Materials for nonlinear optics : semicontinuous gold films and fast saturable absorbers

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2008.In this thesis I have investigated two types of materials that have the potentiality, in different ways, to contribute to practical applications of nonlinear optics: semicontinuous gold films and fast saturable absorbers. Metals have extremely high values of the third-order nonlinear optical susceptibility (they are approximately 106 times more nonlinear than silica) and a very fast response time (on the order of 2 ps); this high nonlinearity makes them attractive candidates for applications such as all-optical switching. On the other hand, as everyday experience teaches, they do not let light pass through. One way to make light penetrate more deeply into metals is mixing them with dielectrics. In particular in my thesis I have studied the nonlinear properties of collection of gold nanoparticles embedded into silica or deposited onto it. These nanoparticle systems possess special resonances, the "plasmon resonances", that are absent from either bulk metal or bulk dielectric but only arise when the two are combined at the nanoscale. In the linear optical response these resonances produce extinction bands that for gold in silica are situated in the visible. At the resonance wavelength the electric field is concentrated inside the nanoparticles, and this field concentration produces an enhanced nonlinear optical response. In my work I have experimentally characterized the nonlinear absorption in these kind of samples over a wide range of composition and wavelength, and have proposed an explanation for the fact that under certain conditions these materials are saturable absorbers while gold itself is an optical limiter. A separate part of my work has examined the use of fast saturable absorbers to slow down light using the mechanism of coherent population oscillations. Using quantum dots as saturable absorbers I have demonstrated the slow propagation of pulses of ≈ 30 ps duration