Abstract-The Role of Photo-Induced Exciton in the Transient Terahertz Conductivity of Few-Layer WS2 Laminate

Xiao Xing, Litao Zhao, Zeyu Zhang, Xiankuan Liu, Kailin Zhang, Yang Yu, Xian Lin, Huaying Chen, Jinquan Chen, Zuanming Jin, Jianhua Xu, and Guo-Hong Ma

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b05345?journalCode=jpccck

The exciton effect in two-dimensional (2D) transition metal dichalcogenides (TMDs) plays a dominated role in describing the optical and optoelectronic properties. However, the interplay between the excitons and free carriers has yet to be understood upon the photo excitation in 2D TMDs. Here we first present a study of the dynamical interplay of excitons and unbound electron-hole pairs using time-resolved terahertz (THz) spectroscopy (TRTS) in a few-layer WS2 laminate. Our experimental results demonstrate that the Auger recombination is observed only in the relaxation process of the mobile charge carriers rather than that of excitons upon photoexcitation. The transient complex THz photoconductivity spectroscopy of WS2 is well described by Lorentz oscillator model of free carriers modulated by the exciton polarization field. Our results provide a comprehensive understanding of non-equilibrium carrier kinetics (both excitons and free carriers) in WS2 laminate, and should be applicable to other 2D system.

Abstract-Switchable metamaterial for enhancing and localizing electromagnetic field at terahertz band

Junxing Liu, Kailin Zhang, Xiankuan Liu, Zeyu Zhang, Zuanming Jin, Xiaoyong He, and Guohong Ma

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-13-13944

In this article, a novel metamaterial is designed aimed at generating a single electromagnetic hot spot, in order to realize the localization of the incident electromagnetic field at terahertz band, and this kind of metastructure is an ideal candidate for many research fields such as spintronics, nonlinear magnetic response, near-field optics, and optical antenna, etc. The specially tailored metamaterial takes the shape of diabolo with a metal triangle pair connected by a cubic gallium arsenide (GaAs) gap. We demonstrated by simulation that both electric- and magnetic-field of incident THz pulse can be confined in the small GaAs gap when a synchronized femtosecond laser pulse is illuminated. The numerical simulation results show that 2 orders of magnitude of field enhancement can be obtained for a 1-by-1 μm GaAs gap, and the field enhancement factor can also be further improved by tailoring the GaAs gap down to nanometer scale.

© 2017 Optical Society of America

Abstract-Terahertz broadband modulation in a biased BiFeO3/Si heterojunction

Xiankuan Liu, Zeyu Zhang, Xian Lin, Kailin Zhang, Zuanming Jin, Zhenxiang Cheng, and Guohong Ma

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-23-26618

A new terahertz (THz) modulator based on bias-driven carrier conductivity change in a heterojunction was proposed. BiFeO3 film and silicon were selected as building blocks for fabricating the THz modulator. THz nonlinear transmission as a function of bias voltage was studied systematically. THz peak transmission as a function of bias shows a similar tendency as the current-voltage response of the heterojunction: the forward bias leads to the exponential enhancement of THz transmission, and in contrast, the reverse bias shows no observable changes in THz transmission. The modulation depth and modulation bandwidth of THz pulse can reach up to 42% and 1.0 THz with forward bias of 4.8 V, respectively. The observed bias dependent THz transmission in the BFO/Si heterojunction is well-interpreted by the proposed model: the diffused carriers across the heterojunction are localized in BFO thin film with applied forward bias. Our finding provides great potential for applications in designing all electrical broadband THz modulators.

© 2016 Optical Society of America

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