Xin-Hua Deng

Tunable THz absorption in graphene-based heterostructures

We investigate THz absorption properties of graphene-based heterostructures by using characteristics matrix method based on conductivity. We demonstrate that the proposed structure can lead to perfect THz absorption because of strong photon localization in the defect layer of the heterostructure. The THz absorption may be tuned continuously from 0 to 100% by controlling the chemical potential through a gate voltage. By adjusting the incident angle or the period number of the two PCs with respect to the graphene layer, one can tailor the maximum THz absorption value. The position of the THz absorption peaks can be tuned by changing either the center wavelength or the thicknesses ratio of the layers constituting the heterostructure. Our proposal may have potentially important applications in optoelectronic devices.

Gate-tunable nearly total absorption in graphene with resonant metal back reflector

The gate-tunable absorption of graphene layers with a resonant metal back reflector (RMBF) is theoretically investigated. We demonstrate that the absorption of graphene with RMBF can vary from nearly negligible to nearly total by tuning the external gate voltage within the terahertz (THz) spectra range. Total THz absorption is less affected by the incident angle of THz beams. This peculiar nearly total THz absorption can be attributed to the Fabry-Perot cavity effect, which enhances the absorption and reduces the reflection of graphene. The absorption spectra of the graphene-RMBF structure can also be tailored in bandwidth and center frequency by changing the thickness and dielectric constant of the spacer layer. These findings can lead to the development of tunable THz photonic devices and have potential applications in studies on the ultrafast dynamics of Dirac fermions in graphene.

Enlargement of polarization-independent omnidirectional band gaps in the photonic heterostructures containing single-negative materials

We show that the frequency range of the zero effective phase gap in a photonic heterostructure containing single-negative materials can be enlarged owing to the property that its lower and upper frequency edges depend on the thickness ratio of the epsilon-negative and mu-negative materials. Compared to the zero effective phase gap of a single photonic crystal, the frequency range of the zero effective phase gap in a photonic heterostructure can be notably enlarged. Moreover, it is shown that the band edges of the zero effective phase gap is insensitive to incident angle and polarization of light.

Optical field modulation on the group delay of chiral tunneling in graphene

The influences of optical fields on the group delay of chiral tunneling in graphene are investigated in real time using the finite-difference time-domain method. The group delay of tunneling electrons irradiated by an optical field is significantly different from that observed in traditional quantum tunneling. We found that when the barrier width increases, the group delay becomes constant for the reflected wave packet, but increases linearly for the transmitted wave packet. This peculiar tunneling effect can be attributed to current leakage in a time-dependent barrier generated via the optical Stark effect.Jiang-Tao Liu, ∗ Fu-Hai Su, Hai Wang, and Xin-Hua Deng Department of Physics, Nanchang University, Nanchang 330031, China Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China The Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Department of Physics, Capital Normal University, Beijing 100037, China (Dated: July 23, 2010)

Broadband ultra-high transmission of terahertz radiation through monolayer MoS2

In this study, the terahertz (THz) absorption and transmission of monolayer MoS2 with different carrier concentrations were investigated theoretically. The calculation shows that the THz absorption of monolayer MoS2 is very low even under high carrier concentrations and large incident angles. The sum of reflection and absorption losses of monolayer MoS2 is lower than that of graphene by one to three orders of magnitude. The transmission of monolayer MoS2 is higher than that of two-dimensional electron gases in traditional GaAs and InAs. The field-effect tube structure formed by monolayer MoS2-insulation-layer-graphene is also studied. The THz absorption of graphene can reach saturation under low voltage by tuning the voltage between MoS2 and graphene layers in the structure. The maximum THz absorption of monolayer MoS2 is approximately 5%. Thus, monolayer MoS2 is a promising candidate for THz transparent electrodes.

Perfect light trapping in nanoscale thickness semiconductor films with a resonant back reflector and spectrum-splitting structures

The optical absorption of nanoscale thickness semiconductor films on top of light-trapping structures based on optical interference effects combined with spectrum-splitting structures is theoretically investigated. Nearly perfect absorption over a broad spectrum range can be achieved in <100 nm thick films on top of a one-dimensional photonic crystal or metal films. This phenomenon can be attributed to interference induced photonic localization, which enhances the absorption and reduces the reflection of the films. Perfect solar absorption and low carrier thermalization loss can be achieved when the light-trapping structures with a wedge-shaped spacer layer or semiconductor films are combined with spectrum-splitting structures.

Power splitter based on photonic crystal waveguides with an air holes array

We propose a new type of 3 dB optical power splitter by splitting reflected field in photonic bandgaps (PBGs) based on photonic crystal waveguides (PCWs) with a triangular lattice of air holes. By employing PBGs effect, the propagation fields with frequencies under the cutoff frequency of guided modes in a modulated PCW (MPCW) are reflected back and output from two symmetrically placed waveguides. As a consecutive section of input PCW, the MPCW is realized by varying the size of the border holes adjacent to the PCW core. A nearly full transmission with a wide bandwidth is achieved only by introducing a defect hole in the MPCW. It provides a new method and a compact model to split input power in PCW devices and can find practical applications in future photonic integrated circuits.

Bloch oscillations in one-dimensional coupled multiple microcavities containing negative-index materials

We propose a one-dimensional (1D) coupled multiple microcavities (MMCs) structure containing negative-index materials. We obtain the photonic Wannier–Stark ladders (WSLs) both in the ordinary and the well-known omnidirectional band gaps by modulating the widths of the cavities. Due to the omnidirectional gaps having novel characteristics compared with the Bragg gaps, it is meaningful to study the WSL that lies in the omnidirectional gap. We study the temporal–spatial evolutions of Gaussian pulses whose central frequencies are in both gaps as passing through the proposed structure. We show for the first time that the electromagnetic Bloch oscillations can occur in such photonic structures containing negative-index materials.

The influence of the optical Stark effect on chiral tunneling in graphene

The influences of intense coherent laser fields on the transport properties of a single-layer graphene are investigated by solving the time-dependent Dirac equation numerically. Under an intense laser field, the valence band and conduction band states mix via the optical Stark effect. The chiral symmetry of Dirac electrons is broken and the perfect chiral tunneling is strongly suppressed. These properties might be useful in the fabrication of an optically controlled field-effect transistor.

Proposal for the momentum-resolved and time-resolved optical measurement of the current distribution in semiconductors

The two-color optical coherence absorption spectrum (QUIC-AB) of semiconductors in the presence of a charge current is investigated. We find that the QUIC-AB depends strongly not only on the amplitude of the electron current but also on the direction of the electron current. Thus, the amplitude and the angular distribution of current in semiconductors can be detected directly in real time with the QUIC-AB.The two-color optical coherence absorption spectrum (QUIC-AB) of GaAs quantum well in the presence of a charge current is investigated. We find that the QUIC-AB depends strongly not only on the amplitude of the electron current but also on the direction of the electron current. Thus, the amplitude and the angular distribution of scattering current in the scatter process can be detected directly in real time with the QUIC-AB. The phase shift of scattered waves and the details of the scattering potential can also be determined.