Photoacoustic chopping frequency studies on uncoated zinc selenide.

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.

Helmholtz resonance effect in photoacoustic cells.

Some types of photoacoustic spectroscopy sample cells exhibit acoustic resonances which are not explained by organ pipe or Bessel mode analysis. It is demonstrated here that one type of acoustic resonance encountered is explained by a Helmholtz resonator analysis. Its properties are studied as a function of the dimensions of the channel connecting the sample chamber and the microphone housing chamber.

Effects of two-photon absorption on degenerate four-wave mixing in solutions of diphenyl polyenes.

We have observed double-peaked phase-conjugate pulses in the degenerate four-wave mixing of nanosecond laser pulses in solutions of diphenyl polyenes. We attribute this to the superposition of fast and slow gratings, where the slow grating is due to two-photon absorption. This is supported by the results of time delay and polarization experiments as well as evidence of a second spatial harmonic grating.

Improved crystal quality and harmonic generation in GaSe doped with indium.

GaSe crystals were doped with indium, and improvements in the mechanical properties and second-harmonic efficiency over pure crystals were obtained. Both effects are due to an improvement in the crystal quality of the material, and it was shown that doping with low levels of indium did not alter the intrinsic value of the nonlinear d coefficient.