Metasurfaces have shown great potential to generate Airy beams, but the available schemes with dielectric and plasmonic metasurfaces suffer from a lack of amplitude modulation and/or low transmission efficiency. Huygens’ metasurfaces have the capability of manipulating electromagnetic wave fronts without reflection, and hence have offered a robust approach to exploit empowered functionalities, such as reflecting, refracting, and focusing plane waves. Here we propose a Huygens’ metasurface structure comprised of a metal capacitor sandwiched between double-pair U-shaped ring resonators, which is capable of independently controlling the electric and magnetic dipole responses. The electric admittance and magnetic impedance can be tuned within a wide range, which enables us to achieve nearly arbitrary transmission amplitude and phase for the transmitted waves. The meta-atom of Huygens’ metasurface can be designed to have ultrahigh transmission amplitude (nearly 100% for the maximum value), and the resultant Airy-beam generators assembled with the Huygens’ metasurfaces present high transmission efficiency. We theoretically design transmissive Airy-beam generators that require polynary amplitude and binary phase distributions, and experimentally verify their excellent performances in terms of transmission efficiency. Our results, based on the independent control of electric admittance and magnetic impedance, can stimulate making high-efficiency wave-front manipulation devices with diversified functionalities in different frequency domains.

This work shows that, counterintuitively, adding extremely small ($<10$ nm in dimension) fully absorbing metal nanoparticles to a material can notably enhance its light emission. It also shows that the mystery behind the unexpected large enhancements in surface-enhanced Raman spectroscopy, even greater than a factor of ${10}^{10}$, is the tunneling out of photons from the strongly absorbing metal surface. Dynamics in a strongly coupled emitter-matter quantum system demands such behavior in the case of limiting small metal nanoparticles and proximal metal surfaces. This effect can be exploited further in light generation, optical sensing, and radiative heat transfer.

We measure the temperature dependence of the two-photon absorption and optical Kerr nonlinearity of a silicon waveguide over a range of temperatures from 5.5 to 300 K. Measurements are taken at a wavelength of 1.55 $\mu \mathrm{m}$ in the technologically important telecom $\mathrm{C}$ band. We observe a near halving (45% reduction) of the two-photon absorption coefficient at low temperature, whereas a smaller reduction in the Kerr nonlinearity of 25% is found. The increased ratio of Kerr to absorptive nonlinearity at low temperatures indicates an improved operation of integrated photonic devices that make use of a nonlinear phase shift, such as optical switches or parametric photon-pair sources. As an example, we examine how the heralding efficiency of a photon-pair source will change at low temperatures and predict a modest improvement in source performance. In addition, the modeling and experimental techniques developed can readily be extended to other wavelengths or materials of interest.

Nonlinear phenomena are a frequent occurrence in nature and inherent to many optical systems. Nevertheless, nonlinearities are often avoided or suppressed in optical resonators. Here we demonstrate the application of nonlinear phenomena to pump-probe spectroscopy by using period doubling in a fiber-feedback optical parametric oscillator. On the basis of an analysis of the optical single-pulse spectra and successive second-harmonic spectra, we introduce a method to enhance the pulse-energy modulation depth to the level of conventional high-frequency modulators. We find that the modulator is suitable for high-speed, low-noise stimulated Raman scattering imaging with performance equal to that of acousto-optic modulation. The concept removes the need for electronic synchronization, reduces the detection effort and artifacts, and has great potential in terms of scalability of the modulation frequency.

Self-assembly of colloids into superstructures is very important for fabricating functional devices. Placement of colloids on predesigned positions, however, remains a major challenge. We report a colloidal system consisting of liquid-crystal droplets dispersed in an isotropic glycerol fluid. The liquid-crystal droplets possess anisotropic dielectric and optical properties and can respond to external stimuli such as electric field and temperature. At high temperature, the two components mix homogeneously to form a single isotropic phase. When temperature is decreased, the liquid-crystal phase separates to form isolated droplets. We discover that the liquid-crystal droplets self-assemble into periodic superstructures in cells with patterned Indium-Tin-Oxide (ITO) electrodes due to the different surface tensions of the liquid crystal and glycerol on ITO and glass. This self-assembly of the liquid-crystal droplets enabled by patterned ITO electrodes opens the door to many alternative applications in functional devices such as switchable photonic crystals, sensors, and microlens arrays.

