Large Enhancement of Nonlinear Terahertz Absorption in Intrinsic GaAs by Plasmonic Nano Antennas

We present our preliminary study on nonlinear THz effects in GaAs and their huge enhancement by plasmonic nano-antennas. We fabricated nano-antenna arrays on a 500-$\mu$ m-thick, intrinsic (100) GaAs wafer, using an electron beam lithography technique. THz pulses were generated by tilted-pulse-front optical rectification in LiNbO$_{3}$. The THz field amplitude (central frequency, 1 THz; bandwidth, 1 THz) varies from 20 to 120 kV/cm. We measured the transmitted THz pulses using a L-He cooled Si:Bolometer to obtain either spectrally-integrated total THz transmitted power or transmission spectra via Michelson interferometry. We observe (1) a transmission decrease ($\Delta $T/T) of about 5 {\%} at around 100 kV/cm incident field strength in bare GaAs wafers and (2) a transmission decrease of more than 30 {\%} over the incident field amplitude range from 40 to 120 kV/cm in nano-antenna-on-GaAs samples. Our experimental study demonstrates that strong THz pulses induce nonlinear THz absorption in intrinsic GaAs. The nonlinear THz effects are intensified by the field enhancement in a nano-antenna array.