ABSTRACT
This chapter focuses on applications of nanoparticles in enhanced optical nonlinearities. The role of acoustic resonance of nanoparticles in nonlinear optical response, where the electrostrictive force excites the acoustic modes of nanoparticles and produces a traveling periodic variation in refractive index of the sample, is investigated. In these measurements, even a weak laser can produce extraordinarily strong four-wave mixing signal above a critical threshold intensity, which means extremely high-quality factors for the nanoparticle vibrations, as if the water damping disappears entirely. Interestingly, it can be seen in the measurement that a smaller bandwidth corresponds to smaller polystyrene nanoparticles due to the fact that larger particles have the resonances approaching to the collision rate for water of 20 GHz , leading to the higher damping and a larger bandwidth. Acousto-optics is a branch of physics that deals with the interaction of sound and light. The diffraction of light by acoustic waves of short wavelengths was first postulated by Leon Brillouin in 1922.
