Understanding and controlling chirality are important because of its ubiquity in nature. In recent years, we have witnessed the rapid development of chiral metamaterials with exceptional light-manipulating capabilities. Here, we present a design for photonic heterostructures in chiral photonics, demonstrating that photonic heterostructures stacked from achiral metasurfaces can create distinctive chiroptical functionalities. Analyses based on the symmetry group and microscopic dipolar interactions at two-dimensional boundaries reveal the implicit mechanism of the interlayer coupling in photonic heterostructures. By flip stacking, the photonic heterostructure gives rise to an asymmetric chiral phase from achiral composites, and hence, provides the full spin-flipped scattering of incident photons. Incident electromagnetic waves with arbitrary polarization orientations can be efficiently separated into two orthogonal circularly polarized ones with orthogonal trajectories, leading to an ideal circular beam splitter. This effect is experimentally demonstrated at microwave frequencies via the measurement of a scattering matrix. Due to its generality and scalability, we expect the methodology of photonic heterostructure might open up an alternative pathway for exploring intriguing and advanced metamaterials over a broad wavelength range.

A two-dimensional Stampfli-triangle photonic crystal (2D S-T PC), which arranges the basic structural unit of a Stampfli-type photonic quasicrystal (PQC) in a triangular lattice, is proposed. The relationship between the structural parameters and the topological trivial or nontrivial state generated in the 2D S-T PC and the relationship between the structural parameters and the frequency of the one-way propagation are analyzed. One-way propagation based on a U-shaped waveguide composed of 2D S-T PCs, with topological trivial and nontrivial states sharing a common band gap, is realized. The topological edge state, which satisfies $C_6$ symmetry, is realized by changing just the diameters of the cylinders in the 2D S-T PC. This provides an idea for the generation of a topological edge state based on a 2D PQC and increases the diversity and adjustability of the method to realize the topological edge state.

Indistinguishable single photons are key ingredients for a plethora of quantum-information-processing applications, ranging from quantum communications to photonic quantum computing. A mainstream platform to produce indistinguishable single photons over a wide spectral range is based on biphoton generation through spontaneous parametric down-conversion in nonlinear crystals. The purity of the biphotons produced is, however, limited by their spectral correlations. Here we present a design recipe, based on a machine-learning framework, for the engineering of biphoton joint spectral amplitudes over a wide spectral range. By customizing the poling profile of the ${\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}\mathrm{O}}_4$ crystal, we show, numerically, that spectral purities of 99.22%, 99.99%, and 99.82%, respectively, can be achieved in the 1310-, 1550-, and 1600-nm bands after applying a moderate 8-nm filter. The machine-learning framework thus enables the generation of near-indistinguishable single photons over the entire telecommunication band without resorting to the ${\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}\mathrm{O}}_4$ crystal’s group-velocity-matching wavelength window near 1582 nm.

We experimentally demonstrate a photon-pair source, based on continuous-wave pumped spontaneous four-wave mixing in a silicon microring resonator, which efficiently generates photon pairs with high coincidence counts, high brightness, and high coincidence-to-accidental ratio at multiple resonance wavelengths matching a standard International Telecommunication Union (ITU) frequency grid. The signal-idler joint spectral intensity is measured with a very high resolution of $2\mathrm{pm}$ over multiple wavelengths by an optical spectrum analyzer through probe-swept stimulated four-wave mixing, allowing an accurate quantification of the spectral correlation. Strong spectral correlation and uniform degree of quantum correlation are observed from the source, and the measured amount of spectral modes agrees well with that obtained by unheralded second-order correlation measurements.

Sensitive detection of phase-locked terahertz (THz) pulses is routinely achieved using electro-optic sampling (EOS), a technique that harnesses second-order optical nonlinearities in a crystalline semiconductor. The thickness and electro-optic coefficient of such a crystal set the detection sensitivity in EOS, while its linear optical properties often determine the accessible detection bandwidth through the phase-matching conditions. Here, we demonstrate how a periodically patterned structure on the incident surface of an EOS crystal can be used to overcome bandwidth limitations by enabling noncollinear propagation between the THz waves and a near-infrared (NIR) gating pulse. The concept is demonstrated with phase gratings etched on a 1-mm-thick gallium phosphide ($\mathrm{Ga}\mathrm{P}$) semiconductor. The use of a diffracted NIR gating pulse to retrieve the THz waveform results in a sensitivity enhancement of a factor of approximately 2 with regard to the spectral maximum at 2 THz and exceeding an order of magnitude at 5 THz. This extends the spectral detection bandwidth up to 5 THz, which is twice as large as that achievable in a similar configuration with collinear THz and NIR pulses, facilitating the broadband detection of weak signals in THz spectroscopy.

