Abstract-Terahertz Surface Emission from Layered MoS2 Crystal: Competition Between Surface Optical Rectification and Surface Photocurrent Surge

Yuanyuan Huang, Lipeng Zhu, Zehan Yao, Longhui Zhang, Chuan He, Qiyi Zhao, Jintao Bai, and Xin Long Xu


http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b09723?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry–breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary condition. The maximum of surface depletion field is estimated to be 1.45×104 V/cm with 130 nm in depth under -40oincidence. Interestingly, when the incident angle is tuned from -40o to 0o, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs (100). The results afford not only comprehensive understanding of THz radiation from layered materials like MoS2, but also put forward THz emission spectroscopy for characterizing the surface and interface properties of two-dimensional materials.

Abstract-Enhanced Polarization-sensitive Terahertz Emission from Vertical Grown Graphene by Dynamical Photon drag Effect

Lipeng Zhu,  Yuanyuan Huang,  Zehan Yao,  Baogang Quan,  Longhui Zhang ,  Junjie Li,  changzhi gu,  Xin Long Xu  and  Zhaoyu Ren 

http://pubs.rsc.org/en/content/articlelanding/2017/nr/c7nr02227a#!divAbstract

Improving terahertz (THz) emission from graphene is a challenge for graphene-based THz photonics as graphene demonstrates a weak light-matter interaction. With an unique ultra-black surface structure, vertical grown graphene (VGG) is proposed to enhance the light-matter interaction and further enhance THz emission. Herein, enhanced THz radiation is observed by THz time-domain emission spectroscopy from VGG compared with single-layer graphene. The radiated THz amplitude shows a linear dependence on the pump power, which demonstrates a second order nonlinear effect. Considering the symmetry of VGG on substrate, we can exclude the optical rectification effect and photogalvanic effect (PGE) by the D6h point group with centrosymmetry. Thus we analyze the transient photocurrent related to THz emission only by the photon drag effect (PDE). The polarization-sensitive THz radiation signals are wave-vector reliance and demonstrate cos2φ and sin2φ dependence on the polarization angles of the pump laser. This is consistent with the theoretical analysis of PDE. Our results show the enhanced, ultrafast, broadband THz radiation property of VGG, which paves the way for high performance of THz emitter and THz detector based on graphene materials.

Abstract-Anisotropic terahertz response of stretch-aligned composite films based on carbon nanotube-SiC hybrid structures

Ruili Wu,   Weilong Li,   Yun Wan,   Zhaoyu Ren,   Xin Long Xu and  Yixuan Zhou  

RSC Adv., 2015, Accepted Manuscript

DOI: 10.1039/C4RA14871A
Received 19 Nov 2014, Accepted 09 Mar 2015
First published online 10 Mar 2015
http://pubs.rsc.org/en/content/articlelanding/2015/ra/c4ra14871a#!divAbstract

Well-organized CNT-SiC hybrid structures were prepared by floating catalytic chemical vapor deposition (CCVD) process. The aligned CNTs-based composite materials were also fabricated by simply stretching. Gelatin and CNT-SiC hybrid structures were taken as the matrix and fillers, respectively. The alignment of CNT-SiC hybrids and anisotropic properties of the composite materials were discussed by the detection of THz time-domain spectroscopy technology. The results indicated that the anisotropic properties of CNT-SiC hybrids based composite films are better than the case of pure MWCNTs. The unique structure of hybrids not only largely favors the dispersion of CNTs in polymer matrix, but also favors the alignment of CNTs in the composites by stretching. The CNTs-based composite films with further improving the anisotropy are highly desirable for various modern applications in the electrical and optical industry (such as the polarizer).

