Heat generation by optical resonance in a layered photoacoustic ultrasound generator

Laser light irradiated onto a layered photoacoustic (PA) ultrasound generator is absorbed by a highly light-absorptive thin metal film, swiftly converted into and released as heat. Subsequently, through heat conduction, the adjacent polymer layer with excellent thermoelastic property experiences temperature rise, generating elastic/acoustic waves. Therefore, the photothermal effect significantly influences the performance of the PA generator. In this study, theoretical analyses of M (thin metal film)-P (polymer layer) and M-P-M* layered structures were performed to understand the basic operating principle how optical resonance induced by a Fabry-Perot optical cavity formed of two thin metal films can enhance heat generation. To this end, the Maxwell Helmholtz equation was solved in the frequency (Fourier) domain to achieve the complex electric field and then predict the heat generation by assuming that the PA generator is one-dimensional in space and follows a sinusoidal variation in time. Results show that the introduction of another thin metal film obviously induces optical resonance between the two metal films, which may further improve heat generation. Moreover, it is found that heat generation depends largely on the thicknesses and materials of the layered structure forming the optical cavity. Specifically, to maximize heat generation by optical resonance, the thickness of the polymer layer must be approximately an odd number multiple of the half wavelength of the transmitted light.