Nanomaterials - 18

Abstract

Hyperbolic metamaterials feature extraordinary optical properties of unequal and oppositely signed in-plane and out-of-plane dielectric permittivities. Recent research on natural hyperbolic metamaterials [1] [2] unveiled photonic thermal transport rate competitive to the phononic one. Hyperbolic properties can be effectively obtained by stacking two unlike planar materials of sub-wavelength thicknesses, which have much tunability compared to natural hyperbolic materials. In this work, two kinds of free-standing hyperbolic metamaterials were investigated for intrinsic photonic thermal transport by fluctuational electrodynamics modeling [3]. The first type is nanolaminates of cBN and GaN. The other type is nanolaminates of Mo and Si. It was found that, compared to Mo/Si nanolaminates, cBN/GaN nanolaminates feature greatly enhanced spectral heat flux by 3 – 4 orders of magnitude. This phonon induced enhancement can be attributed to the out-of-plane transport of hyperbolic phonon polaritons, the electromagnetic modes due to coupling of phonons and photons with large magnitude wavevectors. Furthermore, compared to conduction, radiative heat flow occupies a significant fraction when the driving temperature difference across two ends is small. This work paves the way on the development of wide range of all-solid-state emerging thermo-optical devices relying on photons as the major energy carriers, like rectifiers [4], transistors [5] and memories [6].