Acoustic waves undetectable by transient reflectivity measurements

In laser ultrasonics, ultrashort light pulses generate coherent acoustic pulses of picosecond duration via multiple possible physical mechanisms, involving optoacoustic conversion processes. These wide-band GHz acoustic signals are optically detected at the sample surfaces by ultrafast time-delayed probe light pulses. When the coherent acoustic pulses in GaAS are detected via the Brillouin scattering of probe light pulses of 400 nm wavelength, certain spectral components of the acoustic pulses remain invisible. The theoretical analysis relates this observation to the existence of zeros in the spectral transformation function of acousto-optic conversion. The phenomenon of zero sensitivity of the optical reflectivity to coherent acoustic phonons of particular frequencies is rather general and depends on both the probe light wavelength and the evaluated material. These findings substantially contribute to picosecond ultrasonics and laser-based nondestructive testing.