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High-order diffraction (HOD) from optical microstructures is undesirable in many applications because of the accompanying ghosting patterns and loss of efficiency. In contrast to suppressing HOD with subwavelength structures that challenge the fabrication of large-scale devices, managing HOD is less developed due to the lack of an efficient method for independently manipulating HOD. Here, we report independent manipulation of HODs, which are unexploited for subdiffraction-limit focusing in diffractive lenses, through an analytical formula that correlates the diffraction order and the width of each zone. The large spatial frequencies offered by the HODs enable our lenses to reduce the lateral focal size down to 0.44 λ even without any subwavelength feature (indispensable in most high-NA diffractive lenses), facilitating large-scale manufacture. Experimentally, we demonstrate high-order lens-based confocal imaging with a center-to-center dry resolution of 190 nm, the highest among visible-light confocal microscopies, and laser-ablation lithography with achieved direct-writing resolution of 400 nm (0.385 λ). The authors introduce the independent manipulation of high-order diffraction with a high-order diffractive lens for sub-diffraction limit focusing in confocal microscopy and laser ablation lithography. [ABSTRACT FROM AUTHOR]

Solid-state quantum emitters coupled to integrated photonic nanostructures are quintessential for exploring fundamental phenomena in cavity quantum electrodynamics and are used in a wide range of photonic quantum technologies. One of the most exciting prospects for integrated photonics is the potential for massive production of miniaturized devices on a single chip. However, the efficiency and reproducibility of light–matter coupling are hindered by the spectral and spatial mismatch between the single solid-state quantum emitters and the optical modes supported by the photonic nanostructures. Here we develop a platform and method for hyperspectral imaging of solid-state quantum emitters to address this long-standing issue. Spatially distributed and spectrally broadened InAs quantum dots are embedded in a GaAs/AlGaAs one-dimensional (1D) planar cavity that consists of two distributed Bragg reflectors acting as mirrors. By exploiting the extended mode of the dispersive 1D cavity and the way it shapes the out-of-plane emission from the quantum dots, we extract the spatial position and emission wavelength of each dot from a single wide-field photoluminescence image, with a spatial and spectral accuracy down to 15 nm and 0.4 nm, respectively. We then fabricate quantum light sources by etching the 1D confined planar cavity into 3D confined micropillars. Extension of this technique using an open planar cavity can be exploited for a variety of compact quantum photonic devices with expanded functionalities for large-scale integration. Our technology is particularly appealing for quantum photonic applications that involve the spatial and spectral characterization of a large number of solid-state quantum emitters. The position and emission wavelengths of single quantum dots embedded in a one-dimensional planar cavity can be simultaneously determined from a single photoluminescence image with 15 nm spatial accuracy and subnanometric spectral accuracy in the near-infrared. [ABSTRACT FROM AUTHOR]

Ultracompact entangled photon sources are pivotal to miniaturized quantum photonic devices. Van der Waals (vdW) nonlinear crystals promise efficient photon-pair generation and on-chip monolithic integration with nanophotonic circuitry. However, it remains challenging to generate maximally entangled Bell states of photon pairs with high purity, generation rate, and fidelity required for practical applications. Here, we realize a polarization-entangled photon-pair source based on spontaneous parametric down conversion in an ultrathin rhombohedral tungsten disulfide (3R-WS2) crystal. This vdW entangled photonic source exhibits a high photon-pair purity with a coincidence-to-accidental ratio of above 800, a generation rate of 31 Hz, and two maximally polarization-entangled Bell states with fidelities exceeding 0.93 and entanglement degree over 0.97. These results stem from scalable optical nonlinearity, enhanced second-order susceptibility by electronic transitions, and a well-defined symmetry-enabled selection rule inherent in 3R-WS2. Our polarization entangled photon source can be integrated with photonic structures for generating more complex entangled states, thus paving an avenue for advanced quantum photonic systems toward computation and metrology. [ABSTRACT FROM AUTHOR]

Anisotropic optical crystals can exhibit a hyperbolic response within the Reststrahlen band (RB) and support directional polaritonic propagations when interacting with light. Most of the reported low-symmetry optical crystals showcase the evolution from hyperbolic to elliptic dispersion topologies, largely owing to their adjacent RBs being either overlapped or separated. Here, we report an exceptional Reststrahlen point (ERP) in rare-earth oxyorthosilicate Y2SiO5, at which two neighboring RBs almost kiss each other. Consequently, we observe the direct hyperbolic-to-hyperbolic topological transition: the hyperbolic branches close and reopen along with the rotating transverse axis (TA). At such ERP, the TA merges to the direction orthogonal to its proximate phonon mode, mainly due to the interplay between these two non-orthogonal phonon modes. We also find that even with the existence of only one single RB, the TA can rotate in-plane. Our findings are prevalent in isostructural rare-earth oxyorthosilicates, such as Lu2SiO5. The universally underlying physics of ERP and its corresponding special class of rare-earth oxyorthosilicates may offer playgrounds for continuously tuning phonon polariton propagation direction, and broadband controlling light dispersion of polaritonic nanodevices. The authors reveal hyperbolic-to-hyperbolic transitions where two nearby Reststrahlen bands just touch each other. Such phenomena are prevalent in isostructural rare-earth oxyorthosilicates and unfolds vast opportunities for broadband on-chip light control. [ABSTRACT FROM AUTHOR]

