The alphaphone - - a method for measuring thin-film absorption at laser wavelengths.

A sensitive infrasonic method for measuring the absorption of thin films at laser wavelengths has been demonstrated. A fraction of the heat absorbed from the beam by the film passes through a thin layer of gas to the back wall of the Alphaphone, while the remaining absorbed energy heats the supporting window. The gas temperature and pressure rise is then measured with a capacitance microphone. Sensitivity as great as 1.5 x 10(-7) absorption per surface could be achieved with 10 W input by allowing irradiation to continue until the window is saturated with heat. However, the waiting time for saturation even with thin windows would be about 5 min, and with lock-on amplifier demodulation the total measurement time would be several hours. If the chopping speed is increased to 1.5 sec for irradiation and 1.5 sec for cooling, adequate sensitivity of 10(-4) absorption with a signal-to-noise ratio of 9 is achieved with a total measurement time of 1 min. The samples we tested were KRS-5 substrates coated with layers one quarter-wave thick at 10.6 microm. One sample was a single layer of calcium fluoride. Other samples had a layer of zinc sulfide over a binder layer of thorium fluoride. The instrument may be calibrated absolutely with a thin-film heater described large as several percent.