Plasmon Dephasing and Heat Dissipation in Gold Nanostructures

This thesis examines two topics, energy loss mechanisms for surface plasmon modes in metal nanostripes, and heat diffusion in photothermal experiments on gold particles and how it affects microscopy images. Electron motions which occur at the surfaces of conducting materials that have a definitive momentum are known as propagating surface plasmon polaritons (PSPPs).For a metal structure supported on a dielectric substrate, there exists two types of PSPP modes: the bound mode at the interface between the metal and dielectric and the leaky mode at the glass-air interface. The leaky mode for photolithographically made gold nanostripes were examined by a combination of leakage radiation microscopy (LRM) and back focal plane (BFP) imaging. The results were compared to finite element simulations performed in COMSOL Multiphysics. The experimentally determined wavevector, k_SPP, and propagation length, L_SPP, for the leaky mode was found to be in excellent agreement with simulations. Plotting the frequency versus k_SPP, the group velocity, v_g, is obtained from the slope of the line. The v_g was found to be about 90% of the speed of light. The lifetimes for the leaky mode, obtained via T₁ = L_SPP/v_g, were found to be an order of magnitude longer than typical lifetimes for the localized surface plasmon resonance (LSPR) modes of gold nanorods.

Leakage radiation microscopy has been used to examine chemical interface damping (CID) for the (PSPP) modes of Au nanostripes, nanofabricated structures with heights of 40 or 50 nm, widths between 2 and 4 µm, and 100 µm lengths. The difference in T₁ times between bare and thiol coated nanostripes was used to determine the dephasing rate due to CID, Γ_CID for the adsorbed thiol molecules. A variety of different thiol molecules were examined, as well as nanostripes with different dimensions. The values of Γ_CID are similar for the different systems and are an order-of-magnitude smaller than the typical values observed for LSPRs of Au nanoparticles. Scaling the measured Γ_CID values by the effective path length for electron-surface scattering shows that the CID effect for the PSPP modes of the nanostripes is similar to that for the LSPR modes of nanoparticles. This is somewhat surprising given that PSPPs and LSPRs have different properties: PSPPs have a well-defined momentum, whereas LSPRs do not. The magnitude of Γ_CID for the nanostripes could be increased by reducing their dimensions, principally the height of the nanostructures. However, decreasing dimensions for the leaky PSPP mode increases radiation damping, which would make it challenging to accurately measure ΓCID.

The effect of heat diffusion in photothermal experiments on gold nanoparticles was also examined. In photothermal heterodyne imaging (PHI) experiments a time-modulated pump beam heats the sample, creating a thermal lens that is detected by a non-resonant probe. This technique is very sensitive, and has been used to study a variety of systems. The extent of heat diffusion in the system depends on the timescale for the experiment, which is determined by the pump beam modulation frequency. In this paper the way the spatial resolution in PHI microscopy depends on frequency was studied through experiments and heat transfer simulations on gold nanoparticles. The experiments were performed with both focused and widefield pump beams. For a focused pump beam, changing modulation frequency had no effect on the spatial resolution. A small frequency effect was observed for a widefield pump, but the magnitude was much less than that expected from the thermal diffusion lengths in the system. The simulations also showed a small frequency dependence for the spatial extent of heating, consistent with the measurements. This arises because the system rapidly reaches a steady-state condition in these experiments, where the rate of optically heating the particle matches the rate of heat dissipation. In this limit the temperature profile around the particle simply decays inversely with distance, and is independent of the thermal diffusion length.

Traditionally, microscopy techniques are performed on nanoscale structures. This is due to the fact that when structures become larger than the point-spread-function (psf) of the beam, extra scattering effects are observed. With the onset of Infrared PHI, there has been an expansion of materials studied to include biological tissues as well as large polymer beads. These structures are now larger than a typical psf for a visible/near-infrared probe beam. The effect of large objects on the PHI signal is examined using 3 and 6 µm dye-doped polymer beads. A widefield 532nm pump beam is used to excite over the whole particle, and a focused 636nm probe beam is used to detect the PHI signal. It’s been observed that the PHI signal evolves as the probe beam focus is stepped through the polymer beads and shows a very distinct “Newton’s rings” structure. Understanding this effect is important for implementing PHI in complex systems.