Study of the Physical Mechanism of Air Breakdown Using Picosecond Long-Wavelength Infrared Laser Pulses

Recent demonstration of self-guiding of terawatt-power picosecond long-wavelength infrared (LWIR) laser pulses in air has renewed interest of ionization physics in this spectral range. The measured laser intensity is around 10¹² W/cm² in the 10 µm filament, much lower than tunneling threshold for N₂/O₂ and therefore avalanche ionization is thought to dominate. However, dynamics of avalanche ionization for such short and intense pulses is yet to be understood. Here we report a detailed study of the avalanche breakdown process in air using space and time resolved visible light interferometry. First, we observed that air breakdown does not occur for intensities below 100 GW/cm², i.e. the breakdown threshold is much higher than that reported for nanosecond CO₂ lasers. Next, the velocity of the backward propagating “breakdown wave” was measured to be >10⁸ cm/s, an order of magnitude faster than the optical detonation effect associated with nanosecond pulses. Finally, in a large diameter plasma channel, air impurities are seen to initially form localized, hot, high-density plasma beads that drive shock waves travelling around 10⁵ cm/s in the air. These hot regions eventually coalesce to form a centimeter-scale, hot gas channel that persists over millisecond time scales.