Papers

We develop a perturbative framework for constructing the fields of electromagnetic beams beyond the paraxial approximation, in which the vectorial correction is shared symmetrically between the electric and magnetic fields and separated from the isotropic scalar correction. The method is based on a direct iterative derivation from Maxwell equations and combines complementary electric-first and magnetic-first constructions, thereby removing the construction-dependent assignment of the vectorial correction to either the electric or magnetic field. In addition, the scalar envelope correction is obtained independently from the isotropic co-polar contribution to the solutions of the higher-order Helmholtz equation. The resulting corrected fields apply to an arbitrary paraxial envelope and input Jones polarization.

Molecular aggregates are at the basis of a broad range of functional systems, including photosynthetic light-harvesting complexes, organic photovoltaics, optoelectronic devices, and molecular sensors. Their structural organization, however, is often difficult to resolve because intrinsic heterogeneity. In this context, circular dichroism (CD) spectroscopy is particularly powerful because of its sensitivity to the three-dimensional organization of molecular building blocks. Extending CD measurements from ensembles to individual objects therefore offers a unique approach to probing supramolecular architecture in heterogeneous, non-crystallizable systems. However, without spatial and orientational averaging, measurements are highly sensitive to imbalances between left- and right-circularly polarized excitation, deviations from ideal circular polarization, and polarization imperfections coupled to linear dichroism, which creates spurious CD signals. Here, we present a systematic theoretical and experimental framework for understanding, quantifying, and controlling artifacts in single-object CD spectroscopy. We develop a mathematical description of the principal artifact mechanisms, identify their origins in practical implementations, including Pockels-cell-based polarization modulation, and establish strategies for their minimization. We validate this framework using an experimental implementation capable of acquiring CD spectra from individual molecular aggregates over 690-785 nm. Our results demonstrate that reliable single-object CD measurements are achievable when polarization-related artifacts are rigorously characterized and controlled. This Tutorial Review provides a unified theoretical framework and practical guide for single-object CD spectroscopy and its application to single-particle chirality, supramolecular photonics, and biophotonics.

Whispering gallery mode (WGM) resonators with broken rotational symmetry exhibit phenomena that are generally absent in conventional symmetric cavities, including directional emission. While such effects have been extensively investigated in solid and planar resonators, they remain largely unexplored in hollow, three-dimensional cavities, where optical confinement is fundamentally altered by the thin wall geometry. Here, we demonstrate controlled fabrication of asymmetric silica microbubble resonators through anisotropic expansion during the microbubble formation process. X-ray tomography confirms the resulting three-dimensional geometry and allows us to quantitatively characterize the cavity deformation. Despite the broken rotational symmetry, the resonators maintain whispering gallery modes with high loaded Q-factors exceeding $10^5$ for moderate deformations. Optical characterization reveals that only certain resonances exhibit directional emission, whereas neighboring modes retain conventional isotropic behavior. Two-dimensional numerical simulations reproduce the observed emission characteristics and indicate that deformation-induced leakage of higher-order modes provides a plausible mechanism for the directional radiation. These results establish asymmetric hollow microbubble resonators as a platform for investigating light transport in three-dimensional asymmetric whispering gallery cavities while preserving the high-Q performance required for photonics applications.

The ability to generate ultranarrow-band perfect absorption and bound states in the continuum (BICs) in the visible spectrum is highly useful and desired for many advanced optical and nanophotonic applications. However, achieving perfect absorption and BICs with a high quality-factor in the visible region remains challenging due to the material background absorption, radiative losses, and fabrication constraints. Here, we propose and demonstrate a dielectric nanophotonic platform that simultaneously achieves perfect absorption (99.0% at 603 nm) and multiple symmetry-protected BICs in the visible by integrating a dielectric metasurface with a distributed Bragg reflector (DBR). The coupling between the metasurface's localized dielectric resonances and the DBR-assisted photonic mode suppresses radiative leakage and enables ultranarrow-band resonances (full-width at half-maximum ~ 0.65 nm) with strong electromagnetic field (|E|) enhancement (up to 16.4 folds) within the DBR stopband. In addition, the metasurface's asymmetric design enables a magnetic multipole-assisted quasi-BIC resonance with polarization-dependent mode degeneracies, and the periodic structure enables Mie resonance-assisted multiple BICs. Furthermore, the tunability of design parameters provides additional degrees of freedom to control the resonances and their spectral positions. The coexistence of perfect narrowband absorption and BICs within a compact dielectric nanophotonic platform in the visible establishes a promising route toward next-generation low-loss, high-performance meta-optical devices and technologies.

