Multi-milliwatt average power two-color air-plasma terahertz source at 100 kHz repetition rate

Ultrafast THz transients with large spectral bandwidths are commonly generated in the two-color air-plasma scheme, typically driven by high-pulse-energy laser sources, operating at low repetition rates up to a few kHz. The low repetition rate of these sources is a strong limiting factor to reaching high dynamic ranges in measurements involving long integration times or multi-dimensional scans. The advent of high-power Yb-based laser sources in combination with nonlinear temporal compression schemes opens the door to air-plasma THz sources at significantly higher repetition rates up to hundreds of kHz and beyond. In this study, we investigate for the first time repetition rate scaling in the two-color filament plasma scheme at repetition rates ranging from 1 kHz up to 100 kHz. We study the influence of repetition rate on THz yield and observe a correlation between the decrease of the optical-to-THz conversion efficiency and the gas density depletion in the filamentation region that we measure using interferometry. The experimentally observed optical-to-THz conversion efficiencies are in approximate agreement with a simple one-dimensional photocurrent model we developed. These findings set the foundation for future repetition rate scaling experiments towards the MHz region, using the broadband and affordable two-color air-plasma scheme. At 100 kHz, we generate THz radiation with a maximum average power of 2.8 mW and peak electric fields reaching 165 kV/cm. Using electro-optic sampling, we detect frequencies up to 23 THz, showing the broadband nature of the source. This source is a promising alternative both for future linear and nonlinear ultra-broadband spectroscopic investigations at greatly accelerated measurement times.