Infrared Absorption by Coupled Surface-Phonon-Surface-Plasmon Modes in Microcrystals of CdO

The first observation of surface-phonon-surface-plasmon coupling in small particles is reported. Thin layers of semiconducting CdO microcrystals with various free-carrier concentrations were used as samples. The coupling manifests itself in the infrared-absorption spectra through two absorption maxima corresponding to two resonances of the coupled two-oscillator system and through a pronounced absorption minimum near the transverse-optical-phonon frequency ${\omega }_T$. The experimental results are discussed on the basis of a theoretical approach recently developed by Genzel and Martin. Structure in the absorption spectra near the longitudinal-optical-phonon frequency ${\omega }_L$ is attributed to the strong polar-optical scattering of free carriers in CdO.