Waveguides consisting of artificial media based on periodic structures at the subwavelength scale are a major open topic in contemporary applied physics and engineering. Recent research efforts focus on the properties of guided modes in artificial structures. However, as the cornerstone of applications, matching techniques between these interesting waveguides and traditional waveguides deserve more attention. We report a broadband adiabatic mode-matching technique for efficient coupling between conventional microwave transmission lines (a coplanar waveguide and a slotline) and one-dimensional (1D) interface waveguides consisting of transverse-electric (TE) and transverse-magnetic (TM) artificial impedance surfaces. The transverse electromagnetic (TEM) mode is adiabatically transformed to the line wave mode at the 1D impedance-interface. Proposed matching techniques open up avenues for applications of various impedance-interface waveguides.

In recent years different setups based on external cavities have been developed, in order to enhance the performance of pulsed semiconductor mode-locked lasers, namely the peak power and width of the emitted pulses. Depending on the operating conditions and the cavity configuration, pulse cluster solutions emerge with a nonidentical temporal interpulse spacing, which limit the performance of such devices. In this work we present a system of multidelay differential equations to describe the dynamics of a passively mode-locked vertical-external-cavity surface-emitting laser with V-shaped cavity geometry, that allows for an effective modeling and detailed studies of parameter dependencies. We apply numeric integration as well as path-continuation methods to understand the underlying bifurcation scenarios and hence the parameter regions of stable operation. Our investigations indicate that pulse cluster solutions emerge along the fundamental periodic solution branch with a critical influence of the cavity round-trip time on the number of pulses in the cluster. We find regions of multistability of higher-order pulse clusters and predict how different types of dynamics can be favored by tuning the cavity geometry.

Recently, metamaterials-inspired diffraction gratings (or metagratings) have demonstrated unprecedented efficiency in wavefront manipulation by means of relatively simple structures. Conventional one-dimensional (1D) gratings have a profile modulation in one direction and a translation symmetry in the other. In 1D metagratings, the translation invariant direction is engineered at a subwavelength scale, which allows one to accurately control polarization line currents and, consequently, the scattering pattern. In bright contrast to metasurfaces, metagratings cannot be described by means of surface impedances (or local reflection and transmission coefficients). In this paper, we present a simulation-based design approach to construct metagratings in the “unit cell by unit cell” manner. It represents an analog of the local periodic approximation that has been used to design space-modulated metasurfaces and allows one to overcome the limitations of straightforward numerical optimization and semianalytical procedures that have been used to date to design metagratings. Electric and magnetic metagrating structures responding to, respectively, transverse electric and transverse magnetic incident plane waves are presented to validate the proposed design approach.

In this work, our first-principles calculations reveal that direct wave-vector excitation (i.e., interband transitions between different wavenumbers without phonon assistance) can occur in the indirect-band-gap semiconductor silicon. The wave-vector excitation is successfully induced by irradiation of a silicon thin film with an optical near-field (ONF). As a result, the absorption-band-edge energy $E_{\mathrm{edge}}$ shifts to a lower photon energy of 1.6 eV for ONF excitation from $E_{\mathrm{edge}}$ of 2.1 eV for the conventional excitation by propagating far-field light. The direct wave-vector excitation is caused by the sufficiently large components of wave vectors inherent in the ONF, and thus does not require phonon assistance. For a realistic silicon system, it is clarified that the wave-vector excitations are determined by the energy difference between the valence and conduction bands and occur irrespective of the initial and final wave vectors.

We experimentally demonstrate a random distributed feedback fiber laser generating cylindrical vector beams via a mode-selective coupler and a fiber ring resonator with 50-m-long single-mode fiber. Because of random distributed feedback provided by the single-mode fiber, multimode interactions are suppressed in the cavity, leading to modeless output. Azimuthally and radially polarized beams can both be obtained by adjustment of the polarization controllers. Speckle contrast measurements are made to confirm the effectiveness of the designed laser in reducing laser speckle. Apart from the low cost and simple technology of such a laser, the cylindrical vector beams with modeless behavior may have applications in areas such as laser radars, free-space communication, remote sensing, and biomedical imaging.

Controlling spin-to-orbital angular momentum (OAM) conversion plays an important role in many optical and wireless communication applications. While independent control of different circular-polarized (CP) wavefronts could offer an ultimate degree of freedom in designing advanced multifunctional spin devices, most metasurfaces only provide dependent wavefront control. We propose a reflective dual-helicity decoupled coding metasurface to completely realize independent control of OAM vortices for two orthogonal helicities. The element combines both the propagation phase and geometric phase, thus overcoming the inherent limitation encountered by conventional geometric phase elements whose phase responses are constrained to be opposite values for different CP wavefronts. Several design examples for independently generating OAM vortices in the microwave region are presented where a free combination of different OAM modes, helicity, as well as complex spatial beam editing can be fully achieved. Experiments show good agreements with the full-wave simulations, successfully verifying the design theory. The proposed method could offer an alternative platform for designing high-performance devices with independent CP wavefront manipulations, which may enhance the information capacity of metasurfaces and trigger versatile electromagnetic (em) wave function integrations for advanced compact systems.

