title: Theory of low-power ultra-broadband terahertz sideband generation in bi-layer graphene.
abstract: In a semiconductor illuminated by a strong terahertz (THz) field, optically excited electron-hole pairs can recombine to emit light in a broad frequency comb evenly spaced by twice the THz frequency. Such high-order THz sideband generation is of interest both as an example of extreme nonlinear optics and also as a method for ultrafast electro-optical modulation. So far, this phenomenon has only been observed with large field strengths (~10 kV cm(-1)), an obstacle for technological applications. Here we predict that bi-layer graphene generates high-order sidebands at much weaker THz fields. We find that a THz field of strength 1 kV cm(-1) can produce a high-sideband spectrum of about 30 THz, 100 times broader than in GaAs. The sidebands are generated despite the absence of classical collisions, with the quantum coherence of the electron-hole pairs enabling recombination. These remarkable features lower the barrier to desktop electro-optical modulation at THz frequencies, facilitating ultrafast optical communications.
authors.affiliation: Department of Physics and Centre for Quantum Coherence, Chinese University of Hong Kong, Shatin, Hong Kong, China.
authors.forename: J A
authors.initials: JA
authors.lastname: Crosse
authors.affiliation: Department of Physics, Department of Material Science and Engineering, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Xu
authors.affiliation: Department of Physics, Institute for Terahertz Science and Technology, University of California at Santa Barbara, Santa Barbara, California 93106, USA.
authors.forename: Mark S
authors.initials: MS
authors.lastname: Sherwin
authors.affiliation: Department of Physics and Centre for Quantum Coherence, Chinese University of Hong Kong, Shatin, Hong Kong, China.
authors.forename: R B
authors.initials: RB
authors.lastname: Liu