12 - Refraction and reflection at solid/solid interfaces – Experiment

Summary

Having examined the theoretical foundations for phonon scattering at an interface, we now turn to a variety of experiments that involve the interface between two solid materials. It is no surprise that the transmission and reflection of phonons at the boundary of two media have great practical significance, ranging from the dissipation of heat generated by an electrical device to the coupling of phonons into a thin-film detector.

As previously discussed, less-than-perfect interfaces can display more, or less, thermal transmission than that expected from the acoustic-mismatch theory. For the crystal/liquid-He and crystal/rare-gas-film cases, the thermal transmission is much greater than predicted. This is because diffuse phonon scattering at the interface increases the chances of transmission across the boundary.

In view of this, one might wonder if the detailed predictions developed in Chapter 11 for specular refraction and mode conversion at a solid/solid interface can be observed in practice. In particular, can one observe critical cones? … total internal reflection? … mode conversion? In this chapter, we examine phonon-imaging experiments that indeed show such effects of specular phonon scattering at a boundary. And, of course, we find that nature contains some surprises that our limited imaginations did not anticipate. Six different experimental systems are considered: (a) refraction at a specially prepared Ge/MgO interface, (b) transmission through an ordinary sapphire/metal-film interface, (c) internal reflection from high-quality sapphire and Si surfaces, with the crystals immersed in liquid He, and (d) two examples of transmission through internal interfaces: semiconductor superlattices and ferroelectric domains.