Abstract-Optimized nonlinear terahertz response of graphene in a parallel-plate waveguide

Parvin Navaeipour, Marc M.Dignam

https://arxiv.org/abs/1806.08008

Third harmonic generation of terahertz radiation is expected to occur in monolayer graphene due to the nonlinear relationship between the crystal momentum and the current density. In this work, we calculate the terahertz nonlinear response of graphene inside a parallel-plate waveguide including pump depletion, self-phase, and cross-phase modulation. To overcome the phase mismatching between the pump field and third-harmonic field at high input fields due to self-phase and cross-phase modulation, we design a waveguide with two dielectric layers with different indices of refraction. We find that, by tuning the relative thicknesses of the two layers, we are able to improve phase matching, and thereby increase the power efficiency of the system by more than a factor of two at high powers. With this approach, we find that dispite the loss in this system, for an incident frequency of 2 THz, we are able to achieve power efficiencies of 75% for graphene with low Fermi energies of 20 meV and up to 35% when the Fermi energy is 100 meV.

Abstract-Polarization-resolved terahertz third-harmonic generation in a single-crystal NbB superconductor: Dominance of the Higgs mode beyond the BCS approximation

Ryusuke Matsunaga, Naoto Tsuji, Kazumasa Makise, Hirotaka Terai, Hideo Aoki, and Ryo Shimano

https://journals.aps.org/prb/accepted/06073Oa1P3bEc21dc38d310718d5f15931783f2ce

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in }{{s}}{-wave superconductors, where charge-density fluctuations (CDF) have been shown to also contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable polarization-angle dependence relative to the crystal axis. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is the retardation effect in the phonon-mediated pairing interaction beyond BCS.

Abstract-Electrically tunable, plasmon resonance enhanced, terahertz third harmonic generation via graphene

H. Nasari*^a and   M. S. Abrishamian^a  

RSC Adv., 2016,6, 50190-50200

DOI: 10.1039/C6RA08086C
Received 29 Mar 2016, Accepted 07 May 2016
First published online 23 May 2016
http://pubs.rsc.org/en/Content/ArticleLanding/2016/RA/C6RA08086C?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+rss%2Fra+%28RSC+-+RSC+Adv.+latest+articles%29#!divAbstract

In this study, we demonstrate how field enhancement due to plasmonic resonances can noticeably improve the efficiency of third harmonic generation (THG) from graphene sheets on a grating substrate under normal illumination of terahertz (THz) waves. We explore how different parameters of graphene and the substrate can affect the strength of the resonance and, thus, the third harmonic (TH) level. Interestingly, we show that the gate voltage tunability of the linear and nonlinear conductivities of graphene and, thus, the resonant frequency of the structure, enable the achievement and control of the plasmonic resonance enhanced THG over the THz frequency band. Based on our finite difference time domain (FDTD) numerical simulations, we reveal that a more than 5 orders of magnitude improvement in the TH level would be achievable by placing high quality graphene samples on a grating substrate under resonant conditions instead of placing them on a flat substrate. We hope that these findings can pave the way for the development of a plethora of valuable applications in the THz frequency band, such as generating new frequencies, spectroscopy, and signal processing.