Theory of coherent acoustic phonons in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{GaN}$ multiple quantum wells

A microscopic theory for the generation and propagation of coherent LA phonons in pseudomorphically strained wurtzite (0001) ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{GaN}$ multiple quantum well pin diodes is presented. The generation of coherent LA phonons is driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump laser and is treated theoretically by using the density matrix formalism. We use realistic wurtzite band structures taking valence-band mixing and strain-induced piezoelectric fields into account. In addition, the many-body Coulomb interaction is treated in the screened time-dependent Hartree-Fock approximation. We find that under typical experimental conditions, our microscopic theory can be simplified and mapped onto a loaded-string problem that can be easily solved.