Photoacoustics of Gold Nanorods under Low Frequency Laser Pulses in Optical Hyperthermia

When aqueous suspensions of gold nanorods are irradiated with a pulsing laser (808 nm), pressure waves appear even at low frequencies (pulse repetition rate of 25 kHz). We found that the pressure wave amplitude depends on the dynamics of the phenomenon. For fixed concentration and average laser current intensity, the amplitude of the pressure waves shows a trend of increasing with the pulse slope and the pulse maximum amplitude. We postulate that the detected ultrasonic pressure waves are a sort of shock waves that would be generated at the beginning of each pulse, because the pressure wave amplitude would be the result of the positive interference of all the individual shock waves.