ALT The spectral characteristics of ultrashort laser pulses offer unique possibilities to control interactions in space and time as well as the subsequent material responses. In particular, selecting the wavelength predefines the fundamental mechanisms governing laser-matter interaction, and thus, how energy is deposited. This Chapter provides an overview on how the essential physical effects in ultrafast laser-matter interaction scale with the wavelength, and the consequences for bandgap material processing applications. First, we recall the main spectral properties of bandgap materials and discuss the important question of the expected processing resolutions depending on wavelength. Then, recent studies on the impact of the wavelength in ultrafast laser-dielectric interaction are presented. Finally, we present how the latest developments in infrared ultrafast laser sources can be advantageously utilized to internally process narrow-gap materials despite challenges related to increased no

ALT Counting the emitters within a diffraction-limited region is a prerequisite for localizing overlapping molecules. Under pulsed excitation, each molecule emits at most one photon per pulse, so photon-number-resolved detection can count emitters without resolving them. But the required number of pulses grows steeply with the emitter number. Here we show that polarization, conventionally used to measure the orientations of dipole emitters, also carries information on their number. Because mean polarization-resolved signals are unchanged when every emitter is replaced by multiple dimmer copies, this information resides in the photon statistics within each pulse, which polarization reshapes in two ways. Linearly polarized excitation at three angles increases the sub-Poissonian deficit of the photon count by a factor of 1.5 for any emitter orientation, and detection in four polarization channels reveals the emitter number through how the photons of multiphoton pulses are distributed among th

ALT Quantitative phase imaging (QPI) has become a standard approach for label-free microscopy of transparent specimens, mapping the optical path length. However, the isotropic phase is blind to the molecular order underlying optical anisotropy. Implementations that also add quantitative contrast to birefringence, rely on multi-frame illumination or polarization-state scanning, or on dedicated interferometric hardware, that trade away acquisition speed or the simplicity of a standard microscope. We present Quantitative Phase and Polarimetric Imaging (QP2I), a computational imaging framework that converts a standard differential interference contrast (DIC) microscope with a polarization camera into a single-shot, video-rate, dual-contrast imaging system. We developed a new globally regularized phase reconstruction that eliminates artifacts of prior DIC-to-QPI approaches, providing quantitative phase information and used per-pixel Stokes-based reconstruction to obtain quantitative anisotropic

ALT Next-generation on-chip photonic devices require compact, energy-efficient components integrating photodetection and light emission on silicon platforms. Achieving broadband photodetection combined with efficient emission in a single nanoscale device remains challenging. Here, we demonstrate elliptical nanopillar arrays monolithically integrated as III-V-on-Si templates, exhibiting broadband photoresponse across visible to near-infrared wavelengths, dynamic neuron-like graded responses under modulated light, and pronounced electroluminescence at around 865 nm under forward bias. The nanostructured elliptical geometry approach enhances optical absorption and carrier dynamics within an active area significantly smaller than commercial detectors, enabling responsivities up to 0.55 A/W at 830 nm in compact nanoarrays. This work advances the development of miniaturized active nanophotonic devices suitable for integrated sensing, emitting and communication circuits, and neuromorphic processi

ALT Chemical characterization of microscopic samples often requires resolving multiple, widely separated vibrational absorption bands, including their positions and spectral shapes. Photothermal microspectroscopy provides a route to localized vibrational measurements by detecting absorption-induced thermal responses with a separate optical probe. However, rapid acquisition of broadband photothermal spectra remains challenging when spectral information is collected through sequential wavelength tuning. Here, we develop a supercontinuum (SC)-driven Fourier-transform photothermal (FT-PT) microspectroscopy platform that combines broadband near-infrared excitation with rapid-scan interferometric acquisition for localized vibrational measurements. In this approach, an interferometrically modulated SC pump induces wavelength-dependent photothermal responses in the sample, which are detected through a co-propagating visible probe beam and reconstructed by Fourier transformation. Using a 1.65-2.35

