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The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 μs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Organic molecular vibrations, typically occurring in the terahertz (THz) regime, can resonate with a metastructure. A hallmark Rabi splitting occurs when the coupling strength is sufficiently strong. In this work, we observe the strong coupling of localized surface plasmons (LSPs) and intermolecular vibration mode at THz on a metasurface spin-coated with organic molecule α-lactose monohydrate. Excited by transverse-electric THz waves, dispersive localized surface plasmons interact with nondispersive intermolecular vibrations and form two vibro-polariton modes. The angle-resolved transmission spectra of the coupled system are detected by using a terahertz time-domain spectrometer, demonstrating an anti-crossing effect with a clear Rabi splitting. By retrieving the coupling strength and Hopfield coefficients of polariton bands from the measured data, we further verify that these two bands originate from the strong coupling between LSPs and molecular vibration mode. Moreover, we show that it is possible to implement molecular concentration sensing based on this strong coupling effect. This study demonstrates a unique approach to investigate vibro-polaritons at the terahertz regime and provides a testbed for future applications of strong coupling effects in chemical detection and biosensing.

We investigate the electronic properties and valley physics of Janus monolayer WSSe on a CrI3 substrate layer based on first-principles calculations. It is shown that the K and K′ valley degeneracy can be lifted which leads to valley polarization (VP) in the WSSe due to the magnetic proximity coupling to a magnetic substrate. The magnitude of VP is highly sensitive to the interfacial electronic properties and can be tuned by varying the stacking configurations of the heterostructure. Interestingly, the direction of VP can be altered by manipulating the layer alignment without reversing the magnetism orientation of the magnetic substrate CrI3. We suggest that the hybridization between the bands of WSSe and the substrate plays an important role. Meanwhile, the charge distributions have been mapped out to uncover the microscopic origin of the direction variable VP. In addition, large VP can be achieved by adjusting the interlayer spacing. Our investigations may have potential applications in the design of valleytronic devices.

It is known that most of metastructures cannot dynamically tune the optoelectric properties once their structures are fabricated. Naturally, developing active materials and dynamically tunable devices becomes especially desired and necessary. In this work, we have made several active metasurfaces and dynamically-tunable metadevices based on phase change material VO2, which includes: i) the active nanoantennas from thermal to electrical tuning; ii) the electrically-driven tunable broadband waveplates, and iii) the electrically-tuned flexible active absorbers. The investigations here can be applied in constructing unique dynamically-tuning metamaterials, and are expected to promote the further development of new-generation active optoelectronic devices.

Photonic quantum information processing relies on operating the quantum state of photons, which usually involves bulky optical components unfavorable for system miniaturization and integration. Here, we report on the transformation and distribution of polarization-entangled photon pairs with multichannel dielectric metasurfaces. The entangled photon pairs interact with metasurface building blocks, where the geometrical-scaling-induced phase gradients are imposed, and are transformed into two-photon entangled states with the desired polarization. Two metasurfaces, each simultaneously distributing polarization-entangled photons to spatially separated multiple channels M (N), may accomplish M×N channels of entanglement distribution and transformation. Experimentally we demonstrate 2×2 and 4×4 distributed entanglement states, including Bell states and superposition of Bell states, with high fidelity and strong polarization correlation. We expect this approach paves the way for future integration of quantum information networks.

Flexible optoelectronic devices attract considerable attention due to their prominent role in creating novel wearable apparatus for bionics, robotics, health care, and so forth. Although bulk single-crystalline perovskite-based materials are well-recognized for the high photoelectric conversion efficiency than the polycrystalline ones, their stiff and brittle nature unfortunately prohibits their application for flexible devices. Here, we introduce ultrathin single-crystalline perovskite film as the active layer and demonstrate a high-performance flexible photodetector with prevailing bending reliability. With a much reduced thickness of 20 nm, the photodetector made of this ultrathin film can achieve a responsivity much higher than that of recently reported flexible perovskite photodetectors. The demonstrated 0.2 MHz 3 dB bandwidth further paves the way for high-speed photodetection.