Fano resonances, characterized by a unique, sharp, and asymmetric spectral line shape arising from the interference between a narrowband discrete resonant state and a broadband continuum state, provide rich opportunities to control and manipulate light-matter interactions at the nanoscale. Here, we propose and demonstrate a nanophotonic architecture that integrates a multi-layered dielectric cavity into a one-dimensional grating to achieve an outstanding range of Fano resonance tunability by design, spanning 194 nm across the visible (from 500 to 694 nm), even with an ultra-thin layer of cavity (80 nm thick), offering the potential of highly compact photonic devices. The precise tailoring of the Fano spectral position through the adjustment of the cavity's structural parameters enables strongly localized field accumulation (at a maximum of 26 folds) within the cavity region. Additionally, the proposed architecture allows for bright and saturated structural colors in the visible, promising a route toward advanced digital display and printing technologies. Importantly, the experimental performance achieved with a simple, etch-free fabrication process results in a significant reduction in fabrication complexity and process-induced optical degradation. Our work showcases a robust platform for possible applications in Fano resonance-based low-loss nanolasers, on-chip photonic devices, optical communication components, and next-generation quantum technologies in the visible range.

We present a lower bound on the total physical thickness of a lossless, non-dispersive dielectric mirror for a given minimum reflectance across a wavelength band. The bound is based on a causality-based sum rule translated from a known result in acoustics, that expresses the wavelength integral of the logarithmic transmission of any lossless one-dimensional refractive-index profile as a function of the total thickness of each material comprising the profile, independent of how the layers are arranged. Calculating the thickness bound for a dielectric mirror, given some incident medium and substrate, requires only the lowest and highest refractive indices used in the thin-film stack and the minimum desired reflectance for a given wavelength span. We find that (1) the bound is set by the wavelength span so, for example, a 400-700 nm and an 800-1100 nm mirror have the same bound; (2) the figure of merit for materials minimizing the necessary thickness is $(n_H-n_L)^2/(n_H+n_L)$, where $n_H$ and $n_L$ are the indices of the high- and low-index materials; and (3) each additional "nine" of reflectance (e.g., from 99% to 99.9%) requirement adds a fixed amount of additional thickness to the bound. Non-exhaustive numerical calculations show that the readily achievable thickness is about twice our bound.

The design and control of wave scattering, that is, of the reflection and transmission parameters of a device, is of ubiquitous importance. These parameters generally change with varying frequency, though certain frequency-independent linear relations may exist between them. Reciprocity and geometric symmetry (reflections, rotations, etc.) are classic and well-known examples that are present in many devices and significantly ease their design. In this work, we go beyond these and introduce a new class of relations that cannot be induced by reciprocity or geometric symmetry. Choosing networks of waveguides as our workhorse, we discuss the conditions and consequences of such novel behaviour and showcase suitable example setups. We further experimentally test our predictions using coaxial cables and find excellent agreement in the broad frequency range between 0 and 1 GHz. Our work not only deepens the theoretical understanding of waveguide network dynamics, but also opens new avenues for applications in broadband signal processing, quantum information, and integrated photonics.

