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High-quality (Q)-factor optical resonators with extreme temporal coherence are of both technological and fundamental importance in optical metrology, continuous-wave lasing, and semiconductor quantum optics. Despite extensive efforts in designing high-Q resonators across different spectral regimes, the experimental realization of very large Q-factors at visible wavelengths remains challenging due to the small feature size that is sensitive to fabrication imperfections, and thus is typically implemented in integrated photonics. In the pursuit of free-space optics with the benefits of large space-bandwidth product and massive parallel operations, here we design and fabricate a visible-wavelength etch-free metasurface with minimized fabrication defects and experimentally demonstrate a million-scale ultrahigh-Q resonance. A new laser-scanning momentum-space-resolved spectroscopy technique with extremely high spectral and angular resolution is developed to characterize the record-high Q-factor as well as the dispersion of the million-Q resonance in free space. By integrating monolayer WSe2 into our ultrahigh-Q meta-resonator, we further demonstrate laser-like highly unidirectional and narrow-linewidth exciton emission, albeit without any operating power density threshold. Under continuous-wave laser pumping, we observe pump-power-dependent linewidth narrowing at room temperature, indicating the potential of our meta-optics platform in controlling coherent quantum light-sources. Our result also holds great promise for applications like optical sensing, spectral filtering, and few-photon nonlinear optics.

Fano resonances in photonics arise from the coupling and interference between two resonant modes in structures with broken symmetry. They feature an uneven and narrow and tunable lineshape, and are ideally suited for optical spectroscopy. Many Fano resonance structures have been suggested in nanophotonics over the last ten years, but reconfigurability and tailored design remain challenging. Herein, we propose an all-optical pick-and-place approach aimed at assemble Fano metamolecules of various geometries and compositions in a reconfigurable manner. We study their coupling behavior by in-situ dark-field scattering spectroscopy. Driven by a light-directed opto-thermoelectric field, silicon nanoparticles with high quality-factor Mie resonances (discrete states) and low-loss BaTiO3 nanoparticles (continuum states) are assembled into all-dielectric heterodimers, where distinct Fano resonances are observed. The Fano parameter can be adjusted by changing the resonant frequency of the discrete states or the light polarization. We also show tunable coupling strength and multiple Fano resonances by altering the number of continuum states and discrete states in dielectric heterooligomers. Our work offers a general design rule for Fano resonance and an all-optical platform for controlling Fano coupling on demand.

Distinct selectivity to the spin angular momenta of photons have garnered significant attention in recent years, for their relevance in basic science and for imaging and sensing applications. While nonlocal metasurfaces with strong chiral responses to the incident light have been reported, these responses are typically limited to a narrow range of incident angles. In this study, we demonstrate a nonlocal metasurface that showcases strong chirality, characterized by circular dichroism larger than 0.6, over a wide range of incident angles $\pm5^o$. Its quality factor, circular dichroism and resonant frequency can be optimized by design. These findings pave the way to further advance the development of valley-selective optical cavities and augmented reality applications., Comment: 6 pages, 5 figures

The skin effect, where bulk modes collapse into boundary modes, is a key phenomenon in topological non-Hermitian systems, has been predominantly studied in spinless systems. Recent studies illustrate the magnetic suppression of the first-order skin effect while ignoring spin. However, the physical significance of a magnetic field in non-Hermitian skin effect with spin remains elusive. Here, we systematically explore non-Hermitian spinful systems based on generalized Hatano-Nelson models with SU(2) gauge potential fields. In an open one-dimensional lattice, the spin-up and spin-down states can be uniquely separated and localized at the two boundaries without magnetic field. When an external magnetic field is applied, the skin effect exhibits a smooth transition from bidirectional to unidirectional. Remarkably, we demonstrate that the first-order skin effect can be anomalously induced by a magnetic field in a topologically trivial non-Hermitian spinful system without any skin effect at zero field. The direction of such magnetically induced skin modes can be controlled by simply changing the amplitude and polarity of the magnetic field. In addition, we demonstrate a transition between non-Bloch PT and anti-PT symmetries in the system, and uncover the spindependent mechanism of non-Bloch PT symmetry. Our results pave the way for the investigation of non-Hermitian skin effect with spin degrees of freedom.

