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.

Semiconductor metasurfaces have emerged as an effective platform for all-optical modulation thanks to their resonant light confinement and the enhancement of ultrafast optical effects like the Kerr effect and free-carrier generation. However, their narrow resonance features also restrict the available bandwidth for efficient modulation. Here, we demonstrate that a kink-like transmission spectral profile of a silicon metasurface could enable high-contrast modulation of wide-bandwidth pulses. We studied the all-optical ultrafast transmission modulation of the silicon metasurface induced by free-carrier excitation in a pump-probe configuration at different polarizations. We achieved ultrafast modulation of 28% (10%) with a speed of 25 ps and a bandwidth of up to 14 nm (49 nm) for y-and x-polarizations, respectively. Our results open up new opportunities for high-contrast, all-optical modulation of short pulses, with applications in optical communications and computing.

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.

Spin defects in silicon carbide (SiC) are promising candidates for integrated quantum photonics, offering long-lived spin states and near-infrared emission suitable for low-loss photonic integration and fibre-based quantum communication. However, light extraction from these defects remains challenging due the relatively high refractive index of SiC. Metalenses offer a compact approach to enhance light collection by engineering the wavefront directly at the material interface. Here, we design and fabricate monolithic metalenses from SiC bulk material that simultaneously operate at 860 and 1240 nm, matching with emission from the nitrogen vacancy and silicon vacancy colour centers. By independently engineering the phase response at both wavelengths, the metalens enables collection and polarization manipulation of the emitted light. We further employ the metalenses to demonstrate optically detected magnetic resonance of both defects simultaneously. These multifunctional metalenses provide a compact optical interface for scalable integrated SiC photonic devices.

Nonlinear optics underpins a broad range of photonic technologies, from classical and quantum light sources to emerging nonlinear photonic neural networks. Yet, conventional nonlinear optical devices exhibit static functionality: their transfer characteristics and emission profiles are dictated by the intrinsic nonlinear process and locked by fabrication, limiting adaptability. Here, we introduce an ultra-thin metasurface platform that enables dynamic reconfiguration of nonlinear functionality in a contact-less fashion. By leveraging all-optical control of the optical torque exerted on liquid crystal molecules infiltrating a resonant metasurface, we achieve tunable polynomial nonlinear transfer functions based on third-harmonic generation process. This mechanism further allows real-time modulation of nonlinear weighting across different diffraction orders, revealing a previously unexplored interplay between mode structure and nonlinear emission. Our approach opens up a pathway toward field-programmable nonlinear photonic systems, offering unprecedented flexibility for reconfigurable nonlinear signal processing and adaptive photonic computing.

Integrated quantum photonics has become a burgeoning field of research that encompasses quantum light sources, nonlinear materials, photonic resonators, optical interconnects and detectors. There is also a growing need for programmable devices that enable rapid reconfiguration of individual components in integrated quantum photonic chips. In this Review we present our vision for programmable quantum photonics and explain why we regard it as the next frontier in the field of quantum nanophotonics. We discuss state-of-the-art reconfigurable and tunable elements (for example, phase shifters and quantum light sources) and highlight the emergence of materials that offer a new toolkit for tunability and control (for example, van der Waals crystals). Programmable quantum circuitry will play a pivotal role in transitioning quantum optics from proof-of-concept demonstrations to robust technological solutions for the second quantum revolution. This Review covers state-of-the-art reconfigurable and tunable optical components and highlights the emergence of a set of materials that offer a new toolkit for tunability and control.

