Spurious surface finish improvement of the infrared extinction coefficient of alpha quartz

Infrared extinction coefficients obtained using quartz samples lapped with a 3 mum abrasive compound were found to be much smaller than anticipated. The improvement in extinction coefficient is traceable to the irregular surface finish. To demonstrate this improvement, polished quartz samples were evaluated and compared to evaluations after lapping the same sample with 3 mum and 12 mum abrasives. In one sample, a 3 mum finish resulted in an almost 30% improvement in the extinction coefficient evaluated at 3500 cm(-1). Lapping of the sample to a 12 mum finish resulted in an almost 90% improvement. These improvements in alpha correspond to improvements in Q(IR ) (3500) from 2.58 million (polished) to 3.03 million (3 mu) to 4.71 million (12 mum). The improvement is caused by wavelength dependent transmission losses due to surface irregularities. The surface transmission losses can be reduced by polishing the optical surfaces or by applying index matching fluids to 3 mum lapped surfaces.