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• Multi-phonon absorption
The design of any optically transparent device requires the selection of materials based upon knowledge of their properties and limitations. The lattice absorption characteristics observed at the lower frequency regions (mid-infrared to far-infrared wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching) in the far-infrared spectral region (>10 µm).
The concepts of temperature and thermal equilibrium associated with ionic solids are based on individual atoms and molecules in the system possessing vibrational motion. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies. These thermal vibrational modes are associated with atomic and molecular displacements, producing both longitudinal and transverse waves of atomic and molecular displacement.
In the longitudinal (or acoustic) mode, the displacement of particles from their positions of equilibrium coincides with the propagation direction of the wave. Mechanical longitudinal waves have been also referred to as compression waves. For transverse (or optical) modes, individual particles move perpendicular to the propagation of the wave.