Spontaneous parametric down-conversion (SPDC) is one of the most widely used sources of polarization-entangled photon pairs, and understanding the generated biphoton state is essential for realizing high-brightness and high-purity entangled-photon sources. In particular, Type-II SPDC produces a biphoton wavefunction with a complex coupling between the spatial and polarization degrees of freedom owing to birefringence. In this study, we calculate the real-space distribution of the two-photon polarization state from the biphoton wavefunction of Type-II SPDC generated in a e̱ta}-barium borate (BBO) crystal. By identifying the spatial correlation direction between polarization and real-space coordinates, we designed aperture shapes that restrict the collection along this correlation direction. Through both numerical simulations and experiments, we demonstrate that such correlation-aligned apertures simultaneously achieve higher entanglement purity and improved photon collection efficiency. These results establish a practical design principle for optimizing aperture geometries based on the real-space biphoton wavefunction, providing a new approach to realizing high-purity and high-brightness SPDC entangled-photon sources.

We study graded arrays of highly dispersive subwavelength resonators through their volume-potential matrix reduction. The reduction gives a finite-dimensional nonlinear resonance problem whose diagonal entries encode the local dispersive response and whose off-diagonal entries encode volume-mediated coupling between distinct resonators. We show that monotone material grading turns this matrix into an analytically controllable frequency sorter. When graded detuning dominates coupling in the reduced characteristic matrix, the characteristic vectors localize near individual resonators; under an explicit half-lattice-site localization condition, this localization becomes a monotone frequency-position map. We also prove a constructive inverse-design statement: prescribed monotone target frequencies can be realized by choosing the local dispersive parameters, with robustness under sufficiently small grading errors. Numerical experiments, whose parameters and algorithms are recorded in an appendix, illustrate localized resonances, designed frequency-position maps, disorder stability, spacing dependence, and a resolved physical Lippmann-Schwinger validation beyond the leading logarithmic quadrature model.

Exceptional deficiency is a newly discovered broadband non-Hermitian condition, at which the system's spectrum is entirely composed of exceptional points and all eigenvectors pairwise coalesce. Here, we analyze the steady-state and time-domain responses of non-Hermitian lattices at exceptional deficiency by considering their frequency and time-domain Green's functions. We show that the frequency-domain Green's functions are characterized by the emergence of second-order poles across the entire spectrum, producing broadband second-order super-Lorentzian line shapes, enhanced magnitude, and sequential 2pi phase shifts across each resonance. These second-order poles also underpin unconventional responses to global dissipation, by which we uncover a loss-revival of defective skin effect: the skin modes that are "missing" due to the system's defectiveness at exceptional deficiency can re-emerge in the responses. In the time-domain responses, a linear-in-time amplification factor is identified in the evolution kernel, which can enhance skin-effect dynamics that would otherwise be suppressed by global loss. The amplification strength scales with the degree of defectiveness of the system and diminishes upon spectral detuning between subsystems. Our work establishes a theoretical framework for analyzing steady-state and dynamic signatures of exceptional deficiency, and opens new routes for dissipation-controlled broadband exceptional responses.

Coarse-mode aerosols (CMAs), including pollen, spores, and dust, remain difficult to characterize in situ because conventional instruments rarely resolve particle geometry and number concentration together. We present HoloCMA, which combines digital inline holography, numerical reconstruction, and deep learning-assisted analysis for particle-resolved measurement. Its optical system targets particles from approximately 5.0 um to the millimeter scale. HoloCMA supports active sampling at up to 30.0 L/min and converts to open-path operation by removing the sampling module. With an NVIDIA GeForce RTX 5070 Laptop GPU and 8 GB VRAM, HoloCMA reports equivalent circular diameter (ECD) and number concentration in real time without sustained queue buildup up to 11.4 particles/cm3. Additional computing could extend real-time analysis to the workflow limit of 100.0 particles/cm3. We evaluated HoloCMA using polystyrene latex spheres, sodium chloride and ammonium sulfate crystals, and oleic acid droplets with nominal diameters of 4.9-11.8 um, with an Aerodynamic Particle Sizer (APS) for comparison. Across 12 comparisons, absolute differences between HoloCMA and corrected APS geometric-mean diameters ranged from less than 0.1 to 1.0 um, with a mean absolute difference of 0.5 um and a mean absolute relative difference of 8.4%. HoloCMA measures geometric size directly from reconstructed contours, whereas converting APS aerodynamic diameter requires material-specific assumptions. The APS-to-HoloCMA concentration ratio generally decreased with particle size, consistent with size-dependent transport and counting losses in the APS, although the cause could not be determined conclusively. HoloCMA thus enables continuous CMA measurements that combine geometric size, number concentration, and retained particle images, supporting classification and long-term atmospheric, environmental, and indoor-air monitoring.

