Abstract-Negative terahertz photoconductivity in 2D layered materials

Jing Sun, Hongwei Liu, Junpeng Lu, 

http://iopscience.iop.org/article/10.1088/1361-6528/aa8c28/meta;jsessionid=A36CC6F1D9F2CC0B818FE889E0E02BC8.c4.iopscience.cld.iop.org

The remarkable qualities of 2D layered materials such as wide spectral coverage, high strength and great flexibility mean that ultrathin 2D layered materials have the potential to meet the criteria of next-generation optoelectronic devices. Photoconductivity is one of the critical parameters of materials applied to optoelectronics. In contrast to traditional semiconductors, specific ultrathin 2D layers present anomalous negative photoconductivity. This opens a new avenue for designing novel optoelectronic devices. It is important to have a deep understanding of the fundamentals of this anomalous response, in order to design and optimize such devices. In this review, we provide an overview of the observation of negative photoconductivity in 2D layered materials including graphene, topological insulators and transitional metal dichalcogenides. We also summarize recent reports on investigations into the fundamental mechanism using ultrafast terahertz (THz) spectroscopies. Finally, we conclude the review by discussing the existing challenges and proposing the possible prospects of this direction of research.

Abstact-Tunable terahertz reflection of graphene via ionic liquid gating

Yang Wu, Xuepeng Qiu, Hongwei Liu, Jingbo Liu, Yuanfu Chen, Lin Ke, Hyunsoo Yang

(Submitted on 1 Feb 2017)

https://arxiv.org/abs/1702.00114

We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to 3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1 - 1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components.

Abstract-Tunable terahertz reflection of graphene via ionic liquid gating

Yang Wu1,2,3, Xuepeng Qiu1, Hongwei Liu3, Jingbo Liu4, Yuanfu Chen4, Lin Ke3 and 
Hyunsoo Yang1,2

http://iopscience.iop.org/article/10.1088/1361-6528/aa57ad

We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ±3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1–1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components.

Abstract-Transient Photoconductivity of Ternary CdSSe Nanobelts as Measured by Time-Resolved Terahertz Spectroscopy

Junpeng Lu , Hongwei Liu , Xiaodai Lim , Sing Hai Tang , Chorng Haur Sow , and Xinhai Zhang

http://pubs.acs.org/doi/pdf/10.1021/jp4043599

The frequency and fluence dependent transient photoconductivity in ternary CdSSe nanobelts is investigated using time-resolved terahertz spectroscopy. Carrier density and mobility are extracted by modeling the measured complex photoconductivity using Drude-Smith model. Within the first few picoseconds of excitation, both carrier density and mobility reach their maximum values and then decay gradually over tens to hundreds of picoseconds. The decay of free carriers is mainly attributed to fast surface trapping and structural-defect mediated recombination. The surface trapping saturates rapidly with increasing excitation fluence attributable to the low trapping density on the nanobelt surface caused by self-passivation of surface defects during the growth process.