Probing Radiative Thermal Transport at the Nanoscale.

Thermal radiative emission from a hot to a cold surface plays an important role in many applications, including energy conversion, thermal management, lithography, data storage, and thermal microscopy. While thermal radiation at length scales larger than the dominant wavelength is well understood in terms of Planck’s law and the Stefan-Boltzmann law, near-field thermal radiation is not. With constantly advancing micro- and nanofabrication techniques and ever smaller devices a substantial need for a better and more reliable understanding of the fundamental physics governing nanoscale radiative heat transfer has arisen. Unfortunately, and in stark contrast to the abundance of theoretical and numerical work, there have only been limited experimental efforts and achievements. The central challenge in the field is to accurately and unambiguously characterize radiative heat transport between well-defined surfaces across nanometer distances. The key scientific and technological questions that I have experimentally addressed during my doctoral study include: How does radiative heat transfer between an emitter and a receiver depend on their spatial separation (gap size), and does the radiative heat flux increase by over five orders of magnitude as the gap size is reduced to a few nanometers, as theoretically predicted? Can polar dielectric and metallic thin films support substantial near-field heat flow enhancement? For single-digit nanometer gaps, is the widely used theoretical framework of fluctuational electrodynamics (still) applicable? To address these challenging questions in gap sizes as small as tens of nanometers, we developed a nanopositioning platform to precisely control the gap between a microfabricated emitter device and a suspended receiver/calorimeter device which enables simultaneous measurement of the radiative heat flow across the gap. Further, we employed an atomic force microscope (AFM) in conjunction with stiff custom-fabricated scanning thermal microscopy (SThM) probes to explore the extreme near-field characterized by gaps of a few nanometers. In both approaches, high vacuum, vibration isolation and temperature control are implemented for accurate thermal measurements and for maintaining a stable gap. Finally, we performed state-of-the-art fluctuational electrodynamics-based calculations and analysis to compare theoretical predictions with experimental observations.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116634/1/baisong_1.pd

Regularization of fields for self-force problems in curved spacetime: foundations and a time-domain application

We propose an approach for the calculation of self-forces, energy fluxes and waveforms arising from moving point charges in curved spacetimes. As opposed to mode-sum schemes that regularize the self-force derived from the singular retarded field, this approach regularizes the retarded field itself. The singular part of the retarded field is first analytically identified and removed, yielding a finite, differentiable remainder from which the self-force is easily calculated. This regular remainder solves a wave equation which enjoys the benefit of having a non-singular source. Solving this wave equation for the remainder completely avoids the calculation of the singular retarded field along with the attendant difficulties associated with numerically modeling a delta function source. From this differentiable remainder one may compute the self-force, the energy flux, and also a waveform which reflects the effects of the self-force. As a test of principle, we implement this method using a 4th-order (1+1) code, and calculate the self-force for the simple case of a scalar charge moving in a circular orbit around a Schwarzschild black hole. We achieve agreement with frequency-domain results to ~ 0.1% or better.Comment: 15 pages, 12 figures, 1 table. More figures, extended summar

Hybrid method for understanding black-hole mergers: Inspiralling case

We adapt a method of matching post-Newtonian and black-hole-perturbation theories on a timelike surface (which proved useful for understanding head-on black-hole-binary collisions) to treat equal-mass, inspiralling black-hole binaries. We first introduce a radiation-reaction potential into this method, and we show that it leads to a self-consistent set of equations that describe the simultaneous evolution of the waveform and of the timelike matching surface. This allows us to produce a full inspiral-merger-ringdown waveform of the l=2, m=±2 modes of the gravitational waveform of an equal-mass black-hole-binary inspiral. These modes match those of numerical-relativity simulations well in phase, though less well in amplitude for the inspiral. As a second application of this method, we study a merger of black holes with spins antialigned in the orbital plane (the superkick configuration). During the ringdown of the superkick, the phases of the mass- and current-quadrupole radiation become locked together, because they evolve at the same quasinormal-mode frequencies. We argue that this locking begins during the merger, and we show that if the spins of the black holes evolve via geodetic precession in the perturbed black-hole spacetime of our model, then the spins precess at the orbital frequency during the merger. In turn, this gives rise to the correct behavior of the radiation, and produces a kick similar to that observed in numerical simulations