Polarization entanglement holds significant importance for photonic quantum technologies. Recently emerging subwavelength nonlinear quantum light sources, e.g., GaP and LiNbO3 thin films, benefiting from the relaxed phase-matching constraints and volume confinement, has shown intriguing properties, such as high-dimensional hyperentanglement and robust entanglement anti-degradation. Van der Waals (vdW) NbOCl2 crystal, renowned for its superior optical nonlinearities, has emerged as one of ideal candidates for ultrathin quantum light sources [Nature 613, 53 (2023)]. However, polarization-entanglement is inaccessible in NbOCl2 crystal due to its unfavorable nonlinear susceptibility tensor. Here, by leveraging the twist-stacking degree of freedom inherently in vdW systems, we showcase the preparation of tunable polarization entanglement and quantum Bell states. Our work not only provides a new and tunable polarization-entangled vdW photon-pair source, but also introduces a new knob in engineering the entanglement state of quantum light at the nanoscale., Comment: 16 pages,4 figures

We build Floquet-driven capactive circuit networks to realize topological states of matter in the frequency domain. We find the Floquet circuit network equations of motion to reveal a potential barrier which effectively acts as a boundary in frequency space. By implementing a Su-Shrieffer-Heeger Floquet lattice model and measuring the associated circuit Laplacian and characteristic resonances, we demonstrate how topological edge modes can nucleate at such a frequency boundary., Comment: 4 pages, 3 figures, supplement

This paper explores the interplay of momentum transfer and nonlinear optical processes through moir\'e phenomena. Momentum transfer plays a crucial role in the interaction between photons and matter. Here, we study stacked metasurfaces with tailored dispersion and rotated against each other with varying twisted angles. The stacking introduces interlayer interactions, which can be controlled by the relative angle between metasurfaces, significantly enriching the resulting response compared to the single layer counterpart. By focusing on second-harmonic generation (SHG) from these twisted metasurfaces, we delve into the realm of nonlinear moir\'e photonics. Through experimental observations, we unveil the emergence of intricate far-field SHG radiation patterns, showing their effective tuning by varying the twisted angles. These findings offer a fresh perspective to explore nonlinear wavefront shaping through moir\'e phenomena, opening new avenues for nonlinear information processing, optical steering, and nonlinear optical switching., Comment: 10 pages, 5 figures

Physical neural networks (PNNs) are a class of neural-like networks that leverage the properties of physical systems to perform computation. While PNNs are so far a niche research area with small-scale laboratory demonstrations, they are arguably one of the most underappreciated important opportunities in modern AI. Could we train AI models 1000x larger than current ones? Could we do this and also have them perform inference locally and privately on edge devices, such as smartphones or sensors? Research over the past few years has shown that the answer to all these questions is likely "yes, with enough research": PNNs could one day radically change what is possible and practical for AI systems. To do this will however require rethinking both how AI models work, and how they are trained - primarily by considering the problems through the constraints of the underlying hardware physics. To train PNNs at large scale, many methods including backpropagation-based and backpropagation-free approaches are now being explored. These methods have various trade-offs, and so far no method has been shown to scale to the same scale and performance as the backpropagation algorithm widely used in deep learning today. However, this is rapidly changing, and a diverse ecosystem of training techniques provides clues for how PNNs may one day be utilized to create both more efficient realizations of current-scale AI models, and to enable unprecedented-scale models., Comment: 29 pages, 4 figures

