MHz-ultrasound generation by chirped femtosecond laser pulses from gold nano-colloidal suspensions.

Abstract

Strong absorption of femtosecond laser pulses in Au nano-colloidal suspensions was used to generate coherent ultrasound signals at 1-20 MHz frequency range. The most efficient ultrasound generation was observed at negative chirp values and was proportional to the pulse duration. Maximization of a dimensionless factor A ≡ αc0tp defined as the ratio of pulse duration tp and the time required for sound at speed c0 to cross the optical energy deposition length (an inverse of the absorption coefficient α) given by 1/(αc0). Chirp controlled pulse duration allows effective enhancement of ultrasound generation at higher frequencies (shorter wavelengths) and is promising for a high spatial resolution acoustic imaging.