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

Summary form only given. 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)). As shown in the example given in figure 1-(b), when the coupler is excited at 990 MHz and no plasma is present, there is no signal observed at this frequency (black curve), whereas a clear emission peak appears when the laser guided discharge is generated (red curve). RF energy coupling in the plasma has consequently been achieved, which resulted in the plasma column behaving as an emitting antenna. It is worth to note that the emission level from a copper rod with a length similar to that of the plasma is approximately equal to four times the plasma signal strength, demonstrating the applicability of this technology in real situations.