Vortex wave and plane wave, as two most fundamental forms of wave propagation, are widely applied in various research fields. However, there is currently a lack of basic mechanism to enable arbitrary conversion between them. In this paper, we propose a new paradigm of extremely anisotropic acoustic metasurface (AM) to achieve the efficient conversion from 2D vortex waves with arbitrary orbital angular momentum (OAM) to plane waves. The underlying physics of this conversion process is ensured by the symmetry shift of AM medium parameters and the directional compensation of phase. Moreover, this novel phenomenon is further verified by analytical calculations, numerical demonstrations, and acoustic experiments, and the deflection angle and direction of the converted plane waves are qualitatively and quantitatively confirmed by a simple formula. Our work provides new possibilities for arbitrary manipulation of acoustic vortex, and holds potential applications in acoustic communication and OAM-based devices. [ABSTRACT FROM AUTHOR]

Achieving unity quantum efficiency in single-photon emitters (SPEs) is a holy grail in quantum information science. Through plasmonic coupling it is possible to increase the quantum efficiency of SPEs by increasing the radiative decay rate, but to approach unity quantum efficiency, non-radiative decay must be mitigated. Here we show that non-radiative decay in two-dimensional WSe2 quantum emitters can be electrically suppressed through charge depletion by using dual gate configurations under a large electric field. In this condition, for site-controlled SPEs in WSe2 coupled to gold nanogaps, the SPE transition quantum efficiency after gating is increased to 76.4 ± 14.6% on average, with some SPEs reaching near-unity (more than 90%) quantum efficiency. This study provides a new approach for tuning SPEs with an applied gate voltage and motivates further theoretical and experimental studies of SPE enhancement on vertically aligned nanogaps. Non-radiative decay in two-dimensional WSe2 quantum emitters is electrically suppressed through charge depletion using dual gate configurations. The single-photon emitter transition quantum efficiency after gating is increased to 76.4 ± 14.6% on average. [ABSTRACT FROM AUTHOR]

Chirality, defined by Lord Kelvin, refers to the geometric symmetry property of an object that cannot be superposed onto its mirror image using rotations and translations. The material's chirality can be probed with light as the optical activity: optical rotary dispersion (ORD) and circular dichroism (CD). It is still challenging to yield extremely sensitive ORD and CD for very weak chirality and measure both simultaneously. Cavity ringdown polarimetry has been reported to improve ORD detection sensitivity with the absence of equally important CD signature, at the price of high cavity finesse near 400, frequency-locking sophistication, and large magnetic field. Here, we report a unique recipe to demonstrate the simultaneous measurement of ORD and the CD by separately observing the chiral eigenmode spectra from a bowtie optical cavity with a finesse about 30, without resorting to frequency locking or magnetic field. We obtain a sensitivity of ∼ 2.7 × 10 - 3 deg / Hz for ORD, ∼ 8.1 × 10 - 6 / Hz for CD, and a spectral resolution of 0.04 pm within a millisecond-scale measurement. We present a cost-effective yet ultrasensitive account for chiral chromatography, the conformational dynamics and chiroptical analysis of biological samples which particularly exhibit weak and narrow spectral signals. [ABSTRACT FROM AUTHOR]

Intensity, polarization and wavelength are intrinsic characteristics of light. Characterizing light with arbitrarily mixed information on polarization and spectrum is in high demand1–4. Despite the extensive efforts in the design of polarimeters5–18 and spectrometers19–27, concurrently yielding high-dimensional signatures of intensity, polarization and spectrum of the light fields is challenging and typically requires complicated integration of polarization- and/or wavelength-sensitive elements in the space or time domains. Here we demonstrate that simple thin-film interfaces with spatial and frequency dispersion can project and tailor polarization and spectrum responses in the wavevector domain. By this means, high-dimensional light information can be encoded into single-shot imaging and deciphered with the assistance of a deep residual network. To the best of our knowledge, our work not only enables full characterization of light with arbitrarily mixed full-Stokes polarization states across a broadband spectrum with a single device and a single measurement but also presents comparable, if not better, performance than state-of-the-art single-purpose miniaturized polarimeters or spectrometers. Our approach can be readily used as an alignment-free retrofit for the existing imaging platforms, opening up new paths to ultra-compact and high-dimensional photodetection and imaging.By combining spatial and frequency dispersive thin-film interfaces with deep residual learning, a miniature photodetector allowing the acquisition of high-dimensional information on light in a single-shot fashion is described. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM), with its unique physical properties and vast application prospects, has attracted widespread attention in various fields. Nonetheless, the development of valid and practical acoustic OAM detection methods continues to be a challenging endeavor. In this paper, we propose a novel construction method of dual-layer metasurfaces to achieve a double-conversion process for the waveform reshaping and differentiated focusing of two-dimensional vortex sources with different OAMs. Specifically, by utilizing a concise formula, a one-to-one correspondence is established between the OAM of incident vortex waves and different imaging points. The fundamental principle of this special conversion relationship is rigorously constrained by the directional compensation of phase and the material parameters of dual-layer metasurfaces with different quadratic phase distributions. More importantly, the highly consistent results between numerical demonstrations and acoustic experiments further confirm the feasibility and effectiveness of the proposed OAM detection scheme. Our work provides a new perspective on the precise manipulation for the phase of vortex fields, holding potential applications in super-resolution imaging and the design of acoustic OAM-based devices. [ABSTRACT FROM AUTHOR]

Caustics occur in diverse physical systems, spanning the nano-scale in electron microscopy to astronomical-scale in gravitational lensing. As envelopes of rays, optical caustics result in sharp edges or extended networks. Caustics in structured light, characterized by complex-amplitude distributions, have innovated numerous applications including particle manipulation, high-resolution imaging techniques, and optical communication. However, these applications have encountered limitations due to a major challenge in engineering caustic fields with customizable propagation trajectories and in-plane intensity profiles. Here, we introduce the "compensation phase" via 3D-printed metasurfaces to shape caustic fields with curved trajectories in free space. The in-plane caustic patterns can be preserved or morphed from one structure to another during propagation. Large-scale fabrication of these metasurfaces is enabled by the fast-prototyping and cost-effective two-photon polymerization lithography. Our optical elements with the ultra-thin profile and sub-millimeter extension offer a compact solution to generating caustic structured light for beam shaping, high-resolution microscopy, and light-matter-interaction studies. Caustics, as a unique type of singularity in wave phenomena, occur in diverse physical systems. Here, the authors realize multi-dimensional customization of caustics with 3D-printed metasurfaces. This arbitrary caustic engineering is poised to bring new revolutions to many domains. [ABSTRACT FROM AUTHOR]

