Surface ultrasonic envelope solitons and wave collapse in solid film-substrate systems

An excitation of ultra-high frequency (100 MHz - 1 GHz) nonlinear envelope solitary acoustic waves, propagating along the interface between a solid film and a solid substrate, is theoretically analyzed. Both the quadratic nonlinearity and the cubic one are important in the case of the envelope waves. When generation of higher harmonics is reduced due to essential waveguide dispersion and the cubic nonlinearity due to the induced zero harmonic is dominating, a possibility of the envelope solitary pulse propagation and the spatial-temporal wave collapse exists, as demonstrated. When the cubic material nonlinearity reduces the associated cubic nonlinear term, there also exists a possibility to observe a wave collapse, if the initial focusing of the input pulse at the first harmonic is applied.