Hyperbolic Metamaterials as an Analog of a Blackbody in the Near Field

Abstract

We study the near-field heat exchange between hyperbolic materials and demonstrate that these media are able to support broadband frustrated modes which transport heat by photon tunnelling with a high efficiency close to the theoretical limit. We predict that hyperbolic materials can be designed to be perfect thermal emitters at nanoscale and derive the near-field analog of the blackbody limit.PACS numbers: 44.40.+a;81.05.Xj A black body is usually defined by its property of having a maximum absorptivity and therefore also a maximum emissivity by virtue of Kirchhoff's law [1]. The energy transmission between two black bodies having different temperatures obey the well-known Stefan-Boltzmann law. This law sets an upper limit for the power which can be transmitted by real materials, but it is itself a limit for the far-field only, since it takes only propagating modes into account. In terms of the energy transmission between two bodies the black body case corresponds to maximum transmission for all allowed frequencies ω and all wave vectors smaller than ω/c, where c is the vacuum light velocity. This means that all the propagating modes are perfectly transmitted across the separation gap.In the near-field regime, i.e., for distances smaller than the thermal wavelength λ th = c/k B T (2π is Planck's constant, k B is Boltzmann's constant, and T is the temperature) the radiative heat flux is not due to the propagating modes, but it is dominated by evanescent waves [2-4] and especially surface polaritons as confirmed by recent experiments [5][6][7][8][9][10][11]. The common paradigm is that the largest heat flux can be achieved when the materials support surface polaritons which will give a resonant energy transfer restricted to a small frequency band around the surface mode resonance frequency [3,4,12,13]. Many researchers have tried to find materials enhancing the nanoscale heat flux due to the contribution of the coupled surface modes by using layered materials [14,15] In the present work the aim is twofold: (i) We show, that materials supporting a broad band of evanescent frustrated modes can outperform the heat flux due to surface modes. This provides new possibilies for designing materials giving large nanoscale heat fluxes which could be used for thermal management at the nanoscale for instance. (ii) We derive a general limit for the heat flux carried by the frustrated modes and show that it is, in fact, the near-field analog of the usual black body limit. For the evanescent modes a near field analog of a black body can be defined in the sense that the energy transmission coefficient must be equal to one for all frequencies and all wave vectors larger than ω/c. With today's nanofabrication techniques it is possible to manufacture artificial materials such as photonic band gap materials and metamaterials which exhibit very unusual material properties like negative refraction [23]. Due to such properties they are considered as good candidates for perfect lensing [24,25], for repulsive Casimir forces [26][27][28][...

Near-field heat transfer between graphene/hBN multilayers

“…In the hyperbolic regions, the isofrequency surfaces become a hyperboloid instead of a sphere or an ellipsoid, and thus such multilayer structures can support resonance modes with unbounded tangential wavevectors. These multilayer structures have found exciting applications in subwavelength imaging [21][22][23] and near-field radiative heat transfer [24,25]. For heat transfers in particular, materials with hyperbolic responses can provide substantial enhancement of heat transfer over a broad frequency region [24].…”

Effect of shape in near-field thermal transfer for periodic structures

“…This is because of the fact that effective medium theories [19][20][21][22][23][24] become accurate at these small periodicities. Note that this does not hold regarding the part of thermal transfer that radiates from the substrate and transmits through the array beams.…”

“…In addition to a variety of exact numerical techniques, there also exists a set of approximation methods for thermal transfer calculations. Among them, the most popular are the effective medium theory that is used for subwavelength periodic structures [19][20][21][22][23][24] , the proximity approximation and its modified version [25][26][27] , and the coupled mode theory 9,28,29 .…”

Highly Efficient Broadband Light Absorber Based on Nonuniform Hyperbolic Metamaterial Film

“…Unique properties of HM can lead to a wide variety of applications; among these are negative refraction [10,11], subwavelength imaging [12][13][14][15][16][17], spontaneous and thermal emission engineering [18][19][20][21], and broadband light trapping [22,23]. Very high density of photonic states in HM [24][25][26] ensures effective radiative heat transfer [4][5][6] and allows for a nanosized heating/cooling devices.…”