Femtosecond white-light filaments: a new tool in atmospheric research

The need for an atmospheric sensor that combines the advantages of Fourier-transform infrared spectroscopy (FTIR), differential optical absorption spectroscopy (DOAS), and light detection and ranging (LIDAR) method resulted in the idea of producing a remote "white lamp" by generating a laser-induced plasma focus in the atmosphere. Recent advances in ultra-fast lasers have shown that high-power laser light can, under certain power and focusing conditions, create extended regions of ultra-intense illuminations. To reach the required power at the remote location, an experiment is designed to send a slightly focused, high-power femtosecond laser pulse (100 fs, 3 TW) into the atmosphere with an initial negative chirp, that is, with the shorter wavelength emitted before the longer ones. Instead of a small spot of a plasma focus, an extended white-light channel is observed, similar to previous observations on shorter scales in the laboratory. Unlike the fundamental infrared laser wavelength (approx. 800 nm), the white-light channel is clearly visible to the naked eye. The phenomenon also occurred with a spatially unfocused laser beam. The spectrum of the emitted light covered the entire visible range from the ultraviolet (UV) to the infrared (IR), and its signal could be detected from altitudes beyond 10 km. This white light exhibits a directional behaviour with enhanced backward scattering, and is detected from an altitude of more than 20 km. This light source opens the way to white-light and non-linear light detection and ranging applications for atmospheric trace-gas remote sensing or remote identification of aerosols. Air ionisation inside the filaments also opens promising perspectives for laser-induced condensation and lightning control. The mobile femtosecond-terawatt laser system, Teramobile, has been constructed to study these applications.