Thermal nanophotonics has undergone a revolution in recent decades, benefiting from in-depth studies in nano-optics and breakthroughs in nanotechnologies. The majority of thermal devices are constrained by Kirchhoff's law of thermal radiation, whereby spectral directional absorptivity and emissivity are identical according to Lorentz reciprocity. This restriction introduces an intrinsic loss in a plethora of energy technologies and prevents us from controlling emission and absorption independently. Recently, there has been considerable progress in realizing nonreciprocal radiation to overcome these limitations. Here we summarize the fundamental physics of nonreciprocal radiation, lay out different approaches to generalize Kirchhoff's law, and highlight several promising nonreciprocal thermal applications. This comprehensive overview of nonreciprocal thermal photonics may reveal new physics, unprecedented nonreciprocal effects and broader potential applications. This Review discusses the physics of nonreciprocal radiation and Kirchhoff's law generalization in the context of nanophotonics-enabled nonreciprocal thermal applications. [ABSTRACT FROM AUTHOR]

The dynamic control of electromagnetic waves is a persistent pursuit in modern industrial development. The state-of-the-art dynamic devices suffer from limitations such as narrow bandwidth, limited modulation range, and expensive features. To address these issues, we fuse origami techniques with metamaterial design to achieve ultra-wideband and large-depth reflection modulation. Through a folding process, our proposed metamaterial achieves over 10-dB modulation depth over 4.96 – 38.8 GHz, with a fractional bandwidth of 155% and tolerance to incident angles and polarizations. Its ultra-wideband and large-depth reflection modulation performance is verified through experiments and analyzed through multipole decomposition theory. To enhance its practical applicability, transparent conductive films are introduced to the metamaterial, achieving high optical transparency (>87%) from visible to near-infrared light while maintaining cost-effectiveness. Benefiting from lightweight, foldability, and low-cost properties, our design shows promise for extensive satellite communication and optical window mobile communication management. The researchers fuse metamaterials and origami technical to achieve ultra-wideband and large-depth reflection modulation. Flexible electronics amplify its lightweight, transparency, and cost-effectiveness, making it ideal for satellite communications. [ABSTRACT FROM AUTHOR]

Ultrasonic imaging is crucial in the fields of biomedical engineering for its deep penetration capabilities and non-ionizing nature. However, traditional techniques heavily rely on impedance differences within objects, resulting in poor contrast when imaging acoustically transparent targets. Here, we propose a compact spatial differentiator for underwater isotropic edge-enhanced imaging, which enhances the imaging contrast without the need for contrast agents or external physical fields. This design incorporates an amplitude meta-grating for linear transmission along the radial direction, combined with a phase meta-grating that utilizes focus and spiral phases with a first-order topological charge. Through theoretical analysis, numerical simulations, and experimental validation, we substantiate the effectiveness of our technique in distinguishing amplitude objects with isotropic edge enhancements. Importantly, this method also enables the accurate detection of both phase objects and artificial biological models. This breakthrough creates new opportunities for applications in medical diagnosis and nondestructive testing. High-contrast ultrasonic imaging holds significant importance in biomedical and engineering applications. Here, the authors present a compact spatial differentiator tailored for underwater isotropic edge-enhanced imaging, facilitating the realization of high-contrast ultrasonic imaging. [ABSTRACT FROM AUTHOR]

Extensive investigations on the moiré magic angle in twisted bilayer graphene have unlocked the emerging field—twistronics. Recently, its optics analogue, namely opto-twistronics, further expands the potential universal applicability of twistronics. However, since heat diffusion neither possesses the dispersion like photons nor carries the band structure as electrons, the real magic angle in electrons or photons is ill-defined for heat diffusion, making it elusive to understand or design any thermal analogue of magic angle. Here, we introduce and experimentally validate the twisted thermotics in a twisted diffusion system by judiciously tailoring thermal coupling, in which twisting an analog thermal magic angle would result in the function switching from cloaking to concentration. Our work provides insights for the tunable heat diffusion control, and opens up an unexpected branch for twistronics -- twisted thermotics, paving the way towards field manipulation in twisted configurations including but not limited to fluids. Authors control heat transfer through twisting moiré conductive thermal metasurface, showcasing the potential for manipulating thermal conductivity and temperature gradients with imitated magic angles, thereby realizing multifunctional thermal metadevices. [ABSTRACT FROM AUTHOR]

Vectorial optics with fine inhomogeneous polarization control are highly desired. Metasurfaces have been captivated a promising candidate, but their static post-fabrication geometry largely limits the dynamic tunability. Liquid crystal (LC) is usually employed as an additional index-changing layer together with metasurfaces. Unfortunately, most of the reported LCs only impart a varying but uniform phase on top of that from the metasurface, which we term "scalar" LC optics. Here, we pixelate a single-layer LC to display versatile and tunable vectorial holography, in which the polarization and amplitude could be arbitrarily and independently controlled at varying spatial positions. Furthermore, the subtle and vectorial LC-holography highlights the broadband and electrically-switchable functionalities. Our vectorial LC holography reveals significant opportunities for advanced cryptography, super-resolution imaging, and many other applications. [ABSTRACT FROM AUTHOR]