Dielectric metamaterials with high refractive indices may have an incredible capability to manipulate the phase, amplitude, and polarization of the incident light. Combining the high refractive index and the excellent electrical characteristics of the hybrid organicinorganic perovskites (HOIPs), we demonstrate that metamaterial made of HOIPs can trap visible light and realize effective photonto-electron conversion. The HOIP metamaterials are fabricated by focused ion beam milling on a solution-grown single-crystalline HOIP film. The optical absorption is significantly enhanced at the visible regime compared to that of the flat HOIP film, which originates from the excited Mie resonances and transverse cavity modes with inhibited interface reflection. Our results point to the potential application of HOIP metamaterials for high-efficiency light trapping and photon-to-electron conversion.

In this work, we intentionally explored the heavily p-doped graphene stacks by degenerate femtosecond pump-probe spectroscopy, and observed an excitation enhancement of hot electrons at weak pump fluence. Physically, both Auger processes and population inversion are suppressed in this system, yet it becomes possible for the conduction bands to be effectively evacuated within the pulse duration through the ultrafast cooling of hot electrons, which may lead to an enhanced excitation of hot electrons. This excitation enhancement can be further strengthened by multiple layer-stacking processes or a thermal annealing pretreatment. Furthermore, large modulation depth is achieved in graphene stacks with small variation of pump fluence. We suggest that this effect can have potential applications on harvesting energy from excited hot electrons, and may provide a unique way to achieve high-speed modulators, photodetectors, solar cells, and photocatalysts.

Traditional optical elements, such as waveplates and polarization beam splitters, are essential for quantum state tomography (QST). Yet, their bulky size and heavy weight are prejudicial for miniaturizing quantum information systems. Here, we introduce nondiffractive silicon metasurfaces with high transmission efficiency to replace the traditional optical elements for QST of polarization-entangled states. Two identical silicon metasurfaces are employed, and each metasurface comprises four independent districts on a micrometer scale. The unit cell of each district consists of two silicon nanopillars with different geometrical sizes and orientation angles, and the interference of the scattered waves from the nanopillars leads to a single output beam from the district with a specific polarization state with a transmission efficiency above 92%. When the two-photon polarization-entangled state shines on different districts of two metasurfaces, each photon of the photon pair interacts with the local nanopillars within the district, and the two-photon state is projected onto 16 polarization bases for state reconstruction. We experimentally demonstrate the reconstruction of four input Bell states with high fidelities. This approach significantly reduces the number of conventional optical components in the QST process and is inspiring for advancing quantum information technology.

In this paper, we propose a subwavelength closed surface consisting of radial gradient grooves for achieving localized acoustic rainbow trapping. Similar to conventional rainbow trapping based on a plane structured surface, we demonstrate that rainbow trapping can also be realized by a closed surface in the deep-subwavelength scale. In particular, the trapped spatial positions on the closed surface can be freely tuned by changing the structural parameters. Based on the advantages of the multiband response and subwavelength dimension of this structure, we propose an acoustic barrier consisting of an array of closed surfaces to achieve multiband sound insulation and high ventilation simultaneously. Investigated results indicate that this kind of acoustic barrier possesses the features of subwavelength scale, and multiband and omnidirectional responses for blocking incident acoustic waves, yet has high ventilation. Finally, we discuss the influence of the channel width on the magnitude and bandwidth of the transmission loss spectrum. The experimental results agree with our theoretical prediction that the material losses can broaden the response bandwidth, and form broadband sound insulation. The acoustic barrier consisting of localized rainbow trapping structures may have many applications in infrastructure requiring air-permeability and sound-proofing.

