Abstract-Terahertz generation from reduced graphene oxide

Huan Wang, Yixuan Zhou, Zehan Yao, Lipeng Zhu, Yuanyuan Huang, Xinlong Xu, Zhaoyu Ren,

https://www.sciencedirect.com/science/article/pii/S0008622318303609


We firstly investigate the generation of terahertz (THz) wave from reduced graphene oxide(RGO) illuminated with femtosecond near-infrared laser pulse. Experiment results show that the THz generation from RGO can be enhanced by increasing the reduction degree and reducing the film thickness. The former can be attributed to the increase of sp2 carbon region, which has much smaller band gap and graphene-like photoelectric properties. The latter is due to the suppression of the light-induced lateral currents in the surface of RGO layers. The linear dependency of the THz electrical field on the pump power confirms that the THz emission from RGO is governed by second-order nonlinear properties. When exciting laser irradiates from opposite sample sides, π phase shift of the generated THz wave has been observed, suggesting the transient photocurrent related to THz emission is induced by the photon drag effect. The conclusion has been further confirmed by the well fitting of the experiment and theoretical calculation based on the symmetry of RGO. This work makes it clear the THz generation mechanism of RGO and paves a way for developing new THz sources.

Abstract-Terahertz generation from reduced graphene oxide

Huan Wang, Yixuan Zhou, , Zehan Yao, Lipeng Zhu, Yuanyuan Huang, Xinlong Xu, Zhaoyu Ren,

https://www.sciencedirect.com/science/article/pii/S0008622318303609

We firstly investigate the generation of terahertz (THz) wave from reduced graphene oxide (RGO) illuminated with femtosecond near-infrared laser pulse. Experiment results show that the THz generation from RGO can be enhanced by increasing the reduction degree and reducing the film thickness. The former can be attributed to the increase of sp² carbon region, which has much smaller band gap and graphene-like photoelectric properties. The latter is due to the suppression of the light-induced lateral currents in the surface RGO layers. The linear dependency of the THz electrical field on the pump power confirms that the THz emission from RGO is governed by second-order nonlinear properties. When exciting laser irradiates from opposite sample sides, π phase shift of the generated THz wave has been observed, suggesting the transient photocurrent related to THz emission is induced by the photon drag effect. The conclusion has been further confirmed by the well fitting of the experiment and theoretical calculation based on the symmetry of RGO. This work makes it clear the THz generation mechanism of RGO and paves a way for developing new THz sources.

Abstract-Terahertz emission from vertically aligned multi-wall carbon nanotubes and their composites by optical excitation

Shan Huang, Weilong Li.  Lipeng Zhu, Mi He, Zehan Yao, Yixuan Zhou, Xinlong Xu, Zhaoyu Ren,  Jinbo Bai,

https://www.sciencedirect.com/science/article/pii/S000862231830191X

Terahertz (THz) emission has been successfully observed from vertically aligned multi-wall carbon nanotube (VAMCNT) materials excited with polarized laser pulse in a reflection configuration. The effects of experimental parameters on THz generation are investigated systematically. The results indicate that the linear dependence of THz electric field on the pump power shows a typical second-order nonlinear optical effect and the transient photocurrent for generating THz waves is mainly originated from photon drag effect. By comparing with THz emission from polymer-coated VAMCNT composites, the detailed generation process of THz waves from VAMCNT materials is clarified: the free carriers in VAMCNT films could transport both along single nanotube and between the adjacent nanotubes, while free carriers in VAMCNT composite films transport mainly along the tube axis due to the isolated carrier transport between the adjacent nanotubes by the coated polymer. Thus, transient photocurrent for generating THz emission from VAMCNT composite films is only along the tube axis and its intensity depends on the component of the pump light electric field along the tube axis. In addition, THz emission from VAMCNT composites shows more regular response than that from pure VAMCNT films, which paves a way for stable performance of VAMCNT-based THz emitter.

Abstract-Terahertz surface emission of d-band electrons from a layered tungsten disulfide crystal by surface field

Longhui Zhang, Yuanyuan Huang, Qiyi Zhao, Lipeng Zhu, Zehan Yao, Yixuan Zhou, Wanyi Du, and Xinlong Xu

https://journals.aps.org/prb/accepted/ad078Od4Tf91333d54c29f0414038d0387a0cecfe

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide (WS2). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of MoS2\thinspace based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in WS2. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12{\%} approximately. All these suggest the main THz mechanism from WS2 is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of WS2 with the maximum surface depletion field of approximate 1.2 \texttimes 105 V/cm. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.