Light-matter interactions are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non-invasive spectroscopy. While the border between modulation and detection is often assumed to be sharp and well-defined, there are experiments where the boundaries fade. Here, the interplay between bandgap modulation and non-invasive spectroscopy is measured and explained in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor.A clear deviation from the typical quadratic power scaling of second-harmonic generation near an exciton resonance is reported, and this unusual result is explained based on all-optical modulation driven by the intensity-dependent optical Stark and Bloch-Siegert shifts in the +/- K valleys of the Brillouin zone. The experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which the resonant transition dipole moments and dephasing times of the sample are extracted. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light-matter interactions can modify the band structure of a crystal, even in the weak-field regime. Furthermore, the results strengthen the understanding of ultrafast all-optical control of electronic states in 2D materials, with potential applications in valleytronics, Floquet engineering, and light-wave electronics.

Selective control of the emission pattern of valley-polarized excitons in monolayer transition metal dichalcogenides is essential for advancing valleytronic, quantum information, and optoelectronic devices. Although substantial progress has been made in directionally routing photoluminescence from these materials, key challenges persist: specifically, establishing how observed routing effects relate to the degree of valley polarization and distinguishing genuine valley-dependent routing from spin-momentum coupling, an optical scattering effect unrelated to the emitter. In this work, we address these challenges by experimentally and numerically demonstrating a direct link between excitonic valley polarization and the resulting farfield emission pattern, enabling quantitative evaluation of valley-selective emission routing. We report valley-dependent manipulation of the angular emission pattern of monolayer tungsten diselenide using gold nanobar dimer antennas at cryogenic temperatures. By probing the emission under opposite circularly polarized excitation, we observe a valley-selective asymmetry in the photoluminescence circular dichroism of 2%. These measurements are supported by a reciprocity-based numerical framework that enables modeling of valley-selective emission in periodic systems. Our calculations further reveal that the observed valley-dependent directionality is a symmetry-protected property of the nanoantenna array arising from its extrinsic chirality at oblique emission angles, and that it can be substantially enhanced by tailoring the emitter distribution. Together, these results establish our nanoantenna platform as a robust route toward valleytronic signal processing.

Transition metal dichalcogenides are promising quantum materials because of unique exciton-photon interactions. These interactions can be enhanced by coupling with resonant photonic structures, especially in the nonlinear light emission processes like second-harmonic generation (SHG). However, excitonic absorption may dampen SHG. Here, we demonstrate tunable SHG enhancement using virtual coupling effects between quasi-bound state in the continuum (qBIC) optical resonances and tunable excitons in high-index WS2 metasurfaces with crescent meta-atoms. These metasurfaces promote a magnetic-type qBIC resonance, enabling control over nonlinear optical processes in the visible spectrum. The used qBIC resonance at half the exciton energy increases SHG efficiency by 98-fold compared to monolayer WS2 and by four orders of magnitude relative to an unpatterned WS2 film. The enhancement is tunable with temperature and incident light polarization, allowing the dynamic control of virtual coupling and SHG efficiency, thereby paving the way for next-generation reconfigurable metaoptics devices.

Two-dimensional (2D) semiconductors are emerging as a versatile platform for nanophotonics, offering unprecedented tunability in optical properties through exciton resonance engineering, van der Waals heterostructuring, and external field control. These materials enable active optical modulation, single-photon emission, quantum photonics, and valleytronic functionalities, paving the way for next-generation optoelectronic and quantum photonic devices. However, key challenges remain in achieving large-area integration, maintaining excitonic coherence, and optimizing amplitude-phase modulation for efficient light manipulation. Advances in fabrication, strain engineering, and computational modeling will be crucial to overcoming these limitations. This Perspective highlights recent progress in 2D semiconductor-based nanophotonics, emphasizing opportunities for scalable integration into photonics.

Light‐matter interactions are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non‐invasive spectroscopy. While the border between modulation and detection is often assumed to be sharp and well‐defined, there are experiments where the boundaries fade. Here, the interplay between bandgap modulation and non‐invasive spectroscopy is measured and explained in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor.A clear deviation from the typical quadratic power scaling of second‐harmonic generation near an exciton resonance is reported, and this unusual result is explained based on all‐optical modulation driven by the intensity‐dependent optical Stark and Bloch–Siegert shifts in the ±K valleys of the Brillouin zone. The experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which the resonant transition dipole moments and dephasing times of the sample are extracted. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light‐matter interactions can modify the band structure of a crystal, even in the weak‐field regime. Furthermore, the results strengthen the understanding of ultrafast all‐optical control of electronic states in 2D materials, with potential applications in valleytronics, Floquet engineering, and light‐wave electronics.