Metamaterials have generated considerable research interest in manipulating light polarization states, but many proposals suffer from the issue of narrow bandwidth and/or low transmission efficiency. We show that a classical multiple-beam interference mechanism can be employed to modulate the phase dispersion of transmitted waves in metamaterials, which motivates us to establish a general strategy to achieve a constant cross-polarization phase difference by independently controlling the phase dispersion for the two orthogonal polarizations. We derive a criterion under which the cross-polarization phase difference across the metamaterial devices can be maintained at a nearly constant value over a wide frequency band. The codesigned metamaterial quarter-wave plate presents an excellent polarization conversion performance over a broad frequency band. We also employ full-wave simulations to demonstrate a broadband quarter-wave plate at telecom and terahertz frequencies based on the proposed design strategy. Our findings can stimulate making high-performance broadband optical devices based on various metamaterials in different frequency domains, and impact numerous photonic applications.

Combining solid-state single-photon emitters (SPEs) with nanophotonic platforms is a key goal in integrated quantum photonics. In order to realize functionality in potentially scalable elements, suitable SPEs have to be bright, stable, and widely tunable at room temperature. In this work, we show that selected SPEs embedded in a few-layer hexagonal boron nitride ($h$-BN) meet these demands. In order to show the wide tunability of these SPEs we employ an atomic force microscope (AFM) with a conductive tip to apply an electrostatic field to individual $h$-BN emitters sandwiched between the tip and an indium-tin-oxide-coated glass slide. A very large and reversible Stark shift of $(5.5\pm 0.3)\mathrm{nm}$ at a zero-field wavelength of 670 nm is induced by applying just 20 V, which exceeds the typical resonance linewidths of nanodielectric and even nanoplasmonic resonators. Our results help to further understand the physical origin of SPEs in $h$-BN as well as for practical quantum photonic applications where wide spectral tuning and on/off resonance switching are required.

A model of nonlinear optical interaction in the external cavity of a semiconductor disk laser is derived and numerically studied. The model is formalized in a system of differential equations with time delay (TD). The steady-state operation point, its stability, and transient dynamics of intracavity singly resonant optical parametric oscillators (ICSROs) are analyzed in the framework of this model. Also, the same quantities are studied with the help of the normal-mode expansion (NME) method. Both models result in the same steady-state solution, while the stability and dynamic behavior appear to be quite different. The characteristic equation defining the steady-state stability of the TD dynamic system is shown to have an infinite number of roots that may be grouped into two sets of complex-conjugate pairs. For the TD model, the Hopf bifurcation occurs when the primary pump exceeds the bifurcation value. The characteristic equation corresponding to the NME model in the single-mode approximation has only three roots with the real parts not exceeding zero; i.e., the steady-state operation point maintains stability for all values of the primary pump used in the simulation.

Scalable spin-to-photon interfaces require quantum emitters with strong optical-transition dipole moment and low coupling to phonons and stray electric fields. It is known that particularly for coupling to stray electric fields, these conditions can be simultaneously satisfied for emitters that show inversion symmetry. Here, we show that inversion symmetry is not a prerequisite criterion for a spectrally stable quantum emitter. We find that identical electron density in ground and excited states can eliminate the coupling to the stray electric fields. Further, a strong optical-transition dipole moment is achieved in systems with altering sign of the ground and excited wave functions. We use density-functional perturbation theory to investigate an optical center that lacks inversion symmetry. Our results show that this system close to ideally satisfies the criteria for an ideal quantum emitter. Our study opens an additional rationale in seeking promising materials and point defects towards the realization of robust spin-to-photon interfaces.

