Optics Papers

ALT In this paper we present a gauge-invariant formalism for describing the spin and orbital angular momenta of scalar and vector optical vortices. The correction term, arising from consistent incorporation of the transverse vector potential's nonlocality, restores gauge invariance and makes it possible to separate the total angular momentum into spin and orbital components. We propose a method for calculating the orbital and spin angular momenta of multimode vector vortex fields and show that the orbital angular momentum of such beams contains two qualitatively different components. The polarization-independent contribution to the orbital angular momentum is related to the topological charge that determines the order of the phase singularity, while the polarization-dependent one is proportional to the helical topological charge that determines the spatial inhomogeneity of the polarization. We also discuss the quantization procedure for fields with spatially inhomogeneous polarization in t

ALT Topological photonic structures provide a powerful means of confining and directing light through interface states formed between photonic phases with distinct topological properties. Van der Waals (vdW) materials offer complementary opportunities for nanophotonics: their optical properties can be selected from a wide family of layered crystals, while their atomically smooth surfaces and lack of lattice-matching constraints enable complex photonic heterostructures to be assembled layer by layer. This flexibility allows topological photonic confinement and strong excitonic coupling to be engineered separately within the same structure. Here, patterned low-loss dielectric hexagonal boron nitride (hBN) defines the photonic modes, while an encapsulated WSe2 monolayer provides the excitonic resonance that couples directly to the confined optical field. We use this approach to realise Jackiw-Rebbi exciton-polaritons in a one-dimensional hBN double-grating heterostructure. Strong coupling bet

ALT Sommerfeld's half-plane problem, describing diffraction of a monochromatic wave by a semi-infinite perfectly electric conducting (PEC) infinitesimally thin plate, is one of the cornerstones of classical electromagnetic scattering theory. Here we study its temporal analogue: a semi-infinite medium with the Drude dispersion, adjacent to air, whose plasma frequency is rapidly reduced to zero, thus turning it into air, while a monochromatic wave with frequency below the plasma frequency illuminates it under normal incidence. Unlike the PEC plate, the Drude medium supports evanescent penetration of the field prior to its temporal transition; the temporal interface here acts as the analogue of the sharp spatial boundary in the Sommerfeld problem, but now separates two time intervals rather than two spatial half-planes. We show that this transition forces the standing wave in air in front of the Drude medium and the evanescent fields inside it to propagate in both directions, while preserving

ALT As cubic GaN is a promising material for optoelectronic applications, its layers need to be precisely doped to realize the desired properties. While contact-based methods are established to measure the realized charge carrier density, contact-free techniques are more advantageous for this purpose. We show terahertz time-domain spectroscopy measurements of cubic GaN layers with thicknesses between 0.6 μm and 2 μm with doping densities in the range from 10¹⁸ cm⁻³ to 10²⁰ cm⁻³. From these measurements, we determine the charge carrier densities and verify the results with time-of-flight secondary-ion mass spectrometry measurements. The results are in accordance with simulations of the doping ranges which the technique is expected to be sensitive in. In preparation for the terahertz time-domain spectroscopy measurements, we validated the layer thicknesses by Fourier-transform infrared spectroscopy measurements. With this, a wide range of doped cubic GaN samples can be addressed by optical m

ALT Spatial wave localization at boundaries is conventionally restricted to exponential decay. Here, we observe a scale-free boundary mode exhibiting anomalous polynomial decay, highlighted by strictly linear spatial envelopes, driven by topological multicriticality in acoustic metamaterials. We establish a universal geometric bulk-boundary correspondence rooted in singularity theory by mapping n-th order topological multicriticality onto Arnold An catastrophe. We prove that defectiveness in the spatial transfer matrix enforces a (n-1)-th power spatial decay of boundary modes. At a topological tri-critical point, the singularity of winding function manifests as an A2 fold catastrophe. This dictates a strictly linear spatial decay, marking a sharp departure from the conventional exponential scaling paradigm. Using acoustic metamaterial platforms tuned to a tricritical point, we experimentally demonstrate this linear decay envelope in 1D, and further generalize it to two-dimensional systems

