experimental demonstration of optical parametric amplification (OPA) of STOVs. The pulse energy of the STOV signal is enhanced by a factor of 60, while its spatiotemporal structure is robustly preserved. The generated idler inherits the phase-conjugated STOV topology with the opposite topological charge, governed by simultaneous conservation of energy, linear momentum, and spatiotemporal orbital angular momentum. The intrinsic wavelength-conversion capability of OPA further provides a route towards spectrally tunable STOV generation, spanning from visible to near-infrared wavelength regimes. This work paves the way toward further development of multi-stage amplification architectures, facilitating applications of wavelength-tunable spatiotemporally structured light at ultrahigh intensities.
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Optics

Feedback-Controlled Self-Trapping of Solitons

Raonaqul Islam, Pradyoth Shandilya, Yunxiang Song et al. (+5 more)

Chip-scale microresonator-based frequency combs, commonly referred to as microcombs, are promising sources for field-deployable metrology applications. However, intrinsic microresonator noise can make it difficult to generate microcombs with sufficiently low noise for applications. A widely used approach to overcome this limitation is to trap the DKS in the microresonator by phase-modulating the pump laser, although integrating a low-noise modulation reference in a fully on-chip platform remains challenging. Here, we propose an optoelectronic self-trapping scheme in which the impact of intrinsic microresonator noise on microcomb repetition-rate jitter is suppressed through feedback-controlled phase modulation of the driving laser so that the intrinsic noise of the laser sets the ultimate performance limit. This modification eliminates the need for an external low-phase-noise RF source, thereby enabling chip integration. We develop both analytical and numerical models for the proposed system and benchmark its performance against conventional externally referenced bulk implementations, showing that the proposed approach achieves comparable microcomb noise suppression while maintaining chip-scale compatibility.
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All authors Raonaqul Islam, Pradyoth Shandilya, Yunxiang Song, Alioune Niang, Gary Carter, Thomas Carruthers, Curtis Menyuk, Ergun Simsek

Optics

Experimental Demonstration of 7-Dimensional N-Vector Model with Dimensional Inflation and Annealing

Daniele Veraldi, Marcello Calvanese Strinati, Paolo Charlesworth et al. (+4 more)

We report the first experimental realization of a physical system described by a high-dimensional Hamiltonian comprising up to seven dimensions. Our setup is based on a holographic optical computer, which realizes a dimensional inflation by spatial and temporal multiplexing of lower dimensional systems. We demonstrate an $N$-vector model, with applications in field theory and computing. The approach is scalable and enables table-top implementation of high-dimensional theoretical models, and novel methods for combinatorial optimization. We experimentally demonstrate dimensional annealing: by dynamically reducing the number of dimensions, the ground state of an Ising model is found in complex graphs by an adiabatic evolution from high-dimensional spaces.
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All authors Daniele Veraldi, Marcello Calvanese Strinati, Paolo Charlesworth, Matteo Spaziani, Silvia Gentilini, Davide Pierangeli, Claudio Conti

Optics

Chirality Inversion and Giant Optical Activity in Scalable Self-Assembled Metamaterials

Antonio Carone, Aso Rahimzadegan, Birgül Kalkan et al. (+8 more)

Chiral materials manipulate light in ways that hold promise for sensing, imaging, and photonics, but producing such materials with strong and reproducible optical activity at large scale remains challenging. Here, we develop a scalable strategy to assemble hierarchical helicoidal metamaterials from metallic nanoparticles and chitin nanocrystals, a biosourced polysaccharide. Spontaneous co-assembly produces ordered chiral structures with strong plasmonic optical activity, reaching a dissymmetry factor of 0.55. The unprecedented ability to vary the helicoidal pitch allows control of the optical response, enabling inversion of optical handedness without changing the handedness of the template, effectively decoupling geometrical chirality from optical chirality. Remarkably, the gold nanoparticles can also be recovered from dry films and redispersed as colloids, introducing an appealing circularity concept for sustainable chiral plasmonic materials.
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All authors Antonio Carone, Aso Rahimzadegan, Birgül Kalkan, Thomas G. Parton, Tony Jin, Zihao Lu, Clemens Schmitt, Emeline Raguin, Bruno Frka-Petesic, Kevin Vynck, Silvia Vignolini