Abstract-Solution-processable reduced graphene oxide films as broadband terahertz wave impedance matching layers

Yixuan Zhou,   Yiwen E,   Zhaoyu Ren,   Haiming Fan,   Xin Long Xu,  Xinliang Zheng,   Dangyuan Lei,   Weilong Li,   Li Wang and  jintao bai  

J. Mater. Chem. C, 2015, Accepted Manuscript

http://pubs.rsc.org/en/content/articlelanding/2015/tc/c4tc02930e#!divAbstract
DOI: 10.1039/C4TC02930E

The potential of solution-processable reduced graphene oxide (rGO) films as wave impedance matching layers has been examined in a broad terahertz (THz) spectral bandwidth. The THz sheet conductivities of rGO fims measured by THz time-domain spectroscopy were observed to be tunable and sensitive to the film thicknesses and reduction degrees, which can be efficiently controlled by our solution-processable fabrication method. Remarkable broadband impedance matching was achieved with suitable rGO film, showing as the suppression of the internal reflected THz pulses from the substrate in the spectra. The underlying mechanisms have been revealed in both experiment and theory. This work paves the way for developing rGO-based broadband and large-scale anti-reflection layers for THz components.

Abstract-Improving Terahertz Sheet Conductivity of Graphene Films Synthesized by Atmospheric Pressure Chemical Vapor Deposition with Acetylene

Mei Qi , Yixuan Zhou , Fangrong Hu , Xin Long Xu ,Weilong Li , Anran Li , Jintao Bai , and Zhao Yu Ren

J. Phys. Chem. C, Just Accepted Manuscript

DOI: 10.1021/jp502260k

Publication Date (Web): June 17, 2014

Copyright © 2014 American Chemical Society
http://pubs.acs.org/doi/abs/10.1021/jp502260k

Graphene has shown great potential for terahertz (THz) applications in recent years. THz sheet conductivity of graphene is essential to assess the high performance of THz devices such as modulators based on graphene. In this work, THz sheet conductivity of graphene grown with different temperatures, along with the effects of chemical doping by HNO3, were studied in detail. Graphene films were synthesized on Cu surface by atmospheric pressure chemical vapor deposition with C2H2. Different samples with growth temperature from 850 to 1030 oC were characterized by Raman spectroscopy, transmission electron microscope, and UV-Vis spectroscopy. THz time-domain spectroscopy was used to study the THz sheet conductivity of the samples before and after HNO3 doping. The results show that graphene grown at 1000 oC has the highest THz sheet conductivity. Compared with the sample grown at 850 oC, the value enhances 600%. In addition, after HNO3 doping, the THz sheet conductivity of the sample grown at 1000 oC becomes 2.42 mS, which enhances 44%. These indicate that both the optimization of the growth temperature and chemical doping can improve the THz sheet conductivity of graphene significantly. Combining with the characterization of the material, we have attributed the effect of the growth temperature to the influence of carrier momentum scattering time in graphene, and the chemical doping to influence of the carrier concentration in graphene. This work advances the understanding of improving THz sheet conductivity by in-situ growth and post-growth and paves the way for efficient THz components with graphene.

Abstract-Tunable magnetoplasmons for efficient terahertz modulator and isolator by gated monolayer graphene

Yixuan Zhou ,  Xin Long Xu ,  Haiming Fan ,  Zhaoyu Ren ,  jintao bai and Li Wang
http://pubs.rsc.org/en/content/articlelanding/2013/CP/C3CP43994A

Terahertz (THz) technology has been a promising tool for sensing, spectroscopy, imaging, and communication. However, only few devices have shown efficient performance for future THz technology. Herein, we propose a device based on tunable magnetoplasmons in gated monolayer graphene for THz wave modulation and isolation. The relative transmission and the Faraday rotation angle of the device have been calculated by combining the Fresnel method with the voltage-dependent Drude model. Our results suggest that a superior modulation depth and giant Faraday rotation due to the cyclotron effect in the classical regime by intraband transitions in graphene offer an effective, uniform, and flexible tunability for THz wave. And these manipulations by graphene can range from 0 THz to 2 THz with an electron-hole asymmetry originating from variable scattering rate. Moreover, the thickness effect of the thin substrate is also studied for better performance of the device, taking advantage of the unavoidable Fabry-Perot (F-P) effect. This work demonstrates a pathway for an efficient THz modulator and isolator based on the magneto-optical polarization effect in graphene.