SIGNATURES OF PHONON COHERENT STATES

Creation and detection of states of the phonon field are discussed. I briefly compare phonon coherent states (PCS) with Fock and thermal sta tes in two harmonic systems, the diatomic molecules and a non-conducti ng crystal with local centers. I analyze some of the phenomenology sug gesting the cases where PCS are generated and report on their unmistak able fingerprint: the Poisson distribution among the Fock states and t he oscillating Gaussian of the displaced oscillator generated in ultra fast processes. The PCS created in the ''pump and probe'' femtosecond spectroscopy are specially commented. The motion of PCS is seen to be responsible of the persistent oscillations discovered at ultrashort ti mes, but also, in rare cases, of a very fast decay when a true acousti cal PCS is created by the ultrafast excitation.