Dynamic encircling of exceptional points has attracted significant interest in recent years, as it can facilitate chiral transmission selectivity due to a nontrivial eigenstate evolution. Recently, multi-state systems have been explored, associated with more complex topologies supporting a larger number of exceptional points, but chiral switching among multiple eigenstates has remained elusive in experiments. Here, we overcome this challenge by dividing the eigenstate space into multiple subspaces by controlling the adiabaticity. The eigenstates in different subspaces can evolve without crosstalk, and chiral switching occurs as the eigenstates within each subspace are subject to a non-adiabatic transition while they encircle exceptional points. We experimentally demonstrate this phenomenon by reporting chiral switching for two groups of optical modes at telecom wavelengths in a four-state optical system, and theoretically demonstrate that our approach can be extended to higher-order systems. Our findings pave new avenues for studying chiral dynamics based on exceptional-point physics in multi-state systems, and offer opportunities to develop multiplexed photonic devices.

Polar dielectrics with low crystal symmetry and sharp phonon resonances can support hyperbolic shear polaritons - highly confined surface modes with frequency-dependent optical axes and asymmetric dissipation features. So far, these modes have been observed only in bulk natural materials at mid-infrared frequencies, with properties limited by available crystal geometries and phonon resonance strength. Here we introduce hyperbolic shear metasurfaces: ultrathin engineered surfaces supporting hyperbolic surface modes with symmetry-tailored axial dispersion and loss redistribution that can maximally enhance light-matter interactions. By engineering effective shear phenomena in these engineered surfaces, we demonstrate geometry-controlled, ultra-confined, low-loss hyperbolic surface waves with broadband Purcell enhancements, applicable across a broad range of the electromagnetic spectrum., Comment: 27 pages, 13 figures

Handedness-selective chiral transport is an intriguing phenomenon that not only holds significant importance for fundamental research, but it also carries application prospects in fields such as optical communications and sensing. Currently, on-chip chiral transport devices are static, unable to modulate the output modes based on the input modes. This limits both device functionality reconfiguration and information transmission capacity. Here, we propose to use the incident polarization diversity to control the Hamiltonian evolution path, achieving polarization-dependent chiral transport. By mapping the evolution path of TE and TM polarizations onto elaborately engineered double-coupled waveguides, we experimentally demonstrate that different polarizations yield controllable modal outputs. This work combines Multiple-Input Multiple-Output and polarization diversity concepts with chiral transport, and challenges the prevailing notion that the modal outputs are fixed to specific modes in chiral transport, thereby opening pathways for the development of on-chip reconfigurable and high-capacity handedness-selective devices.

Nonlinear intersubband polaritonic metasurfaces support one of the strongest known ultrafast nonlinear responses in the mid-infrared frequency range across all condensed matter systems. Beyond harmonic generation and frequency mixing, these nonlinearities can be leveraged for ultrafast optical switching and power limiting, based on tailored transitions from strong to weak polaritonic coupling. Here, we demonstrate synergistic optimization of materials and photonic nanostructures to achieve large reflection contrast in ultrafast polaritonic metasurface limiters. The devices are based on optimized semiconductor heterostructure materials that minimize the intersubband transition linewidth and reduce absorption in optically saturated nanoresonators, achieving a record-high reflection contrast of 54% experimentally. We also discuss opportunities to further boost the metrics of performance of this class of ultrafast limiters, showing that reflection contrast as high as 94% may be realistically achieved using all-dielectric intersubband polaritonic metasurfaces.

Nonlocal metasurfaces have recently enabled an ultra-compact, low-power and high-speed platform to perform analog image processing. While several computational tasks have been demonstrated based on this platform, most of the previous studies have focused only on spatial operations, such as spatial differentiation and edge detection. Here, we demonstrate that metasurfaces with temporal nonlocalities - that is, with a tailored dispersive response - can be used to implement time-domain signal processing in deeply subwavelength footprints. In particular, we show a passive metasurface performing first-order differentiation of an input signal with high-fidelity and high-efficiency. We also demonstrate that this approach is prone to scalability and cascaded computation. Our work paves the way to a new generation of ultra-compact, passive devices for all-optical computation, with applications in neural networks and neuromorphic computing.