Single-pixel imaging is suitable for low light-level scenarios because a bucket detector is used to maximally collect the light from an object. However, one of the challenges is its slow imaging speed, mainly due to the slow light-modulation technique. We here demonstrate 1.4-MHz video imaging based on computational ghost imaging with a red-green-blue light-emitting diode array having a full-range frame rate up to 100 MHz. With this method, the motion of a high-speed propeller is observed. Moreover, by use of single-photon detectors to increase the detection efficiency, this method is developed for ultrahigh-speed imaging under a low light level.

Anticounterfeiting is an effective method to decrease the harmful effects of counterfeit goods. Optical metasurfaces can tailor light's amplitude, phase, and polarization at subwavelength resolution, enabling compact optical devices with unusual functionalities that can outperform conventional bulky components. We propose and experimentally demonstrate a multichannel metasurface device that can reconstruct helicity multiplexed holographic images and hide a grayscale image in the polarization profile of a light beam, which can be used for high-level anticounterfeiting. A dielectric metasurface consisting of nanopillars with spatially variant orientations is used to realize distinct functionalities in multiple channels. The two holographic images can be switched by changing the helicity of the incident light, while the encoded image in the polarization profile of the light beam can be revealed by a polarizer. Our work demonstrates the feasibility of the multifunctional device based on the independent control of phase and polarization in different channels, which may open an alternative window for compact optical devices with high density of functionalities for anticounterfeiting.

We study the reconstruction-algorithm effect on positive and negative ghost imaging (GI). By introducing three GI algorithms (logarithmic GI, exponential GI, and trigonometric GI), we perform numerical simulations and experiments with different GI algorithms, which demonstrate that a positive or negative ghost image can be recovered by modulating the monotonicity of the object signal function. Our work broadens the GI reconstruction algorithm and deepens the understanding of positive and negative GI principle.

Terahertz (THz) radiation sources ranging from 0.3 to 10 THz are the last frontier of the light spectrum because of the relative lack of physical systems that can operate in this regime, the well-known “THz gap.” We propose a mechanism for frequency down-conversion to produce THz radiation covering most of the THz gap by utilizing the approximately equidistant helical edge states in topological insulator quantum dots. If the edge-state level separation inside the phonon gap is chosen to suppress phonon-induced nonradiative decay, a topological insulator quantum dot array in a resonant cavity can form a continuous-wave THz laser operating at room temperature, with an output power reaching 28 mW at $\sim 3$ THz and an energy conversion efficiency of $\sim 14$%. Our work may pave the way to applications of topological insulator quantum dots for THz emission.

Nonreciprocal devices that allow unidirectional wave propagation are indispensable in many scientific fields, including, for example, optics, acoustics, electronics, and thermodynamics. Here, we propose to realize the nonreciprocal transmission of a microwave field in a two-cavity magnonics system consisting of two microwave cavities and a yttrium iron garnet (YIG) sphere, which is placed in one of them. We discover that magnon Kerr nonlinearity leads to the occurrence of magnon bistability and different magnonic responses for the forward and backward propagation of a microwave field. The different magnonic responses play a crucial role in the nonreciprocal transmission, which essentially breaks the space-reversal symmetry of transmission of the microwave field. More importantly, by adjusting the external bias magnetic field, which determines the frequency of the magnon, we can obtain a variable isolation of the microwave field or even its transmission in only one direction, which may be utilized to achieve light diodes. In addition, by increasing the strength of a microwave source applied to drive the magnon mode, stronger nonreciprocal transmission can be observed. These results may open a route to exploiting various nonreciprocal effects in cavity magnonics systems.

