Dissertation Defense - Lang Zhang

TITLE: Maximal Heat and Light Extraction from the Optical Near Field

ABSTRACT:  The optical near field comprises large-amplitude evanescent waves that, through suitable interactions with patterned materials, can be harnessed and utilized for optical imaging, microscopy, thermophotovoltaics, and optical communications. This dissertation centers around two cornerstones of the near field: radiative heat transfer (between near-field bodies), and radiation enhancement (from near field to far field).  Recent state-of-the-art approaches, including silica-silica designs for radiative heat transfer and dielectric bowtie antennas for radiation enhancement, originate from longstanding far-field or microwave-frequency theories. Are these approaches really optimal for near-field electromagnetism at optical frequencies?

In this dissertation, I describe novel materials and design methods for maximizing heat and light extraction from the optical near field, revolving around new theoretical frameworks. In both cases, I find unique aspects of the optical near field that offer the possibility for significant improvement over the state of the art.

In radiative heat transfer, I explore the design space of all causality-allowed materials and derive a “near-field Wien’s law” that predicts significant red shifts relative to its far-field counterparts. Alternative plasmonic materials can offer 10X enhancements beyond the silica-silica standard, which are validated in a recent record-setting experimental collaboration. Moreover, the planar-planar configurations of these materials may be the very best designs, over all possible materials in any geometric configuration. We justify this claim through a new framework for probing optimal broadband response, in which the full scattering matrix of any passive linear electromagnetic scatterer is subject to a representation of matrix-valued oscillators. Through this representation we identify general bounds to near-field radiative heat transfer, tightening previous theories by at least two orders of magnitude, and confirming the new near-field Wien’s law. Beyond radiative heat transfer, our scattering-matrix formalism offers the possibility for insights into maximum broadband response and spectral control across wave physics, including acoustics, quantum scattering, and beyond.

Radiation enhancement uses a different mechanism—transduction of near-field energy to far-field radiation—that requires a different theoretical approach. We identify optimal scaling laws for far-field radiation, and show that dielectric bowtie designs fall significantly short of optimal. In fact, any positive-contrast-dielectric design must exhibit sub-optimal scaling laws as a function of minimum feature size. I propose a new class of design: by using two materials, one with positive contrast and a second with negative contrast (but not polaritonic), with an “alignment/anti-alignment” layout, one can approach the optimal scaling laws. These approaches extend to any multipolar amplification or enhancement, with applications to imaging, spectroscopy, nonlinear optics, and even potentially acoustic and mechanical “antennas.”

Event time: 
Wednesday, April 10, 2024 - 10:00am
Applied Physics
Owen Miller