Influences of point defects on electron transport of two-dimensional gep semiconductor device.

The quantum transport properties of defective two-dimensional (2D) GeP semiconductor nanodevice consisting of typical point defects, such as antisite defect, substitutional defect, and Schottky defect, have been studied by using density functional theory combined with non-equilibrium Green's function calculation. The antisite defect has indistinctive influences on electron transport. However, both substitutional and Schottky defect have introduced promising defect state at the Fermi level, indicating the possibility of improvement on the carrier transport. Our quantitative quantum transport calculations ofI-Vbbehavior have revealed that the electrical characters are enhanced. Moreover, the P atom vacancy could induce significant negative differential resistance phenomenon, and the physical mechanism is unveiled by detailed analysis. The transfer characteristic properties could be prominently improved by substitutional defect and vacancy defect. Most importantly, we have proposed a computational design of GeP-based electronic device with improved electrical performance by introducing vacancy defect. Our findings could be helpful to the practical application of novel 2D GeP semiconductor nanodevice in future.

Simulation of the nonlinear Kerr and Raman effect with a parallel local time-stepping DGTD solver.

In this paper, an efficient discontinuous Galerkin time-domain (DGTD) method is proposed to solve Maxwell's equations for nonlinear Kerr or Raman media. Based on our previous work, an arbitrary high-order derivatives DGTD method with a local time-stepping scheme is introduced for simulating dynamic optical responses in nonlinear dispersive media such that the nonlinear effects do not impose constraints on the stability conditions for linear subdomains. Therefore, the scheme enables the simulations in the nonlinear and linear media regions with independent time-stepping increments, which greatly improves the efficiency of the time-domain analysis. Moreover, by applying an iteration solution scheme, the proposed method preserves the intrinsic local features, which is favorable for the realization of highly parallelized algorithms. Numerical examples demonstrate the accuracy and the efficiency of our proposed method. We believe the proposed method provides an effective tool for numerical analysis of nonlinear optical phenomena.

Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces.

Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high Q-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative Q-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter α of the system. Furthermore, high Q-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a Q-factor of ∼8×104, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of 3.3kW/cm2. This suggests that such high Q-factor LN metasurfaces may be good candidates for practical blue-ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.

Vanadium dioxide based broadband THz metamaterial absorbers with high tunability: simulation study.

With their unprecedented flexibility in manipulating electromagnetic waves, metamaterials provide a pathway to structural materials that can fill the so-called "THz gap". It has been reported that vanadium dioxide (VO2) experiences a three orders of magnitude increase in THz electrical conductivity when it undergoes an insulator-to-metal transition. Here, we propose a VO2 based THz metamaterial absorber exhibiting broadband absorptivity that arises from the multiple resonances supported by a delicately balanced doubly periodic array of VO2 structures and numerically demonstrate that the corresponding absorption behavior is highly dependent on the VO2's THz electrical properties. Considering the phase transition induced dramatic change in VO2's material property, the proposed metamaterial absorbers have the potential for strong modulation and switching of broadband THz radiation.

Discontinuous Galerkin time domain analysis of electromagnetic scattering from dispersive periodic nanostructures at oblique incidence.

An efficient discontinuous Galerkin time domain (DGTD) method with a generalized dispersive material (GDM) model and periodic boundary conditions (PBCs), hereto referred to as DGTD-GDM-PBCs, is proposed to analyze the electromagnetic scattering from dispersive periodic nanostructures. The GDM model is utilized to achieve a robust and accurate universal model for arbitrary dispersive materials. Using a transformed field variable technique, PBCs are introduced to efficiently truncate the computational domain in the periodic directions for both normally and obliquely incident illumination cases. Based on the transformed Maxwell's equations with PBCs, the formulation of the DGTD method with a GDM model is derived. Furthermore, a Runge-Kutta time-stepping scheme is proposed to update the semi-discrete transformed Maxwell's equations and auxiliary differential equations (ADEs) with high order accuracy. Numerical examples for periodic nanostructures with dispersive elements, such as reflection and transmission of a thin film, surface plasmon at the interfaces of a metallic hole array structure, and absorption properties of a dual-band infrared absorber are presented to demonstrate the accuracy and capability of the proposed method.

Continuous-discontinuous Galerkin time domain (CDGTD) method with generalized dispersive material (GDM) model for computational photonics.

The discontinuous Galerkin time domain (DGTD) method and its recent flavor, the continuous-discontinuous Galerkin time domain (CDGTD) method, have been extensively applied to simulations in the radio frequency (RF) and microwave (MW) regimes due to their inherent ability to efficiently model multiscale problems. We propose to extend the CDGTD method to nanophotonics while exploiting its advantages which have already been established in the RF and MW regimes, such as domain decomposition, non-conformal meshing, high-order elements, and hp-refinement. However, at optical frequencies many materials are highly dispersive, so the modeling of nanophotonic devices requires accurate handling of different dielectric functions, including those of plasmonic elements, dielectrics, and tunable materials. In this paper, we propose a CDGTD method that incorporates a generalized dispersive material (GDM) model which is an efficient way to implement a wide range of optical dispersion models with a universal analytic function. Physics-based dispersion models, such as the Drude, Debye, Lorentz, and critical points as well as more complicated behavior founded on ab-initio principles can all be obtained as special cases of the universal GDM approach. The accuracy and convergence of this GDM-incorporated CDGTD are verified by numerical examples. The CDGTD method, equipped with the GDM model, paves the way to the efficient design and optimization of large scale photonic devices with a diverse range of optical dispersive materials.