Within the context of spin-related optical phenomena, near-field directionality is generally understood from the viewpoint of the quantum spin Hall effect of light, according to which the transverse spin of surface or guided modes is locked to the propagation direction. Most previous works have been focused on the spin properties of circularly polarized dipolar sources. However, in near-field optics, higher-order multipole sources (e.g., quadrupole, octupole, and so on) might become relevant, so a more in-depth formulation would be highly valuable. Building on the angular-spectrum representation, we provide a general, analytical, and ready-to-use treatment in order to address the near-field directionality of any multipole field, particularizing to the electric quadrupole case. Besides underpinning and upgrading the current framework on spin-dependent directionality, our results may open up interesting perspectives for engineering light-matter coupling at the nanoscale.

Electromagnetic waves exhibiting vortex properties from orbital angular momentum (OAM) modes that are inherently orthogonal show promising potential for a wide range of applications. However, classic OAM-wave generators in the radio-frequency band are generally limited by a narrow operation bandwidth, complicated feeding structures, or high losses. In this work, we propose a method for OAM-wave generation using the concept of spatial transformation at microwave frequencies. A reflection-type vortex-beam generator is designed and simulated. Then an all-dielectric proof-of-concept prototype for a +1 topological charge is physically implemented and the complete characterization of the device is reported. Both full-wave simulations and experimental measurements successfully validate the spiral-shaped phase fronts of the vortex wave. Furthermore, the all-dielectric implementation of the device enables operation over a broad frequency bandwidth. The proposed method provides an efficient approach to generate vortex waves carrying OAM modes and illustrates the practicality of using spatial transformations to achieve an alternative class of optical devices at microwave frequencies.

Quantum memories for light are important components for future long-distance quantum networks. We present on-chip quantum storage of telecommunication-band light at the single-photon level in an ensemble of erbium-167 ions in an yttrium orthosilicate photonic crystal nanobeam resonator. Storage times of up to $10\mu \mathrm{s}$ are demonstrated with an all-optical atomic-frequency-comb protocol in a dilution refrigerator under a magnetic field of 380 mT. We show this quantum-storage platform to have high bandwidth, high fidelity, and multimode capacity, and we outline a path toward an efficient erbium-167 quantum memory for light.

The information-theoretic security promised by quantum key distribution (QKD) holds as long as the assumptions in the theoretical model match the parameters in the physical implementation. The superlinear behavior of sensitive single-photon detectors represents one such mismatch and can pave the way to powerful attacks hindering the security of QKD systems, a prominent example being the after-gate attack. A long-standing tenet is that trapped carriers causing delayed detection can help mitigate this attack, but despite intensive scrutiny, it remains largely unproven. Here we approach this problem from a physical perspective and find evidence to support a detector’s secure response. We experimentally investigate two different carrier-trapping mechanisms causing delayed detection in fast-gated semiconductor avalanche photodiodes, one arising from the multiplication layer and the other arising from the heterojunction interface between absorption and charge layers. The release of trapped carriers increases the quantum bit error rate measured under the after-gate attack above the typical QKD security threshold, thus favoring the detector’s inherent security. This represents a significant step to avert quantum hacking of QKD systems.

Magnetfree circulators are crucial devices to enable full-duplex communications with the goal of enhancing the spectral efficiency of wireless radios compared to currently deployed half-duplex systems. Nevertheless, implementations proposed to date at radio- and millimeter-wave frequencies have been based on spatiotemporally modulating a discrete number of nonlinear elements, such as varactors or switched capacitors, which, despite achieving strong nonreciprocity, suffer from spurious emission in adjacent channels and limited power handling. These problems jeopardize the practical use of these components in many applications, including wireless communications and quantum computing, and make them less attractive compared to harmonicfree magnetic devices. We address these problems by introducing N-way circulators, which consist of N spatiotemporally modulated angular-momentum (STM AM) biased units connected either in series or in parallel and modulated through a rotating phase pattern. We show that these circuits, for a sufficiently large N, effectively impart an overall linear and continuous angular-momentum bias, which significantly reduces spurious emission and increases the overall power handling. This, in turn, strengthens the argument of magnetless circulators compared to their magnetic counterparts, and opens exciting venues to address the realistic needs of practical systems.