Metasurfaces have exhibited unprecedented degree of freedom in manipulating electromagnetic (EM) waves and thus provide fantastic front-end interfaces for smart systems. Here we show a framework for perception enhancement based on vision-driven metasurface. Human’s eye movements are matched with microwave radiations to extend the humans’ perception spectrum. By this means, our eyes can “sense” visual information and invisible microwave information. Several experimental demonstrations are given for specific implementations, including a physiological-signal-monitoring system, an “X-ray-glasses” system, a “glimpse-and-forget” tracking system and a speech reception system for deaf people. Both the simulation and experiment results verify evident advantages in perception enhancement effects and improving information acquisition efficiency. This framework can be readily integrated into healthcare systems to monitor physiological signals and to offer assistance for people with disabilities. This work provides an alternative framework for perception enhancement and may find wide applications in healthcare, wearable devices, search-and-rescue and others.Authors showcase a framework for visual perception enhancement system based on vision-driven metasurfaces. It can help humans obtain information in multiple frequency bands and allows the metasurface platform with more interesting functions. [ABSTRACT FROM AUTHOR]

Singularities ubiquitously exist in different fields and play a pivotal role in probing the fundamental laws of physics and developing highly sensitive sensors. Nevertheless, achieving higher-order (≥3) singularities, which exhibit superior performance, typically necessitates meticulous tuning of multiple (≥3) coupled degrees of freedom or additional introduction of nonlinear potential energies. Here we propose theoretically and confirm using mechanics experiments, the existence of an unexplored cusp singularity in the phase-tracked (PhT) steady states of a pair of coherently coupled mechanical modes without the need for multiple (≥3) coupled modes or nonlinear potential energies. By manipulating the PhT singularities in an electrostatically tunable micromechanical system, we demonstrate an enhanced cubic-root response to frequency perturbations. This study introduces a new phase-tracking method for studying interacting systems and sheds new light on building and engineering advanced singular devices with simple and well-controllable elements, with potential applications in precision metrology, portable nonreciprocal devices, and on-chip mechanical computing. The authors report a controllable third-order cusp singularity in the phase-tracked closed-loop oscillation of two coupled mechanical modes. This finding addresses the challenge of constructing and controlling higher-order singularities. [ABSTRACT FROM AUTHOR]

Dispersion decomposes compound light into its monochromatic components, which is detrimental to broadband imaging but advantageous for spectroscopic applications. Metasurfaces provide a unique path to modulate the dispersion by adjusting structural parameters on a two-dimensional plane. However, conventional linear phase compensation does not adequately match the meta-unit's dispersion characteristics with required complex dispersion, hindering at-will dispersion engineering over a very wide bandwidth particularly. Here, we propose an asymptotic phase compensation strategy for ultra-broadband dispersion-controlled metalenses. Metasurfaces with extraordinarily high aspect ratio nanostructures have been fabricated for arbitrary dispersion control in ultra-broad bandwidth, and we experimentally demonstrate the single-layer achromatic metalenses in the visible to infrared spectrum (400 nm~1000 nm, NA = 0.164). Our proposed scheme provides a comprehensive theoretical framework for single-layer meta-optics, allowing for arbitrary dispersion manipulation without bandwidth restrictions. This development is expected to have significant applications in ultra-broadband imaging and chromatography detection, among others. The authors present a comprehensive framework for on-demand dispersion control with a single-layer metasurface, particularly in an ultra-broad bandwidth. An achromatic metalens spanning the visible and near-infrared spectra is experimentally demonstrated. [ABSTRACT FROM AUTHOR]

We introduce a new paradigm for generating high-purity vortex beams with metasurfaces. By applying optical neural networks to a system of cascaded phase-only metasurfaces, we demonstrate the efficient generation of high-quality Laguerre-Gaussian (LG) vortex modes. Our approach is based on two metasurfaces where one metasurface redistributes the intensity profile of light in accord with Rayleigh-Sommerfeld diffraction rules, and then the second metasurface matches the required phases for the vortex beams. Consequently, we generate high-purity LGp,l optical modes with record-high Laguerre polynomial orders p = 10 and l = 200, and with the purity in p, l and relative conversion efficiency as 96.71%, 85.47%, and 70.48%, respectively. Our engineered cascaded metasurfaces suppress greatly the backward reflection with a ratio exceeding −17 dB. Such higher-order optical vortices with multiple orthogonal states can revolutionize next-generation optical information processing. The authors demonstrate an efficient way to generate high-purity vortex beams by applying optical neural networks to cascaded phase-only metasurfaces. Specifically, they present record-high-quality Laguerre-Gaussian (LGp,l) optical modes with polynomial orders p = 10 and l = 200 with purity in p, l and relative conversion efficiency of 96%, 85%, and 70%, respectively. [ABSTRACT FROM AUTHOR]

Planar diffractive lenses (PDLs) with optimized but disordered structures can focus light beyond the diffraction limit. However, these disordered structures have inevitably destroyed wide-field imaging capability, limiting their applications in microscopy. Here, we introduce information entropy S to evaluate the disorder of an objective chip by using the probability of its structural deviation from standard Fresnel zone plates. Inspired by the theory of entropy change, we predict an equilibrium point S 0 = 0.5 to balance wide-field imaging (theoretically evaluated by the Strehl ratio) and subdiffraction-limit focusing. To verify this, a N A = 0.9 objective chip with a record-long focal length of 1 mm is designed with S = 0.535 , which is the nearest to the equilibrium point among all reported PDLs. Consequently, our fabricated chip can focus light with subdiffraction-limit size of 0.44 λ and image fine details with spatial frequencies up to 4000 lp/mm experimentally. These unprecedented performances enable ultracompact reflective confocal microscopy for superresolution imaging. The optimized and disordered structures of planar diffractive lenses enable sub-diffraction limit focusing but destroy wide-field imaging. Here, the authors introduce the information entropy to evaluate the disorder of PDL and achieve good balance between super-focusing and imaging. [ABSTRACT FROM AUTHOR]