Analog computation with passive optical components can enhance processing speeds and reduce power consumption, recently attracting renewed interest thanks to the opportunities enabled by metasurfaces. Basic image processing tasks, such as spatial differentiation, have been recently demonstrated based on engineered nonlocalities in metasurfaces, but next-generation computational schemes require more advanced capabilities. Here, we tailor nonlocalities in space and time to design a metasurface that can perform mixed spatio-temporal differentiation of an input image, realizing event-based edge detection with a passive ultrathin silicon-based structured film compatible with standard fabrication techniques. The metasurface detects the object edges only when the object moves, and its design can be tailored to selectively enhance objects moving at desired speeds. Our results point towards fully-passive processing of spatio-temporal signals, for highly compact neuromorphic cameras.

Polaritons are a hybrid class of quasiparticles originating from the strong and resonant coupling between light and matter excitations. Recent years have witnessed a surge of interest in novel polariton types, arising from directional, long-lived material resonances, and leading to extreme optical anisotropy that enables novel regimes of nanoscale, highly confined light propagation. While such exotic propagation features may also be in principle achieved using carefully designed metamaterials, it has been recently realized that they can naturally emerge when coupling infrared light to directional lattice vibrations, i.e., phonons, in polar crystals. Interestingly, a reduction in crystal symmetry increases the directionality of optical phonons and the resulting anisotropy of the response, which in turn enables new polaritonic phenomena, such as hyperbolic polaritons with highly directional propagation, ghost polaritons with complex-valued wave vectors, and shear polaritons with strongly asymmetric propagation features. In this Review, we develop a critical overview of recent advances in the discovery of phonon polaritons in low-symmetry crystals, highlighting the role of broken symmetries in dictating the polariton response and associated nanoscale-light propagation features. We also discuss emerging opportunities for polaritons in lower-symmetry materials and metamaterials, with connections to topological physics and the possibility of leveraging anisotropic nonlinearities and optical pumping to further control their nanoscale response.

Optical metasurfaces have been enabling reduced footprint and power consumption, as well as faster speeds, in the context of analog computing and image processing. While various image processing and optical computing functionalities have been recently demonstrated using metasurfaces, most of the considered devices are static and lack reconfigurability. Yet, the ability to dynamically reconfigure processing operations is key for metasurfaces to be able to compete with practical computing systems. Here, we demonstrate a passive edge-detection metasurface operating in the near-infrared regime whose image processing response can be drastically modified by temperature variations smaller than 10{\deg} C around a CMOS-compatible temperature of 65{\deg} C. Such reconfigurability is achieved by leveraging the insulator-to-metal phase transition of a thin buried layer of vanadium dioxide which, in turn, strongly alters the nonlocal response of the metasurface. Importantly, this reconfigurability is accompanied by performance metrics - such as high numerical aperture, high efficiency, isotropy, and polarization-independence - close to optimal, and it is combined with a simple geometry compatible with large-scale manufacturing. Our work paves the way to a new generation of ultra-compact, tunable, passive devices for all-optical computation, with potential applications in augmented reality, remote sensing and bio-medical imaging., Comment: Main text (18 pages, 5 figures), followed by high-resolution vector-graphic versions of the figures and by the Supplementary Information

State transfer and photon detection are fundamental processes that have direct implications in fields such as quantum computing and photonic circuits. However, while naturally emitted photons decay exponentially in time, to perfectly capture a photon its envelope should increase exponentially to match the time-reversed response of the absorbing cavity. Here we show that a cavity at a virtual absorbing exceptional point captures additional temporal orders of an incoming waveform, resulting in efficient passive state transfer and photon detection. This approach paves the way for state transfer at optical frequencies and efficient detection of a spontaneously emitted photon.