Energetic photoinduced hot electrons have been attracting increased scientific attention owing to their potential applicability in numerous photoelectrical and photochemical processes. Normally, the energy of electrons quickly converts into heat by ultrafast cooling, which is considered as the bottleneck for high-efficiency utilization of hot electrons. In this work, we explore intentionally heavily $p$-doped graphene stacks by degenerate femtosecond pump-probe spectroscopy, and observe an excitation enhancement of hot electrons at weak pump fluence. The time scale of hot-electron excitation is of the same order as that of fast decay via electron-electron and electron-optical-phonon scattering in our experiments. Physically, both Auger processes and population inversion are suppressed in this system, yet it becomes possible for the conduction bands to be effectively evacuated within the pulse duration through the ultrafast cooling of hot electrons, which may lead to an enhanced excitation of hot electrons. This excitation enhancement can be further strengthened by multiple layer-stacking processes or a thermal annealing pretreatment. The optical absorption of graphene stacks increases correspondingly, exhibiting a value larger than the linear limit at low pump fluence. Furthermore, the absorption modulation depth can reach approximately 0.79% in monolayer graphene for a small pump-fluence change ($l20\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{J/cm}}^{2}$), further increasing to approximately 1.2% and approximately 2.5% in double- and triple-stacking graphene layers, respectively, indicating that the absorption can be sufficiently altered by an extremely small variation of pump fluence. These outcomes can be applied for low-cost pulse operations. We suggest that this effect can have potential applications to harvesting energy from excited hot electrons, and may provide a unique way to achieve high-speed modulators, photodetectors, solar cells, and photocatalysts.

Most polarization-sensitive photodetectors detect either linearly polarized (LP) or circularly polarized (CP) light. Here, we experimentally demonstrate a multiple-polarization photodetector based on a hybrid organic–inorganic perovskite (HOIP) metasurface, which is sensitive to both LP and CP light simultaneously. The perovskite metasurface is composed of a HOIP antenna array on a single-crystal HOIP film. Owing to the antenna anisotropy, the absorption of linearly polarized light at the metasurface depends on the polarization angle; also, due to the mirror asymmetry of the antenna elements, the metasurface is also sensitive to different circular polarizations. Polarization-dependent photocurrent responses to both LP and CP light are detected. Our results highlight the potential of perovskite metasurfaces for integrated photoelectric applications.

The interplay between topology and correlation lies at the forefront of modern condensed matter physics. In this paper, we study the extended fermion-Hubbard model, including the on-site as well as the nearest-neighbor repulsive interactions, on a topological square lattice that supports the Chern insulator. Within the mean-field method, we find that the spontaneous symmetry breaking (SSB) charge-density wave or antiferromagnetic insulator dominates the system when the onsite or nearest-neighbor interactions are strong enough. It is interesting that the antiferromagnetic Chern insulator will appear in the phase diagram when there is an explicitly nonvanishing sublattice potential. In addition, we explore how a finite-size ribbon structure affects the phase diagram and point out that the critical interaction for SSB occurs with weaker strength than the bulk system.

Optical nanoantennas have attracted significant attention over the past decades, owing to their exceptional capabilities in terms of light manipulation and versatile optical applications. Recently, active nanoantennas have been developed by introducing phase change materials, to achieve specific tunable electromagnetic responses. However, most of these attempts only function with ‘ON’/‘OFF’ states or switch in a few discrete states, significantly restricting the application in dynamic tunability. Thus far, the continuous and reversible modulation of optical nanoantennas has not been sufficiently explored. In this article, we experimentally demonstrate a continuously and reversibly electro-tunable optical nanoantenna, by integrating an asymmetric gold nanodisk dimer array with a vanadium dioxide (VO2) film and graphene thin film. By accurately controlling the applied electrical current, the Joule heat generated in the graphene film excites the metal-insulator phase transition of VO2, and the refractive index of VO2 exhibits a relatively large variation. When VO2 is in the insulating phase, we observe an apparent resonance dip in the reflection spectrum, which is attributed to a hybrid mode originating from the gap plasmon in the dimers and localized surface plasmon (LSP) resonance excited at the edge of the nanodisks. Meanwhile, owing to the coupling between two asymmetric LSP resonances in the neighboring nanodisks, the reflected peak based on the Fano effect is realized. However, once VO2 is in the metallic phase, the hybrid mode becomes weaker and red-shifted, and the Fano effect disappears. Thereafter, the continuous and reversible electro-modulation of the nanoantenna features, including the resonant wavelength, resonant intensity, and quality factors (Q), are experimentally verified in the optical communication region, by varying the applied electrical current in the hybrid structure. To further increase the modulation range of these properties, we scan the gap size and structural asymmetry parameter of the nanodisk dimer. The results show that, with a smaller gap size, the resonant intensity of the nanoantenna is stronger. When the structural asymmetry parameter increases, the resonant wavelength is redshifted. We expect that such continuously and reversibly electro-tunable nanoantennas will stimulate various applications in optical communication systems, tunable photoelectric sensors, and beyond.

