Tunable broadband THz emission from air plasma pumped by femtosecond pulses composed of a fundamental frequency with its detuned second harmonic

Abstract

We demonstrate both theoretically and experimentally that the broadband THz pulses emitted by air plasma can be tuned in the spectrum domain by employing pump pulses composed of a fundamental femtosecond pulse and its detuned second harmonic (SH). Based on the plasma photocurrent model, our numerical simulations show that the central frequency of the broadband THz emission can be shifted from ∼4 THz to much higher frequency (∼38 THz) by red-shifting the second harmonic pulse. The relative phase of the two optical frequencies is found to present a critical role on the yield and spectrum of the THz emission. Experimentally, we obtained detuned second harmonic of the fundamental 800 nm femtosecond pulse by tilting the second harmonic generation (SHG) nonlinear crystal and achieved a tuning range from 391 nm to 403 nm. The central frequency of the corresponding THz emission was observed to shift from 4 THz to higher frequency side, in agreement with the simulation. This method therefore provides a simple mean to tune the spectrum of the broadband THz obtained from air plasma. We believe that even larger tunability can be achieved with a wavelength tunable Optical Parametric Amplification (OPA) driving laser.