Plasma column from laser filamentation in air as a virtual radio-frequency antenna

Plasma antennas, where plasma replaces metal as the conducting element, have long been known and used [1]. However, most designs use low-pressure plasmas confined inside solid dielectric vessels. We experimentally demonstrated a functional plasma antenna in air, which brings about many advantages like tunability in a large frequency range (100 MHz-1 GHz), stealth when de-activated and quick reconfiguration capabilities [2]. This antenna is based on the low-density (n_e ≈ 10¹⁶ cm^-3) plasma column created by an ultrashort laser pulse (700 fs, 300 mJ @ 800 nm) undergoing filamentation in atmospheric air. However, such plasma has a very short lifetime (less than 1 ns). To deal with this problem, we used the filament to guide a high-voltage electric discharge generated by a compact Tesla coil (output voltage of 350 kV), extending the plasma lifetime to at least 100 ns [3]. Radio-frequency (RF) power was then injected in the plasma by means of an inductive coupler in the form of a hollow metallic cylindrical cavity, fed by a 35 W solid-state RF amplification chain. Radio emission was then detected by means of a remote patch antenna with a 100 MHz-1 GHz bandwidth (figure 1-(a)).