Optical metasurfaces have been recently explored as ultrathin analog image differentiators. By tailoring the momentum transfer function, they can perform efficient Fourier-filtering - and thus potentially any linear mathematical operation - on an input image, replacing bulky 4f systems. While this approach has been investigated in different platforms, and several techniques have been explored to achieve the required angular response, little effort has been devoted so far to tailor and control also the polarization response of an image-processing metasurface. Here, we show that edge-detection metasurfaces can be designed with tailored polarization responses while simultaneously preserving an isotropic response. In particular, we demonstrate single-layer silicon metasurfaces yielding efficient Laplacian operation on a 2D image with either large polarization asymmetry, or nearly polarization-independent response. In the former case, we show that a strongly asymmetric polarization response can be used to unlock more sophisticated on-the-fly image processing functionalities, such as dynamically tunable direction-dependent edge detection. In parallel, metasurfaces with dual-polarized response are shown to enable efficient operation for unpolarized or arbitrarily polarized images, ensuring high efficiency. For both devices, we demonstrate edge detection within relatively large numerical apertures, with excellent isotropy and intensity throughput. Our study paves the way for the broad use of optical metasurfaces for sophisticated, massively parallel analog image processing with zero energy requirements., Comment: 8 pages, 4 figures in main text. 6 pages, 3 figures in supplemental material

Diffractive nonlocal metasurfaces have recently opened a broad range of exciting developments in nanophotonics research and applications, leveraging spatially extended (yet locally patterned) resonant modes to control light with new degrees of freedom. While conventional grating responses are elegantly captured by temporal coupled mode theory (TCMT), TCMT is not well equipped to capture the more sophisticated responses observed in the nascent field of nonlocal metasurfaces. Here, we introduce spatio-temporal coupled mode theory (STCMT), capable of elegantly capturing the key features of the resonant response of wavefront-shaping nonlocal metasurfaces. This framework can quantitatively guide nonlocal metasurface design, and is compatible with local metasurface frameworks, making it a powerful tool to rationally design and optimize a broad class of ultrathin optical components. We validate this STCMT framework against full-wave simulations of various nonlocal metasurfaces, demonstrating that this tool offers a powerful semi-analytical framework to understand and model the physics and functionality of these devices, without the need for computationally intense full-wave simulations. We also discuss how this model may shed physical insights into nonlocal phenomena in photonics and into the functionality of the resulting devices. As a relevant example, we showcase STCMT's flexibility by applying it to study and rapidly prototype nonlocal metasurfaces that spatially shape thermal emission., Comment: 85 pages, 15 figures

Following the discovery of moir\'e-driven superconductivity in twisted graphene multilayers, twistronics has spurred a surge of interest in tailored broken symmetries through angular rotations, enabling new properties from electronics to photonics and phononics. Analogously, in monoclinic polar crystals a nontrivial angle between non-degenerate dipolar phonon resonances can naturally emerge due to asymmetries in their crystal lattice, and its variations are associated with intriguing polaritonic phenomena, including axial dispersion, i.e., a rotation of the optical axis with frequency, and microscopic shear effects that result in asymmetric loss distributions. So far these phenomena were restricted to specific mid-infrared frequencies, difficult to access with conventional lasers, and fundamentally limited by the degree of asymmetry and the strength of light-matter interactions available in natural crystals. Here, we leverage twistronics to demonstrate giant axial dispersion and loss asymmetry of hyperbolic waves in elastic metasurfaces, by tailoring the angle between coupled pairs of anisotropic metasurfaces. We show extreme control over elastic wave dispersion via the twist angle, and leverage the resulting phenomena to demonstrate reflection-free negative refraction, as well as the application of axial dispersion to achieve diffraction-free non-destructive testing, whereby the angular direction of a hyperbolic probe wave is encoded into its frequency. Our work welds the concepts of twistronics, non-Hermiticity and extreme anisotropy, demonstrating the powerful opportunities enabled by metasurfaces for tunable, highly directional surface acoustic wave propagation, of great interest for applications ranging from seismic mitigation to on-chip phononics and wireless communications, paving the way towards their translation into emerging photonic and polaritonic metasurface technologies.

