Diamond, beryllia, and cubic zirconia are potential optical window materials that feature high strength, hardness, and high
melting temperatures. Diamond, in particular, is receiving considerable attention concerning the fabrication of polycrystalline
films and windows. To fully appreciate the potential of these materials as infrared windows, the optical properties
must be accurately known.
Optical phonons, both single and multiple, determine the intrinsic absorption properties and infrared-active single-phonon
transitions contribute to the index of refraction. Both beryllia and cubic zirconia are composed of ionic bonds. This means
that dipole moments will exist for some of the vibrational modes and they will be infrared active. These modes can be
observed in reflection spectra. The higher harmonics and combination bands of the one-phonon band form the multiphonon
band absorption. The region of the three-phonon band determines the beginning of infrared transparency in these materials.
Intrinsic diamond is composed of covalent bonds with a high degree of symmetry and hence has no infrared active one phonon
bands. However, lattice defects produce weak one-phonon absorption bands in the region of 1000 cm. Also,
multiphonon combination bands of inactive one-phonon and Raman bands produce significant absorption beyond 1800 cm.
Experimental reflection spectra of beryllia and cubic zirconia from 200 to 1200 cm, and experimental transmission spectra
of intrinsic type ila diamond from 500 to 5000 cm are presented. These experimental data are used to determine the
necessary optical parameters for models of the complex index of refraction as a function of frequency and temperature.