Abstract

Photon echoes are the optical analogue of spin echoes and are usually described by a model of two-level atomic system. Many experiments so far have been explained in this framework. The phase relaxation times T₂ of condensed matter like solid and liquid are generally very short even at very low temperatures and often fall in the picosecond or femtosecond region. By the recent progress in the technology of ultra-short pulse lasers, the photon echo experiments in a material with such a short T₂ have become possible. Even without ultra-short pulses, picosecond or femtosecond photon echo experiments are now available by using temporally-incoherent excitation light[1,2]. In these experiments, measured values of T₂ are sometimes in the picosecond or femtosecond region even at very low temperatures. We cannot attribute such a short T₂ to thermal fluctuations, since it violates the energy conservation law in the system.

© 1988 Optical Society of America