Photon echoes in lithium vapor with the use of angled excitation beams

Two-pulse photon echoes on the 2²S1/2-2²P3/2 transition in lithium vapor are generated using laser excitation pulses with angular separations of 0.70 and 1.40 mrad. The observed degradation of echo intensity with angling of the excitation pulses is in agreement with theory. The largest echoes radiated by the lithium sample contained 10⁹ photons; the smallest echoes detected contained only an average of 0.04 photons. This intensity range of greater than 10¹⁰ represents measurements made over more than 15 natural lifetimes of the 2²P3/2 state despite the degradation due to angling. Modulation of the echo signal due to the hyperfine structure of the 2²P3/2 state is also observed.