CAREER: Fluctuation-Induced Phenomena in Microstructured Media

Princeton University, Princeton NJ

Investigators

Abstract

NON-TECHNICAL SUMMARY Light can exert a force and carry heat. Although these statements are uncontroversial today, their confirmations were remarkable triumphs a little over a century ago, only a couple of centuries after scientists had finally managed to show that light is a wave and moves at a finite speed, as opposed to appearing instantaneously. Today's extraordinary ability to confine and manipulate laser light at the nanoscale (billionths of a meter) is enabling unprecedented control over the state of small particles and the guiding of information over long distances, leading to breakthroughs in a variety of technological areas, from medical imaging to telecommunications. The increasing miniaturization of small mechanical devices, from accelerometers in mobile phones to pressure sensors in modern tires, is pushing these small systems into regimes where even tiny effects of light brought about by quantum and thermal fluctuations of electric currents, the jittery motion of matter, become important. Most famously, the vibrations of matter give rise to thermal radiation, or the familiar glow of hot objects that underlies many naturally occurring phenomena (such as the heating of the earth by the sun), and emerging technologies (such as energy-harvesting devices which convert solar heat to electricity). The same fluctuations carry momentum and lead to so-called "dispersion" forces that enable geckos to climb walls and also cause static friction between small devices with movable parts. This project will support theoretical and computational research as well as educational initiatives aimed at studying fluctuation effects in unexplored regimes of nano-scale systems. In particular, the research team will develop theoretical methods to understand the role and impact of designable materials on heat transport and quantum forces between bodies with nano-metric features and separations. At these tiny length scales, the fundamental description of structured materials and their interaction with radiation must be modified to account for the increasing role of atomic effects and strong light-matter interactions. This paves the way for the study of emerging experimental and technological regimes in which, like laser light before it, fluctuation effects are used to control the state of nano-devices. Recent and ongoing studies of these phenomena at larger length scales (millionths of a meter) all point to the unavoidable impact of fluctuations on these systems, from heat radiation that is many times larger than that of far-separated objects, to repulsive forces that can be used to levitate objects in vacuum. In addition to increasing the fundamental understanding of many naturally occurring processes, progress in modeling fluctuation interactions will promote technical developments that benefit society, by guiding improvements and applications in nanotechnology, such as micro-electromechanical systems, thin-film microfluidics, thermophotovoltaic energy conversion, and nano-scale cooling. Furthermore, the research team will incorporate the resulting computational techniques in freely available simulation-software packages that promote teaching and learning by students, new researchers, and researchers from developing nations who could not otherwise afford the high start-up costs of developing new numerical tools from scratch. TECHNICAL SUMMARY Quantum and thermal fluctuations of electromagnetic fields lead to a variety of important phenomena, including radiative heat transport and Casimir forces between neutral, macroscopic objects. These effects play a central role in many naturally occurring processes at everyday length scales, including the radiation emitted from the sun and absorbed by the earth, but become particularly pronounced at sub-micron scales where they can reach atmospheric pressures. Recent theoretical developments in modeling fluctuation phenomena between complex, nanostructured surfaces are enabling rigorous explorations of these interactions. While calculations are extremely challenging and come in a bewildering variety of flavors, from formulations based on statistics of fluctuations to expressions based on path integrals and scattering matrices, at its core the problem of modeling fluctuation interactions can be reduced to the calculation of a large and cumbersome number of classical electromagnetic scattering problems. Existing theoretical techniques have been applied at both microscopic and atomistic scales, but fail to account for important effects arising at intermediate scales. This project aims to fill this gap by introducing novel theoretical and computational tools applicable in emerging experimental regimes where the increasing role of atomistic and nonlinear light-matter interactions cannot be ignored. Research thrusts include the study of (1) non-local effects caused by smearing of the electronic response of materials, (2) temperature gradients induced by external stimuli or material anisotropy, and (3) nonlinearities arising from strong light-matter interactions. In particular, this project aims to study heat transport between various classes of nanostructured surfaces with features and separations at the nanometer scale, where non-local effects and temperature gradients can be significant. Techniques based on the volume integral equation of electromagnetic scattering which can handle non-local and inhomogeneous media will be developed and applied to study thermal radiation as well as other important fluctuation effects, including fluorescence and Raman scattering. At high temperatures, the statistics of fluctuations can be significantly modified due to the presence of strong light-matter interactions. The research team will study the impact of such material nonlinearities on the radiation spectra of bodies at and out of equilibrium, where material-mediated coupling between photons at different frequencies can lead to a variety of unusual effects, including phase transitions and line-shape alterations. Finally, the impact of geometry and non-additivity on wetting phenomena in highly non-planar surfaces will be explored, bringing new perspectives and tools to the field of microfluidics. In addition to increasing the fundamental understanding of many naturally occurring processes, progress in modeling fluctuation interactions will promote technical developments that benefit society, by guiding improvements and applications in nanotechnology, such as micro-electromechanical systems, thin-film microfluidics, thermophotovoltaic energy conversion, and nano-scale cooling. The highly interdisciplinary nature of this research, spanning methods and ideas from statistical physics, numerical linear algebra, electromagnetism, and microfluidics, will be a great source of motivation and material to engage students and the general scientific community. Finally, the techniques outlined above will form the core of a free, well-documented, and portable software package enabling study of fluctuation interactions spanning multiple length scales. Such user-friendly packages promote teaching and learning.