Photoacoustic signal enhancements from gold nano-colloidal suspensions excited by a pair of time-delayed femtosecond pulses

Photoacoustic signal enhancements were observed with a pair of time-delayed femtosecond pulses upon excitation of gold nanosphere colloidal suspension. A systematic experimental investigation of photoacoustic intensity within the delay time, ∆t = 0 to 15 ns, was carried out. The results revealed a significant enhancement factor of ∼ 2 when the pre-pulse energy is 20-30% of the total energy. Pre-pulse and main pulse energy ratios, E_p⁽¹⁾:E_s⁽²⁾, were varied to determine the optimal ratio that yields to maximum photoacoustic signal enhancement. This enhancement was ascribed to the initial stage of thermalization and bubble generation in the nanosecond time scale. Pre-pulse scattering intensity measurements and numerical finite-difference time-domain calculations were performed to reveal dynamics and light field enchancement, respectively.