Single-Shot Measurement Of The Nonlinear Refractive Index Of Air At 9.2 Mu M With A Picosecond Terawatt Co2 Laser

We developed a simple, accurate single-shot method to determine the nonlinear refractive index of air by measuring the evolution of the spatial shape of a laser beam propagating through the atmosphere. A distinctive feature of this new method, which relies on a modified Fresnel propagation model for data analysis, is the use of a hard aperture for producing a well-defined, high-quality beam from a comparatively non-uniform quasi-flat-top beam, which is typical for high-peak-power lasers. The nonlinear refractive index of air for a very short (2 ps) long-wave infrared (LWIR) laser pulse was measured for the first time, to the best of our knowledge, yielding n(2) = 3.0 x 10(-23) m(2)/W at 9.2 mu m. This result is 40% lower than a corresponding measurement with longer (200 ps) LWIR pulses at a similar wavelength. (C) 2021 Optical Society of America