Experimental investigations of the role of proximity approximation in near-field radiative transfer
The nature of thermal radiative transfer changes significantly as the nominal gap between two objects becomes comparable to or smaller than the characteristic wavelength given by Wien's displacement law. At larger gaps, conventional theory of blackbody radiation is sufficient to describe the radiative transfer; at smaller gaps, however, wave effects such as evanescent wave tunneling, interference and diffraction render the classical theory invalid. The change in radiative transfer between two objects is most dramatic when they can support electromagnetic surface polaritons because of the high local density of states at the interface between the object and vacuum. When two objects of polar dielectric materials are close enough, the enhanced near-field radiation due to surface phonon polariton tunneling can exceed the blackbody limit by several orders of magnitude. This enhanced radiation at nanoscale has potential applications in energy transfer, heat assisted magnetic recording and near-field radiative cooling. In recent years, several experiments measuring the enhanced near-field radiation between a micro-sphere and a plane substrate have been reported. To measure the radiative transfer, the magnitude of which can be less than 10 nW, the sensor of choice is the bi-material micro-cantilever. My thesis has focused on two aspects of near-field radiative transfer between a micro-sphere and a substrate: (1) to enable quantitative comparison between experimental measurement and theoretical/numerical prediction of near-field radiative transfer. (2) to develop a comprehensive thermal model for the experimental measurement procedure. To enable the first task, an improved experimental apparatus to measure the near-field radiation between a micro-sphere and a substrate has been developed. In previous experimental apparatuses, radiative transfer was measured between a micro-sphere and a truncated plane surface. This was necessary because of the optical configuration. Our new apparatus overcomes this drawback with a newly designed optical path. With this new apparatus, the experiments are truly between a micro-sphere and an infinite plane. Measurements for micro-spheres with wide range of radii from 2.5 µ; to 25 µ; have been conducted. The experimental measurements are compared to the numerical prediction using the modified proximity proximation. In contrast to van der Waals force and Casimir force measurements in which the proximity approximation agree better when applied to larger spheres, in radiative heat transfer measurements, the modified proximity approximation agree better for smaller spheres. This surprising finding is explained by the difference in nature of radiative transfer and forces. To go along with the improved apparatus, we have also modified the method of data acquisition, calibration procedures and the thermal model for the experiment. In terms of data collection, we can now eliminate the effects of spurious forces; the second change we have implemented in the experiment is that the substrate is translated at a constant velocity, as opposed to discrete steps. We have developed a thermal model for the new experimental procedure.
Mechanical engineering, Electrical engineering, Physics
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