Materials with large third-order nonlinear optical susceptibilities, X⁽³⁾, are essential for light-controlled phase and refractive index modulation for future optical device applications. This thesis aims to experimentally study the enhancement of the optical nonlinearity in metal nanocluster-dispersed dielectric composites. Three kinds of gold:dielectric composite films: Au:SiO₂, Au:Al₂O₃, and Au:TiO₂ with high Au concentration were prepared by reactive co-sputtering and post-annealing, and their properties were characterized by various techniques. The task of this thesis is to systematically examine the dependence of X⁽³⁾ enhancement on metal concentration, structure and distribution of metal clusters, dielectric constant of matrix, as well as the pulse width and wavelength of probing...[ Read more ]

Materials with large third-order nonlinear optical susceptibilities, X⁽³⁾, are essential for light-controlled phase and refractive index modulation for future optical device applications. This thesis aims to experimentally study the enhancement of the optical nonlinearity in metal nanocluster-dispersed dielectric composites. Three kinds of gold:dielectric composite films: Au:SiO₂, Au:Al₂O₃, and Au:TiO₂ with high Au concentration were prepared by reactive co-sputtering and post-annealing, and their properties were characterized by various techniques. The task of this thesis is to systematically examine the dependence of X⁽³⁾ enhancement on metal concentration, structure and distribution of metal clusters, dielectric constant of matrix, as well as the pulse width and wavelength of probing lasers.

The value of X⁽³⁾ in these samples, measured using both forward and backward geometry of degenerate four-wave mixing (DFWM), was found highly dependent on Au composition, and it reached a maximum value of 4x10^-6 esu near 40% Au for Au:SiO₂ (measured by a probe laser with a 70ps pulse-width). It was found, for the first time, that between 10%Au - 25%Au, instead of a linear relation, the X⁽³⁾ follows a cubic-power law with Au concentration, indicating the importance of metal particle-particle interactions at high concentration. The X⁽³⁾ is also dependent on matrix material used, for instance, the X⁽³⁾ in Au:TiO₂ composites was found about 5 times larger than that in Au:SiO₂ near their own surface plasmon resonance, due to a large dielectric constant of Au:TiO₂. Another important finding is that the X⁽³⁾ is critically dependent on the wavelength and pulse-width of the probing laser (three kinds of lasers with a pulse-width of 70ps, 35ps and 200fs were used). The X⁽³⁾ measured by a 70ps laser is 30 times larger than that measured by a 200fs laser, and the maximum of X⁽³⁾ as well as the figure of merit, X⁽³⁾/α (α: absorption coefficient) always occurs at the surface plasmon resonant frequency of the composites.

The time-resolved conjugate signal revealed that the contributions to the enhancement of optical nonlinearity in these heavily Au-doped dielectric composites are from different microscopic sources in different time-scales. In femtosecond regime, interband electron-dipole transition has a full contribution, and as the time increases, hot electrons starts to dominate in picosecond regime, and thermal effect gradually takes part in the nonlinear process as the interaction time further increases and it eventually becomes dominant at nanosecond regime and beyond.