Ultrafast excitation and detection of acoustic phonon modes in superlattices

We have performed theoretical and experimental studies on the surface and bulk acoustic phonons in superlattices excited with ultrafast lasers. Analytical expressions for the detectability and excitability of normal modes are derived from the elastic continuum theory. They are formulated in the form of overlap integrals, which evaluate the similarity between the normal modes and the optical sensitivity or thermal stress pattern. The calculation is simple, yet it offers physical insight into the generation and detection of normal modes. Marked differences are observed between Si∕Mo and Be∕Mo superlattices due to their distinct thermal properties. The amplitudes and line shapes of surface and bulk phonon modes are well explained by the theory.