Local variations and temperature dependence of optical absorption coefficient in natural IIa type and CVD diamond optical windows

We used the photothermal deflection (PTD) technique working in a collinear mode to measure optical absorption coefficient at the CO2 laser wavelength (10.6 mum) of polished CVD and natural type IIa diamond optical windows. Th e CO2 pump beam is focused to a spot size of about 100 mum, enabling the lo calized detection of variations in the optical absorption coefficient, For the first time. we present detailed 2D absorption maps of CVD diamond optic al 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 i ncreases almost one decade at 900 K in natural type IIa single crystal diam ond. Our results indicate that the intrinsic optical absorption at 10.6 mun is caused by two-phonon acoustic-acoustic interactions.