Time-harmonic electromagnetic plane waves in anisotropic media can exhibit complex-valued wavevectors (with nonzero real and imaginary parts) even in the absence of material dissipation. These peculiar modes, usually referred to as "ghost waves," hybridize the typical traits of conventional propagating and evanescent waves, displaying both phase accumulation and purely reactive exponential decay away from the direction of power flow. Their existence has been predicted in several scenarios, and has been recently observed experimentally in the form of surface phonon polaritons with complex-valued out-of-plane wavevectors propagating at the interface between air and a natural uniaxial crystal with slanted optical axis. Here, we demonstrate that ghost waves can arise also in lower-dimensional flat-optics scenarios, which are becoming increasingly relevant in the context of metasurfaces and in the field of polaritonics. Specifically, we show that planar junctions between isotropic and anisotropic metasurfaces can support "ghost line waves" that propagate unattenuated along the line interface, exhibiting phase oscillations combined with evanescent decay both in the plane of the metasurface (away from the interface) and out-of-plane n the surrounding medium. Our theoretical results, validated by finite-element numerical simulations, demonstrate a novel form of polaritonic waves with highly confined features, which may provide new opportunities for the control of light at the nanoscale, and may find potential applications in a variety of scenarios, including integrated waveguides, nonlinear optics, optical sensing and sub-diffraction imaging., Comment: 19 pages, 5 figures. Some typos fixed

We study pressure acoustic propagation in asymmetric spoof-fluid-spoof acoustic waveguides and its potential application in acoustic gas sensors. First, a stable and efficient analytical method is established for fast calculation of the dispersion curves based on spectral expansion and enforcement of continuity between segments at suitable collocation points. The analysis is validated by a commercial finite element software. The geometric design of the waveguide is then optimized for the emergence of a nearly-flat dispersion curve associated with vertical geometric asymmetry. The waveguide is fabricated using 3D printing technology and the measurement results corroborate the numerical simulations. Based on the nearly-flat dispersion curve supported by this waveguide, a CO$_2$ sensor is proposed allowing to relate the phase difference measured between two points in the waveguide to the composition of the gas in the waveguide. The proposed sensor is experimentally validated in a controlled environment and the measurement results match the computational predictions well. The sensor is robust with respect to noise and signal-recording duration due to fast phase measurements and shows high sensitivity to gas concentration due to reliance on the second, nearly-flat, dispersion curve. In addition, the sensor is label-free and low-cost, while exhibiting rapid response, low-maintenance requirements and potential for measurements in a wide range of CO$_2$ concentrations without saturation issues., Comment: 21 pages, 12 figures

Weyl semimetals hold great promise in revolutionizing nonreciprocal optical components due to their unique topological properties. By exhibiting nonreciprocal magneto-optical effects without necessitating an external magnetic field, these materials offer remarkable miniaturization opportunities and reduced energy consumption. However, their intrinsic topological robustness poses a challenge for applications demanding tunability. In this work, we introduce an innovative approach to enhance the tunability of their response, utilizing multilayered configurations of twisted anisotropic Weyl semimetals. Our design enables controlled and reversible isolation by adjusting the twist angle between the anisotropic layers. When implemented in the Faraday geometry within the mid-IR frequency range, our design delivers impressive isolation, exceeding 50 dB, while maintaining a minimal insertion loss of just 0.33 dB. Moreover, the in-plane anisotropy of Weyl semimetals eliminates one or both polarizers of a conventional isolator geometry, significantly reducing the overall dimensions. These results set the stage for creating highly adaptable, ultra-compact optical isolators that can propel the fields of integrated photonics and quantum technology applications to new heights.