We introduce a phase retrieval method based on harmonic field representations generated by intensity-only spatiotemporal modulation. A periodic amplitude angular modulation encodes phase information into temporal intensity harmonics, enabling wavefront reconstruction without interferometry, reference beams, or a calibrated phase modulator. We consider a disordered system that can be modeled as a phase plate and measure the intensity pattern in the far field for a rotating excitation pattern generated by a digital micromirror device (DMD). We extract the harmonic components of the spatiotemporal speckle and exploit a gradient-based optimization algorithm to retrieve both the optical phase in the plane of the camera corresponding to a plane-wave illumination and the effective phase plate that represents the disordered system. We demonstrate experimentally the accuracy of the approach in quantitatively estimating and compensating for phase distortion ranging from low-spatial-frequency aberrations to high-frequency distortions of a ground-glass diffuser. The proposed approach establishes a new route toward calibration-free phase retrieval using intensity-only measurements.

Quantum systems are invariably coupled to a surrounding environment. A common theoretical method to simplify the problem is to trace over the environment under a Markov approximation, which assumes that the environment holds no memory over the timescales that the system evolves under. In interacting many-body systems, it is often not trivial to extract which of the multiple collective timescales are relevant. We consider a time-delayed waveguide QED setup with a single excitation. By fixing the maximum propagation time and increasing the number of emitters, we show that, even for small delay times, collective effects are sufficient to cause non-Markovianity. The impact of memory is intrinsically state-dependent. For superradiant states, the relevant system timescale is the superradiant lifetime while the relevant bath timescale is the end-to-end delay time. For subradiant states, both timescales depend on the structure of the specific state. Our results demonstrate the importance of prudently making Markov approximations in quantum many-body systems, and highlight potential pitfalls to avoid in scaling up quantum devices to large system sizes.

Metasurface design increasingly requires fast models that can operate across structurally distinct device families, rather than retraining a separate surrogate for every geometry class. Conventional neural network surrogates often depend on fixed-dimensional descriptors, family-specific output formats, and repeated architecture tuning, which limits their scalability across heterogeneous meta-atoms. Here, we present a unified large language model (LLM) workflow for multi-family metasurface modeling and inverse-design. Geometries, design parameters, and optical response channels were converted into a shared instruction-following text format and used to fine-tune Gemma-2-9B across 8 metasurface families. Compared with single-family baselines, the joint model simultaneously predicted the optical responses of all metasurface families while reducing the MSE for each family by an average of 56.5%. The same representation was also used for inverse design. These results show that a shared sequence-based LLM interface can provide a practical route to cross-family metasurface design while reducing the need for task-specific surrogate architectures.

While thermal runaway occurs across diverse physical disciplines, its presence in subwavelength phase-change photonics remains unexplored because existing thermo-optical models break down near sharp optical transitions. Here, we introduce an iterative multiphysics framework coupling full-wave electrodynamics with heat transfer, and we discover optically induced thermal runaway in vanadium dioxide (VO2) nanostructures. The runaway switching threshold depends on the illumination wavelength, ambient temperature, and underlying substrate, with external thermal biasing significantly reducing the optical intensity required for runaway ignition. Our results push forward the modeling of sharp photothermal transitions and provide essential design principles for active metasurfaces, neuromorphic photonic devices, and nanoscale thermal management.

