The linear and third-order nonlinear optical response of thin-films of the transition metal dichalcogenide hafnium disulfide (HfS2) is investigated. Varying angle spectroscopic ellipsometry measurements are performed to obtain the material's in-plane and out-of-plane refractive indices in the - wavelength range. HfS2 is found to exhibit a strong, highly anisotropic linear optical response. In particular, it is shown that the material's in-plane refractive index exceeds a value of 3 throughout the visible wavelength range, while simultaneously offering a remarkably wide transparency window with for . The absolute value of the in-plane third-order nonlinear susceptibility is derived from third-harmonic generation (THG) measurements for fundamental wavelengths of to and is found to range from to , respectively. The obtained values significantly exceed those of conventional high-index materials, such as silicon or gallium phosphide. Furthermore, the efficiency of the THG process is found to be controllable by varying both the film thickness and the dielectric environment. These findings establish HfS2 as a highly promising candidate for nonlinear optical applications, surpassing the performance of conventional high-index materials.

High-reflectivity coatings for precision interferometry must simultaneously minimize optical losses and thermally driven displacement noise. Dielectric Bragg mirrors provide robust high reflectance but rely on thick multilayer coatings, whereas metasurface mirrors provide high reflectance and low noise but are sensitive to fabrication-induced deviations. A fabrication-aware hybrid mirror concept is introduced that combines a resonant single-layer metasurface, an etch-stop layer, an antiresonant spacer, and a reduced Bragg reflector. Fabrication effects, including geometric tolerances and line-edge roughness, are explicitly considered in the optical design. Full-wave electromagnetic simulations are combined with a truncated-Gaussian Monte Carlo analysis to determine the performance distribution under the assumed fabrication conditions. The ideal metasurface design exceeds 99.999% reflectance. After roughness-aware reoptimization, the modeled non-reflected power remains below 2.88·10^-4 at 95% fabrication yield. For the cryogenic ETpathfinder gravitational-wave testbed, three Bragg layer pairs reduce the non-reflected power of the complete stack to approximately 6.4 ppm. The estimated thermal displacement-noise amplitude spectral density is 8.4·10^-21 m Hz^-1/2 at 100 Hz. The architecture connects fabrication robustness, optical performance, and thermal-noise reduction within a single design framework.

Barium titanate (BaTiO3) is a promising material for integrated photonics due to its large electro-optic and second-order nonlinear coefficients. Crystal ion slicing (CIS) enables the fabrication of thin BaTiO3 films, but ion implantation introduces crystal damage and strain that can degrade their structural and optical quality. We investigate bulk BaTiO3 subjected to low- and high-dose implantation as well as exfoliated CIS-processed flakes following thermal annealing. In bulk, ion implantation reorients ferroelectric domains from partially out-of-plane to fully in-plane polarization. Low doses relax the initial surface strain but do not enable exfoliation, whereas higher doses allow BaTiO3 thin film release at the cost of increased crystalline disorder. Raman spectroscopy reveals that phonon coherence and crystallinity, key to electro-optic performance, are restored only after thermal recovery, while second-harmonic generation microscopy shows that CIS-exfoliated flakes retain ferroelectric order and chi(2) activity even before annealing. Thermal annealing further reorganizes the ferroelectric domain structure underlying the chi(2) nonlinear response and returns the optical dispersion into close agreement with that of bulk BaTiO3. The results suggest that optimized CIS processing can yield optically nonlinear and electro-optically active films suitable for integrated photonics, with an accessible thickness range that surpasses what is achievable with epitaxial growth methods.

Photonic bound states in the continuum (BICs) have emerged as a versatile tool for enhancing light-matter interactions by strongly confining light fields. Chiral BICs are photonic resonances with a high degree of circular polarisation, which hold great promise for spin-selective applications in quantum optics and nanophotonics. Here, we demonstrate a novel application of a chiral BIC for inducing strong coupling between the circularly polarised photons and spin-polarised (valley) excitons (bound electron-hole pairs) in atomically-thin transition metal dichalcogenide crystals (TMDCs). By placing monolayer WS2 onto the BIC-hosting metasurface, we observe the formation of intrinsically chiral, valley-selective exciton polaritons, evidenced by circularly polarised photoluminescence (PL) at two distinct energy levels. The PL intensity and degree of circular polarisation of polaritons exceed those of uncoupled excitons in our structure by an order of magnitude. Our microscopic model shows that this enhancement is due to folding of the Brillouin zone creating a direct emission path for high-momenta polaritonic states far outside the light cone, thereby providing a shortcut to thermalisation (energy relaxation) and suppressing depolarisation. Moreover, while the polarisation of the upper polariton is determined by the valley excitons, the lower polariton behaves like an intrinsic chiral emitter with its polarisation fixed by the BIC. Therefore, the spin alignment of the upper and lower polaritons (↑↓ and ↑↑) can be controlled by σ+ and σ- circularly polarised optical excitation, respectively. Our work introduces a new type of chiral light-matter quasi-particles in atomically-thin semiconductors and provides an insight into their energy relaxation dynamics.