Tuning the composition of perovskites to approach the ideal bandgap raises the single-junction Shockley-Queisser efficiency limit of solar cells. The rapid development of narrow-bandgap formamidinium lead triiodide-based perovskites has brought perovskite single-junction solar cell efficiencies up to 26.1%. However, such compositional engineering route has reached the limit of the Goldschmidt tolerance factor. Here, we experimentally demonstrate a resonant perovskite solar cell that produces giant light absorption at the perovskite band edge with tiny absorption coefficients. We design multiple guide-mode resonances by momentum matching of waveguided modes and free-space light via Brillouin-zone folding, thus achieving an 18-nm band edge extension and 1.5 mA/cm2 improvement of the current. The external quantum efficiency spectrum reaches a plateau of above 93% across the spectral range of ~500 to 800 nm. This resonant nanophotonics strategy translates to a maximum EQE-integrated current of 26.0 mA/cm2 which is comparable to that of the champion single-crystal perovskite solar cell with a thickness of ~20 μm. Our findings break the ray-optics limit and open a new door to improve the efficiency of single-junction perovskite solar cells further when compositional engineering or other carrier managements are close to their limits. Further extending the band edge of perovskite approaching the ideal bandgap of single-junction solar cell is essential to improve device efficiency. Here, the authors integrate optical resonances with perovskite solar cells to extend the band edge, achieving EQE-integrated current of 26.0 mA/cm2. [ABSTRACT FROM AUTHOR]

Photonic crystals (PhCs) are a kind of artificial structures that can mold the flow of light at will. Polaritonic crystals (PoCs) made from polaritonic media offer a promising route to controlling nano-light at the subwavelength scale. Conventional bulk PhCs and recent van der Waals PoCs mainly show highly symmetric excitation of Bloch modes that closely rely on lattice orders. Here, we experimentally demonstrate a type of hyperbolic PoCs with configurable and low-symmetry deep-subwavelength Bloch modes that are robust against lattice rearrangement in certain directions. This is achieved by periodically perforating a natural crystal α-MoO3 that hosts in-plane hyperbolic phonon polaritons. The mode excitation and symmetry are controlled by the momentum matching between reciprocal lattice vectors and hyperbolic dispersions. We show that the Bloch modes and Bragg resonances of hyperbolic PoCs can be tuned through lattice scales and orientations while exhibiting robust properties immune to lattice rearrangement in the hyperbolic forbidden directions. Our findings provide insights into the physics of hyperbolic PoCs and expand the categories of PhCs, with potential applications in waveguiding, energy transfer, biosensing and quantum nano-optics. Photonic crystals (PhCs) are artificial periodic materials that can be used to manipulate the flow of light. Here, the authors report the realization of asymmetric PhCs based on in-plane hyperbolic phonon polaritons in perforated α-MoO3, showing low-symmetry deep-subwavelength Bloch modes that are robust against lattice rearrangement in specific directions. [ABSTRACT FROM AUTHOR]

Broadband and efficient terahertz absorbing films are crucial to high-performance terahertz detectors, which are in demand for next-generation wireless communications, astronomy, security screening, medical imaging and so on. Recent studies reported a series of two-dimensional materials for enhanced light absorption in terahertz waves, such as graphene, transition metal dichalcogenides and topological insulators, among others. However, it is still challenging to achieve the intrinsic thin-film absorption limit (50%) across the whole ultrabroad terahertz band. Here we demonstrate that ultrathin 10.2-nm-thick (~λ/30,000) Ti3C2Tx MXene assemblies can reach the intrinsic thin-film absorption limit across the entire 0.5–10 THz band. Such intriguing phenomena are attributed to the highly concentrated free electrons (~1021 cm−3), short relaxation time (~10 fs) and unique intra- and interflake (hopping) electron transport properties in Ti3C2Tx MXenes. Our results are validated by alternating current impedance theory using the Drude–Smith model, rather than classic direct current impedance matching. We believe that our findings will stimulate more studies of broadband terahertz technologies with MXenes and beyond, providing a route to developing compact, supercontinuum terahertz optoelectronic or photothermoelectric devices. Ultrathin 10.2-nm-thick (~λ/30,000) Ti3C2Tx MXene assemblies that offer an absorption of 49.2%, which is close to the theoretical limit of 50%, in the range of 0.5–10 THz are reported, benefiting terahertz optoelectronic and photothermoelectric devices. [ABSTRACT FROM AUTHOR]

Controlling the direction of light emission on the subwavelength scale is a key capability for quantum information processing and optical imaging in photonics and biology1–5. The spin–orbit interaction of light is widely adopted for dynamically tuning the direction of circularly polarized photoluminescence (PL), but this is challenging to accomplish for many luminescence materials, which typically generate unpolarized luminescence. Here, we report optical control of unidirectional emission of unpolarized luminescence in a spatially symmetric nanopillar lattice assisted by photonic orbital angular momenta. The orientation of anti-Stokes shift PL emissions can be controlled by the helicity of incident vortex beams. The experimental directionality of unpolarized PL emission in the one-dimensional nanopillar lattice can reach 0.59, which is markedly stronger than that possible by a spin mechanism. Our findings thus offer a new promising approach for tuneable nanoscale optical control of emission. The use of a nanopillar lattice and orbital angular momentum provides control over the directionality of light emission on the nanoscale. [ABSTRACT FROM AUTHOR]

