Nonlinear optics of light-induced structural transitions in confined Gallium

MacDonald, Kevin (2002) Nonlinear optics of light-induced structural transitions in confined Gallium. University of Southampton, Doctoral Thesis, 113pp.

Record type: Thesis (Doctoral)

Abstract

An ultra-high-vacuum system has been constructed to facilitate atomic-beam deposition of gallium on cryogenically cooled substrates, including optical fibre tips. Alongside this, a fibre-optic pump-probe diagnostic system, based on semiconductor lasers, has been developed to perform in-situ measurements of the linear and transient nonlinear optical properties of gallium nanostructures, both during and after deposition. This unique combination of deposition and optical diagnostic techniques has provided a new means of studying the growth and optical characteristics of gallium nanostructures under highly controlled conditions.

The linear and nonlinear optical properties of a new material structure, namely gallium/glass interfaces prepared by ultrafast pulsed laser deposition (UPLD), have been studied for the first time. The reflectivity characteristics of these high-quality interfaces were measured under varying conditions of temperature and light intensity at 810 nm: At temperatures several degrees below gallium's melting point Tm, excitation intensities of just a few kW.cm^-2 were seen to induce reflectivity changes of more than 30%. Experiments performed with a nanosecond optical parametric oscillator have illustrated that UPLD gallium/silica interfaces show a nonlinear response to optical excitation in the 440-680 nm wavelength range: Fluences of less than 10 mJ.cm^-2 were seen to induce reflectivity changes of up to 35%, even at temperatures 15° below T_m

It has been found that low power (17 μW average) laser illumination of cryogenically cooled substrates during atomic-beam deposition of gallium leads to the formation of uniformly sized gallium nanoparticles. This phenomenon is believed to be the result of a non-thermal light-assisted particle self-assembly process.

Gallium nanoparticles have been seen to show a strongly temperature-dependent nonlinear response to low intensity, infrared (1550 nm) optical excitation; 1 μs pulses with peak intensities in the kW.cm^-2 range induced reversible reflectivity changes with a magnitude of as much as several percent (in the vicinity of the phase transition temperatures) and a typical relaxation time of ~0.5 μs.

These various experiments have illustrated that the modification of gallium's transitional properties, brought about by confinement, facilitates the achievement of a large optical nonlinearity via light-induced structural transformations in the metal. The studies of UPLD interfaces have shown that bulk gallium's nonlinearity is exceptionally broadband, that its response time can be as short as a few picoseconds, and that its relaxation time is typically in the nano- to microsecond range. Furthermore, the data collected have enabled the development and quantitative testing of theories to describe the thermal and non-thermal metallization mechanisms underlying the nonlinearity. The nanoparticles' nonlinear response shares certain characteristics with that of the bulk interfaces but the experimental data suggest that, in contrast to the bulk nonlinearity, it is not derived from a structural transition involving gallium's α phase.