Light-matter interactions in crystals are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non-invasive spectroscopy. While we often assume that the border between the two regimes of modulation and detection is sharp and well-defined, there are experiments where the boundaries fade. The study of these transition regions allows us to identify the real potentials and inherent limitations of the most commonly used optical spectroscopy techniques. Here, we measure and explain the co-existence between bandgap modulation and non-invasive spectroscopy in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor. We report a clear deviation from the typical quadratic power scaling of second-harmonic generation near an exciton resonance, and we explain this unusual result based on all-optical modulation driven by the intensity-dependent optical Stark and Bloch-Siegert shifts in the ±K valleys of the Brillouin zone. Our experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which we extract the resonant transition dipole moments and dephasing times of the used sample. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light-matter interactions can modify the band structure of a crystal, even in the weak-field regime. Furthermore, our results strengthen the understanding of ultrafast all-optical control of electronic states in two-dimensional materials, with potential applications in valleytronics, Floquet engineering, and light-wave electronics.

Transition metal dichalcogenides (TMDCs) have demonstrated significant potential as versatile quantum materials for light absorption and emission. Their unique properties are primarily governed by exciton-photon interactions, which can be substantially enhanced through coupling with resonant photonic structures. For example, nonlinear light emission, such as second harmonic generation (SHG) is doubly enhanced when the incident wave is resonant simultaneously with the excitonic and photonic resonance. However, the excitonic absorption of incident waves can significantly dump the SHG emission. Here, we propose and demonstrate a tunable enhancement of SHG by leveraging virtual coupling effects between quasi-bound states in the continuum (qBIC) optical resonances and tunable excitons in arrays of high-index WS2 crescent metaatoms. These crescent metaatoms excites a pure magnetic type qBIC resonance, enabling dynamic control and enhancement of nonlinear optical processes in visible spectrum. Our findings demonstrate that an array of WS2 crescent metaatoms, exhibiting qBIC resonance at half the exciton energy, enhances SHG efficiency by more than 98-fold compared to monolayer WS2 (1L-WS2) and four orders of magnitude relative to unpatterned WS2 film. This substantial SHG enhancement is tunable as a function of temperature and polarization angle of incident light, allowing us to obtain control of the virtual coupling and SHG efficiency in the visible spectrum (600-650 nm). Our work opens new avenues toward next-generation reconfigurable meta-optics devices.

Polarisation imaging is used to distinguish objects and surface characteristics that are otherwise not visible with black-and-white or colour imaging. Full-Stokes polarisation imaging allows complex image processing like water glint filtering, which is particularly useful for remote Earth observations. The relatively low cost of small-satellites makes their use in remote sensing more accessible. However, their size and weight limitations cannot accommodate the bulky conventional optics needed for full-Stokes polarisation imaging. We present the modelling of an ultra-thin topology-optimised diffractive metasurface that encodes polarisation states in five different diffraction orders. Positioning the metasurface in a telescope's pupil plane allows the diffraction orders to be imaged onto a single detector, resulting in the capability to perform single-shot full-Stokes polarisation imaging of the Earth's surface. The five rectangular image swaths are designed to use the full width of the camera, and then each successive frame can be stitched together as the satellite moves over the Earth's surface, restoring the full field of view achievable with any chosen camera without comprising the on-ground resolution. Each set of four out of the five orders enables the reconstruction of the full polarisation state, and their simultaneous reconstructions allow for error monitoring. The lightweight design and compact footprint of the polarisation imaging optical system achievable with a metasurface is a novel approach to increase the functionality of small satellites while working within their weight and volume constraints.