We propose a hybrid mirror design that places an embedded metasurface beneath a Bragg reflector, combining the low coating Brownian noise of metasurface mirrors with the low optical loss of conventional multilayer coatings. Unlike previous hybrid designs, which place the metasurface at the input surface, our design positions it below the Bragg stack, reducing the optical field incident on the loss-prone resonant layer. For a design operating at 1064 nm, we show that this reduces absorption from 24 ppm for a bare metasurface to 7 ppm for the hybrid. We introduce a general method for calculating the coating Brownian noise of hybrid metasurface-multilayer mirrors at arbitrary coupling strength, extending prior methods to coherently combine a beam-scale mean-field contribution with the periodic, sub-wavelength-scale response of the metasurface. Applying this to our design, we find a reduction in Brownian noise amplitude spectral density relative to an equivalent-reflectance Bragg mirror by a factor of 1.6. Finally, we show that this design has higher angular tolerance than a bare metasurface mirror, maintaining higher reflectance over an angular range of 1 degree. This work provides a practical path towards incorporating low-noise metasurface mirrors into precision optical cavities, while minimizing optical loss and angular sensitivity.

I propose a simple and progressive way of designing and fabricating complex multilayered optics, even as we scale to large numbers of layers. The approach relies on successive layers of two-by-two blocks in which light only flows in the forward direction. It exploits a proposed trapezoidal architecture that can also support self-configuration in programmable structures, with the desired linear function or matrix set up using input vectors that correspond to the matrix rows, without any iteration other than successive single-parameter power maximizations. A key required component is a layer of 2 by 2 beamsplitters. The approach can support functions corresponding to a broad range of banded diagonal matrices. Example applications include a camera that also measures phase gradients and the direct convolution of a line of inputs with a kernel based on wavelets.

Localized surface plasmons (LSPs) supported by metallic nanostructures exhibit a great tunability of their resonance frequencies and associated spatial field distributions. In particular, nanoscale gaps and sharp corners exhibit promising near-field enhancements for applications such as biochemical sensing and nanoantennas. In this work, we investigate the optical properties of bowtie nanowire dimers excited by linearly polarized light, including polarization orthogonal to the dimer axis. To distinguish dimer-separation-induced and monomer-shape-induced effects, we compare bowtie, circular-cylindrical dimer and triangular wires. We emphasize the advantages of a Bézier-type corner parametrization with tunable curvature and employ the discontinuous-Galerkin time-domain finite-element method for numerical calculations. This enables the identification of hybrid resonances and the systematic analysis of the role of geometrical parameters. For all investigated geometries, we compare the spatially local Drude model with the spatially nonlocal Halevi model. For the specific case of silver, we demonstrate quantitative curvature-dependent LSP line shifts arising from the longitudinal nonlocality captured by the Halevi model. For higher-order LSPs, this nonlocality gives rise to a sequence of resonances that may be exploited for light-harvesting applications. Furthermore, we investigate the field enhancement associated with LSP resonances and, motivated by the occurrence of both hot and cold spots, introduce geometry-dependent measures for assessing the ability of nanostructures to enhance nonlinear optical effects, such as through the optimal positioning of SERS-active molecules. To facilitate the simultaneous study these measures, we propose the use of suitable radar charts.

Bright quantum emitters are key components for quantum communication systems. Radiative point defects in gallium nitride (GaN) are promising candidates for room-temperature single-photon emission, operating from the visible to the telecom O-band. Despite their potential, their integration into photonic structures has remained limited in the literature, with experimental photon extraction efficiencies far below simulated predictions. To identify the origin of this limitation, we investigate visible and near-infrared quantum emitters in GaN epilayers grown on $c$-plane sapphire substrates exhibiting narrow linewidths ($\sim$4 nm), high photon count rates ($ > $2 MHz), and strong antibunching, reaching $g^{(2)}(0)$ values as low as 0.06 at room temperature. We find that these emitters are located near the GaN/substrate interface, explaining their limited coupling to optical modes. Building on this observation, we show that the insertion of a thin low-temperature GaN interlayer enables the formation of quantum emitters at arbitrary depths with sub-60 nm accuracy, independent of the substrate. The intentionally introduced emitters retain optical properties comparable to naturally occurring ones, including narrow linewidths ($\sim$6 nm), saturation count rates exceeding 1.5 MHz, high Debye-Waller factors (0.69-0.98), and strong antibunching. The resulting epilayer fully coalesces within less than 250 nm ensuring compatibility with GaN-based cavity fabrication and enabling emitter placement within intrinsic regions of p-i-n diode architectures. These results mark a decisive step toward efficient emitter-cavity coupling, enabling the realization of cavity-enhanced quantum emission in GaN.

