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

Area of Science:

• Optics
• Materials Science
• Acoustics

Background:

• Accurate measurement of thin film optical properties is crucial for various applications.
• Existing methods for measuring thin film absorption can be time-consuming or lack sensitivity.
• Laser-induced heating and acoustic detection offer a potential pathway for sensitive absorption measurements.

Purpose of the Study:

• To demonstrate a novel infrasonic method for measuring the absorption of thin films at laser wavelengths.
• To evaluate the sensitivity and measurement time of the developed infrasonic technique.
• To assess the applicability of the method for characterizing optical coatings.

Main Methods:

• Utilized an Alphaphone setup where absorbed laser energy heats a thin film and a supporting window.
• Employed a capacitance microphone to detect gas pressure and temperature changes induced by heat transfer.
• Investigated different chopping speeds for laser irradiation and cooling to optimize sensitivity and measurement duration.

Main Results:

• Achieved high sensitivity, reaching 1.5 x 10(-7) absorption per surface with prolonged irradiation.
• Demonstrated a practical measurement time of 1 minute with adequate sensitivity (10(-4) absorption) by optimizing chopping speed.
• Successfully tested various thin film samples, including calcium fluoride and zinc sulfide on thorium fluoride.

Conclusions:

• The infrasonic method provides a sensitive and efficient approach for measuring thin film absorption at laser wavelengths.
• Optimizing the chopping frequency balances sensitivity and measurement time, making the technique practical for optical coating characterization.
• The method shows promise for absolute calibration using a thin-film heater.