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

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 ≡ αc₀t_p defined as the ratio of pulse duration t_p 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.