Advanced photonic quantum technologies -- from quantum key distribution to quantum computing -- require on-chip sources of entangled photons that are both efficient and readily scalable. In this theoretical study, we demonstrate the generation of polarization-entangled Bell states in structurally simple waveguides by exploiting the intrinsic properties of nonlinear crystals. We thereby circumvent elaborate phase-matching strategies that commonly involve the spatial modulation of a waveguide's linear or nonlinear optical properties. We derive general criteria for the second-order susceptibility tensor that enable the generation of cross-polarized photon pairs via spontaneous parametric down-conversion in single-material waveguides. Based on these criteria, we systematically categorize all birefringent, non-centrosymmetric crystal classes in terms of their suitability. Using coupled mode theory, we then numerically analyze cuboid waveguides made from two materials that are highly relevant to integrated photonics: lithium niobate, a well-established platform, and barium titanate, an emerging alternative. We find that barium titanate consistently outperforms lithium niobate by providing a higher nonlinear efficiency and high concurrence over a significantly broader spectral range. These findings outline a practical route toward highly efficient, fabrication-friendly, and scalable sources of polarization-entangled photons for integrated quantum photonic circuits.

The coherent, periodic energy transfer between light- and matter excitations characterizes the strong coupling regime of cavity exciton-polaritons, resulting, in the simplest case, in a Rabi-doublet in the spectral domain. We demonstrate a peculiar regime of strong light-matter coupling, which arises when photonic cavity modes couple to an ultra-thin excitonic mirror. We embed a 12 nm J-aggregated thin film in an open microcavity and tune the coupling strength from weak to the onset of ultrastrong coupling. At resonance, the excitonic mirror selectively changes dielectric to metallic field boundary conditions adding a 2π phase, which links optical cavity modes of different order. Our work gives an exciting perspective to ultra-fast cavity switches and photonic devices based on excitonic optical elements.

Monolayers of transition metal dichacogenides (1L-TMDs) show strong second-order nonlinearity and symmetry-driven selection rules from their threefold lattice symmetry. This process resembles the valley-contrasting selection rules for photoluminescence (PL) in these materials. However, the underlying physical mechanisms fundamentally differ since second harmonic generation (SHG) is a coherent process, whereas PL is incoherent, leading to distinct interactions with photonic nanoresonators. In this study, the far-field circular polarization properties of SHG from MoS2 monolayers resonantly interacting with spherical gold nanoparticles were investigated. The results indicate that the coherence of the second harmonic allows its polarization to be mostly preserved, unlike in an incoherent process, where the polarization is scrambled. These findings provide important insights for future applications in valleytronics and quantum nanooptics, where both coherent and incoherent processes can be probed in such hybrid systems without altering sample geometry or operational wavelength.

We present a tunable, single-mode-optical-fiber-based source of polarization entangled photon pairs for the near-infrared telecommunication band that is deployable in standard infrastructure. The photon pairs are generated via spontaneous parametric down-conversion (SPDC) in a submicron-scale thin film of the inversion-broken rhombohedral polytype of the transition metal dichalcogenide molybdenum disulfide (3R-MoS_2), located between two fiber connectors. By exploiting the intrinsic symmetries of the second-order nonlinear susceptibility tensor of 3R-MoS_2, this hybrid approach offers control over the generated two-photon polarization state through the incident pump polarization. Most notably, two of the four maximally entangled Bell states, as well as fully co-polarized pairs can be produced. This represents a substantial improvement in terms of tunability and simplicity over established fiber-integrated sources, which require additional optical elements, precise alignment, or careful engineering of design parameters. Additionally, a nonlinear drop in the background photoluminescence signal of 3R-MoS_2 is observed at low pump powers, allowing us to reach a coincidences-to-accidentals ratio (CAR) of (8.3±1.8)×10^3, the highest value recorded for SPDC in van der Waals materials to date.

