Coherent production of phonons from time-dependent strains

A simple model of a classical oscillator, with time-dependent frequency, is applied to the normal modes of condensed systems. Taking advantage of the analogy with the stationary Schrodinger equation in one dimension, the existence of special modes (s modes), with exponentially increasing amplitude, is proven, both for disordered and for periodic time-dependent strains. Anharmonic effects are then accounted for in terms of absorption processes for the excess phonons (s phonons), coherently produced by the strain. A nonmetallic continuum lattice is used as a reference system for applications. Possible macroscopic effects, such as the resonant activation of optical modes and the production of time-controlled shock waves, are outlined.