Coherent phonons and their properties

The optical phonons in semimetals, semiconductors, and superconductors were studied by the light reflection techniques with femtosecond time resolution and by the method of spontaneous Raman scattering. During measurements in the time domain, the phonon system is converted into a coherent state by the first ultrashort laser pulse and then probed at a variable delay by the second pulse. In this case, the phonons are shown to occur in a nonclassical state in which their fluctuational properties, different in various quadratures, are described by periodic functions of time. A comparison of the results obtained in the time and frequency domains gives evidence that the energies of thermal and coherent phonons coincide, while their dephasing and energy relaxation times are different.