ALT Nematic liquid crystals confined in cylindrical pores adopt multiple stable director topologies — radial, escaped, and arches — each imprinting a distinct optical signature on transmitted circularly polarized light, with topological charge set by the projected director winding number, ℓ = 2m. Using Landau–de Gennes and Jones-calculus modeling validated against micron-scale experiments, we show that this mechanism persists as the pore diameter shrinks toward the optical wavelength. However, full-wave Maxwell simulations of periodic sub-wavelength pore arrays reveal that the resulting vortex is a near-field phenomenon only: each topology imprints a distinguishable phase-singular signature — differing in core multiplicity, core width, and ring amplitude — directly below the pore array, but this signature does not survive propagation into the far field, where diffraction and inter-pore interference erase it within about one wavelength. Landau–de Gennes simulations further demonstrate that

ALT Absolute distance interferometry requires an unlimited non-ambiguity range to be truly absolute. Tunable synthetic wavelength interferometry allows for the construction of pseudo-supersynthetic wavelengths using a limited number of optical sources and a standard interferometer setup. This approach facilitates the creation of a series of consecutively deployable synthetic wavelengths, ultimately combining macroscopic unambiguity with microscopic interferometric accuracy. We have implemented and demonstrated this concept in the TeleYAG-II interferometer, designed as an SI-traceable standard for the calibration of geodetic baselines. The system employs two Nd:YAG lasers as optical sources, with their frequency difference controlled by a versatile optical phase-locked loop. Specifically engineered for robustness in field conditions, the heterodyne interferometer was validated through an outdoor measurement campaign. The results demonstrate the efficacy of the method, achieving an expanded

ALT The linear behavior of micro- and nanomechanical devices has been the topic of intense study over the past years. Their nonlinear behavior, on the other hand, continues to provide new and interesting dynamics. This work leverages a cavity opto-mechanical system whose very high sensitivity and low dissipation rate allows to capture the dynamics of fast transients that occur when strongly-driven nonlinear resonators relax to their equilibrium state. We experimentally study the ringdown dynamics of a nanomechanical Duffing resonator in both the time domain and in phase space. The resonator exhibits very different ringdown dynamics depending on whether it is subject to one or two tone driving. In the former case the ringdown follows a regular exponential decay, whereas the latter case shows exotic dynamics such as libration, slowing-down while scattering in phase-space, and ghost slowing-down. We demonstrate that this distinct difference can be attributed to the structure of the phase spac

ALT Optical manipulation offers a powerful non-contact approach to precise positioning, directional transport and three-dimensional (3D) imaging of microscale objects. However, positional fluctuations during optical confinement introduce spatial sampling errors that compromise imaging quality and acquisition speed. Here, we demonstrate a scattering-force-driven optical manipulation imaging method that replaces the stepwise scanning of conventional optical trapping with continuous directional transport of microparticles. An axial scattering force generated by a low-numerical-aperture focused beam propels microparticles through a light sheet, while their positions and fluorescence signals are simultaneously recorded to establish direct spatial correspondence between particle motion and tomographic images. Experiments demonstrate that this approach reduces the effects of positional fluctuations on 3D imaging, achieving an axial sampling interval of 0.1 μm and an imaging rate of 250 Hz. The me

ALT Distributed acoustic sensing (DAS) transforms optical fibres into dense arrays of vibration sensors for continuous, long-range monitoring with high spatial resolution. Yet converting these large-scale measurements into actionable information with compact models that retain task performance remains challenging. Existing artificial-intelligence approaches are typically optimized for specific sensing environments and tasks, but their computational requirements can hinder deployment in compact, power-constrained field systems. Here, we introduce DAS-NanoNet, a framework that automatically constructs lightweight neural networks for different DAS scenarios and tasks, together with OmniDAS, an open benchmark spanning six representative environments. Using lightweight temporal operators tailored to DAS signals, an adaptive neural architecture search strategy derives a compact network for each scenario-task pairing. Across classification, localization, compression, and denoising tasks in divers

