Heterodyned photoacoustic effect generated by irradiation of single particles by two laser beams modulated at different frequencies

•A Heterodyned photoacoustic effect is reported.•The heterodyning depends on change in Grueneisen constant with temperature.•The amplitude and phase of the heterodyned signal is shown to change with frequency.•Determination of the distance between two chromophores is discussed. When a modulated light beam is absorbed by an incompressible particle, the photoacoustic effect can take place through heat diffusion into the surrounding fluid followed by thermal expansion and the generation of sound. When two laser beams modulated at different frequencies irradiate a particle in aqueous solution, the effect of one laser is to modulate the thermal expansion coefficient of the solution in the proximity of the particle at its modulation frequency. Heat diffusion into the same region of fluid from absorption of radiation from the second laser takes place in fluid where the thermal expansion coefficient is modulated so that the photoacoustic effect is produced at the sum and difference frequencies of the two lasers. Here, a theory for the photoacoustic effect at the heterodyne frequency for a single particle and the corresponding experiments with carbon particles of different sizes are reported.