Dynamically Temperature-Voltage Controlled Multifunctional Device Based on VO2 and Graphene Hybrid Metamaterials: Perfect Absorber and Highly Efficient Polarization Converter.

Vanadium dioxide (VO2) is a temperature phase change material that has metallic properties at high temperatures and insulation properties at room temperature. In this article, a novel device has been designed based on the dielectric metasurface consisting of VO2 and graphene array, which can achieve multiple functions by adjusting temperature and voltage. When the temperature is high (340 K), the device is in the absorption state and its absorptivity can be dynamically controlled by changing the temperature. On the other hand, the device is in the polarization state under room temperature, and the polarization of electromagnetic waves can be dynamically controlled by adjusting the voltage of graphene. This device can achieve a broadband absorber (the maximum absorptance reaches 99.415% at wavelengths ranging from 44 THz to 52 THz) and high polarization conversion efficiency (>99.89%) in the mid-infrared range, which has great advantages over other single-function devices. Our results demonstrate that this multifunctional device may have widespread applications in emitters, sensors, spatial light modulators, IR camouflages, and can be used in thermophotovoltaics and wireless communication.

Negative Differential Conductance Assisted by Optical Fields in a Single Quantum Dot with Ferromagnetic Electrodes.

In a single quantum dot (QD) system connected with ferromagnetic electrodes, the electron transport properties, assisted by the thermal and Fock state optical fields, are theoretically studied by the Keldysh nonequilibrium Green's function approach. The results show that the evolution properties of the density of state and tunneling current assisted by the Fock state optical field, are quite different from those of the thermal state. The photon sideband shift decreases monotonously with the increase in the electron-photon coupling strength for the case of the thermal state, while the shift is oscillatory for the case of the Fock state. Negative differential conductance (NDC) appears obviously in a QD system contacted with parallel (P) and antiparallel (AP) magnetization alignment of the ferromagnetic electrode leads, assisted by the Fock state optical field in a wide range of electron-photon interaction parameters. Evident NDC usually only arises in an AP configuration QD system assisted by the thermal state optical field. The results have the potential to introduce a new way to actively manipulate and control the single-electron tunneling transport on a QD system by the quantum states of the optical field.

Full Counting Statistics of Electrons through Interaction of the Single Quantum Dot System with the Optical Field.

In this paper, using the particle-number-resolved master equation, the properties of full counting statistics (FCS) are investigated for a single quantum dot (QD) system interacting with optical fields in the thermal state, Fock state, coherent state, and coherent state with random phase. In these diverse quantum states of optical fields, average tunneling currents have different step shoulder heights at a lower bias voltage with the same light intensity, and a staircase-shaped current can be induced unexpectedly in vacuum state optical field. The characteristics of the Fano factor and skewness in the coherent state differ from those in all of the other cases. For avalanche-like transport at a lower bias voltage, the mechanism is a dynamical channel blockade in a moderate electron⁻photon interaction regime. There is a pronounced negative differential conductance that results from tuning the phase of the coherent state optical field in a symmetric QD system.

Analogue of electromagnetically induced absorption with double absorption windows in a plasmonic system.

We report the observation of an analog of double electromagnetically induced absorption (EIA) in a plasmonic system consisting of two disk resonators side-coupled to a discrete metal-insulator-metal (MIM) waveguide. The finite-difference time-domain (FDTD) simulation calculations show that two absorption windows are obtained and can be easily tuned by adjusting the parameters of the two resonance cavities. The consistence between the coupled-model theory and FDTD simulation results verify the feasibility of the proposed system. Since the scheme is easy to be fabricated, our proposed configuration may thus be applied to narrow-band filtering, absorptive switching, and absorber applications.

Active plasmonic band-stop filters based on graphene metamaterial at THz wavelengths.

Active plasmonic band-stop filters based on single- and double-layer doped graphene metamaterials at the THz wavelengths are proposed and investigated numerically by using the finite-difference time-domain (FDTD) method. The metamaterial unit cell structure is composed of two parallel graphene nanoscale ribbons. Simulated results exhibit that significant resonance wavelength shifts can be achieved with a slight variation of the doping concentration of the graphene ribbons. Besides, the asymmetry double-layer graphene metamaterial device has two apparent filter dips while the symmetry single-, double-layer and asymmetry single-layer graphene metamaterial devices just only one. The metamaterials with symmetry single-layer and asymmetry double-layer graphene can be used as a high-sensitivity refractive sensor with the sensitivity up to 5100 nm/RIU and a two-circuit switch, respectively. These prospects pave the way towards ultrafast active graphene-based plasmonic devices for THz applications.

Double plasmonic nanodisks design for electromagnetically induced transparency and slow light.

An analog of plasmonic system for electromagnetically induced transparency (EIT), in which a small nanodisk with a big side-coupled-nanodisk is directly coupled to the metal-insulator -metal (MIM) waveguide, has been proposed and investigated theoretically and numerically. When the resonant frequencies of the two nanodisks differ not too much, a powerful EIT-like effect can be obtained, and the transparency window can be easily tuned by adjusting the radii of the two nanodisks. The plasmonic device can be used as a high-performance EIT-like filter with transmission over 80% and full width at half-maximum (FWHM) less than 30nm, besides, the novel structure shows a high group index over 355. The system paves a new way toward highly integrated optical circuits and networks, especially for wavelength-selective, ultrafast switching, light storage and nonlinear devices.

Asymmetric band-pass plasmonic nanodisk filter with mode inhibition and spectrally splitting capabilities.

A compact wavelength band-pass filter based on metal-insulator-metal (MIM) nanodisk cavity is proposed and numerically investigated by using Finite-Difference Time-Domain (FDTD) simulations. It is found that the transmission characteristics of the filter can be easily adjusted by changing the geometrical parameters of the radius of the nanodisk and coupling distance between the nanodisk and waveguide. By extending the length of input/output waveguides, the filter shows the resonant mode inhibition function. Basing on this characteristic, a two-port wavelength demultiplexer is designed, which can separate resonant modes inside the nanodisk with high transmission up to 70%. The waveguide filter may become a potential application for the design of devices in highly integrated optical circuits.

Plasmonic filters and optical directional couplers based on wide Metal-Insulator-Metal structure.

The wide Metal-Insulator-Metal (WMIM) structure is proposed and its characteristics are analyzed numerically using finite-difference time-domain (FDTD) method. Simulations show that power can be periodically transferred between its two Metal-Insulator (MI) interfaces while power is injected asymmetrically. Novel plasmonic filters and optical directional couplers (ODCs) based on WMIM structure are proposed, which work similarly as traditional dielectric devices. Due to the simple structures without thin metal gaps, our result may provide an alternative way to realize the fabrication of nanoscale optical devices.