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.

中文翻译：

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两束不同频率调制的激光束照射单个粒子所产生的外差光声效应

当调制光束被不可压缩的粒子吸收时，光声效应可以通过热扩散到周围流体中，然后热膨胀和声音的产生而发生。当以不同频率调制的两束激光束照射水溶液中的粒子时，一束激光的作用是以调制频率调制粒子附近溶液的热膨胀系数。由于吸收来自第二个激光器的辐射，热扩散到流体的同一区域发生在流体中，其中热膨胀系数被调制，从而在两个激光器的和频和差频处产生光声效应。这里，