  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(1)p-pol. :E(2)s-pol. 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.
