Optical breakdown of air triggered by femtosecond laser filaments

Experiments on gas breakdown by powerful optical pulses date back to the early days of lasers. The following development resulted in the generation of dense plasma columns up to several tens of meters in length. Pulsed neodymium-glass and CO2 lasers used in these demonstrations had pulse energies of several hundred Joules. Optical breakdown with high-energy nanosecond (or longer) laser pulses is a threshold-like process with respect to the peak intensity of the pulse. Consequently, the only way of controlling the longitudinal position of the breakdown region that does not rely on adding dust or aerosols locally into the propagation medium is through focusing of the laser beam. In the last decade, an alternative mechanism of freecharge generation in gases, through femtosecond laser filamentation, has been extensively studied4−6. Differently from breakdown with nanosecond pulses, femtosecond laser filamentation allows for a substantial degree of control over plasma generation in the ambient air. The longitudinal position of the filament can be manipulated by temporally chirping the laser pulse. The placement of the plasma channels within the transverse beam profile can be controlled by beam shaping. Longitudinally extended plasma channels generated in air through femtosecond laser filamentation could be useful in numerous potential applications. However, the density of free electrons in femtosecond filaments is only about 1022m−3 which limits their application space. It has been suggested that the dilute plasma in femtosecond filaments can be densified through avalanche ionization driven by an additional energetic laser pulse with the duration in the nanosecond range. In this approach, the femtosecond filament acts as a trigger that can be relatively easily controlled, while the nanosecond laser pulse provides sufficient energy for the densification of the plasma channel. In the present Letter, we report the proof-of-principle realization of that proposal. The combined femtosecond-nanosecond excitation of plasma in gases has been previously used for the generation of dense plasma channels for table-top particle acceleration and for Raman amplification of laser pulses. The experimental geometries used previously resulted in the generation of, at most, several centimeterlong plasma channels and were not straightforwardly applicable to plasma generation at range.