The nonlinear beam deflection (BD) technique is used to directly measure and time-resolve the nonlinearly-induced phase shift in a variety of materials. In this technique, a weak probe beam is spatially overlapped, while slightly displaced, with a strong excitation beam while the temporal delay is scanned. The excitation-induced index gradient, which for 3rd-order nonlinearities is proportional to the nonlinear refractive index 16 n2"> of the medium, deflects the weak probe beam. This deflection is determined using a position sensitive segmented detector after propagation to the far field. In this dissertation, we expand our previous work on BD theory to include the effects of the Gaussian spatial beam profile of the excitation, as opposed to a constant index gradient. We also explore the BD signal as we allow the spatial size and relative position of the probe with respect to the excitation beam, r, at the sample to vary to maximize the calculated signal. While the analysis requires numerical solutions, we find a simple empirical fitting function for the BD signal that allows determination of the nonlinear phase shift and thus the nonlinear refraction. We performed BD experiments at near-degenerate photon energies for various spot size ratios which resulted in very good agreement with our simulation results. In order to examine our empirical function the BD signal for various r (0.2-0.6) is measured while keeping the phase shift relatively constant. This helped us isolate the effect of spot size ratio on the BD signal. Our results showed the correct trend for the growth of BD signal as r increases, which is what is expected from our model.
We also studied nondegenerate two-photon absorption (ND-2PA) in bulk silicon. We present the results of spectroscopic pump-probe measurements of ND-2PA in silicon across the indirect-gap (1.12 eV). We observed enhancement of the 2PA coefficient as the degree of degeneracy of pump and probe photon energies increased, and the dispersion compares favorably with our recently-developed semi-empirical theoretical model for the dispersion of indirect ND-2PA in silicon.
Additionally, we experimentally investigated WTe2 which is a Weyl semimetal. Here, we prepared very thin flakes (10s of microns thick) of WTe2 and investigated the possibility of observing circular dichroism (CD) in pump-probe measurements, pumping at near IR and probing at mid-IR. Although we did not observe any CD, we believe this is because our pump photon energy is far from Weyl nodes and that we need to pump at mid-IR range.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd2023-1298 |
| Date | 01 January 2024 |
| Creators | Faryadras, Sanaz |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Thesis and Dissertation 2023-2024 |
| Rights | In copyright |
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