Optothermal Energy Conversion and Transport in Plasmonic Structures
Plasmonic structures can efficiently interact with electromagnetic waves at their interfaces, leading to strong light confinement and field enhancement, which has been crucial for many nanotechnology applications in manufacturing, lithography, data storage, biosensing, spectroscopy, molecular trapping and other molecular level studies. Due to the intrinsic losses of metallic structures, the strong light-matter interaction also generates significant energy dissipation, making the thermal management of the devices very challenging. The purpose of this dissertation is to study the optothermal energy conversion and transport in plasmonic structures, especially in structures at the nanoscale. We first investigate the general optothermal responses of plasmonic structures by developing a theoretical framework where the coupled optical and thermal responses can be numerically simulated. Nonlocal electromagnetic responses are considered since the nonlocal effect plays a very important role in small structures with a characteristic length of ~10 nm or even smaller. Meanwhile, non-diffusive (ballistic) thermal transport is included since the characteristic length is comparable to or even smaller than the mean free paths of thermal carriers. Using a newly developed integrated diffusion model (IDM), the ballistic property can be calculated with a accuracy of the linear Boltzmann transport equation by solving commercial software compatible diffusion equations. Besides, when the structures are at the nanoscale, energy conversion and energy transfer can be strongly affected by the interfaces and the outside environment. Our theoretical and numerical results suggest wave amplification may happen near interfaces where nonlocal responses of electrons are strong. The strong electron interactions may drive current opposite to local electric field, resulting in negative optical energy absorption, which is unexpected from the perspective of local electromagnetic responses. The effect of interfaces is also measured experimentally. We simultaneously measure the responses both inside and outside small gold nanoparticles (AuNPs) using ultrafast pump-probe methods on a newly proposed platform. The platform consists of AuNPs in solution mixed with fluorescent molecules. These molecules serve as sensitive probes the measure the energy transfer from the plasmon-induced hot electrons in photoexcited AuNPs based on stimulated-emission depletion (STED). Together with the traditional transient absorption spectroscopy measurement, STED signal gives a comprehensive description of the inside and outside responses of AuNPs after photoexcitation with a sub-picosecond temporal resolution. The measured data suggest that direct energy transfer from hot electrons to the outside environment may occur before electrons thermalize with inside phonons. To our knowledge, this is the first time to probe ultrafast optothermal energy conversion at both sides of the nanoscale metallic interfaces. Besides, fluorescence thermometry is also studied since it may be integrated into STED spectroscopy and offer an opportunity to directly measure temperature on a picosecond time scale. For that purpose, two new thermometries based on molecular rotation are proposed and developed. These fluorescence thermometries potentially can provide more detailed information than the current transient absorption measurement and help understand energy conversion and energy transfer processes in nanoscale plasmonic structures. As stand-alone thermometries, they have the advantages of compatibility with polarizing materials since the measured ratiometric parameters essentially characterize the inequality in fluorescence intensity along different directions, as well as the stability against intensity variation, suitability of two-dimensional mapping and rapidity in readout rate.
Pan, Purdue University.
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