Abbe diffraction limit fundamentally bounds the resolution of conventional optical imaging and spectroscopic systems. Along the years, several schemes have been introduced to overcome this limit, each offering opportunities and trade-offs. Metamaterials have been proposed as a promising platform in this context, in principle enabling superlenses capable of drastic resolution enhancements. However, their performance is severely hindered by material loss and nonlocal phenomena, which become increasingly more detrimental as we attempt to image more subwavelength details. Active metamaterials have been explored to overcome these challenges, however material gain introduces other obstacles, e.g., instabilities, nonlinearities and noise. Here, we demonstrate that the temporal excitation of passive superlenses using signals oscillating at complex frequencies compensates material loss, leading to resolution enhancement. Our results demonstrate that virtual gain stemming from tailored forms of excitation can tackle the impact of loss in superlenses. More broadly, our work opens promising avenues for loss compensation in metamaterials, with a broad range of applications extendable to optics and nanophotonic systems., Comment: 19 pages, 4 figures

Symmetry-driven phenomena arising in nonlocal metasurfaces supporting quasi-bound states in the continuum (q-BICs) have been opening new avenues to tailor enhanced light-matter interactions via perturbative design principles. Geometric phase concepts - observed in many physical systems - are particularly useful in nonlocal metasurfaces, as they enable to locally pattern the q-BIC scattering rate and phase across the metasurface aperture without affecting the delocalized nature of the q-BIC resonance. However, this control typically comes with stringent limitations in terms of efficiency and/or of polarization operation. Here, we unveil a new form of geometric phase control, accumulated along a continuous contour of geometric perturbations that parametrically encircle a singularity associated with a bound state. This response is obtained regardless of the chosen polarization state, which can be rationally specified by a judicious choice of the perturbation geometry. Our findings extend geometric phase concepts to arbitrary polarizations, including linear and elliptical states, with near-unity scattering efficiency. This polarization-agnostic geometric phase offers new opportunities for wavefront manipulation, multifunctional operation and light emission, with applications in augmented reality, secure communications, wireless systems, imaging, lasing, nonlinear and quantum optics., Comment: 52 pages, 14 figures

Thermal emission from a hot body is ubiquitous, yet its properties remain inherently challenging to control due to its incoherent nature. Recent advances in thermal emission manipulation have been unveiling exciting phenomena and new opportunities for applications. In particular, judiciously patterned nanoscale features over their surface have been shown to channel emission sources into partially coherent beams with tailored directionality and frequency selectivity. Yet, more sophisticated forms of control, such as spin-selective and unidirectional thermal emission have remained elusive. Here, we experimentally demonstrate single-layer metasurfaces emitting unidirectional, narrowband thermal light in the infrared with arbitrary polarization states - an operation enabled by photonic bound states in the continuum locally tailored by a geometric phase controlling the temporal and spatial coherence of emitted light. The demonstrated platform paves the way to a compactification paradigm for metasurface optics, in which thermal emission or photoluminescence can feed arbitrarily patterned beams without the need of external coherent sources., Comment: 29 pages, 13 figures

Controlling the properties of quantum materials with light is of fundamental and technological importance. While high-power lasers may achieve this goal, more practical strategies aim to exploit the strong coupling of light and matter in optical cavities, which has recently been shown to affect elemental physical phenomena, like superconductivity, phase transitions, and topological protection. Here we report the capacity of strong light-matter coupling to modify and control the magneto-optical properties of magnets. Tuning the hybridization of magnetic excitons and cavity photons allows us to realize distinct optical signatures of external magnetic fields and magnons in the archetypal van der Waals magnetic semiconductor CrSBr. These results highlight novel directions for cavity-controlled magneto-optics and the manipulation of quantum material properties by strong light-matter coupling.

Nonreciprocity originating from classical interactions among nonlinear scatterers has been attracting increasing attention in the quantum community, offering a promising tool to control excitation transfer for quantum information processing and quantum computing. In this work, we explore the possibility of realizing largely nonreciprocal total cross sections for a pair of quantum metasurfaces formed by two parallel periodic arrays of two-level atoms. We show that large nonreciprocal responses can be obtained in such nonlinear systems by controlling the position of the atoms and their transition frequencies, without requiring that the environment in which the atoms are placed is nonreciprocal. We demonstrate the connection of this effect with the population of a slowly-decaying dark state, which is critical to obtain large nonreciprocal responses.

