Diamond is transparent from the UV (225 nm) to the far infrared. Only minor absorption bands exist resulting from two-phonon absorption between 2.5 and 6.5 µm. This makes diamond an ideal material for multispectral optical applications. Diamond’s optical properties are isotropic with an index of refraction of 2.4 in the visible region.
A celebrated early use of a diamond window was for the IR emission sensor of the Venus explorer. Diamond’s particular combination of very low absorption coefficient and its very high thermal conductivity (up to 2,000 W/mK at room temperature) facilitates it’s widespread use as a window for high power lasers. Conventional materials (a) absorb too much of the laser energy, (b) may become nonlinear under the intense energy levels, and (c) cannot dissipate the heat from the center of the window to the outer periphery where a cooling apparatus may be attached. The use of diamond not only reduces absorption by several orders of magnitude, but the absorbed energy is quickly conducted to the window periphery where it may be dissipated safely. Optical mirrors are also similarly vulnerable to high energy laser beams. Diamond mirrors or diffraction gratings have been calculated to reduce the length of a free electron laser 100-fold by reducing the need to diverge the beam to lower power density.
Seitz, R., Laser systems with Diamond Optical Elements, U.P. Office, Editor. 1975: USA.