Local Variations and Temperature Dependence of Optical Absorption Coefficient in Natural IIa Type and CVD Diamond Optical Windows

Abstract

We used the photothermal deflection (PTD) technique working in a collinear mode to measure optical absorption coefficient at the CO 2 laser wavelength (10.6 mm) of polished CVD and natural type IIa diamond optical windows. The CO 2 pump beam is focused to a spot size of about 100 mm, enabling the localized detection of variations in the optical absorption coefficient. For the first time, we present detailed 2D absorption maps of CVD diamond optical windows. The regions of enhanced absorption are visualized. The enhanced absorption above 0.03 cm --1 at 300 K is caused by non-diamond inclusions ("black spots"). The residual absorption coefficient 0.03 cm --1 at 300 K increases almost one decade at 900 K in natural type IIa single crystal diamond. Our results indicate that the intrinsic optical absorption at 10.6 mm is caused by two-phonon acoustic-acoustic interactions.

Factors affecting the optical performance of CVD diamond infrared optics

“…In another study, Thomas et al [17] also found the same phenomena in a single-crystal diamond type IIa sample. The temperature dependency of the absorption coefficient for type IIa diamond has been measured using photo thermal deflection measurements on type IIa diamond by Rames et al [19] and they report a one decade increase from α = 0.03 cm −1 at 300 K to α = 0.3 cm −1 at 900 K. All this leads to the conclusion that it is an intrinsic property of diamond. Thus, the underlying temperature-dependent phenomenon is diamond type, CVD diamond grade and surface state-independent and it is not an extrinsic property exclusive to CVD diamond.…”

Optical properties of epitaxial single-crystal chemical-vapor-deposited diamond

“…Very few absorption measurements in diamond are available in the literature, and most of them have been performed at 10.6 µm, where high-order phonons processes produce a weak absorption band 18 . To the best of our knowledge, the only published measurements around 1 µm are Mollart's et al 19 and Godfried's et al 6 .…”