Abstract-Electrically Controllable Terahertz Second‐Harmonic Generation in GaAs

Joo Kang, Won Tae Kim, Hyeon‐Don Kim, Soojeong Baek, Kwang Jun Ahn, Bumki Min,Fabian Rotermund

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202000359?af=R

Terahertz radiation and its nonlinear optical manipulation may possess potential for a variety of applications in next‐generation electronics and optics. Pioneering studies have shown that the nonlinearity of carrier drift in semiconductors and graphene can be utilized for nonlinear optical processes at terahertz frequencies. However, because of the symmetric response of carriers to the terahertz field direction, most experiments have confirmed only the presence of odd‐order nonlinear processes. In this study, electric‐field‐induced terahertz second‐harmonic generation (SHG) in photoexcited gallium arsenide is demonstrated, where an applied bias field breaks the directional symmetry of the drift transport of electrons. The amplitudes of odd‐ and even‐harmonic waves are found to be highly controllable using the bias field. The measured conversion efficiency of SHG reaches beyond 10−5, substantially higher than the value previously reported. This terahertz harmonic generation platform with electrical controllability may be useful for future nonlinear applications at terahertz frequencies.

Abstract-Terahertz optical bistability of graphene in thin layers of dielectrics

Kwang Jun Ahn and Fabian Rotermund

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-8-8484

We theoretically studied in terahertz frequency regime optical bistability of graphene placed at the interface between thin dielectric layers. We solved self-consistently the nonlinear wave equations containing the third-order optical conductivity of graphene in four-layer structures and obtained hysteresis response of the transmitted power as a function of the incident power. We numerically observed that the critical powers for the up and down transitions and the Fermi-energy of graphene required for terahertz optical bistability can be reduced by carefully choosing material properties and the thicknesses of dielectric layers. Furthermore, these values can be substantially decreased when graphene as a randomly stacked multilayer structure is asymmetrically located in thin dielectric layers.

© 2017 Optical Society of America

Abstract-Plasmon Enhanced Terahertz Emission from Single Layer Graphene

Young-Mi Bahk , Gopakumar Ramakrishnan ,Jongho Choi , Hyelynn Song , Geunchang Choi ,Yong Hyup Kim , Kwang Jun Ahn , Dai-Sik Kim , and Paul C. M. Planken

ACS Nano, Just Accepted Manuscript

DOI: 10.1021/nn5025237

Publication Date (Web): August 19, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nn5025237

We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, ‘non-resonant laser-pulse-induced photon drag currents’ appear to be responsible for the relatively weak emission of both s- and p-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the p-polarized terahertz electric field is enhanced, whereas the s-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, ‘surface-plasmon-enhanced optical rectification’, made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin and more efficient terahertz photonic devices.