What is Photoacoustics

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Experimental setup

The gas detection method: light into sound

The original paper by Alexander Graham Bell.

Alexander Graham Bell (1880) reported the photoacoustic effect for the first time writing: [1] "..thin disks of very many substances emitted sounds when exposed to the action of a rapidly interrupted beam of sunlight.." The sound production from those solid samples resulted from the absorption of the infrared part of the solar energy spectrum.

First application of the effect. Can you hear the light? Bell's 'photophone'

The photoacoustic effect is based on the generation of acoustic waves as a consequence of light absorption [2,3]. Absorption of an infrared photon excites a molecule into a higher ro-vibrational state. Collisions transfer the ro-vibrational energy to translational energy, i.e. heat. Modulation of the light intensity (turning the light on and off) causes the temperature of the sample to rise and fall periodically. For a gas sample in a closed volume this temperature variation is accompanied by a pressure variation which creates a sound wave that can be detected with a sensitive microphone.

The pressure amplitude, and subsequently the microphone signal, is proportional to the number of absorbing molecules present in the gas (i.e. the trace gas concentration). It is also proportional to the absorption strength of the molecules at a specific light frequency; the 'fingerprint-like' absorption pattern over the infrared region can identify a specific molecule. The microphone signal can be enhanced by increasing the laser power, the use of efficient acoustic amplifiers and by employing sensitive microphones.

An artist view of the Experimental Setup.

What is Photoacoustics