Thermo-plasmonics is an emerging field in photonics which aims at harnessing the kinetic energy of light to generate nanoscopic sources of heat. Localized surface plasmons (LSP) supported by metallic nanostructures greatly enhance the interactions of light with the structure. By engineering the size, morphology and composition of metallic nanostructures, the absorption of light can be maximized, resulting in a substantial temperature elevation in a nanoscopic volume.
Applications of these nanoscopic sources of heat can be found in various contexts including localized cancer therapy, drug delivery, nano-surgeries and thermo-transportations. Apart from generating well-controlled temperature increase in functional thermo-plasmonic devices, thermo-plasmonics can also be used in understanding complex phenomena in thermodynamics by creating drastic temperature gradients which are not accessible using conventional techniques.
In this thesis, we present novel experimental and numerical tools to characterize thermo-plasmonic devices in a biologically relevant environment, and explore the thermodiffusion properties and measure thermophoretic forces for particles in temperature gradients ranging from 1-100 K/μm.
Chapter 2 presents the relevant theoretic background in thermo-plasmonics and a literature review on available experimental methods for temperature mapping around a nanoscopic source of heat.
Chapter 3 discusses two numerical methods employed in this thesis for thermo-plasmonic simulations as well as the ImageJ program “Mosaic”, used for single particle tracking.
Chapter 4 presents the experimental details of the lipid bilayer based temperature mapping technique based on a lipid bilayer containing fluorophores with a phase dependent partitioning. This assay allowed quantification of particle temperatures by simple detection of the phase boundary located far away from the particle. Two types of nanostructures were investigated using this assay: colloidal gold nanoparticles (rods and spheres) and e-beam printed metallic composite nanostructures.
Chapter 5 presents the quantifications of temperature increase around irradiated gold colloidal particles. We show that the temperature profile of an irradiated gold nanorod is highly dependent on its orientation with respect to the laser polarization vector. The magnitude of the temperature increase on the gold nanorod aligned in parallel to the laser polarization vector is found to be significantly higher than a spherical gold nanoparticle having a 100 times larger volume. Furthermore, we show that an irreversible surface melting occurred for gold nanorods when their surface temperature increase exceeded 200 °C above room temperature. Our results are supported by theoretical simulations of the expected absorbance of gold nanorods and spheres using the discrete dipole approximation (DDA).
In Chapter 6, we present direct experimental measurements paralleled by FEM based numerical calculations of the heating properties of irradiated complex e-beam composite nanostructures, these including discs, triangles, stars and a dimer. The highest surface temperature elevation occurs on the nanostructure with the highest absorption efficiency at the laser irradiation wavelength, regardless of the size or the morphology. We also demonstrate that substantial heat in e-beam printed nanostructures illuminated from the bottom is generated in the titanium adhesive layer sandwiched between gold and glass. This contribution of the Ti layer has been largely ignored in literature, thus leading to a severe underestimation of the heat generated by e-beam printed nanostructures.
Chapter 7 demonstrates our ongoing attempts in measuring the thermodiffusion properties and thermophoretic forces of particles subject to temperature gradients in liquids, at a single particle level by utilizing state-of-the-art confocal microscopy techniques and dual-trap optical tweezers. We have measured the thermophoretic forces on a polystyrene particle (radius = 500 nm) as a function of its distance to the center of the nanoscopic source of heat. With the temperature gradient ranging from 1 - 100 K/μm, the thermophoretic force was on the order of tens of femtonewton and decreased with an increase of the distance to the center of the hotspot.
Thermo-plasmonics is a young but fast growing field. With the remarkable consistency between our numerical models and our experimental results, we believe that both can be used to design and study a large variety of complex thermo-plasmonic systems