ALT Scalable quantum platforms require optical systems that deliver well-defined polarisation without relying on macroscopic bulk optics or manual alignment, motivating chip-integrated approaches capable of shaping both the spatial mode and the polarisation of the emitted field. By developing a full three-dimensional analytical model of the scattering process together with numerical simulations to design and evaluate the emitted field, we show that tilted waveguide Bragg gratings can act as integrated polarisation beam shapers. We find that scattering two co-propagating, orthogonally polarised pumps on a single grating is fundamentally limited: waveguide birefringence and the difference in scattering efficiency prevent full polarisation control. We overcome this with an orthogonal superimposed-grating architecture in which two gratings driven independently by two pumps propagating perpendicular to each other succeed in generating any desired polarisation output apart from directions in a n

ALT Optical spatiotemporal vortices carrying transverse photonic orbital angular momentum have become an intense research topic in the recent study of structured light fields. Despite being mainly studied within the framework of fully coherent optical pulses, the complex topologies of the light field are also investigated with partially coherent light sources, offering an alternative pathway in applications. In this work, we theoretically and experimentally investigate partially temporally coherent spatiotemporal Laguerre-Gaussian (STLG) field with independently controllable radial and azimuthal indices. The spatiotemporal topological information, carried by the structured amplified spontaneous emission source, are well preserved in the presence of inherent instantaneous random temporal phase fluctuations. Under combined spatiotemporal astigmatism and group velocity dispersion, the partially temporally coherent STLG field can be converted into spatiotemporal Hermite-Gaussian (STHG) field.

ALT In non-Markovian light-matter interactions, a spectrally structured bath can alter the evolution of a quantum emitter by modifying the vacuum fluctuations experienced by the emitter. This raises the question of how the localization transition and the associated multifractal critical modes induced by a quasiperiodic potential shape the emitter dynamics. We investigate this scenario in the single-excitation sector for a two-level quantum emitter coupled to an Aubry-André-Harper (AAH) photonic lattice. By externally tuning the strength of the quasiperiodic onsite modulation, the lattice is tuned from the extended to the localized regime through a critical point characterized by multifractal modes. Using modal analysis, we show that the quasiperiodic modulation reshapes the local density of states at the coupling site and thereby controls the effective spectral coupling between the emitter and the lattice. The transient dynamics depend strongly on the spatial and spectral overlap between t

ALT Circularly polarized light underpins a broad range of chiroptical and spin-selective photonic technologies, yet extending resonant chirality to nonlinear harmonic emission remains challenging. Here, we experimentally demonstrate a symmetry-governed mechanism that maps the maximal linear chirality of high-Q degenerate resonances onto the polarization of the generated harmonic. Preserved rotational symmetry protects modal degeneracy and polarization-selection rules, while broken mirror symmetries enable a strong chiroptical response. The metasurface thereby filters the pump polarization and purifies the handedness of the nonlinear output, simultaneously achieving third-harmonic circular dichroism of -0.98, a degree of circular polarization of 0.83, and an effective nonlinear Q-factor of 140. Importantly, the nonlinear emission preserves its handedness even under spatially depolarized excitation, demonstrating robustness to variations in the incident polarization state. These results esta

ALT Near-infrared (NIR) fluorescent molecules often suffer from low quantum yields, limiting their applications in sensing and imaging. Their fluorescence can be enhanced by coupling with plasmonic nanoparticles exhibiting resonances in the same spectral region. However, this approach requires precise nanoscale control over the plasmonic near-field. Here, we combine scanning transmission electron microscopy with electron energy-loss spectroscopy (STEM-EELS) and boundary element electromagnetic simulations to engineer silica-coated anisotropic plasmonic nanoparticles. We synthesised three types of nanoparticles, gold nanorods (AuNRs), gold bipyramids (AuBPs), and silver nanorods (AgNRs) with tunable NIR plasmonic resonances. Silica serves as a dielectric spacer, enabling precise control over the distance between the nanoparticle surface and a potential molecular emitter. To assess their suitability for fluorophore attachment, we investigated the dependence of the electric near-field on sili

