Photoacoustic chopping frequency studies on uncoated zinc selenide

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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 CO2 laser operating at 10.6 μm 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°; between 1000 and 500 Hz and 100 Hz, of 6°. Laser rate calorimetry on these samples gave an average βeff = βB + 2βs/l = 4.32 × 10−3 cm−1, where l is the sample thickness, βB is the bulk optical absorption coefficient in cm−1, and βs, 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 ≡ βs/βB, was r = 0.025 cm. Using this value yields βB = 4.06 × 10−3 cm−1 and βs = 0.000102. Thus for these samples 94% of the measured βeff is due to βB. 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.