Thermal Transient Gratings as Probes for Intrinsic Localized Modes

Recent studies on intrinsic localized modes or breathers in extended nonlinear lattices have demonstrated spontaneous thermal generation of nonlinear modes accompanied by very long relaxation time scales. The nonexponential lattice relaxation to equilibrium has been connected to local energy accumulation induced by nonlinearity while the decay law depends in general on the lattice temperature. We propose the use of transient thermal gratings as a means for experimental study of breathers at finite temperatures. We show that the thermal grating signal decays as a power law, in marked contrast to the usual exponential law followed in linear systems. In the models we study, we assume electronic decay much faster than vibrational relaxation, leading to dilferential lattice heating that results in thermal gradients.