Hexagonal boron nitride (h-BN) thin films support volume-confined phonon polariton modes within the bulk material as well as surface-confined modes at the edges of thin films. In this Letter, we theoretically investigate the phonon polaritons in curved h-BN thin films. One-dimensional guided phonon polariton modes are found, which are caused by the curved geometry and do not exist in extended flat films. These modes are guided along a specific direction with relatively low propagation losses. So far, one-dimensional guided phonon polariton modes have only been proposed in nanowire and nanoribbon structures. Our study offers another way with the advantage of keeping the h-BN film intact, which can avoid huge scattering losses due to the structural defects. These investigations may offer an easy and robust approach toward phonon-polariton-based nanophotonic circuitry.

Hexagonal boron nitride supports phonon polaritons in its two Reststrahlen bands. In this paper, we investigate phonon polaritons in cylindrically curved hexagonal boron nitride thin films. The phonon polariton modes in such structure carry orbital angular momentums depending on its azimuthal index. For extremely small-size structures, high order polariton modes show cutoff behaviors; while, for large-size ones, modes with low azimuthal indexes are nearly degenerate, showing similar mode effective indexes. In dimer structures, phonon polariton modes in the neighboring structures are coupled, creating hybrid modes; gap phonon polaritons arise due to such coupling. For large-size dimers, multiple gap phonon polariton modes have been found. Then, cylindrically curved hexagonal boron nitride thin film is placed on a substrate, which also leads to the emergence of multiple gap phonon polariton modes near the touching point. In the end, we vary the geometric parameters of the structures and give some discussions about the phonon polariton modes. Based on these investigations, we may say that the curvature can strongly affect the phonon polariton modes in h-BN thin films.

The plasmonic nanoantenna has attracted intensive attention over the last decades owing to its unique optical response. Although various nanoantennas have been designed, so far very few efforts have been devoted to their dynamic tunability. Here we present a study on dynamically tunable bowtie nanoantennas integrated on a vanadium dioxide thin film with a thermal phase transition. The insulator–metal transition of vanadium dioxide changes its electric feature and permittivity; hence, the resonance of the bowtie nanoantennas is actively tuned by varying the temperature of the device. Further, by adjusting the gap of the bowtie and the edge size of the nanotriangle at a different temperature, the shift of the resonant wavelength of the nanoantenna has been found to increase for a larger triangle edge size, but less dependent on the gap width. The features suggest that VO2-integrated nanoantennas may have applications in dynamically tunable high-harmonic generation, single-molecule fluorescence enhancement, and nanolasers.

In this work, we investigate the coupling of surface plasmons and excited optical modes in metal/dielectric grating stacks theoretically and experimentally. We have observed three kinds of modes in these structures: the cavity mode, the propagated surface plasmon (PSP) mode and the localized surface plasmon (LSP) mode, which can enhance the optical transmission. Firstly, it is shown that the cavity mode is excited in the grating stacks. And the cavity mode has redshift if we enhance the thickness of metal layers, while it has blueshift when we increase the thickness of dielectric layers. The redshift of the cavity mode also occurs when the number of repeating layers is increased. Secondly, the PSP mode is also excited, which can be described by the effective permittivity method. It is found that the PSP modes are coupled with each other, which leads to a modified dispersion relation of surface plasmon polaritons (SPP). The theoretical analysis is in good agreement with the observed transmission enhancement in the grating stacks. And the coupling of PSPs also leads to a blueshift when the number of metal layers is increased. Thirdly, the LSP mode, generated in single metal strip, can also enhance the optical transmission of the grating stacks. Yet the transmission intensity induced by LSP decreases rapidly with increasing the number of metal layers. The investigations here may have potential applications in designing plasmonic metamaterials and subwavelength optical devices.