Optical metasurfaces performing analog image processing - such as spatial differentiation and edge detection - hold the potential to reduce processing times and power consumption, while avoiding bulky 4F lens systems. However, current designs have been suffering from trade-offs between spatial resolution, throughput, polarization asymmetry, operational bandwidth, and isotropy. Here, we show that dispersion engineering provides an elegant way to design metasurfaces where all these critical metrics are simultaneously optimized. We experimentally demonstrate silicon metasurfaces performing isotropic and dual-polarization edge detection, with numerical apertures above 0.35 and spectral bandwidths of 35 nm around 1500 nm. Moreover, we introduce quantitative metrics to assess the efficiency of these devices. Thanks to the low loss nature and dual-polarization response, our metasurfaces feature large throughput efficiencies, approaching the theoretical maximum for a given NA. Our results pave the way for low-loss, high-efficiency and broadband optical computing and image processing with free-space metasurfaces.

Coherent wave control exploits the interference among multiple waves impinging on a system to suppress or enhance outgoing signals based on their relative phase and amplitude. This process inherently requires non-Hermiticity, in order to enable energy exchanges among the waves, and spatial interfaces in order to tailor their scattering. Here we explore the temporal analogue of this phenomenon, based on time-interfaces that support instantaneous non-Hermitian scattering events for photons analogous to mechanical collisions. Based on this mechanism, we demonstrate ultra-broadband temporal coherent wave control and photonic collisions with phase-tunable elastic features, and apply them to erase, enhance and reshape arbitrary pulses by suitably tailoring the amplitude and phase of counterpropagating signals. Our findings provide a pathway to effectively sculpt broadband light with light without requiring spatial boundaries, within an ultrafast and low-energy platform.

The growing field of quantum information technology requires propagation of information over long distances with efficient readout mechanisms. Excitonic quantum fluids have emerged as a powerful platform for this task due to their straightforward electro-optical conversion. In two-dimensional transition metal dichalcogenides, the coupling between spin and valley provides exciting opportunities for harnessing, manipulating and storing bits of information. However, the large inhomogeneity of single layers cannot be overcome by the properties of bright excitons, hindering spin-valley transport. Nonetheless, the rich band structure supports dark excitonic states with strong binding energy and longer lifetime, ideally suited for long-range transport. Here we show that dark excitons can diffuse over several micrometers and prove that this repulsion-driven propagation is robust across non-uniform samples. The long-range propagation of dark states with an optical readout mediated by chiral phonons provides a new concept of excitonic devices for applications in both classical and quantum information technology.

One of the most significant breakthroughs in physics of the last decade has been the discovery that materials with non-trivial topological properties for electronic, electromagnetic, acoustic and mechanical responses can be designed and manufactured at our will through engineered metamaterials (MMs). Here, we review the foundation and the state-of-the-art advances of topological photonics, acoustics and mechanical MMs. We discuss how topological MMs enable nontrivial wave phenomena in physics, engineering, of great interest for a broad range of interdisciplinary science disciplines such as classical and quantum chemistry. We first introduce the foundations of topological materials and the main concepts behind their peculiar features, including the concepts of topological charge and geometric phase. We then discuss the topology of electronic band structures in natural topological materials, like topological insulators and gapless Dirac and Weyl semimetals. Based on these concepts, we review the concept, design and response of topologically nontrivial MMs in photonics and phononics, including topological phases in 2D MMs with and without time-reversal symmetry, Floquet TIs based on spatial and temporal modulation, topological phases in 3D MMs, higher-order topological phases in MMs, non-Hermitian and nonlinear topological MMs and the topological features of scattering anomalies. We also discuss the topological properties emerging in other related contexts, such as the topological aspects of chemical reactions and polaritons. This survey aims at connecting the recent advances in a broad range of scientific areas associated with topological concepts, and highlights opportunities offered by topological MMs for the chemistry community at large.