The coherent coupling of cavity-confined photons and excitonic matter resonances leads to the formation of cavity polaritons, hybrid light-matter quasi-particles. If multiple exciton resonances couple to the same photonic mode, the resulting polariton constitutes a coherent interaction between matter resonances that can be spatially separated without any direct electronic coupling. In this work, we demonstrate the formation of such a coherent coupling at room temperature using an open optical cavity containing two distinct van der Waals materials - monolayer WS2 and layered quasi-2D halide perovskites (HaPs) - separated by 1.5 μ m. The system forms three polariton branches, with the middle branch possessing nearly equal fractions of both excitons and the photonic mode. White-light reflectivity and luminescence measurements are in good agreement with simulations using a coupled harmonic oscillator and a microscopic Wannier-Hopfield framework. Our results lay the foundation to combine highly complementary degrees of freedom in 2D materials in an in-situ tunable fashion to enable new polaritonic functionalities.

Quasi-2D halide perovskites are chemically synthesized realizations of quantum well stacks with giant exciton oscillator strengths, tunable emission spectra, and very large exciton binding energies. While these features render quasi-2D halide perovskites a promising platform for room-temperature polaritonics, bosonic condensation and polariton lasing in quasi-2D perovskites have so far remained elusive at ambient conditions. Here, we demonstrate room-temperature cavity exciton-polariton condensation in mechanically exfoliated crystals of the quasi-2D Ruddlesden-Popper iodide perovskite (BA)2(MA)2Pb3I10 in an open optical microcavity. We observe a polariton condensation threshold of 0.41 µJ cm−2 per pulse and detect a strong non-linear response. Interferometric measurements confirm the spontaneous emergence of spatial coherence across the condensate with an associated first-order autocorrelation reaching 0.6 with 1 ps coherence time and an effective de Broglie wavelength of 13 µm. Our results lay the foundation for a new class of room-temperature polariton lasers based on quasi-2D halide perovskites with great potential for hetero-integration with other van-der-Waals materials and combination with photonic crystals or waveguides. The authors report the experimental observation of room-temperature condensation of exciton polaritons in quasi-2D layered crystals of halide perovskite, integrated into an open optical microcavity. These materials combine van-der-Waals properties with dominant exciton physics at room temperature.

ABSTRACT Versatile, tunable, and potentially scalable single‐photon sources are a key asset in emergent photonic quantum technologies. In this work, a single‐photon source based on WS 2 micro‐domes, created via hydrogen ion irradiation, is realized and integrated into an open, tunable optical microcavity. Single‐photon emission from the coupled emitter–cavity system is verified via the second‐order correlation measurement, revealing a value of . A detailed analysis of the spectrally selective, cavity enhanced emission features shows the impact of a pronounced acoustic phonon emission sideband, which contributes specifically to the non‐resonant emitter–cavity coupling in this system. The achieved level of cavity–emitter control highlights the potential of open‐cavity systems to tailor the emission properties of atomically thin quantum emitters, advancing their suitability for real‐world quantum technology applications.

Niobium oxide diiodide (NbOI2) is an emerging material for photonics and electronics, distinguished by its exceptional second-order nonlinearity and pronounced in-plane ferroelectricity, both originating from its highly anisotropic ABC-stacked crystal structure. Its broken inversion symmetry enables its optical nonlinear efficiency to scale with thickness, making multilayer NbOI2 highly promising for nonlinear frequency conversion like second harmonic generation or and spontaneous parametric down-conversion in bulk or waveguides. However, under ambient conditions NbOI2 degrades into an amorphous oxide within weeks, severely diminishing its nonlinear response. To overcome this, we investigate SiO2 encapsulation via physical vapor deposition to protect NbOI2 multilayers from environmental degradation. Our systematic study reveals that encapsulation preserves structural integrity and nonlinear optical performance, establishing NbOI2 as a stable candidate for heterogeneous integration in foundry-compatible photonic platforms and quantum technologies.

We present a non-destructive, spatially resolved thickness characterization method for rhombohedral (3R) molybdenum disulfide (MoS_2) on polydimethylsiloxane (PDMS) substrates. Unlike broadband spectroscopic approaches, the proposed method reduces the measurement to a small number of discrete intensity images, enabling direct thickness mapping with a conventional microscope architecture and commercially available bandpass filters. Our approach combines a systematic framework for selecting optimal discrete wavelength samples of the material's reflectance with a robust thickness retrieval algorithm based on a multivariate Gaussian probability model. By sampling the reflectance with just five strategically chosen near-infrared bandpass filters, we demonstrate thickness characterization up to 691 nm with a mean 95

