THz photonics of stratified plasmas: Transient Cherenkov radiation from laser wakes

A terawatt laser pulse, propagating in a stratified gas, instantly ionizes it, forming a nonhomogeneous plasma column, concurrently driving a partly electromagnetic wake wave. Ramping up the plasma density along the drive pulse path increases the wake phase velocity beyond the vacuum speed of light, making the wake emit a broadband, positively chirped THz Cherenkov signal. At any locality within the ramp, the wake phase velocity increases continuously, becoming singular within a finite interval of time, changing sign afterwards, eventually becoming sub-luminal by absolute value. In the course of this wake `reversal,' the Cherenkov wave vector rotates by 180^○. The signal in the wave zone is thus an expanding spherical shell, the signal length increasing with the observation angle from almost zero (forward emission) to a few tens of picoseconds (backward emission.) The signal frequency increases in time from the Langmuir frequency at the foot of the column to the one at the top. The THZ energy flow at 90^○, emanating from the infinitely superluminal wake, is the highest, a few kW. Experimentally capturing the details of this transient THz emission must shed light onto the plasma wake dynamics. Approved for public release; distribution is unlimited. PA release approval AFRL-2021-1933