ALT Cameras in the surf zone or behind ship wakes look through turbid, bubbly, moving water close to the lens, yet restoration networks are usually scored only on displayed images. We introduce a measured dataset of a telescope viewing displayed images and physical objects through a 25 mm water layer 45 mm in front of its objective, under seven combinations of flow, bubbles and turbidity, with clear-water recordings as a second reference. Because the suspension scatters light far outside the 1.3textdegree field of view, turbidity mainly removes contrast, 4.4- to 4.8-fold on a Siemens star. Restoring the mean of eight frames improves structural similarity (SSIM) by 0.032 over a single frame, and learned fusion of the frames adds little. One raw frame supports five-category recognition (96.8\%) as well as restored images. Adding synthetic training pairs leaves display-image scores unchanged but raises the correlation of a restored Siemens star with its clear-water recording from below 0.5 to

ALT This paper develops a stochastic model for quantifying the impact of laser phase noise on photonics-assisted radio frequency (RF) beamforming. Unlike conventional deterministic models, the presented theoretical framework captures the statistical randomness associated with the RF array factor of beamforming networks driven by finite-linewidth lasers. Fourier-domain analysis reveals that laser phase noise reshapes the spatial coherence function of such beamforming networks, inducing coherent RF aperture shrinkage, beam-pattern distortion, and directivity degradation. The analysis highlights that beamforming network architectures employing nanosecond-scale optical differential delays can exhibit substantial performance degradation even when driven by commercially available narrow-linewidth (kHz-range) lasers. Furthermore, it is demonstrated that average RF power gain gets downscaled and fluctuations dominate in large-aperture beamforming networks due to phase noise, imposing an upper limi

ALT The confinement of light within optically thin media challenges the conventional view that optical localization requires high-index dielectric materials and total internal reflection. Over the past decades, a broad range of photonic phenomena has demonstrated that electromagnetic energy can also be guided and localized inside low-index regions or nanoscale void cavities. What first appears to be a curiosity is, in fact, highly relevant: The interaction between light and matter lies at the heart of much in optics and photonics. It enables sensing, nonlinear optical phenomena, and optical manipulation, yet it also inevitably introduces absorption, dissipation, and loss. Pushing localized, enhanced, or guided energy into a lower-index space thus offers entirely new possibilities for light-matter interaction and dispersion control. In this review, we discuss some of these counterintuitive structures, starting with light guidance within optically thin media, such as hollow-core photonic cry

ALT Accurate optical characterization of high-albedo scattering media is fundamentally limited by integrating-sphere measurement errors. Scattering events spatially redistribute photons, leading to lateral beam spread and angular divergence that bias measured reflectance and transmittance. Here, we investigate these deviations, which are quantified and interpreted by using the point spread function (PSF), guided by the radiative transfer equation. Experiments and Monte Carlo modelling show that the widely used external-mount integrating-sphere configuration introduces systematic, configuration-dependent errors that become pronounced when scattering-induced lateral spreading is substantial. Using a normalised PSF to quantify photon redistribution, we map the characteristic lateral spreading length across a broad (τs,g) parameter space in the high-albedo limit, providing a basis for evaluating errors in specific measurement geometries. We further quantify the influence of measurement geometr

ALT Optically bistable materials promise technological advances in optical information processing, memristive devices, and multiparametric sensing. However, optical bistability is predominantly observed under cryogenic conditions or is governed by subtle thermo-optical nonlinearities. Here, we report on Ho3+-activated nanocrystals that exhibit cavity-free intrinsic optical bistability (IOB) at ambient conditions. Non-thermal IOB stems from a cross-coupled photon avalanche (XPA) mechanism enabled by a low-phonon-energy double-perovskite host, which reduces nonradiative losses and facilitates excited-state population via XPA, even when the pump intensity is below threshold. Time-resolved luminescence measurements support the proposed XPA, as evidenced by a double-rise profile consistent with two reservoir states interlocked in energy looping. IOB with high-contrast photoswitching (up to 46 dB) is observed at room temperature across multiple visible emission lines, enabling wavelength-divisio