Nanoimprint lithography (NIL) has attracted attention recently as a promising fabrication method for dielectric metalenses owing to its low cost and high throughput, however, high aspect ratio (HAR) nanostructures are required to manipulate the full 2π phase of light. Conventional NIL using a hard-polydimethylsiloxane (h-PDMS) mold inevitably incurs shear stress on the nanostructures which is inversely proportional to the surface area parallel to the direction of detachment. Therefore, HAR structures are subjected to larger shear stresses, causing structural failure. Herein, we propose a novel wet etching NIL method with no detachment process to fabricate flawless HAR metalenses. The water-soluble replica mold is fabricated with polyvinyl alcohol (PVA) which is simpler than an h-PDMS mold, and the flexibility of the PVA mold is suitable for direct printing as its high tensile modulus allows high-resolution patterning of HAR metalenses. The diffraction-limited focusing of the printed metalenses demonstrates that it operates as an ideal lens in the visible regime. This method can potentially be used for manufacturing various nanophotonic devices that require HAR nanostructures at low cost and high throughput, facilitating commercialization. [ABSTRACT FROM AUTHOR]

The quantized bulk quadrupole moment has so far revealed a non-trivial boundary state with lower-dimensional topological edge states and in-gap zero-dimensional corner modes. In contrast to photonic implementations, state-of-the-art strategies for topological thermal metamaterials struggle to achieve such higher-order hierarchical features. This is due to the absence of quantized bulk quadrupole moments in thermal diffusion fundamentally prohibiting possible band topology expansions. Here, we report a recipe for generating quantized bulk quadrupole moments in fluid heat transport and observe the quadrupole topological phases in non-Hermitian thermal systems. Our experiments show that both the real- and imaginary-valued bands exhibit the hierarchical features of bulk, gapped edge and in-gap corner states—in stark contrast to the higher-order states observed only on real-valued bands in classical wave fields. Our findings open up unique possibilities for diffusive metamaterial engineering and establish a playground for multipolar topological physics. Topological transport in thermal diffusion is governed by physical principles that are distinct from those encountered in solid-state or photonic topological systems. Here, the authors demonstrate an experimental strategy for engineering topological thermal phases with bulk, edge and corner modes. [ABSTRACT FROM AUTHOR]

Free-space circularly polarized light (CPL) detection, requiring polarizers and wave plates, is well established, but such a spatial degree of freedom is unfortunately absent in integrated on-chip optoelectronics. The filterless CPL photodetectors reported so far suffer from an intrinsic small discrimination ratio, vulnerability to the non-CPL field components and low responsivity. Here we report a distinct paradigm of geometric photodetectors in the mid-infrared, exhibiting a substantial discrimination ratio of 84, a close-to-perfect CPL-specific response, a zero-bias responsivity of 392 V W−1 at room temperature and a detectivity of ellipticity down to 0.03° Hz−1/2. Our approach makes use of a plasmonic nanostructures array with judiciously designed symmetry, assisted by graphene ribbons, to electrically read their near-field optical information. This geometry-empowered recipe for infrared photodetectors provides a robust, direct, strict and high-quality solution to on-chip filterless CPL detection and unlocks new opportunities for integrated functional optoelectronic devices. A photodetector responding to only circularly polarized light is developed. It has a ring-shaped form, consisting of plasmonic nanostructures on a graphene sheet. Its zero-bias responsivity and detectivity of ellipticity in the mid-infrared at room temperature are 392 V W−1 and 0.03° Hz−1/2, respectively. [ABSTRACT FROM AUTHOR]

Pyroelectricity originates from spontaneous polarization variation, promising in omnipresent non-static thermodynamic energy harvesting. Particularly, changing spontaneous polarization via out-of-plane uniform heat perturbations has been shown in solar pyroelectrics. However, these approaches present unequivocal inefficiency due to spatially coupled low temperature change and duration along the longitudinal direction. Here we demonstrate unconventional giant polarization ripples in transverse pyroelectrics, without increasing the total energy input, into electricity with an efficiency of 5-fold of conventional longitudinal counterparts. The non-uniform graded temperature variation arises from decoupled heat localization and propagation, leading to anomalous in-plane heat perturbation (29-fold) and enhanced thermal disequilibrium effects. This in turn triggers an augmented polarization ripple, fundamentally enabling unprecedented electricity generation performance. Notably, the device generates a power density of 38 mW m−2 at 1 sun illumination, which is competitive with solar thermoelectrics and ferrophotovoltaics. Our findings provide a viable paradigm, not only for universal practical pyroelectric heat harvesting but for flexible manipulation of transverse heat transfer towards sustainable energy harvesting and management. Pyroelectricity promises viable heat harvesting and sensing. Here, the authors identify transverse polarization ripple in pyroelectrics via heat localization and propagation decoupling and offer competitive power output to solar thermoelectrics. [ABSTRACT FROM AUTHOR]

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics1, quantum optics2, topological photonics3 and chiroptics4. Intrinsic chirality is weak in natural materials, and recent theoretical proposals5–7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light–matter interactions. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies8. Therefore, most of the experimental attempts9–11 showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence9,10 or structural anisotropy11. Here we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces in which the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks, to our knowledge, the first observation of intrinsic chiral bound states in the continuum with near-unity circular dichroism of 0.93 and a high quality factor exceeding 2,663 for visible frequencies. Our chiral metasurfaces may lead to a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.Chiral metasurfaces have been produced, with experimental observation of intrinsic chiral bound states in the continuum, which may lead to applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis. [ABSTRACT FROM AUTHOR]

