PROPERTIES OF PHOTOACOUSTIC WAVES IN ONE-DIMENSION, 2-DIMENSION, AND 3-DIMENSION

The time dependence of photoacoustic waves generated by irradiation of fluid bodies with optical radiation can be found by solution of the w ave equation for pressure. Consider the generation of ultrasonic waves where heat deposition is described by a spatial heating function time s a delta function in time. Solution of the wave equation in one dimen sion, shows that the spatial deposition of heat is directly mapped int o the time profile of the photoacoustic wave. In two dimensions, a cyl indrically symmetric deposition of heat gives a photoacoustic wave des cribed by an integral over a spatial source function. In three dimensi ons, a spherically symmetric deposition of heat gives the photoacousti c wave as a mapping of the product of the retarded time with the spati al heating function. For laser pulses long compared with the transit t ime of sound across the irradiated body, the photoacoustic wave-form i s found to be proportional to the zero'th time derivative (the time pr ofile) of the laser pulse in one dimension, the (1/2)'th time derivati ve of the laser pulse in two dimensions and the first time derivative of the laser pulse in three dimensions. Formulae for multipole radiati on in one, two, and three dimensions are derived from the wave equatio n for sources having arbitrary spatial dependencies.