Knowledge Bank

Diamond is a purely covalent material with no fundamental infrared active vibrational modes. Intrinsic diamond is completely transparent below the band gap except for two-, three- and higher-orders multiphonon absorption bands located in the mid-IR. Results on experimental measurements of infrared transmittance on Type lla and CVD diamond as a function of temperature are presented. The measurements show the first observation of the four-phonon band in diamond and the two-phonon red wing out to $900cm-1$. Diamond absorptance near $1000cm-1$ is of particular interest because many infrared sensors operate at this atmospheric window. Multiphonon absorption band models have been successfully developed for ionic materials. This is the first application of such a model on a purely covalent material. To obtain a fit to the experimental data, the phonon density of states function required considerable modification relative to other ionic materials studied. Evidence indicates that the absorption in this region is caused by two-phonon acoustic-acoustic interactions. In a most materials, pure acoustic multiphonon absorption usually in not measurable because it is observed by strong one-phonon optical mode absorption. Diamond has very high acoustic frequencies owing to its strong bonds, and the lack of fundamental absorption unmasks the pure acoustic contribution. This acoustic contribution is modeled by applying a Debye acoustic phonon density of states distribution function. Good agreement with room temperature experimental data is obtained.