We report a gradient metasurface design at microwave bands as an elegant approach to realize the goal of “rainbow trapping” for the storage of waves involving wave localization and absorption phenomena. A longitudinally placed coplanar waveguide is loaded with gradient metasurfaces on both sides, where split-ring resonators (SRRs) are the basic cell. The same SRRs are arranged along the transverse direction to establish magnetoinductive channels. Waves of different frequencies are coupled to corresponding SRRs at different positions in metasurfaces. Resonant trapping with a long oscillation life time enhances the absorption caused by inherent losses of the materials, thereby suppressing reflections. Both simulations and measurements verify the existence of “rainbow trapping.” The proposed strategy enhances the interaction between waves and matter, opening an avenue for further component designs, including absorptive filters, multiplexers, and buffers.

We demonstrate full control of acoustic and thermal periodic deformations at solid surfaces down to subnanosecond time scales and few-micrometer length scales via independent variation of the temporal and spatial phase of two optical transient grating (TG) excitations. For this purpose, we introduce an experimental setup that exerts control of the spatial phase of subsequent time-delayed TG excitations depending on their polarization state. Specific exemplary coherent control cases are discussed theoretically and corresponding experimental data are presented in which time-resolved x-ray reflectivity measures the spatiotemporal surface distortion of nanolayered heterostructures. Finally, we discuss examples where the application of our method may enable the control of functional material properties via tailored spatiotemporal strain fields.

Slow light enables spatiotemporal manipulation of electromagnetic waves at the nanoscale and allows access to a plethora of nonlinear optical phenomena. Although the guided waves in plasmonic waveguides are known to inherently possess a slow energy velocity, their ultimate light-trapping performance remains unknown as the effect of the waveguide's shape alteration has not been considered systematically so far. In this work, we theoretically demonstrate a free-form optimized metal-insulator-metal plasmonic waveguide for light trapping that exhibits a quality factor several times higher than that of the conventional linearly tapered structures. The quality factor of the optimized waveguide saturates to the theoretical limit at a surprisingly short device length, which shows a nontrivial inverse logarithmic dependence on the material loss. The demonstrated design has a quality-factor-to-footprint ratio comparable to that of state-of-the-art photonic cavities.

Room-temperature surface exciton polaritons (SEPs) were successfully demonstrated recently by replacement of the metal thin film in the Kretschmann-Raether configuration of surface plasmon polaritons (SPPs) with a film of the J-aggregate cyanine dye 5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)benzimidazol-2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)benzimidazolium hydroxide (TDBC). Compared with SPPs (i.e., photons coupled to highly mobile free electrons at the metal surface), the coupling of photons to less-mobile excitons supports stronger field confinement at the TDBC surface. This extraordinary field confinement of SEPs beyond SPPs motivated us to explore the feasibility of refractive-index (RI) sensing, in particular, surface RI sensing. We find that the bulk RI sensitivity of ${118}^{\circ }$ (${140}^{\circ }$) per refractive-index unit (RIU) of the proposed SEP sensor in the RI range from 1.0 to 1.001 (from 1.33 to 1.36) for a gaseous analyte (an aqueous solution) is higher than that of a conventional gold-based SPP sensor [${73}^{\circ }/$RIU (${124}^{\circ }/$RIU)]. For surface RI sensing, the SEP sensor has both higher sensitivity and a higher figure of merit than the SPP sensor. These numerical results undoubtedly reveal SEPs to be a promising mechanism for sensing.

Asymmetric absorptivity has recently been demonstrated in phase-gradient metasurfaces. Such an effect varies from an integer m, which is the number of discrete unit cells in a supercell. There is still lack of analytical formulas to reveal the relationship between the absorptivity and the value of m. We enrich the previous studies by considering a subwavelength metallic grating at optical frequencies and present a simple and intuitive semianalytical approach to explain the observed asymmetric absorptivity. Our theory clearly indicates that the phenomenon of m dependent asymmetric absorptivity is very robust and can be observed in any grating with a phase gradient. The mechanism is that the phase gradient ensures an additional process of multiple total internal reflections inside the gratings, leading to more dissipation and an asymmetric diffraction response.