Metasurfaces, artificial 2D structures, have been widely used for the design of various functionalities in optics. Jones matrix, a 2×2 matrix with eight parameters, provides the most complete characterization of the metasurface structures in linear optics, and the number of free parameters (i.e., degrees of freedom, DOFs) in the Jones matrix determines the limit to what functionalities we can realize. Great efforts have been made to continuously expand the number of DOFs, and a maximal number of six has been achieved recently. However, the realization of the ultimate goal with eight DOFs (full free parameters) has been proven as a great challenge so far. Here, we show that by cascading two layer metasurfaces and utilizing the gradient descent optimization algorithm, a spatially varying Jones matrix with eight DOFs is constructed and verified numerically and experimentally in optical frequencies. Such ultimate control unlocks opportunities to design optical functionalities that are unattainable with previously known methodologies and may find wide potential applications in optical fields. The realization of Jones matrix with full eight free parameters is particularly challenging. Here, the authors construct spatially varying Jones matrix with eight free parameters by cascading two-layer metasurfaces and use it for new optical functionalities. [ABSTRACT FROM AUTHOR]

The reciprocity principle governs the symmetry in transmission of electromagnetic and acoustic waves, as well as the diffusion of heat between two points in space, with important consequences for thermal management and energy harvesting. There has been significant recent interest in materials with time-modulated properties, which have been shown to efficiently break reciprocity for light, sound, and even charge diffusion. However, time modulation may not be a plausible approach to break thermal reciprocity, in contrast to the usual perception. We establish a theoretical framework to accurately describe the behavior of diffusive processes under time modulation, and prove that thermal reciprocity in dynamic materials is generally preserved by the continuity equation, unless some external bias or special material is considered. We then experimentally demonstrate reciprocal heat transfer in a time-modulated device. Our findings correct previous misconceptions regarding reciprocity breaking for thermal diffusion, revealing the generality of symmetry constraints in heat transfer, and clarifying its differences from other transport processes in what concerns the principles of reciprocity and microscopic reversibility. The use of time modulation to break reciprocity is well understood for light, sound or charge diffusion, but it's unclear whether it can work for thermal diffusion. Here, the authors answer in the negative by analysing diffusive processes under time modulation, and giving numerical and experimental evidence. [ABSTRACT FROM AUTHOR]

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T′ MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers. 2D transition metal ditellurides exhibit nontrivial topological phases, but the controlled bottom-up synthesis of these materials is still challenging. Here, the authors report the layer-by-layer growth of large-area bilayer and trilayer 1T' MoTe2 films, showing thickness-dependent ferroelectricity and nonlinear Hall effect. [ABSTRACT FROM AUTHOR]

It is widely adopted in thermal metamaterials that mixing different materials could conveniently result in effective thermal conductivities (ETCs) beyond naturally-occurring materials. When multiple materials are isotropically mixed, the ETC is a direct average governed by their filling fractions and given bulk conductivities. That could lead to an inhomogeneous and anisotropic value within the maximal and minimal thermal conductivities of constituent materials. Usually thermal metadevices rely on anisotropic thermal conductivity tensor, whose tensorial elements are frequently inter-dependent and confined within a limited parametric space. It is thus nontrivial to establish a design recipe for advanced thermal metamaterials whose ETCs could cover full-parameter anisotropic space. We demonstrate topological functional cells (TFCs) with copper and polydimethylsiloxane, and show that the anisotropic ETCs traverse their full-parameter space. Such robust scheme based on topology-optimized TFCs unlocks unexplored opportunities for functional thermal metadevices whose parameters may not be reached in previous mixing approaches. This study also sheds light on the developments in emerging acoustic, mechanical and electromagnetic composite materials. [ABSTRACT FROM AUTHOR]

Optical metasurfaces with high quality factors (Q-factors) of chiral resonances can boost substantially light-matter interaction for various applications of chiral response in ultrathin, active, and nonlinear metadevices. However, current approaches lack the flexibility to enhance and tune the chirality and Q-factor simultaneously. Here, we suggest a design of chiral metasurface supporting bound state in the continuum (BIC) and demonstrate experimentally chiroptical responses with ultra-high Q-factors and near-perfect circular dichroism (CD = 0.93) at optical frequencies. We employ the symmetry-reduced meta-atoms with high birefringence supporting winding elliptical eigenstate polarizations with opposite helicity. It provides a convenient way for achieving the maximal planar chirality tuned by either breaking in-plane structure symmetry or changing illumination angle. Beyond linear CD, we also achieved strong near-field enhancement CD and near-unitary nonlinear CD in the same planar chiral metasurface design with circular eigen-polarization. Sharply resonant chirality realized in planar metasurfaces promises various practical applications including chiral lasers and chiral nonlinear filters. Here, the authors employ the physics of chiral bound states in the continuum and suggest planar chiral metasurfaces with simultaneous ultrahigh quality factor and near-perfect circular dichroism in both linear regime and nonlinear regime. [ABSTRACT FROM AUTHOR]

Recent investigations on non-Hermitian physics have unlocked new possibilities to manipulate wave scattering on lossy materials. Coherent perfect absorption is such an effect that enables all-light control by incorporating a suitable amount of loss. On the other hand, controlling heat transfer with heat may empower a distinct paradigm other than using thermal metamaterials. However, since heat neither propagates nor carries any momentum, almost all concepts in wave scattering are ill-defined for steady-state heat diffusion, making it formidable to understand or utilize any coherent effect. Here, we establish a scattering theory for heat diffusion by introducing an imitated momentum for thermal fields. The thermal analogue of coherent perfect absorption is thus predicted and demonstrated as the perfect absorption of exergy fluxes and undisturbed temperature fields. Unlike its photonic counterpart, thermal coherent perfect absorption can be realized for regular thermal materials, and be generalized for various objects. A thermal analogue of coherent perfect absorption would allow to control heat transfer using heat, but the lack of momentum propagation in a thermal field seems to prevent any role for coherence. Here, the authors allow this by introducing an imitated momentum for steady-state heat diffusion. [ABSTRACT FROM AUTHOR]

