Enhanced surface optical phonon in CdTe thin film observed by Raman scattering

Abstract

Surface optical (SO) phonon modes are characteristic atomic vibrations confined within the first several atom layers of the free surface of a polar solid material. Raman scattering of SO modes is hardly to be observed due to extremely weak scattering from few surface atom layers. In principle, the SO modes are associated with a long-range Fröhlich interaction with the optical modes in a polar material and would lead to strong or weak anharmonic coupling with these modes depending on mode polarization. In this report, both the SO and the bulk Raman scattering efficiency were dramatically enhanced in CdTe thin films by employing silver nano-plasmonic enhanced Raman scattering and maximized free surface atomic vibration. Raman scattering phonon overtones of the longitudinal optical (LO) mode up to the fifth order were observed. Strong and rich information on the multi-phonon anharmonic couplings of the SO mode with the LO mode and the overtones was well resolved and clarified.