The phononic band structure of two binary colloidal crystals, at hypersonic frequencies, is studied by means of Brillouin light scattering and analyzed in conjunction with corresponding dispersion diagrams of the single colloidal crystals of the constituent particles. Besides the acoustic band of the average medium, the authors’ results show the existence of narrow bands originating from resonant multipole modes of the individual particles as well as Bragg-type modes due to the (short-range) periodicity. Strong interaction, leading to the occurrence of hybridization gaps, is observed between the acoustic band and the band of quadrupole modes of the particles that occupy the largest fractional volume of the mixed crystal; the effective radius is either that of the large (in the symmetric NaCl-type crystalline phase) or the small (in the asymmetric NaZn13 -type crystalline phase) particles. The possibility to reveal a universal behavior of the phononic band structure for different single and binary colloidal crystalline suspensions, by representing in the dispersion diagrams reduced quantities using an appropriate length scale, is discussed.

The optical response of finite slabs of polar materials, containing two- and three-dimensional periodic structures of air cavities, is studied by means of accurate numerical calculations using the layer-multiple-scattering method. Our results reveal the existence of strong resonant modes, originating from the excitation of flat-surface and cavity phonon-polaritons, which may be useful in terahertz applications.

The present work demonstrates the first application of Brillouin light scattering (BLS) to probe film-guided elastic waves in transparent-substrate supported polymer thin films. In comparison with earlier BLS studies that were restricted to films either free-standing or supported on opaque substrates, the progress made in this work substantially extends the applicability of BLS and permits direct access to the elastic properties of thin films lying on transparent substrates, which is of important practical relevance. A series of thin supported polystyrene and poly(methyl methacrylate) films with thickness in the range of 40−500 nm were explored, and no noticeable trend in elastic properties with thickness has been found, in conformity with earlier BLS results. The first measurement of glass transition temperature, Tg, of supported polymer thin films by BLS is also reported. We observed that the ultrathin (40 nm) films for both polymers exhibit a clear reduction in Tg.