Jiang-Tao Liu

Enhanced absorption of graphene with one-dimensional photonic crystal

The optical absorption of graphene layers prepared on top of a one-dimensional photonic crystal (1DPC) is investigated theoretically. The absorption of graphene with 1DPC is enhanced greatly over a broad spectral range due to photon localization. The absorption of graphene can also be tuned by varying either the incident angle or the distance between the graphene and the 1DPC.

Enhanced absorption of monolayer MoS2 with resonant back reflector

The optical absorption of monolayer MoS2 on top of one-dimensional photonic crystal (1DPC) or metal films with spacer layers is theoretically investigated by extracting the permittivity of monolayer MoS2 from existing experimental results [K. F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010)]. The absorption of graphene with 1DPC across a broad spectral range is substantially enhanced because of the photonic localization at the optical micro-cavity on top of the 1DPC or metal films. The absorption of monolayer MoS2 can be tuned by varying either the distance between the monolayer MoS2 and the back reflector or the thickness of the cover layers.

Omnidirectional bandgaps in Fibonacci quasicrystals containing single-negative materials.

The band structure and bandgaps of one-dimensional Fibonacci quasicrystals composed of epsilon-negative materials and mu-negative materials are studied. We show that an omnidirectional bandgap (OBG) exists in the Fibonacci structure. In contrast to the Bragg gaps, such an OBG is insensitive to the incident angle and the polarization of light, and the width and location of the OBG cease to change with increasing Fibonacci order, but vary with the thickness ratio of both components, and the OBG closes when the thickness ratio is equal to the golden ratio. Moreover, the general formulations of the higher and lower band edges of the OBG are obtained by the effective medium theory. These results could lead to further applications of Fibonacci structures.

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.

Enhanced visibility of graphene: Effect of one-dimensional photonic crystal

We investigate theoretically the light reflectance of a graphene layer prepared on the top of one-dimensional Si/SiO2 photonic crystal (1DPC). It is shown that the visibility of the graphene layers is enhanced greatly when 1DPC is added, and the visibility can be tuned by changing the incident angle and light wavelengths. This phenomenon is caused by the absorption of the graphene layer and the enhanced reflectance of the 1DPC

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.

Tunable giant Faraday rotation of exciton in semiconductor quantum wells embedded in a microcavity

The Faraday rotation of an exciton in a GaAs quantum well (QW) embedded in a microcavity is investigated theoretically. The authors find that the Faraday rotation is enhanced remarkably by the microcavity, with a magnitude about two orders of magnitude larger than that of a single QW without microcavity. The Faraday rotation can be tuned by changing the incident angle of the pump and probe lights, or by varying the temperature or an external electric field. With an appropriate detuning between the cavity mode of the pump and probe lights, the Faraday rotation spectrum displays a strongly asymmetric line shape, which can easily be detected experimentally.

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.