1. From photophone to photoacoustics

Photoacoustic detection involves exciting molecules with light and then detecting their non-radiative de-excitation using an acoustic microphone. The technique is far from new, dating back to 1880, when Alexander Graham Bell discovered this effect in solids and, later, in gases. The process was forgotten and then rediscovered with the invention of the microphone in the 1930s and the laser in the 1970s.

Photoacoustic measurement methods are finding increasing application in spectrometry and in the study of the interaction of light with matter. The photoacoustic effect consists in the formation of acoustic waves in a sample periodically illuminated by light. The origin of this effect is explained by non-radiative transitions transforming part of the energy of the absorbed radiation into thermal energy which, under certain conditions, gives rise to acoustic vibrations....