Frequency encoded photonic qubits promise a scalable path towards high-dimensional quantum information processing, but require efficient components for coherently mixing frequency modes. Coupled cavity modulators provide this functionality by using only a single driving microwave tone to couple hybridized optical supermodes. Here, we demonstrate a heterogeneously integrated thin-film lithium-niobate-on-silicon coupled-cavity modulator that realizes tunable bidirectional frequency mode transformations, including \(50/50\) beam splitting and near complete frequency swapping with \(>20~\mathrm{dB}\) pump extinction at a \(10~\mathrm{GHz}\) supermode splitting. Because the electro-optic film is bonded onto a foundry fabricated silicon photonics platform, the approach is compatible with co-integration of photon pair sources, spectral filters, active tuning elements, and single photon detectors. We also bond thin-film lithium tantalate onto the same coupled-cavity platform, demonstrating material flexibility for scalable integrated frequency bin quantum photonic circuits.

Localized surface plasmons (LSPs) in multi-shell nanostructures provide a versatile route for controlling optical fields at the nanoscale, yet the influence of deviations from concentric geometries remains insufficiently understood. Here, we investigate the impact of shell nonconcentricity on the quasistatic optical response of core-single-shell and core-multi-shell nanowires. Exploiting the conformal properties of bipolar coordinates, we derive analytical solutions for nonconcentric cylindrical interfaces and systematically analyze the evolution of LSP resonances, absorption spectra, and near-field distributions. Starting from single-shell structures, we show that nonconcentricity enables finite coupling of incident radiation to higher-order plasmon modes that are optically inactive in the quasistatic concentric limit. Extending the analysis to multi-shell bull's eye wires, we identify how shell thickness, number of fixed-thickness shell units, each defining a set of a dielectric and metal shell, as well as interface nonconcentricness shape the hybridized plasmon spectrum. Increasing the number of metal-dielectric interfaces broadens the spectral response, while nonconcentric geometries additionally increase the density of accessible resonances and localize electromagnetic fields preferentially within and around the thinner shell sections. Eventually, comparison of concentric with bipolar and Doppler-grating-inspired nonconcentric bull's eye wires based on Mie theory and full-wave Discontinuous Galerkin Time-Domain simulations, respectively, allows to assess the impact of nonconcentricness for typical nanowire dimensions. These results provide insight into geometry-induced plasmon hybridization and suggest routes toward nanoscale control of optical energy localization for applications in active nanophotonics and plasmon-assisted photochemistry.

Cavity quantum electrodynamics (QED) modifies many-body systems by combining cavity-mediated collective interactions with microscopic short-range interactions. The resulting dynamics lies between the local and fully collective limits, such that neither locality nor collectivity alone provides a complete organizing principle. In the clean conceptual limit of dominant collective coupling, we show that the effective dynamics within each energetically isolated subspace is generically controlled by whether that subspace admits a local product-state basis, and identify exceptions imposed by angular-momentum selection rules. Product-state subspaces generically retain the spatial structure of the microscopic interaction. Entangled subspaces instead generically dress local processes with global operators, generating non-local but highly structured dynamics. We illustrate this by deriving the corresponding effective Hamiltonians in two representative cavity-QED spin models. The cavity-isolated subspace thus becomes a resource for generating competing short-range interactions or globally conditioned local processes, opening a class of many-body dynamics without local or fully collective counterparts.

