Wavelength-dependent Faraday-Tyndall effect on laser-induced microbubble in gold colloid

The cavitation microbubbles in dilute gold colloids of different concentrations (2-10. ppm) induced by a focused nanosecond-pulsed laser beam were measured and characterized at different wavelengths by using the passive and active ultrasound measurements. Three colloids with gold nanoparticles (GNPs) of different sizes (10, 45, and 75. nm) were used for experiment. The results show that the lifespan of the microbubble is reduced as the concentration of GNP increases, particularly at the wavelength of 532. nm, the surface plasmon resonance (SPR) of GNP. In contrast, at the off-resonant wavelength (e.g. 700. nm), the lifespan reduction is relatively small. This wavelength-dependent cavitation is attributed to the Faraday-Tyndall effect, a strong light scattering by GNPs. A slight defocusing of the Gaussian beam in gold colloid was proposed. Hence, the waist of the focused beam increases to reduce the optical breakdown in gold colloid. For simplicity, a linear relation between the incremental waist radius of Gaussian beam and the concentration of GNP was assumed. According to this formulation, the theoretical results are consistent with the experimental ones. In addition, the dynamics of the microbubble in gold colloid measured by the active ultrasound method agree with the Rayleigh-Plesset model.