3R-MoS2, a MoS2 polytype with broken inversion symmetry, enables unique light-matter interactions and is promising for linear and nonlinear integrated photonics beyond the monolayer limit. Yet, systematic studies of its thickness-dependent reflectivity and its impact on harmonic generation are still lacking. While AFM can offer atomic-scale resolution, measuring 3R-MoS2 on non-solid substrates like PDMS remains challenging. To address this, a fast, non-destructive optical method is introduced to determine the thickness of 3R-MoS2 flakes from reflectivity measurements with a mean bias of less than 2 nm in the 3-200 nm range. Nonlinear characterization further reveals distinct thickness-dependent maxima in second- and third-harmonic generation (SHG/THG), with the first clear peak at approximate to 200 nm. These maxima arise from Fabry-P & eacute;rot-type phase matching conditions mediated by the film thickness and can further be shaped by absorption. This work thus provides both a practical thickness metrology and new insights for exploiting thickness-dependent 3R-MoS2 nonlinearities in scalable photonic technologies.

Gallium phosphide (GaP) is a promising material platform for integrated photonics because of its high refractive index, broad optical transparency, and strong second-order nonlinear response. Here, we demonstrate GaP-on-insulator thin films fabricated by crystal ion slicing and direct wafer bonding, using fused silica and SiO_2/Si/Si thermally oxidized silicon substrates as representative platforms. Unlike GaP thin-film platforms that rely on heteroepitaxial growth or sacrificial-layer release, the presented approach enables the flexible integration of crystalline GaP thin films, independent of both donor and target substrates. Following post-transfer annealing, the films exhibit near-bulk crystalline quality with low residual strain, smooth surfaces suitable for nanophotonic fabrication, and homogeneous bonding interfaces. Furthermore, annealing restores the linear optical dispersion (n and k) approaching that of epitaxially grown GaP with estimated plane wave absorption loss of 0.9 dB/cm at 1550 nm in the telecom C-band. The demonstrated approach establishes a scalable pathway toward high-quality GaP thin-film photonics compatible with versatile heterogeneous integration and back-end-of-line CMOS processing.

Transition metal dichalcogenides represent a versatile platform to study strong light-matter interactions based on excitons and electrons in ordered lattices. Twist-engineering of moiré structures further enables the manipulation of the polaritonic nonlinearities via engineering the exciton landscape on the nanoscale. In this work, we demonstrate in-situ control of the optical saturation-based nonlinearity of moiré exciton-polaritons by phase space restriction via charge doping. Strong exciton-photon coupling is established in a gate-controllable MoTe_2-MoSe_2 heterobilayer, embedded in a spectrally-tunable open cavity. A small gate voltage can effectively lower the necessary polariton density by one order of magnitude to achieve a similar nonlinear saturation effect as in the charge-neutral case. Our microscopic description successfully explains the observed phenomena in the framework of Pauli blocking for the moiré superlattices with charge preoccupation.

Coating thermal noise in high-reflectivity test-mass mirrors is a major limitation for future gravitational-wave detectors, especially in the 10–300 Hz band. ET-Pathfinder therefore requires mirror coatings that combine very high reflectance at 1.55 micrometer with low thermal noise under cryogenic conditions. Conventional dielectric Bragg mirrors provide high reflectance but require thick coatings, whereas metasurface mirrors can reduce coating-related noise but are limited by fabrication tolerances and line-edge roughness. We present a hybrid metasurface–Bragg mirror concept tailored to ET-Pathfinder. The design combines a fabrication-tolerant one-layer metasurface, an anti-resonant Fabry–Perot spacer, and a reduced dielectric Bragg stack. Optical performance is evaluated using full-wave electromagnetic simulations, while fabrication robustness is assessed with a truncated-Gaussian Monte Carlo analysis. Line-edge roughness is included as a systematic edge-smoothing effect. The resulting reflectance distributions are used to determine the minimum Bragg-stack support required to meet system-level specifications. The ideal metasurface exceeds 99.999

Multispectral full-Stokes polarisation imaging has a broad range of applications, from biological cell imaging to agricultural remote surveying. For such applications, especially involving lightweight unmanned aerial vehicles like drones, it is necessary to have compact, single-shot, efficient optical systems. We present a topology-optimised metasurface design that diffractively separates a scene into spectral and polarimetric measurements, operating for 532 nm and 700 nm in a single-shot imaging system. The polarisation imaging performance of the design is shown in simulation to be robust to critical performance metrics, matching both spectral and angular bandwidth requirements. We fabricated our metasurface design by nanopatterning a TiO_2 thin film, and experimentally demonstrate polarisation reconstruction for two wavelengths with a single metasurface structure.