In this work, we investigate the transmission of light through an isolated subwavelength hole, the hole chain and the hole array with different geometries, respectively. It is shown that that when the light illuminates an isolated subwavelength hole, localized surface plasmons (LSPs) are excited and contribute to transmission of the hole; while the light illuminates the hole chain and array, both LSPs and propagating surface plasmons (PSPs) are excited, and their contributions to the transmission of holes strongly depend on the geometry of the hole chain and array, and also on the polarization of incident light. The theoretical calculations are based on three-dimensional full-vector finite-difference time-domain method. The investigations may have potential applications on designing plasmonic structures and devices.

In this paper, we demonstrate that a series of hybrid Helmholtz resonators, which introduce "acoustic transparent atoms", "acoustic nontransparent atoms", and "acoustic quasitransparent atoms" simultaneously, can generate multimode acoustic transparency and the slow sound effect. Dual-mode acoustic transparency can be achieved by employing a waveguide incorporating three different Helmholtz resonators. Additional modes are introduced by adding further acoustic quasitransparent atoms. This can be explained by the destructive interference among different resonators. Furthermore, slow sound propagation is demonstrated in our multimode acoustic transparency systems by employing time-domain simulations. Our results may have potential applications for sound control in one-dimensional waveguides.

We explore an analogue of optical frequency combs in driven nonlinear phononic systems, and present a mechanism for generating phononic frequency combs through nonlinear resonances. In the underlying process, a set of phonon modes is simultaneously excited by the external driving which yields frequency combs with an array of discrete and equidistant spectral lines of each nonlinearly excited phonon mode. Frequency combs through nonlinear resonance of different orders are investigated, and in particular the possibility of correlation tailoring in higher-order cases is revealed. We suggest that our results can be applied in various nonlinear acoustic processes, such as phonon harvesting, and can also be generalized to other nonlinear systems.

In this work, we demonstrate broadband extraordinary transmission and antireflection in two-dimensional periodic metallic cuboids at optical frequencies. These phenomena originate from nonresonant excitations of surface plasmons, and represent high antireflection simultaneously for a broad spectral band and a wide angular range of incidence with polarization insensitivity. Based on this principle, we further introduce such metallic cuboids arrays into silicon solar cells. It is shown that high performance of light trapping in the cells can be achieved with a significant enhancement of the ultimate quantum efficiency. This study shows promising applications of plasmonic nanostructures to high-efficiency photovoltaic devices.

In this paper, we demonstrate theoretically that an electric-field-dependent multimode quantized thermal conductance can be achieved in a composite polymer in the frame of ballistic phonon transport. The composite polymer consists of three segments, where an ionic polymer is introduced as its central part and the nonpolar polymers are designed on the left and right parts, respectively. By increasing the applied electric field, the dispersion relation of phononic torsion mode is tuned, hence multiple phononic channels are adjusted one by one in the composite polymer. As a result, multiple-step quantized thermal conductance can be manipulated by external electric field. The analysis based on Landauer formula is in good agreement with both the numerical calculations with transfer-matrix method and molecular dynamics simulations. By designing such a three-segment composite polymer, multimode quantized thermal conductance tuned by external electric field becomes an exact analog to multistep quantized electrical conductance tuned by external magnetic field in the quantum Hall effect. The investigations may have potential applications in thermal manipulation and information transfer in mesoscopic phonon systems.

In this work, we demonstrate that resonant full transmission of acoustic waves exists in subwavelength H-shaped metallic gratings, and transmission peaks can be efficiently tuned by adjusting the grating geometry. We investigate this phenomenon through both numerical simulations and theoretical calculations based on rigorous-coupled wave analysis. The transmission peaks are originated from Fabry-Perot resonances together with the couplings between the diffractive wave on the surface and the multiple guided modes in the slits. Moreover, the transmission modes can be efficiently tuned by adjusting the cavity geometry, without changing the grating thickness. The mechanism is analyzed based on an equivalent circuit model and verified by both the theoretical calculations and the numerical simulations. This research has potential application in acoustic-device miniaturization over a wide range of wavelengths.

