Near-Field Radiative Heat Transfer Between Subwavelength Structures

Near-field radiative heat transfer (NFRHT), a regime in which thermal sources are separated by subwavelength vacuum gaps, can exceed the blackbody limit owing to the tunneling of evanescent electromagnetic waves, which include frustrated modes and surface modes. The substantial enhancement of heat transfer in the near field could be exploited in various technologies, such as noncontact localized cooling and thermophotovoltaic power generation.<br/><br/>To date, NFRHT is still a laboratory-scale concept. The viability of potential NFRHT applications directly depends on the ability of enhancing and spectrally controlling the flux with designer materials, such as metamaterials and metasurfaces, made of subwavelength structures. However, the basic physics underlying the interactions between thermally generated evanescent waves and subwavelength structures is not well understood, partially due to the lack of numerically exact framework enabling predictions of NFRHT in complex geometries. The overall objective of this talk is to address this knowledge gap by first reviewing a novel method for solving NFRHT in complex geometries, and by then discussing recent results of NFRHT between two coplanar membranes of subwavelength thickness.<br/><br/>In the first part of the talk, we will introduce the discrete system Green’s function (DSGF) method, which is a volume integral numerical approach based on fluctuational electrodynamics. In the DSGF method, thermal sources are discretized into cubic subvolumes, and all electromagnetic interactions are determined by numerically calculating the system Green’s function between the subvolumes. NFRHT is then computed by simple mathematical operations to the system Green’s functions. The DSGF method is numerically exact, and is applicable to an arbitrary number of finite, 3D thermal sources of any sizes and shapes separated by gaps smaller or larger than the thermal wavelength. We will review the verification of the DSGF method against exact results of NFRHT between two and three spheres, and briefly discuss its application to predict NFRHT between complex-shaped particles.<br/><br/>The second part of the talk will focus on DSGF predictions and experiments of NFRHT between two coplanar SiC membranes with thickness comparable to or smaller than their vacuum gap spacing of 100 nm. The results show that the radiative heat transfer coefficient increases substantially as the membrane thickness decreases. At room temperature, a maximum heat transfer coefficient <i>h</i>_rad of ~800 W/m²K for a membrane thickness of 20 nm is measured and predicted. This <i>h</i>_rad value is ~5 times larger than that predicted between two infinite SiC surfaces (~150 W/m²K) at the same vacuum gap, and is ~1200 times larger than the blackbody limit. A heat transfer coefficient of 800 W/m²K would require a gap spacing of ~30 nm between two infinite SiC surfaces. In addition, the resonance of the heat transfer coefficient is broadened and redshifted as the membrane thickness decreases. We demonstrate via a modal analysis that the enhancement and spectral redshift of the heat transfer coefficient is due to the 2D confinement of the electromagnetic fields. Specifically, SPhP coupling between the membrane parallel edges and between the perpendicular edges through corners generate resonant electromagnetic corner and edge modes dominating NFRHT.<br/><br/>The large enhancement of NFRHT mediated by resonant electromagnetic corner and edge modes could be exploited in a variety of applications, such as localized radiative cooling, thermal management technologies, and energy conversion devices. Multidimensional confinement of SPhPs in arrays of subwavelength structures could enable further modulation and enhancement of NFRHT.