Crossed femtosecond filaments create plasma grating in air

A group at East China Normal University (Shanghai, China) has shown self-channeling filaments created by femtosecond-laser pulses can interact in air to form a 2D plasma grating that diffracts light. The team split 50 fs pulses at a 1 kHz repetition rate from a Ti:sapphire laser into three equal-energy pulses, each of which is focused into an air region so the filaments (and the synchronized pulses) cross each other at angles from 3º to 6º. The resulting interference-induced gratings were enhanced by nonlinear Kerr and plasma effects and formed channels a few millimeters long. The input pulses were guided into the plasma channels; noncollinear pulses at arbitrary incidence angles could be coupled into the channels.

The plasma structures were characterized using time-resolved holographic imaging, with the weak test probe propagating perpendicularly through the plasma grating. From the measured phase profile, the change in refractive index for a single filament, a 1D grating, and a 2D grating were 1.47 × 10^-5, 2.21 × 10^-4, and 3.53 × 10^-4, respectively. Third-harmonic pulses generated in air by the filaments were diffracted by the gratings as per the Bragg condition, confirming the measurements. Potential uses include plasma photonics for pulse compression and shaping. Contact Heping Zang at[email protected].