In this letter, we have demonstrated that serrated metal gratings, which introduce gradient coatings, can give rise to broadband transmission enhancement of acoustic waves. Here, we have experimentally and theoretically studied the acoustic transmission properties of metal gratings with or without serrated boundaries. The average transmission is obviously enhanced for serrated metal gratings within a wide frequency range, while the Fabry-Perot resonance is significantly suppressed. An effective medium hypothesis with varying acoustic impedance is proposed to analyze the mechanism, which was verified through comparison with finite-element simulation. The serrated boundary supplies gradient mass distribution and gradient normal acoustic impedance, which could efficiently reduce the boundary reflection. Further, by increasing the region of the serrated boundary, we present a broadband high-transmission grating for wide range of incident angle. Our results may have potential applications to broadband acoustic imaging, acoustic sensing, and acoustic devices.

In this work, we investigate electronic band structures and transport properties in dimerlike graphene superlattices (DGSLs), where the modulated potentials of square barrier A and well B on graphene are arranged as S(m) = (AB)m(BA)m. Here m is the repeated number of units. It is found that the mirror symmetry of the potential distribution on graphene can induce extra Dirac points (DPs), which originates from the dimerlike positional correlations in the system. The induced DPs, which are exactly located at the energy corresponding to zero averaged wave number, do not exist in the periodic graphene superlattices of (AB)m. The number and the position of DPs in the zero averaged wave number gap of DGSL can be manipulated. Correspondingly, multiple perfect transmissions are observed at the resonant modes. Moreover, the conductance for DGSL presents extra resonant peaks accompanying with the emergence of the induced DPs. The investigations may have potential applications in graphene-based electronic devices.

In this work, we experimentally and theoretically demonstrate that oblique metal gratings with optimal tilt angles can become transparent for broadband terahertz waves under normal incidence. Direct imaging is applied to intuitively prove this broadband transparency phenomenon of structured metals. The transparency is insensitive to the grating thickness due to the non-resonance mechanism, and the optimal tilt angle is determined only by the strip width and the grating period. The oblique metal gratings with broadband transparence may have many potential applications, such as transparent conducting panels, white-beam polarizers, and stealth objects.

In this work, we demonstrate that acoustic waves can achieve extremely flat transmission through a metallic grating under oblique incidence within multiple frequency bands separated by Wood’s anomalies. At the low-frequency band, the transmission of acoustic wave is independent of the frequency and presents a flat curve with the transmission efficiency reaching about 100%; while at high-frequency bands, the transmission decreases to be lower flat curves due to the diffraction effect. The transmission efficiency is insensitive to the thickness of the grating. This phenomenon is verified by experiments, numerical simulations, and an analytical model. The broadband high transmission is attributed to the acoustic impedance matching between the air and the grating. This research may open up a field for various potential applications of acoustic gratings, including broadband sonic imaging and screening, grating interferometry, and antireflection cloaking.

In this work, we investigate electromagnetic responses in multilayered Ag/SiO2 nanoplates at optical frequencies. Electric and magnetic resonances, which originate from localized surface plasmons, are demonstrated by the effective permeability and permittivity, electric and magnetic field distributions, and measured transmission at oblique incidence. Furthermore, electric and magnetic resonances can be tailored by the geometrical parameters of the nanoplates. In a rectangular nanoplate, magnetic resonance only shifts with the width along the incident polarization, and electric resonance shifts obviously as the aspect ratio of the nanoplate changes. The investigation may provide a tunable building block for optical metamaterials.

We theoretically investigate the resonant transmission of phonons and step-like thermal conductance in a weakly nonlinear generalized random n-mer (NGRN) system, where the impurity cluster with short-range correlation is randomly distributed in the host monatomic chain, and the atoms are connected by anharmonic potentials. The weakly anharmonic potential can be reduced to the quasi-harmonic form, which is tuned by external stretching. Due to the delocalization of phonons, resonant transmission is observed and thermal conductance presents a "quantized" feature. By applying external stretching on the NGRN system, the number of resonant modes and their locations are tuned. As a consequence, the "quantized" thermal conductance becomes tunable in this system. This finding may achieve potential applications in designing novel thermal-conducting materials.