Structural color arises from the interaction of light with nanoscale structures and is widespread in nature. As structural complexity increases, the mechanisms governing coloration become progressively less understood. The optical response of periodic photonic crystals with a spatially periodic refractive index is well described by Bloch theory, whereas that of photonic glasses composed of randomly assembled uniform spheres is more subtle yet well studied. In contrast, disordered photonic networks found in many beetles are among the most complex natural photonic architectures, and the fundamental relationships between their structure and color remain unclear. Here, we use an inverse method to identify the structural features encoded in the reflectance spectrum. By comparing the spectrum of an unknown system with a database of simulated spectra from computer-generated photonic networks, we infer its structural properties. Applying this approach to both simulated networks and the biophotonic network responsible for the blue coloration of the weevil Pachyrhynchus congestus mirabilis, we identify rings and pores as the key local scattering motifs. Their characteristic sizes govern the spectral position of the reflectance peak, whereas short-range disorder controls its width. The inverse method reveals clear spectral signatures of short-range order, whereas the influence of hyperuniformity and primitive similarity appears comparatively weak. This suggests that blue structural coloration is governed by local scattering mechanisms rather than photonic band-gap effects. We propose an analogy to an inverse photonic glass, in which pores and rings act as correlated local resonators. This perspective provides new design principles for bio-inspired structural-color materials.

Terahertz (THz) photonics offers a promising platform for next-generation 6G wireless communications, on-chip spectrometers, and non-invasive biomedical sensors. In the near- and mid-infrared, monolithically integrated photodetectors are a cornerstone of photonic integrated circuits (PICs). In the THz range, however, such integration on low-loss dielectric waveguides remains an open challenge: to date, room-temperature on-waveguide detection has been realized only through hybrid flip-chip assembly of discrete elements, which leads to poor mode coupling, larger footprint, and reduced mechanical stability. Here we address this gap by demonstrating a room-temperature graphene sub-THz detector monolithically integrated on a high-resistivity silicon dielectric waveguide for D-band (110-170 GHz) operation. We couple an hBN-encapsulated graphene channel to the guided mode using a tapered slot-line antenna patterned directly onto the silicon surface. The detector achieves a voltage responsivity of 11.5 V/W and a 3 dB bandwidth of 1.94 GHz, with the latter currently limited by parasitic inductance of the readout interconnects rather than by the intrinsic graphene response. This platform provides a practical pathway to fully integrated room-temperature THz photonic circuits, where electrostatic gating and impedance matching can improve both responsivity and bandwidth by roughly an order of magnitude.

We identify single-mode Gaussian probes, generated by displacement and squeezing operations on the vacuum state, which are optimal for the simultaneous estimation of displacement and squeezing operations in continuous-variable quantum systems. Importantly, our results reveal that the best precision at a fixed energy is achieved not by an experimentally costly squeezing resource, but rather by redirecting some of the energy towards displacement, thus allowing for more resource-effective operations. Furthermore, introducing indefinite causal order (ICO) in either the probe preparation or parameter encoding step can surpass the Gaussian precision bound, even though the optimal Gaussian probe state is agnostic to the ordering of the operations. Specifically, we observe that odd-parity superpositions of the two definite orders can enhance precision over optimal Gaussian probes in specific parameter regimes. Further, the observed advantage cannot be attributed solely to non-Gaussianity, as quantified by the relative entropy of non-Gaussianity, highlighting ICO as an independent resource for enhancing multiparameter estimation.

Robust unidirectional edge propagation is conventionally attributed to topological protection. Whether edge states in the continuum (EICs) can exhibit such robustness in non-topological systems remains an open question. Here we demonstrate robust unidirectional EICs in Floquet photonic crystals (PhCs) composed of a honeycomb lattice of helical waveguides, where both time-reversal and spatial inversion symmetries are broken. Within a topologically trivial parameter regime of this system, where the Chern, valley Chern, and winding numbers all vanish, the EIC robustness is decoupled from topology. Instead, the robustness originates from a z-periodic Floquet artificial gauge field geometrically locked to the helical lattice. Numerical simulations show that the EIC survives 120 bent edges, 6% on-site potential noise, and 27% hopping phase noise. This work establishes a paradigm for robust light propagation in non topological systems and broadens the physical basis for unidirectional EICs.

