Picowatt Resolution Calorimetry for Micro and Nanoscale Energy Transport Studies.

Abstract

Precise quantification of energy transport is key to obtaining insights into a wide range of phenomena across various disciplines including physics, chemistry, biology and engineering. This thesis describes development of two types of microfabricated calorimeter devices with single digit picowatt resolution at room temperature. Both devices incorporate two distinct features; an active area isolated by a thermal conductance of less than 1 microwatt per kelvin and a high-resolution thermometer with temperature resolution in the micro-kelvin regime. These features enable measurements of heat currents with picowatt resolution. In the first device the active area is suspended via silicon nitride beams with excellent thermal isolation (~600 nW/K) and a bimaterial cantilever (BMC) thermometer with temperature resolution of ~6 micro-kelvin. Taken together this design enabled calorimetric measurements with 4 pW resolution. In the second device, the BMC thermometer is replaced by a high-resolution resistance thermometry scheme. A detailed noise analysis of resistance thermometers, confirmed by experimental data, enabled me to correctly predict the resolution of different measurement schemes and propose techniques to achieve an order of magnitude improvement in the resolution of resistance thermometers. By incorporating resistance thermometers with temperature resolution of ~30 micro-kelvin, combined with a thermal isolation of ~150 nW/K, I demonstrated an all-electrical calorimeter device with a resolution of ~ 5 pW. Finally, I used these calorimeters to study Near-Field Radiative Heat Transfer (NF-RHT). Using these devices, we studied—for the first time—the effect of film thickness on the NF-RHT between two dielectric surfaces. Specifically, we showed that even a very thin film (~50 nm) of silicon dioxide deposited on a gold surface dramatically enhances NF-RHT between the coated surface and a second silica surface when the gap size is reduced to be comparable to that of the film thickness. This interesting effect is understood on the basis of detailed computational analysis, which shows that the NF-RHT in gaps comparable to film thickness is completely dominated by the contributions from surface phonon-polaritons whose effective skin depth is comparable to the film thickness. These results have important implications for near-field based thermo photovoltaic devices and for near-field based thermal management.