ALT We present RADICAL, the silicon-photonic-enabled Reconfiguration-Aware Direct-Connect AI Cluster that integrates a custom spatial-and-wavelength selective switch (SWSS) with the Linux network stack. RADICAL deploys a packaged 4x4x4λ microring-based SWSS into a four-GPU-server cluster using standard SFP28 optical transceivers, and a reconfiguration-aware host-networking interface to demonstrate a direct-connect reconfigurable optical fabric. The SWSS provides dynamic spatial and wavelength routing with 3.2-6.8 dB path loss, average wavelength and spatial crosstalk isolation exceeding 25 dB, and error-free 25 Gbps signal recovery across routing paths. At the system level, RADICAL validates reconfiguration-aware operation through continuous bulk data transfers totaling 6.4 TBytes over 10 minutes. We further demonstrate multitenant AI training across reconfigurable link topologies, including four ResNet-18 instances and six nanoGPT instances, with the latter sustained for more than five ho

ALT We present an ultrastable laser system based on miniature core components: a self-injection-locked laser comprising a distributed-feedback laser diode and an integrated microring resonator, together with a 3-ml vacuum-gap Fabry-Perot cavity. By carefully addressing technical noise sources, in particular residual amplitude modulation, we demonstrate a thermal-noise-limited fractional frequency instability of 5⋅10⁻¹⁵ for averaging times between 1 and 10 s, which is among the lowest reported for compact cavity-stabilized lasers. This performance is achieved with conventional multilayer mirrors and a moderate cavity finesse of 10⁵. Such stability of the output frequency, combined with the small size of the core components and a possibility to generate a Kerr frequency comb in the very same microring resonator, provides a path towards fully integrated photonic frequency references for transportable optical clocks and low-noise microwave synthesis.

ALT Programmable photonic meshes can perform general linear transformations, but universal transformations on N modes require O(N²) programmable phase shifters, which cannot simultaneously achieve low drive voltage, gigahertz speed, zero static power, lossless modulation, and a compact footprint. Signal routing does not require universal transformations, which allows considerable circuit simplifications. We present a compact reflective circuit architecture that connects all input-output pairs across N modes in N steps using only O(N) programmable phase shifters. The circuit comprises a single passive N× N interferometer and a single column of programmable reflective phase shifters. We demonstrate the architecture by implementing a 3× 3 router in x-cut thin-film lithium niobate with a passive multimode interferometer and phase shifters based on single-sided photonic-crystal nanobeam cavities. Our programmable devices exhibit a 1.92 GHz/V resonance tuning efficiency via the Pockels effect, a

ALT In this work, we present a spatial and temporal light trapping mechanism induced by coherent frequency conversion. We consider a wave-mixing process, where a pumped nonlinearity mediates a frequency-domain beamsplitter interaction between two signal modes. When these wave modes are counterpropagating, temporally trapped eigenmodes emerge at the zero of the nonlinearity profile. We establish that such spatio-temporal trapping has a direct connection to supersymmetric quantum mechanics, which ensures the existence and robustness of the trapped modes and allows us to derive analytic solutions of the bound eigenmodes. Backwards sum-frequency conversion may thus enable a unique class of active photonic memory devices, whose design parameters are compatible with the state of the art in integrated nonlinear photonics. We propose proof-of-principle experiments to study this nonlinear-optical trapping effect on thin-film lithium niobate. In addition, as a case study, we show how the temporal tr

ALT Polarization topology in momentum space is a fundamental characteristic of photonic bound states in the continuum (BICs), where the winding of far-field polarization defines a quantized topological charge. Although magneto-optical (MO) BICs combine polarization topology with magnetically tunable chirality, experimental characterization remains challenging because time-reversal symmetry (TRS) breaking complicates eigenpolarization extraction. Here, we experimentally resolve the momentum-space polarization topology of an MO BIC by reconstructing its two-dimensional far-field eigenpolarization texture. We extract the resonant output polarization using singular value decomposition of the polarization-resolved angle-resolved reflection matrix without invoking reciprocity. The reconstructed polarization azimuth winds counterclockwise by 2π around the Γ-point singularity, yielding a topological charge of q = +1. Moreover, the polarization handedness can be reversed solely by varying the magne

