Description
Laser filamentation is a well-established nonlinear propagation regime in which Kerr self-focusing is dynamically balanced by plasma-induced defocusing, enabling high-intensity pulses to propagate over distances far exceeding the Rayleigh length. Although filamentation at atmospheric pressure has been extensively studied, its behavior in low-pressure environments remains largely unexplored. Recent numerical studies predict that filamentation may still occur at pressures as low as 0.0035 atm (corresponding to ~38.5 km altitude). Accessing this regime is of considerable interest both for fundamental nonlinear optics and for applications such as long-range laser propagation and optical communication at high atmospheric altitudes and in near-space conditions.
At 1 atm in air, the critical power for self-focusing is approximately 5 GW (for 40 fs pulse durations). Since the critical power scales inversely with pressure (Pcrit ∝ 1/p), significantly higher peak powers are required to achieve filamentation at reduced pressures, reaching the 50 TW level for 100 mTorr.
Here we report laboratory-scale experiments of filamentation in Nitrogen at pressures between 50 mTorr (6.6 × 10⁻⁵ atm) and 5 Torr (6.6 × 10⁻³ atm), reaching well below previously explored conditions. Experiments were conducted using the Multi-Terawatt (MTW-OPAL) laser system at the Laboratory for Laser Energetics, delivering femtosecond pulses with peak powers up to 200 TW. Filament formation was characterized through measurements of nonlinear spectral broadening, filament diameter and length. Additionally, the influence of spatial beam profile on Pcrit was studied by comparing Gaussian and flat-top spatial intensity distributions.
Our initial results indicate a transition from linear to filamentary propagation as pressure is varied, with meter-scale filament length observed at pressures orders of magnitude below atmospheric conditions.
Ongoing experiments will extend these studies to low pressure Helium in order to study nonlinear propagation in both atomic and molecular rarefied gases. These investigations will also inform prospects for intense terahertz generation in long filaments at extremely low pressure.