Polarisation imaging is a technique used to enhance and distinguish objects and surface characteristics. In particular, it is used in agricultural drone and small-satellite applications such as for crop classification [1] and monitoring vegetation [2]. Small-satellites, in particular, form an accessible remote-sensing platform for universities, research groups, industry, and government bodies due to their lower costs compared to large satellites. However, their size and weight limit inhibit the use of conventional full-Stokes polarisation imaging methods. We propose that metasurfaces can perform complex imaging in ultra-compact systems and present a metasurface designed for polarisation imaging at two distinct wavelengths.

Metasurfaces have long served as a cornerstone technique to enhance nonlinear processes, enabling frequency conversion, efficient light manipulation and integrated photonic devices. However, traditional bulk materials often suffer from high absorption losses, hindering the second harmonic generation (SHG) efficiency. Here, we develop a novel approach exploiting quasi-bound state in continuum (qBIC) to achieve giant SHG efficiency in metasurfaces utilizing 3R-MoS_2, with high index, superior damage threshold and inherent nonlinearity. The high refractive index of 3R-MoS_2, facilitates the high-quality factor (Q) metasurfaces, leading to reduced radiation leakage and localized light confinement within qBIC resonances, with which a remarkable 2000-fold enhancement in SHG intensity has been experimentally demonstrated. Additionally, the twist angle between the lattice orientation and the metasurface unit geometry exhibits a 120^∘ periodicity in its influence on SHG behaviour. By strategically designing to realize the qBIC and exciton dual resonances and optimized twist angle (30^∘), SHG conversion efficiency was boosted to 1 than those of the best metasurfaces on traditional bulk materials. This approach enables potential applications in various areas of nonlinear optics, including frequency conversion, light manipulation, integrated photonics, and quantum communications.

Emerging flat optical components based on single layers of metamaterials and metasurfaces have introduced a new approach to microlens arrays. These components hold promise for seamless integration with narrow-bandgap semiconductor-based infrared focal plane arrays, aiming to increase operating temperatures and improve imaging sensitivity. This study focuses on the design and rigorous coupled-wave analysis-based simulation of dielectric-transmissive metalens arrays, specifically tailored to optimize the performance of mid-wavelength infrared imaging arrays. Both Si and Ge metalenses are modeled and compared to spherical lenses, with Ge requiring a smaller aspect ratio than Si due to its higher refractive index, simplifying fabrication. Simulations demonstrate that pixel-registered flat metalenses achieve focusing efficiencies comparable to conventional spherical lenses by reducing the pillar pitch to 1 µm, improving phase profile accuracy and minimizing high-order diffraction losses. These findings highlight the potential of customized metalenses to advance infrared imaging technology and provide practical insights for optimizing imaging array performance through integration with microlens arrays.

We measured the cubic nonlinear susceptibility tensor elements (chi(3)) for polarization along the primary crystallographic axis in titanium-indiffused lithium niobate waveguides, assessed through self-phase modulation using picosecond-duration pulses at telecommunication wavelengths. A dominant, highly temperature- and wavelength-dependent contribution from a cascaded second-order nonlinearity is observed. Through careful extraction of the cascaded effect, we quantify the intrinsic third-order susceptibility tensor elements as chi(3) (5.2 +/- 1.3) x 10-21 m2 V2 and chi(3) xxxx (w; w, w, -w) = chi(3)yyyy(w; w, w, -w) = (3.6 +/- 0.8) x 10-21 m2 These measurements underscore the substantial impact of the cascaded nonlinearity in enhancing the effective cubic nonlinearity in lithium niobate and offer precise values essential for the design of nonlinear photonic devices.

We present novel image processing through nonlinear upconversion from infrared to visible light in resonant dielectric metasurfaces. We further discuss the ability to perform image processing beyond linear operations for advanced night vision applications. Full-text article not available; see video presentation