Infrared reflectivity by transverse-optical phonons in (GaAs)m/(AlAs)nultrathin-layer superlattices

We report far-infrared reflectivity measurements on (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$ superlattices (SL’s) with systematically varied layer thicknesses in the range 1≤m,n≤7. Taking advantage of an appropriate choice of the total SL thickness D≤0.3 μm, we measure the frequencies of AlAs-like ${\mathrm{TO}}_1$ confined phonons from the peak of the reststrahlen band. The GaAs-like ${\mathrm{TO}}_1$ frequencies are obtained by fitting reflectivity spectra to the SL dielectric-response-theory model. Microscopic calculations of confined TO frequencies are performed within an ab initio scheme and successfully compared with the experimental data.