Abstract-Enhanced spatial terahertz modulation based on graphene metamaterial

Dandan Sun, Mengqi Wang, Yuanyuan Huang, Yixuan Zhou, Mei Qi, Man Jiang, and Zhaoyu Ren

https://www.osapublishing.org/col/abstract.cfm?uri=col-15-5-051603

The plasmonic mode in graphene metamaterial provides a new approach to manipulate terahertz (THz) waves. Graphene-based split ring resonator (SRR) metamaterial is proposed with the capacity for modulating transmitted THz waves under normal and oblique incidence. Here, we theoretically demonstrate that the resonant strength of the dipolar mode can be significantly enhanced by enlarging the arm-width of the SRR and by stacking graphene layers. The principal mechanism of light–matter interaction in graphene metamaterial provides a dynamical modulation based on the controllable graphene Fermi level. This graphene-based design paves the way for a myriad of important THz applications, such as optical modulators, absorbers, polarizers, etc.

© 2017 Chinese Laser Press

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Abstract-Angular dependent anisotropic terahertz response of vertically aligned multi-walled carbon nanotube arrays with spatial dispersion

• Yixuan Zhou
• , Yiwen E.
• , Xinlong Xu
• , Weilong Li
• , Huan Wang
• , Lipeng Zhu
• , Jintao Bai
• , Zhaoyu Ren
•  & Li Wang

http://www.nature.com/articles/srep38515

Spatial dispersion effect of aligned carbon nanotubes (CNTs) in the terahertz (THz) region has significance for both theoretical and applied consideration due to the unique intrinsically anisotropic physical properties of CNTs. Herein, we report the angular dependent reflection of p-polarized THz wave from vertically aligned multi-walled CNT arrays in both experiment and theory. The spectra indicate that the reflection depends on the film thickness of vertically aligned CNTs, the incident angle, and the frequency. The calculation model is based on the spatial dispersion effect of aligned CNTs and performed with effective impedance method and the Maxwell-Garnett approximation. The results fit well with the experiment when the thickness of CNT film is thin, which reveals a coherent superposition mechanism of the CNT surface reflection and CNTs/Si interface reflection. For thick CNT films, the CNTs/Si interface response determines the reflection at small incident angles, while the CNTs surface effect dominates at large incident angles. This work investigates the spatial dispersion effect of vertically aligned CNT arrays in the THz region, and paves a way for potential anisotropic THz applications based on CNTs with oblique incidence requirements.

Abstract-Dielectric Property of MoS2 Crystal in Terahertz and Visible Region

Xianding Yan, Lipeng Zhu, Yixuan Zhou, Yiwen E, Li Wang, Xinlong Xu

(Submitted on 14 May 2015)

http://xxx.tau.ac.il/abs/1505.03617

Two-dimensional materials such as MoS2 have attracted much attention in recent years due to their fascinating optoelectronic properties. Dielectric property of MoS2 is desired for the optoelectronic application. In this paper, terahertz (THz) time-domain spectroscopy and ellipsometry technology are employed to investigate the dielectric response of MoS2 crystal in THz and visible region. The real and imaginary parts of the complex dielectric constant of MoS2 crystal are found to follow a Drude model in THz region, which is due to the intrinsic carrier absorption. In visible region, the general trend of the complex dielectric constant is found to be described with a Lorentz model, while two remarkable peaks are observed at 1.85 and 2.03 eV, which have been attributed to the splitting arising from the combined effect of interlayer coupling and spin-orbit coupling. This work affords the fundamental dielectric data for the future optoelectronic applications with MoS2.

Comments:	6 pages
Subjects:	Materials Science (cond-mat.mtrl-sci)
Cite as:	arXiv:1505.03617 [cond-mat.mtrl-sci]
	(or arXiv:1505.03617v1 [cond-mat.mtrl-sci] for this version)

Submission history

From: Xinlong Xu [view email] 
[v1] Thu, 14 May 2015 04:34:20 GMT (337kb)

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 ‡, Xinlong Xu *†, Weilong Li †, Anran Li †, Jintao Bai †, andZhaoyu Ren *†

† State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China

‡ School of Electronic Engineering and Automation,Guilin University of Electronic Technology, Guilin 541004, China

J. Phys. Chem. C, Article ASAP

DOI: 10.1021/jp502260k

Publication Date (Web): June 17, 2014

Copyright © 2014 American Chemical Society

*Tel.: +86-29-88303336. Fax: +86-29-88303336. E-mail: xlxuphy@nwu.edu.cn., *Tel.: +86-29-88303336. Fax:+86-29-88303336. E-mail: rzy@nwu.edu.cn.

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 °C 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 °C has the highest THz sheet conductivity. As compared to the sample grown at 850 °C, the value enhances 600%. In addition, after HNO3 doping, the THz sheet conductivity of the sample grown at 1000 °C 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 the influence of the carrier concentration in graphene. This work advances the understanding of improving THz sheet conductivity by in situ growth and postgrowth and paves the way for efficient THz components with graphene.

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.