Photoacoustic chopping frequency studies on uncoated zinc selenide

Abstract

Experimental photoacoustic (PA) signal amplitude and phase data were obtained as a function of chopping frequency on chemical vapor deposition (CVD) grown zinc selenide. The light source was a CO(2) laser operating at 10.6 microm with about a cw 24-W power level. The PA amplitude had an f(-n) dependence with n = 1.04. The PA phase angle plot yielded a phase difference between 1000 and 500 Hz and 50 Hz of 13 degrees ; between 1000 and 500 Hz and 100 Hz, of 6 degrees . Laser rate calorimetry on these samples gave an average beta(eff) = beta(B) + 2beta(s)/l = 4.32 x 10(-3) cm(-1), where l is the sample thickness, beta(B) is the bulk optical absorption coefficient in cm(-1), and beta(s), is the dimensionless surface absorption. The average value of l was 0.79 cm. Theoretical calculations were made using several PA theories and parameters relevant to CVD ZnSe. A modified version of the Rosencwaig-Gersho (RG) theory was derived to explicitly include surface absorption. The RG theory calculations yielded results that differed greatly from experiment. Calculations using the McDonald-Wetsel composite-piston model, an extension of the RG theory, yielded results closer to experiment. A comparison of various calculations using the Bennett-Forman theory indicated that the best consensus value for the surface-to-bulk optical absorption ratio, r identical with beta(S)/beta(B), was r = 0.025 cm. Using this value yields beta(B) = 4.06 x l0(-3) cm(-1) and beta(s) = 0.000102. Thus for these samples 94% of the measured beta(eff) is due to beta(B). This value for r is about an order of magnitude smaller than that obtained from a study on similar samples using the multithickness technique. Calculations using the modified RG theory were almost identical to the results of the Bennett-Forman theory for the same value of r.