Anomalous Absorption of Bulk Phonons by a Two-Dimensional Transient Layer
It is known from the theory of waves in layered structures that a thin nonabsorbing layer on the free surface of a solid results only in a small change in the phase of a total reflection of bulk acoustic waves. The acoustic absorption in the layer leads to the variation (with the wave frequency) also of the reflection amplitude. When the layer posesses both the resonant and absorbing properties the phenomenon of anomalous absorption of bulk acoustic waves by a two-dimensional layer can take place. This phenomenon is characterized by an absence of the reflection and by an almost complete surface absorption of the incident wave. The origin of this effect is a strong enhancement of the resonant oscillations amplitude of a light and weakly bonded thin layer, which in the presence of the finite and relatively low absorption in the layer leads to an anomalously large ”absolute” value of acoustic losses of the incident bulk wave. The width of the resonant absorption curve is proportional to the ratio between the effective elastic impedance of a two-dimensional layer and the acoustic impedance of a solid. This effect can be experimentally revealed on two-dimensional layers of helium (or other noble gases), adsorbed on a solid (see, e.g., ), or on two-dimensional polymer or liquid-crystal Langmuir-Blodgett adsorbed films (see, e.g., ). When a weakly bonded resonant two-dimensional absorbing layer is placed at the interface between two different crystals the phenomenon of anomalous surface absorption can take place for the incident bulk wave in the conditions of a total reflection (in the absence of the transient layer). In this case the phenomenon is characterized by an almost complete absence of both reflected and transmitted bulk waves. All vibrational energy of the incident acoustic wave is absorbed at the interface, and then is reemitted in the adjacent media via the thermal phonons.