Sound velocity study in multilayer ultrathin films using wavelet transform

Ultrafast spectroscopy of thin films reveals mechanisms responsible for relaxation of charge carriers’ energy to lower energy states. In the case of a strong enough excitation pump pulse, a shock wave will travel with the sound velocity, longitudinally in the thin film. Due to the interference effect, the shock wave superimposes a sinusoidal wave on the transient reflectivity, which can be used to calculate the sound velocity. In lower thicknesses, this sinusoidal behavior is not analyzable by Fourier transforms. Wavelet transforms, which use scalable wavelets localized in time and space, are perfect tools to analyze these fast-vanishing oscillations. Wavelet analysis has been applied to the residual sinusoidal TR to extract information related to different oscillatory modes, in multilayer ultrathin films. This approach can isolate the location of various oscillatory modes with close energy ranges, at the surface, interface and in the bulk of the heterostructures. As a result, implementation of wavelet transforms provided information regarding the sound velocity and acoustic phonons in multilayer thin films.