Photon echo

A photon echo is an optical analogue of nuclear magnetic resonance , ^[1] coherent radiation of a medium in the form of a short pulse, due to the restoration of the phase matching of individual emitters after exposure to a medium of a sequence of two or more short pulses of resonant radiation. The photon echo effect is an optical analogue of the spin echo phenomenon known in radio spectroscopy. It occurs when two radiation pulses are transmitted through the medium at a frequency corresponding to the transition between energy levels and allows one to measure the measure of coherence of the excited state.

The first exciting pulse transfers the atoms to an excited coherent state in which all elementary dipoles are phase-coupled (in the optimal case, the area of ​​this pulse is ^π ⁄ ₂ ). At the end of the action of this pulse, the induced macroscopic polarization of the medium gradually decreases. There is a misphasing of the dipole oscillations.

Under the action of the second pulse, the Doppler phases of the oscillators change sign, and the misphasing is replaced by phasing ( π pulse). When all the oscillators are again completely phased, an echo pulse of coherent radiation is formed.

As a rule, a three-pulse measurement is applied using the so-called stimulated photon echo . The process of generating a stimulated photon echo is similar to the formation of a photon echo. A stimulated photon echo is formed by three pulses. As in the case of the photon echo, the first exciting pulse creates a polarization of the medium. The second converts this polarization into a population difference. The third does the inverse transformation and changes the signs of the phases. Such an experiment makes it possible to measure the population of the ground and excited states of the medium. ^[one]