We generalize normal mode expansion of Green’s tensor $\overline{\overline{G}}(\mathit{r},{\mathit{r}}^{\prime })$ to resonators in open systems, resolving a long-standing open challenge. We obtain a simple yet robust formulation whereby radiation of energy to infinity is captured by a complete, discrete set of modes rather than a continuum. This enables rapid simulations by providing the spatial variation of $\overline{\overline{G}}(\mathit{r},{\mathit{r}}^{\prime })$ over both $\mathit{r}$ and ${\mathit{r}}^{\prime }$ in one simulation. Systems with or without material losses can be treated. Few eigenmodes are often necessary for nanostructures, facilitating both analytic calculations and unified insight into computationally intensive phenomena such as Purcell enhancement, radiative heat transfer, van der Waals forces, and Förster resonance energy transfer. We bypass all implementation and completeness issues associated with the alternative quasinormal eigenmode methods by defining modes with permittivity rather than frequency as the eigenvalue. We obtain true stationary modes that decay rather than diverge at infinity, and are trivially normalized. Completeness is achieved both for sources located within the inclusion and the background through use of the Lippmann-Schwinger equation. Modes are defined by a linear eigenvalue problem, readily implemented with any numerical method. We demonstrate its simple implementation with comsol multiphysics using the default inbuilt tools. The results are validated against direct scattering simulations, including analytic Mie theory, attaining arbitrarily accurate agreement regardless of source location or detuning from resonance.

Frequency combs based on nonlinear optical phenomena in integrated photonics are a versatile light source that can explore new applications, including frequency metrology, optical communications, and sensing. We demonstrate robust frequency-control strategies for near-infrared, octave-bandwidth soliton frequency combs created with nanofabricated silicon nitride ring resonators. Group-velocity-dispersion engineering allows operation with a 1064-nm pump laser and generation of dual-dispersive-wave frequency combs linking wavelengths approximately between 767 and 1556 nm. To tune the mode frequencies of the comb, which are spaced by 1 THz, we design a photonic chip containing 75 ring resonators with systematically varied dimensions and we use a thermo-optic tuning range of $50{}^{\circ }\mathrm{C}$. This single-chip frequency-comb source provides access to every wavelength, including those critical for near-infrared atomic spectroscopy of rubidium, potassium, and cesium. To make this possible, solitons are generated consistently from device to device across a single chip with use of rapid pump-frequency sweeps that are provided by an optical modulator.

Energy transport is of central importance in understanding a wide variety of transitions of physical states in nature. Recently, the coherence and noise have been identified for their existence and key roles in energy-transport processes, for instance, in a photosynthesis complex, DNA, and odor sensing etc., of which one may have to reveal the inner mechanics in the quantum regime. Here we present an analog of Newton’s cradle by manipulating a boundary-controlled chain on a photonic chip. Long-range interactions can be mediated by a long chain composed of $21$ strongly coupled sites, where single-photon excitations are transferred between two remote sites via simultaneous control of intersite weak and strong couplings. We observe a high retrieval efficiency in both uniform and defect-doped chain structures. Our results may offer a flexible approach to Hamiltonian engineering beyond geometric limitation, enabling the design and construction of quantum simulators on demand.

We theoretically study a graphene-based plasmonic waveguide composed of graphene, hexagonal boron nitride and active layer heterostructure, which can gate the transmission of a surface plasmon polariton (SPP) localized near the interface between the active layer and graphene. When a gate voltage is applied above a certain critical value, the charge-density modulation in the quasi-2D electron gas formed in the inversion layer can induce a local plasmon resonance. Since the local plasmon resonance is strongly coupled to the SPP, it can suppress the transmission of the SPP. The main advantage of our device lies in the sharpness of the switching line shape, since the operation manifests the underlying resonance phenomenon. By taking the active layer to be the $p$-type $\mathrm{Si}$(100) layer, we calculate the propagation length of the SPP with varying gate voltage. We show that the wavelength of the SPP is reduced to approximately $1/100$ of that of an incident light and the propagation length of the SPP is significantly reduced by a factor of approximately $20$ upon switching. This ensures that our plasmonic waveguide can operate effectively as a plasmonic switch for the SPP.

The conventional approach in transformation optics (TO) starts with a virtual space and determines a complicated material profile in physical space to achieve unconventional phenomena, such as invisibility cloaks. We demonstrate that complicated materials can be effectively fabricated by virtual space based on the reversed process of TO in two dimensions. We first show that a conformal singularity of a refractive-index profile, with either zero or infinity resulting from a power conformal mapping $w=z^{\alpha }$, is equivalent to a topological defect with positive charge or negative charge. The splitting effect and illusion effect are induced by the conformal singularities and related to the two-dimensional topological defects. Based on the equivalence, we fabricate a device of such a topological defect with a positive charge in experiments. Moreover, we observe its related light-bending functionality with laser beams. It has potential important applications in designing on-chip optical devices.