Systematic Study of Two-Color Air Plasma Broadband THz-TDS

The lateral and temporal walk-offs between fundamental wave and second harmonic wave are compensated simultaneously, which enhances THz conversion efficiency by a factor of 3 in our homemade air plasma system. Second harmonic generation efficiency is also optimized for THz radiation. Together with a dual-wavelength half-waveplate, a high THz conversion efficiency of > 0.06% is achieved at 800 nm excitation in the dry air environment, corresponding to an overall enhancement factor of 6. In the detection part, electro-optic sampling and air-biased coherent detection are experimentally investigated. By using our air plasma system, two characteristic peaks (4.85 and 5.8 THz) of the rust sample are identified.