Optoacoustic Sources: a Practical Green Function-Based Model for Thin Film Laser-Ultrasound Generation

The theory of optoacoustic generation of ultrasound in fluids through the thermoelastic effect has been addressed, considering the case of a thin absorbing film used as a target for the laser light. A novel theoretical method, based on the use of the Green function and of the image theorem, has been developed and discussed. The model is mainly carried out in time and space domains, and connects the sound field directly to the laser pulse intensity through equations that show the relevant underlying phenomena. With respect to standard double transform (Laplace + Hankel) techniques, it ensures a simpler physical understanding and appears to be more robust from the computational point of view. Moreover it allows us to identify the approximations that may simplify many practical problems. Many approximate formulae are deduced, in addition to the most significant ultrasonic pressure field parameters, such as amplitude and bandwidth, which are explicitly evaluated in terms of laser excitation and of the materials characteristics.