ALT Mechanically compliant mirrors are key components in cavity optomechanics. However, the requirements to achieve the best optical and mechanical performance are usually in direct competition. A high reflectivity for tightly focused beams entails thick reflectors, whereas ultracoherent flexural nanomechanics necessitates thin membranes. We eliminate this trade-off by introducing high-refractive-index nanopillar metasurfaces on top of a thin membrane. We pattern a 50 micrometer-diameter array of nanopillars to form a photonic crystal. The flat band behavior of guided resonances in the photonic crystal enables a reflectivity of 99.82(1)% for a tightly focused beam of 7 micrometer waist. The combination of two guided resonances creates a 100 nanometer-wide high-reflectivity band around 1550 nanometer. Simultaneously, the nanopillar patterning realizes a density-modulated phononic crystal that isolates an ultracoherent localized mechanical mode with a Q⋅ f-product of 2⋅ 10¹⁴Hz. The demonstra

ALT Narrow-linewidth lasers are essential for various key atom-laser interaction processes and optical setups in quantum coherent control, quantum precision measurement, and quantum information science. The Pound-Drever-Hall (PDH) technique is the standard method for the frequency-stabilization of narrow-linewidth lasers, but the inherent time delay in its feedback loop inevitably induces excessive phase noise, commonly known as servo bumps. Previous solutions primarily rely on additional optical components outside the electronic feedback control loop. Here, we propose and implement a generic method based on the Smith predictor to address the time-delay issue entirely within the electronic part of the PDH feedback control loop. We systematically demonstrate its capability to suppress phase noise and reduce servo bumps by adding only a single Field-Programmable Gate Array (FPGA) controller to the loop. This class of methods is demonstrated to be conceptually concise with minimal hardware ov

ALT Astigmatic transformation is widely used to convert the azimuthal phase structure of Laguerre-Gaussian vortex beams into intensity patterns from which the topological charge can be read out. Here, we show that this phase-to-intensity conversion is not just restricted to the Laguerre-Gaussian radial profile. We derive analytical expressions for the astigmatically transformed high-order Bessel–Gauss and perfect optical vortex fields, and find that, despite their substantially different radial structures, the number of intensity minima is directly set by the accumulated azimuthal phase winding. Numerical simulations and experimental measurements on both beam families confirm this result, establishing astigmatic transformation as a direct probe of the topological charge of scalar structured optical fields. We further extend the approach to non-separable vector Bessel-Gauss beams, in which the spatial and polarization degrees of freedom are coupled, and show that polarization-resolved detec

ALT Thick, dispersion-engineered silicon nitride is a leading platform for integrated third-order nonlinear optics, supporting supercontinuum and frequency comb generation. However, it lacks the second-order nonlinearity required by many applications. Here we bond pre-fabricated x-cut thin-film lithium niobate membranes onto finished silicon nitride waveguides as a back-end step, compatible with commercially fabricated waveguides. A single silicon nitride waveguide generates an octave-spanning supercontinuum and, through the lithium niobate membrane, concurrent second-harmonic, sum- and difference-frequency generation. These results open new possibilities for thick silicon nitride photonics, from visible light generation to on-chip f-2f comb self-referencing, a key step towards miniaturised timing and navigation quantum technologies.

ALT Topological textures of light, such as optical vortices and skyrmions, are attractive for high-density information encoding and light-matter interactions. In the near field, topologies have primarily been realized using polaritonic platforms that inherently tie the platform to the material dispersion, limiting their footprint due to propagation losses and restricting their operating spectral region. Here, we introduce a route to shape near-field topologies through the geometric engineering of a dielectric metasurface supporting a quasi-bound state in the continuum resonance, without the need for polaritonic surface waves. In-plane deformation of our metasurface bars conserves their resonance-driven near-fields locally, so that the metasurface geometry and the polarization of the illuminating light dictate the generated topology. By shaping silicon unit cells into geometries with rotational symmetry, we create optical topologies with controllable topological charge. Using phase-resolved