Photoacoustic signal variations with chopping frequency for ZnSe laser windows

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Abstract
Experimental photoacoustic (PA) signal amplitude and phase angle data were obtained as a function of chopping frequency on a 10.6‐μm antireflective coated ZnSe window using a CO2 laser. The PA signal amplitude had an f−n frequency dependence with n=1.04. The PA phase angle varied by 20° between 1000 and 50 Hz; 7° between 1000 and 100 Hz. Theoretical calculations were made using the Bennett‐Forman, Rosencwaig‐Gersho, and two‐layer Rosencwaig‐Gersho theories. No theory yielded a 20° phase change between 1000 and 50 Hz. The homogeneous sample Rosencwaig‐Gersho theory did not fit either amplitude or phase angle data. Assuming a 1‐μm‐thick ThF4 layer on top of the ZnSe, the two‐layer Rosencwaig‐Gersho theory yielded a coating optical absorption coeffficient, βc =2.7 cm−1 to give a 7° phase change. This then gave an amplitude dependence of n?1.03. The Bennett‐Forman theory agreed with PA amplitude results for r=βsurface /βbulk ≳0.05 cm. A 7° phase change was obtained for r=0.03 cm. The best fit of the Bennett‐Forman and two‐layer Rosencwaig‐Gersho theories gave very similar amplitude and phase results.