ALT We introduce holographic microscopy (HM) as a compact, low-cost, label-free, non-specific alternative to fluorescence microscopy (FM) for quantifying bacterial physiological responses during drug screening. Using the regularized inverse holographic volumetric reconstruction (RIHVR) framework, we extract phase maps from HM images and then invert the reconstructed phase to generate fluorescence-like contrast, in which bright inverted-phase regions correspond to bacterial nucleoid content captured by FM signals. We show that this relationship holds across multiple bacterial species, growth stages, drug treatments, and post-treatment time points, demonstrating that HM can consistently capture nucleoid associated structural information without fluorescent labeling. Because HM phase images contain both whole-cell refractive-index information and internal nucleoid-associated contrast, they enable simultaneous quantification of cell boundaries, cell length, nucleoid size, and nucleoid occupanc

ALT Super resolution optical imaging and nanolithography enable nanoscale observation and fabrication, but high order, spatially varying phase and amplitude aberrations in optically heterogeneous media severely limit their performance at depth. Here we introduce focal-plane extreme adaptive optics (FP-ExAO), which moves beyond conventional phase conjugate correction by directly sensing and controlling the focal-plane complex field. This terminal field captures aberrations accumulated throughout the optical system. A state space formulation directly links deformable mirror commands to the focal field, avoiding explicit plane by plane Fourier optics modelling, while information theoretically optimal probing and closed-form control enable rapid field estimation and correction. Cascaded deformable mirrors further enable FP-ExAO to correct coupled phase-amplitude and spatially varying aberrations. In three-dimensional (3D) single molecule localization microscopy (SMLM), FP-ExAO enables imaging

ALT Absorption-band depth measures the fractional decrease in mineral reflectance below its local continuum. Recovering this feature strength for a small mineral region supports quantitative comparisons between regions, but diffraction mixes its light with the surroundings. Conventional spectral fitting can separate mean contributions; it cannot remove background photon noise already recorded. We develop a quantum-inspired measurement framework that uses spatial-mode demultiplexing (SPADE) to retain the target absorption response while reducing background acceptance before detection. The framework connects finite mineral-region geometry and reference spectra to estimation precision. A loss-dependent design condition identifies when adding SPADE exposure improves on direct imaging (DI) under fixed-response Poisson measurements with unknown scene parameters. Joint inference from initial DI observations and both mode ports estimates target depth together with an unknown, spatially shared back

ALT Exceptional points (EPs) exhibit characteristic square-root spectral responses to weak perturbations, with response magnitudes governed by configuration-dependent prefactors that are often relatively small. Modulating these prefactor while preserving the exceptional characteristics offers an additional degrees of freedom for engineering non-Hermitian sensing responses. Here, we report a nonreciprocal non-Hermitian circuit system with gyrator-meditated coupling that supports a continuous family of EP configurations. Unlike reciprocal coupling, which typically yields a unique EP configuration for balanced gain and loss, nonreciprocal coupling allows the directional forward and backward coupling strengths to be varied while maintaining their product and thus preserving the EP condition and yielding a library of EPs. This additional freedom enables continuous control of the frequency-splitting magnitude without modifying the square-root characteristic dependence on absolute perturbation st

ALT Nanophotonic patterning is a promising approach to enhancing light extraction in scintillator materials, with the potential to improve performance in applications such as medical imaging and particle detection. While 2D surface patterns have been successful in improving scintillator performance, the true potential of nanophotonic patterning lies in the bulk, where photons are emitted, transported, and absorbed. Here, we experimentally demonstrate the enhancement and control of scintillation using 2.5D volumetric nanophotonic patterns. Using a direct femtosecond laser writing approach, we nanofabricate square lattices of air holes into the bulk of inorganic scintillators (YAG:Ce and YAG:Yb), achieving nearly subwavelength periods compared to the peak emission wavelength and depths up to hundreds of microns, which fill a large fraction of the full thickness of the bulk. Under high-energy excitation, the patterns appear up to 3.5× brighter and demonstrate emission enhancement trends consi