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Abstract The non-ionizing and penetrative characteristics of terahertz (THz) radiation have recently led to its adoption across a variety of applications. To effectively utilize THz radiation, modulators with precise control are imperative. While most recent THz modulators manipulate the amplitude, frequency, or phase of incident THz radiation, considerably less progress has been made toward THz polarization modulation. Conventional methods for polarization control suffer from high driving voltages, restricted modulation depth, and narrow band capabilities, which hinder device performance and broader applications. Consequently, an ideal THz modulator that offers high modulation depth along with ease of processing and operation is required. In this paper, we propose and realize a THz metamaterial comprised of microelectromechanical systems (MEMS) actuated by the phase-transition material vanadium dioxide (VO2). Simulation and experimental results of the three-dimensional metamaterials show that by leveraging the unique phase-transition attributes of VO2, our THz polarization modulator offers notable advancements over existing designs, including broad operation spectrum, high modulation depth, ease of fabrication, ease of operation condition, and continuous modulation capabilities. These enhanced features make the system a viable candidate for a range of THz applications, including telecommunications, imaging, and radar systems. Graphical Abstract

As a 2-D full-wave numerical algorithm in the time domain, the compact Finite Difference Time Domain (FDTD) is an efficient algorithm for eigenvalue analysis of optical waveguide system. However, the numerical dispersion accuracy and stability of fast algorithm need to be improved while simulating at high frequency. A novel high-order symplectic compact FDTD scheme is developed and validated for optical waveguide modal analysis. The stability condition and the numerical dispersion of schemes with fourth-order accuracy in temporal and spatial using the symplectic integrator and compact scheme are analyzed. By comparisons with other time-domain schemes, their stable and accurate performance is qualitatively verified. The proposed high-order SC-FDTD method can be used for efficiently simulating electrically large and longitudinally invariant optical devices since the reduction of simulation dimensionality and the novel high-order symplectic algorithm can greatly reduce the memory cost and the numerical dispersive errors.

Random phase screen method was used to simulate the light intensity and phase distribution of the synthesized double vortex beams transmitted in atmospheric turbulence given the light intensity characteristics and phase characteristics of double Laguerre–Gaussian beams (DLGB) generated by coaxial superposition. Results show that the scintillation coefficient decreases initially, and then increases with the topological charge difference of the double vortex beam, and the topological charge difference is related to the number of split spots. When the number of split spots is small, the intensity distribution is concentrated, exhibiting a good penetrating effect on atmospheric turbulence.

All-dielectric metasurfaces offer unconventional optical functional behaviors for manipulating light, due to the strong localization of light with negligible dissipative loss. Here, we propose a compact design for a light emission cavity consist of an all-dielectric grating reflector and an all-dielectric symmetry-breaking metasurface supporting “dark” mode. The “dark” mode of the asymmetric metasurface structure results in high quality factor, and the broadband perfect reflector leads to directional emission. We present a theoretical study of the spectral characterization of the emission cavity, and the emission property of the nanocavity embedded with gain medium is investigated by a Maxwell-Bloch Langevin approach. Strong luminescence enhancement with directive light emission and reduced lasing threshold are observed. Our observations offer enormous potential in controlling light emission and provides an essential step for developing efficiency and directionality LEDs, and ultra-compact lasers.

The perovskite light-emitting diodes (PeLEDs) have attracted considerable research interests in recent years due to their decent and tunable optoelectronic characteristics. Here, we unveil the properties of the emission source (e.g., location, polarization, etc.) and the influences of the incorporated nanopatterns on the light emission of PeLEDs by using the finite difference time domain (FDTD) method. The results indicated that the PeLEDs with nanopatterns not only show substantial improvement of light extraction intensity but also reveal a remarkable directivity. Moreover, the emission angle of the directional PeLEDs can be continuously engineered over 65o by judicious selection of the nanopatterns. This work is of great importance for engineering highly directional and efficient luminescence devices.

All-dielectric metasurfaces have been widely used in nanophotonics because of their properties, such as negligible Ohmic loss, stable properties, and flexible fabrication techniques. We propose an all-dielectric metasurface consisting of subwavelength particles, which supports a “trapped” mode after the introduction of adequate symmetry-breaking in the silicon meta-atoms. Coating the metasurface with monolayer graphene enables the adsorption of nucleic acids such as DNA and RNA. We compare the resonant states in the transmission spectrum and find that the proposed metasurface is ultra-sensitive to the surrounding environment and exhibits excellent biosensing performance. Our conclusions indicate that all-dielectric metasurfaces can be a promising platform to realize ultra-sensitive bio-sensors.

Quasi-bound states in the continuum (quasi-BIC) are a particular resonant state, which can be regulated by the degree of symmetry breaking in nanostructures. Here, we propose a fourfold rotationally symmetric (C4v) metasurface supporting the dual quasi-BIC modes. The Fano characteristics have observed in the near-infrared region. The resonant peaks of the dual quasi-BIC modes can be adjusted flexibly and independently with a simple breaking of the structural symmetry. More importantly, the dual quasi-BIC modes demonstrate the extraordinary capability in controlling the optical activity. This work will offer us more freedom for controlling the resonance and optical activity by the quasi-BIC modes, which is promising to engineer the optical device in displaying and optics communications.

Metallic gratings have drawn great attention over the past few years due to effective light trapping and power conversion efficiency enhancement in organic solar cells (OSCs). Novel OSCs incorporated short-pitch metallic grating into the conventional OSCs as the back electrode is presented. The optical and electrical properties of OSCs are investigated by applying the finite-difference frequency-domain method. It is found that the light absorption at 400-650 nm in the active layer is strongly improved, compared with the standard structure without metallic grating. The underlying physical mechanism is due to the surface plasmon resonance and the interaction between two neighbor gratings, which undoubtedly enhances the electrical properties such as short-circuit current (JSC), fill factor, and power conversion efficiency. Meanwhile, the optical and electrical properties of OSCs with different groove widths are also investigated. With an optimal groove width of 4 nm, the overall performance of OSCs is enhanced. This paper indicates that OSCs incorporated short-pitch metallic grating are promising candidates for high-performance OSCs.

The influence of ohmic (metallic) loss on the complex band structure (BS) and eigenmodes of 2-D plasmonic crystals is investigated. With the help of wave equations and periodic boundary conditions, a finite-difference-based eigenvalue algorithm is proposed to model the plasmonic crystals with arbitrarily lossy and dispersive materials. Given a frequency of interests, the algorithm solves one complex Bloch wavenumber as the eigenvalue via fixing another. Most importantly, the developed eigenvalue analysis could expand the bulk excitation solution with eigenmodes, which satisfies the generalized phase (momentum) matching condition. For a TE polarization with Hz field, the ohmic loss strongly affects the BS and eigenmodes at plasmonic resonance frequencies. Both the fast oscillation of a dispersion curve and strong field confinement of eigenmodes are damped due to the high ohmic loss. For a TM polarization with Ez field, the introduction of ohmic loss twists the vertical dispersion curve at the bandgap and breaks the symmetry of the eigenmodes. For both polarizations, the high ohmic loss lowers the quality factor of the eigenmodes. This paper offers a fundamental and important eigenvalue analysis for designing lossy and dispersive plasmonic crystals.