Abstract-Circular-Photon-Drag-Effect-Induced Elliptically Polarized Terahertz Emission from Vertically Grown Graphene

Lipeng Zhu, Zehan Yao, Yuanyuan Huang, Chuan He, Baogang Quan, Junjie Li, Changzhi Gu, Xinlong Xu, and Zhaoyu Ren

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.12.044063

Circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors yet is less studied in graphene due to the relatively weak light-matter interaction and is submerged by other nonlinear optical effects. We give experimental evidence of CPDE in vertically grown graphene (VGG) by terahertz (THz) emission spectroscopy. The emitted THz polarization states can be tuned to linear, left-handed, and right-handed elliptical polarizations by changing the helicity of the pump laser. Polarity reversal of the THz time-domain signal occurs with the opposite helicity of pump laser excitation due to the CPDE. Theory analysis suggests that both the linear photon drag effect and CPDE-induced transient photocurrents contribute to the THz emission from which the contribution weight of CPDE can be tuned by different elliptical states of the excitation light. The photon-helicity-dependent THz emission from VGG based on CPDE offers an alternative thought of graphene-based polarization sensitive THz sources for chiral analysis in the THz field.

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Abstract-Simultaneous excitation of extremely high-Q-factor trapped and octupolar modes in terahertz metamaterials

Shengyan Yang, Chengchun Tang, Zhe Liu, Bo Wang, Chun Wang, Junjie Li, Li Wang, Changzhi Gu,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-14-15938&origin=search

Achieving high-Q-factor resonances allows dramatic enhancement of performance of many plasmonic devices. However, the excitation of high-Q-factor resonance, especially multiple high-Q-factor resonances, has been a big challenge in traditional metamaterials due to the ohmic and radiation losses. Here, we experimentally demonstrate simultaneous excitation of double extremely sharp resonances in a terahertz metamaterial composed of mirror-symmetric-broken double split ring resonators (MBDSRRs). In a regular mirror-arranged SRR array, only the low-Q-factor dipole resonance can be excited with the external electric field perpendicular to the SRR gap. Breaking the mirror-symmetry of the metamaterial leads to the occurrence of two distinct otherwise inaccessible ultrahigh-Q-factor modes, which consists of one trapped mode in addition to an octupolar mode. By tuning the asymmetry parameter, the Q factor of the trapped mode can be linearly modulated, while the Q factor of the octupolar mode can be tailored exponentially. For specific degree of asymmetry, our simulations revealed a significantly high Q factor (Q>100) for the octupolar mode, which is more than one order of magnitude larger than that of conventional metamaterials. The mirror-symmetry-broken metamaterial offers the advantage of enabling access to two distinct high-Q-factor resonances which could be exploited for ultrasensitive sensors, multiband filters, and slow light devices.

© 2017 Optical Society of America

Abstract-Single-layer plasmonic metasurface half-wave plates with wavelength-independent polarization conversion angle

Zhaocheng Liu, Zhancheng Li, Zhe Liu, Hua Cheng, Wenwei Liu, Chengchun Tang, Changzhi Gu, Junjie Li, Hou-Tong Chen, Shuqi Chen, and Jianguo Tian

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.7b00491

Publication Date (Web): June 30, 2017

Copyright © 2017 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b00491?journalCode=apchd5

Manipulation of polarization state is of great fundamental importance and plays a crucial role in modern photonic applications such as optical communication, imaging and sensing. Metamaterials and metasurfaces have attracted increasing interest in this area because they facilitate designer optical response through engineering the composite subwavelength structures. Here we propose a general methods of designing half-wave plate, and demonstrate in the near-infrared wavelength range an optically thin plasmonic metasurface half-wave plates that rotate the polarization direction of the linearly polarized incident light with a high degree of linear polarization. The half-wave plate functionality is realized through arranging the orientation of the nanoantennas to form an appropriate spatial distribution profile, which behave exactly as in classical half-wave plates but over in a wavelength-independent way.

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-Self-referenced sensing based on terahertz metamaterial for aqueous solutions

http://apl.aip.org/resource/1/applab/v102/i15/p151109_s1?isAuthorized=no

Xiaojun Wu¹, Xuecong Pan¹, Baogang Quan¹, Xinlong Xu^1,2, Changzhi Gu¹, and Li Wang¹

¹Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²Nanobiophotonic Center, State Key Laboratory for Incubation Base of Photoelectric Technology and Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi'an 710069, China 

We demonstrated a self-referenced sensing method in reflection geometry for characterizing aqueous solutions based on terahertz metamaterials. The sensing signal and the reference signal are taken in one measurement from different interfaces of the substrate. For ethanol-water mixture and aqueous solution of NaCl, the line-shape of the modulated response shows distinct polarity, while the peak-valley value near resonant region depends linearly on the solution concentration. These observations result from the variation of dielectric environment near the interface between the metamaterials and the aqueous solutions. This method holds promise for future application in monitoring real aqueous biosystems and ecological water systems.