Photoacoustic Effect
Photoacoustic Cell Design
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Photophone by Bell (1880)

Production of sound by radiant enery, paper by Alexander Graham Bell
















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Photoacoustic Effect

Photoacoustics is the production of acoustic waves by the absorption of light. The history of photoacoustic effect goes back to 1880 when Alexander Graham Bell first observed it while working on a way to transmit sound without any cables. He discovered that if a focused beam of light was rapidly interrupted and allow to fall on a Selenium block, an audible signal could be picked up through a hearing tube. Then the field lay dormant for 50 years until the discovery of the microphone made it possible to enhance the measurements. In 1973 a detailed model for the effect was developed by Allan Rosencwaig and Gersho (R-G Theory).

Photoacoustic Principle

During a photoacoustic measurement the sample is enclosed in a small, tightly closed sample compartment called photoacoustic cell (usually cylindrical in shape).The photoacoustic effect is based on the sensitive detection of acoustic waves launched by the absorption of pulsed or modulated laser radiation via transient localized heating and expansion in a gas, liquid, or solid. When the laser hits the sample, some of the energy is absorbed by the molecules in the samples resulting in a region of higher temperature. The rise in temperature will generate an expanding region and a pressure wave will propagate away from the heat source. The periodic pressure wave can be detected using a pressure transducer in contact with sample (piezo electric)or in contact with contact gas (microphone) in the photoacoustic cell. Please see Fig. (1) for details. The pressure transducer signal is proportional to the amplitude of the pressure wave.

Photoacoustics-Production of signal

The acoustic signal produced by the absorption of pulsed or modulated laser radiation can be amplified by tuning the modulation frequency to one of the acoustic resonance of the photoacoustic cell,(e.g. the radial, longitudinal, or azimuthal mode of a cylinder, if it is a cylindrical cell). In this resonant case the cell works as an acoustic amplifier and the photoacoustic signal is amplified by the Q factor of the resonance with values between 10 and 1000. 

A photoacoustic spectra of the sample can be taken by continuously tuning the wavelength of the exciting light source and taking the photoacoustic signal at each wavelength. If the optical wavelength couples to an energy transition in the sample material, there will be a variation in the photoacoustic signal. The photoacoustic spectra give many valuable information regarding the sample.


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