The influence of surfaces and interfaces on coherent phonons in semiconductors

Experiments have shown that ultrafast optical excitation of semiconductors can produce oscillating changes in the optical properties of the material. The frequency of the oscillations in transmission or reflection usually mat ches one of the phonon modes, typically the q = 0 optical mode. These oscil lations are known as coherent phonons. We discuss the role of surfaces and interfaces on the coherent phonon signal. We show that: (1) the coherent ph onon signal can be used as a probe of the surface depletion field and (2) m ultiple interfaces as in a superlattice, can drastically alter the coherent phonon spectrum: screening of the modes in the superlattices is reduced an d acoustic modes can now be excited. (C) 2000 Academic Press.