Photoacoustic spectroscopy

Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk. The absorbed energy from the light causes local heating, generating a thermal expansion which creates a pressure wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum (i.e., the infrared and the ultraviolet) can also produce sounds. Although Bell discovered the underlying effect, the ability to extract meaningful material information through photoacoustic signals—forming the basis of photoacoustic spectroscopy—was not achieved until nearly a century later. In the 1970s, physicist Allan Rosencwaig developed the theoretical foundations and experimental techniques that established photoacoustic spectroscopy as a powerful analytical tool. His work, including the development of the Rosencwaig–Gersho model, enabled quantitative interpretation of photoacoustic signals in solids and laid the groundwork for practical applications in condensed matter physics, semiconductor diagnostics, and biomedical imaging. These developments are discussed extensively in Rosencwaig’s monograph, Photoacoustics and Photoacoustic Spectroscopy, Wiley, 1980. A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths of the light. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solids, liquids and gases.