Continuous-wave (CW) deep-ultraviolet (DUV) lasers near 230~nm enable laser cooling of AlF, Cd, and Zn, but second-harmonic generation below 237~nm relies in practice on beta-barium borate (BBO), whose walk-off and UV-induced degradation hinder sustained operation. We demonstrate compact, affordable VECSEL-based systems informed by four years of operating 14 DUV cavities in 12 laser systems across six European laboratories. External LBO cavities produce nearly 4~W at 463~nm with 94% cavity efficiency. We compare spherically and elliptically focused Brewster-cut BBO cavities with a normal-incidence AR-coated design. The AR-coated cavity delivers the highest power and efficiency, reaching 1.0~W at 51% cavity and 44% external efficiency; the spherical Brewster cavity reaches 700~mW and maintains constant circulating power over 70~h, while elliptical focusing reduces peak intensity sixfold and improves beam quality, albeit with greater alignment sensitivity. Collaboration-designed DUV optics, AlF spectroscopy, and Cd trapping validate the system.

The rapid growth of generative AI has intensified the need for efficient heat dissipation in large-scale data centers. To control heat flow, thermal metamaterials with layered structures have been widely used, which impart the anisotropic properties of thermal conductivities. However, the conventional effective medium approximation (EMA) often fails to provide accurate predictions in systems with a high thermal conductivity contrast between adjacent layers embedded in a background medium. Here, we generalize the EMA by introducing two corrective coefficients that extend its validity to regimes where the conventional EMA was previously inapplicable, i.e., high-contrast thermal metamaterials with the background medium. Notably, one of these coefficients that we proposed has the same mathematical form as the Fresnel reflection coefficient in optics. This allows us to interpret the "reflection-like" behavior of heat flow as it penetrates adjacent layers with high thermal contrast. Our findings suggest that heat diffusion, traditionally viewed as a purely dissipative process, can be understood intuitively through the framework of ray optics.

The double-indexed geometric phase (DIGP) based on a family of Poincaré spinors was formulated to extend the Pancharatnam phase, commonly used to describe the evolution of the polarization state in an optical system relative to a reference polarization state. Analytical results establish the symmetries of the DIGP under reversal of the index $p\in\mathbb{R}$ for the latitude on the Poincaré sphere, the phase change induced by reversal of the index $q\in\mathbb{R}$ for the longitude on the same sphere, and the periodic dependence on $q$. For closed loops on the Poincaré sphere, the DIGP can be interpreted geometrically through a $p$-dependent mapping of both geographic coordinates and the associated solid angle subtended at the center of the sphere. Although the geometric phase is usually applied to compare two plane waves propagating in the same direction in free space, the DIGP is expected to contain information when comparing two non-co-propagating plane waves, as demonstrated by specular reflection by a planar thin film and far-zone scattering by a three-dimensional object. DIGP maps for specular reflection and transmission by thin films resolve Bragg phenomenons, Fabry–Pérot resonances, structural chirality, anisotropy, and defects with detail not evident in reflectance and transmittance maps. A map of direction-dependent DIGP for plane-wave scattering by a three-dimensional object may reveal strong polar and azimuthal structure absent from the map of differential scattering efficiency. Principal component analysis indicates that a collection of DIGP maps, all calculated from the same set of polarimetric measurements, form a coordinated data family rather than a collection of independent observables. The DIGP concept offers a complementary framework for polarization-state-resolved characterization and inverse scattering.

Frequency-to-time mapping (FTTM)-based photonics-assisted spectrum sensing enables wideband spectrum monitoring for applications such as cognitive radio and electronic warfare. However, FTTM-based photonics-assisted spectrum sensing approaches involve trade-offs among analysis bandwidth, temporal resolution, and frequency resolution. Although channelization has been introduced to alleviate these trade-offs, the limited number of available channels in previously reported systems constrains further improvements in overall performance. Here, we propose a photonics-assisted dual-comb channelization architecture for broadband frequency measurement and time-frequency analysis. The system employs a stepped-frequency pump based on stimulated Brillouin scattering (SBS) to generate an optical filter comb and adopts a single-laser design for the pump, probe, and reference paths to enhance frequency stability while reducing system complexity. Experimental results show a 48-GHz analysis bandwidth in the 16-channel configuration and 52 GHz in the 20-channel configuration, together with microsecond-scale temporal resolution and a frequency resolution better than 80 MHz. The proposed architecture therefore offers a practical and reconfigurable solution for real-time wideband spectrum sensing.