Enhanced photoacoustics from gold nano-colloidal suspensions under femtosecond laser excitation.

Enhanced photoacoustic (PA) intensity from gold nanosphere and nanorod colloidal suspensions in water under tightly-focused femtosecond pulsed laser irradiation was systematically investigated. PA signal amplitudes were measured by ultrasound transducers at frequencies of 5, 10, and 25 MHz. The experimental results revealed a linear-dependence of the relative photoacoustic amplitude on the laser power and the mechanism was attributed to non-radiative relaxation dynamics of surface plasmon oscillations. When gold nanorod with longitudinal absorption/extinction peak at 800 nm coincides with the wavelength of femtosecond laser pulses, the most efficient PA signal is generated. Laser excitation was kept within a thermal stability region of gold nanoparticles, i.e., colloidal suspension can be continuously reused for PA generation.