This thesis reports on ultrafast linear and nonlinear optical probing of nanometer
thick films. Exfoliated graphene and few-layer graphite are probed through optical second harmonic generation (SHG) with 800 nm, 150 fs pulses. Samples of varying thickness from 1 carbon layer to bulk graphite are deposited onto an oxidized silicon substrate. SHG measurements are taken as a function of azimuthal rotation angle of the films. It is found that the SHG from graphene is much weaker than that from bilayer graphene, and has a qualitatively different azimuthal pattern. As the sample thickness increases from bilayer graphene to bulk graphite, the SHG yield generally decreases. Both of these effects are explained in terms of the symmetry of graphene and graphite, and modeled
using multilayer optical transfer matrices, and an identical set of nonlinear susceptibility tensor elements for the front and back surfaces. These tensors are independent of sample thickness. MnAs films of 150 and 190 nm thickness on (001)GaAs are optically excited with 775 nm, 200 fs pump pulses. Specular SHG at 388 nm and first order optical diffraction at ∼ 400 nm are used to probe the samples on timescales up to 2 μs. It is found that the SHG probes the temperature-dependent, spatially averaged, surface strain. This strain reaches a maximum deviation in ∼ 6–100 ps after optical excitation depending on the pump fluence and initial temperature. The strain then recovers in hundreds of picoseconds, a timescale consistent with heat diffusion.
The optical diffraction probes the first Fourier component of the paramagnetic–ferromagnetic stripes inherent to MnAs films in the 10–40◦C temperature range. After
optical excitation, the diffraction data show highly nonthermal behaviour in the MnAs
films. If a sample is excited from the coexistence phase, the diffraction signal shows decaying oscillations with a period of ∼ 335±4 (408±4) ps for the 150 (190) nm films; this is consistent with the release of a standing acoustic wave. Decay occurs on a timescale of ∼ 2 ns consistent with local diffusion through the films. The stripes are restored on a timescale of hundreds of nanoseconds, with a temporal behavior consistent with a diffusion process, possibly thermal in origin.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/35803 |
| Date | 07 August 2013 |
| Creators | Dean, Jesse Jackson |
| Contributors | van Driel, Henry M. |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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