Arvind Narayanaswamy

Surface Phonon Polaritons Mediated Energy Transfer between Nanoscale Gaps

Surface phonon polaritons are electromagnetic waves that propagate along the interfaces of polar dielectrics and exhibit a large local-field enhancement near the interfaces at infrared frequencies. Theoretical calculations show that such surface waves can lead to breakdown of the Plancks blackbody radiation law in the near field. Here, we experimentally demonstrate that surface phonon polaritons dramatically enhance energy transfer between two surfaces at small gaps by measuring radiation heat transfer between a microsphere and a flat surface down to 30 nm separation. The corresponding heat transfer coefficients at nanoscale gaps are 3 orders of magnitude larger than that of the blackbody radiation limit. The high energy flux can be exploited to develop new radiative cooling and thermophotovoltaic technologies.

Surface modes for near field thermophotovoltaics

Thermal radiative energy transfer between closely spaced surfaces has been analyzed in the past and shown not to obey the laws of classical radiation heat transfer owing to evanescent waves and, more recently, electromagnetic surface modes. We have analyzed the energy transfer between layered media, one of the layers being the thermal source, using a Green’s functions method and the fluctuation-dissipation theorem. Based on the analysis, we propose a structure that can utilize the surface modes to increase the power density and efficiency of low temperature thermophotovoltaic generators.

Near-field thermal radiation between two closely spaced glass plates exceeding Planck's blackbody radiation law

This work reports experimental studies on radiative heat flux between two parallel glass surfaces. Small polystyrene particles are used as spacers to maintain a micron-sized gap between two optical flats. By carefully choosing the number of particles and performing the measurement in a high-vacuum environment, the experiment is designed to ensure that the radiative heat flux is the dominant mode of heat transfer. The experimental results clearly demonstrate that the radiative heat flux across micron-sized gaps can exceed the far-field upper limit given by Planck’s law of blackbody radiation. The measured radiative heat flux shows reasonable agreement with theoretical predictions.

Near-field radiative heat transfer between a sphere and a substrate

Near-field force and energy exchange between two objects due to quantum electrodynamic fluctuations give rise to interesting phenomena such as Casimir and van der Waals forces and thermal radiative transfer exceeding Planck’s theory of blackbody radiation. Although significant progress has been made in the past on the precise measurement of Casimir force related to zero-point energy, experimental demonstration of near-field enhancement of radiative heat transfer is difficult. In this work, we present a sensitive technique of measuring near-field radiative transfer between a microsphere and a substrate using a bimaterial atomic force microscope cantilever, resulting in “heat transfer-distance” curves. Measurements of radiative transfer between a sphere and a flat substrate show the presence of strong near-field effects resulting in enhancement of heat transfer over the predictions of the Planck blackbody radiation theory.

Thermal conductance of bimaterial microcantilevers

In this letter, based on the beam theory and the thermal analysis of a bimaterial cantilever, we demonstrate that the effective thermal conductance of the cantilever and the temperature at the tip of the cantilever can be determined by measuring the bending of the cantilever in response to two different thermal inputs: power absorbed at the tip and ambient temperature.

Sub-picowatt resolution calorimetry with a bi-material microcantilever sensor

We have designed and fabricated bi-material microcantilevers with low conductance by minimizing the width and thickness of the cantilevers while keeping them suitable for detection with an optical deflection technique. The conductance of a cantilever is determined experimentally to be 330 ± 20 nWK−1. Using this cantilever, we have measured less than 1 pW of heat flow through the cantilever. The thermal noise-limited resolution of the cantilever is expected to be ≈50 fW. Such cantilevers give us additional tools to probe thermal transport through nanostructures, especially through single molecules where picowatt-level sensitivity is necessary.

A Green's function formalism of energy and momentum transfer in fluctuational electrodynamics

Abstract Radiative energy and momentum transfer due to fluctuations of electromagnetic fields arising due to temperature difference between objects is described in terms of the cross-spectral densities of the electromagnetic fields. We derive relations between thermal non-equilibrium contributions to energy and momentum transfer and surface integrals of tangential components of the dyadic Greens functions of the vector Helmholtz equation. The expressions derived here are applicable to objects of arbitrary shapes, dielectric functions, as well as magnetic permeabilities. For the case of radiative transfer, we derive expressions for the generalized transmissivity and generalized conductance that are shown to obey reciprocity and agree with theory of black body radiative transfer in the appropriate limit.

Proximity Effects in Radiative Heat Transfer

Thoughthedependenceofnear-fieldradiativetransferonthegapbetweentwoplanarobjectsiswellunderstood, that between curved objects is still unclear. We show unequivocally that the surface polariton mediated radiative transfer between two spheres of equal radii R and minimum gap d scales as R/d as the nondimensional gap d/R → 0. We discuss the proximity approximation form that is being used at present to compare with experimental observations and suggest a modified form in order to satisfy the continuity requirement between far-field and near-field radiative transfer between the spheres.

Effects of Periodic Structures on the Coherence Properties of Blackbody Radiation

Radiative properties have been studied for one-dimensional dielectric multilayer structures subjected to blackbody radiation sources. The total hemispherical transmittances are calculated for periodic structures and structures with random variation in layer thickness, using wave-optics and ray-tracing methods

Dyadic Green's functions and electromagnetic local density of states

A formal proof to relate the concept of electromagnetic local density of states (LDOS) to the electric and magnetic dyadic Greens functions (DGF) is provided. The expression for LDOS is obtained by relating the electromagnetic energy density at any location in a medium at uniform temperature T to the electric and magnetic DGFs. The appropriate boundary conditions governing the DGFs are obtained and it is seen that the two types of DGFs are electromagnetic duals of each other. With this the concept of LDOS is also extended to material media. The LDOS is split into two terms—one that originates from the energy density in an infinite, homogeneous medium and the other that takes into account scattering from inhomogeneities. The second part can always be defined unambiguously, even in lossy materials. For lossy materials, the first part is finite only if spatial dispersion is taken into account.