Computational meta-optics brings a twist on the accelerating hardware with the benefits of ultrafast speed, ultra-low power consumption, and parallel information processing in versatile applications. Recent advent of metasurfaces have enabled the full manipulation of electromagnetic waves within subwavelength scales, promising the multifunctional, high-throughput, compact and flat optical processors. In this trend, metasurfaces with nonlocality or multi-layer structures are proposed to perform analog optical computations based on Green's function or Fourier transform, intrinsically constrained by limited operations or large footprints/volume. Here, we showcase a Fourier-based metaprocessor to impart customized highly flexible transfer functions for analog computing upon our single-layer Huygens' metasurface. Basic mathematical operations, including differentiation and cross-correlation, are performed by directly modulating complex wavefronts in spatial Fourier domain, facilitating edge detection and pattern recognition of various image processing. Our work substantiates an ultracompact and powerful kernel processor, which could find important applications for optical analog computing and image processing. Here, the authors propose a Fourier-based metaprocessor to impart customized highly flexible transfer functions for analog computing. Differentiation and cross-correlation are performed to substantiate the ultracompact and high-throughput kernel processor. [ABSTRACT FROM AUTHOR]

Ultrasound focusing in three-dimensional (3D) space is of crucial and enduring significance in a variety of biomedical and industrial applications. Conventional ultrasound focusing based on active phase array or passive geometry of bulky size is unable to realize the 3D arbitrary focusing with subwavelength resolution. Acoustic metamaterial of complex deep-subwavelength microstructure has facilitated the advanced airborne-sound-focusing but is inevitably not applicable for underwater ultrasound, restricted by the law between the multi-modes coupling/thermal viscosity and the feature size of the structure. Here, we aim to circumvent the restriction by increasing the feature size of the metamaterial while keeping the compact overall geometry, and realize the robust subwavelength ultrasound focusing with the sparse metalens of the wavelength-scale meta-atom. We theoretically propose and demonstrate numerically and experimentally the broadband arbitrary ultrasound focusing in 3D space. The axial and off-axis ultrasound focusing with the subwavelength resolution (FWHM<0.58λ) are achieved by the spatially sparse and compact metalens within one-octave bandwidth. With advantages of 3D freewheeling focusing, subwavelength resolution, spatial sparsity, geometric simplicity, and broadband, the sparse metalens would offer more initiatives to advanced researches in ultrasound focusing and empower applications such as precise biomedical imaging and therapy, nondestructive evaluation, integrated and multiplexed ultrasound devices. [ABSTRACT FROM AUTHOR]

Thermal metamaterials have exhibited great potential on manipulating, controlling and processing the flow of heat, and enabled many promising thermal metadevices, including thermal concentrator, rotator, cloak, etc. However, three long-standing challenges remain formidable, i.e., transformation optics-induced anisotropic material parameters, the limited shape adaptability of experimental thermal metadevices, and a priori knowledge of background temperatures and thermal functionalities. Here, we present robustly printable freeform thermal metamaterials to address these long-standing difficulties. This recipe, taking the local thermal conductivity tensors as the input, resorts to topology optimization for the freeform designs of topological functional cells (TFCs), and then directly assembles and prints them. Three freeform thermal metadevices (concentrator, rotator, and cloak) are specifically designed and 3D-printed, and their omnidirectional concentrating, rotating, and cloaking functionalities are demonstrated both numerically and experimentally. Our study paves a powerful and flexible design paradigm toward advanced thermal metamaterials with complex shapes, omnidirectional functionality, background temperature independence, and fast-prototyping capability. Thermal metamaterials can be used to manipulate heat flow but experimental fabrication is challenging. Here, the authors report robustly printable freeform thermal metamaterials to tackle this challenge by topology optimization and 3D printing. [ABSTRACT FROM AUTHOR]

Optical multiplexing1–11 by creating orthogonal data channels has offered an unparalleled approach for information encoding with substantially improved density and security. Despite the fact that the orbital angular momentum (OAM) of light involves physical orthogonal division, the lack of explicit OAM sensitivity at the nanoscale prevents this feature from realizing nanophotonic information encoding. Here we demonstrate the viability of nanoscale information multiplexing utilizing the OAM of light. This is achieved by discovering OAM-dependent polarization ellipses in non-paraxial focusing conditions and hence synthetic helical dichroism resulting from the distinct absorption of achiral nanoparticles to the different order of OAM beams. Leveraging this mechanism, the application of subwavelength-scale focused OAM beams to self-assemble plasmonic nanoaggregates further enables six-dimensional optical information multiplexing, in conjunction with wavelength, polarization and three spatial dimensions. Our results suggest the possibility of multiplexing OAM division as an unbounded degree of freedom for nanophotonic information encoding, security imprinting and beyond. Multiplexing orbital angular momentum states as independent and orthogonal information carriers for data encryption is realized through disorder-induced synthetic helical dichroism in disordered nanoaggregates. [ABSTRACT FROM AUTHOR]

Polaritons are hybrid excitations of matter and photons. In recent years, polaritons in van der Waals nanomaterials—known as van der Waals polaritons—have shown great promise to guide the flow of light at the nanoscale over spectral regions ranging from the visible to the terahertz. A vibrant research field based on manipulating strong light–matter interactions in the form of polaritons, supported by these atomically thin van der Waals nanomaterials, is emerging for advanced nanophotonic and opto-electronic applications. Here we provide an overview of the state of the art of exploiting interface optics—such as refractive optics, meta-optics and moiré engineering—for the control of van der Waals polaritons. This enhanced control over van der Waals polaritons at the nanoscale has not only unveiled many new phenomena, but has also inspired valuable applications—including new avenues for nano-imaging, sensing, on-chip optical circuitry, and potentially many others in the years to come.This Review discusses the state of the art of interface optics—including refractive optics, meta-optics and moiré engineering—for the control of van der Waals polaritons. [ABSTRACT FROM AUTHOR]