A new echo phenomenon in laser induced population grating in gases — LIPG echo

We report the observation of a new type of echo produced by applying two intersecting light pulses with different angles (θ and θ') at t = 0 and τ. The laser induced population grating (LIPG) created by the first excitation and degraded due to the thermal motion of atoms is reformed by the second excitation at t _e= {τ+mτ}/{(αn-M)}, where m and n are integers and α sin ( {θ}/{2})= sin ( {θ'}/{2}) . The experiment was performed in the ground state 3 ²S_{1}/{2} of atomic sodium vapor. This echo is not related to the atomic coherence but to the population distribution, and has novel feature for studying velocity-changing collisions.