Evidence of laser-wavelength effect in picosecond ultrasonics: Possible connection with interband transitions

We have studied the effect of wavelength change on picosecond acoustic pulses generated using a femtosecond laser. For the first time, we show that the pulse shape can be strongly influenced by the laser wavelength. The results are in excellent agreement with a calculation based on a thermoelastic model which connects them to significant changes in the piezo-optical constants. There are similarities between the present study and stress modulation spectroscopy, which allows us to ascribe the observations to interband transitions and suggests thus a new potentiality of picosecond ultrasonics.