Near-field focusing and amplification of tip-substrate radiative heat transfer

TL;DR

The paper addresses active control of spatially resolved near-field radiative heat transfer between a nanoscale tip and a substrate by introducing a thin polar film atop a non-dispersive substrate. It adopts fluctuational electrodynamics in the dipolar regime and computes the z-component of the Poynting vector $S_z$ using Green's functions split into vacuum and scattering parts, with material response described by a Drude-Lorentz model for SiC. A key contribution is showing that film thickness $\delta$ can non-monotonically enhance and narrowly localize the heat flux, due to film-induced surface modes whose dispersion $\omega(k)$ depends on $\delta$ and the substrate permittivity $\varepsilon_{\mathrm{sub}}$, and on the particle–substrate separation $d$. The results indicate that maximum flux and best focusing occur at different $\delta$, highlighting a need for a combined optimization objective for practical design. These insights advance nanoscale thermal management and have implications for HAMR and related technologies, with possible extensions to multi-tip geometries and self-consistent heat diffusion in multilayer substrates.

Abstract

The spatially resolved near-field radiative heat transfer between a nanoscale probe and a substrate is studied in the fluctuational electrodynamics framework within the dipolar approximation. It is shown that the introduction of a thin polar film atop a non-dispersive substrate can lead to both an enhancement and a lateral focusing of the heat exchange. The influence of the probe--substrate separation, film thickness and substrate permittivity is analyzed, revealing that the effect originates from near-field interactions governed by the interplay between film-induced modifications of electromagnetic mode dispersion and the distance-dependent coupling strength. The results highlight a viable route toward the active control of local radiative heat transfer at the nanoscale.

Figures (5)

• Figure 1: Geometry of the system. A small sphere of radius $R$, mimicking the presence of a tip, is placed at distance $d$ from a substrate covered by a SiC film of thickness $\delta$. The limiting case $\delta=0$ corresponds to the absence of film. Both the substrate and the film are translationally invariant along the $x$ and $y$ axes.
• Figure 2: (a) Ratio of the Poynting vector $S_z(0,0,0)$ below the nanoparticle and the one $S_\mathrm{ref}$ in the absence of film ($\delta=0$) as a function of the film thickness $\delta$. The 4 curves correspond to different values of the substrate permittivity (see legend). (b) Corresponding values of the HWHM.
• Figure 3: Profile of the Poynting vector $S_z(x,0,0)$ in normalized (main part) and absolute (inset) units for $\varepsilon_\mathrm{sub}=2$ and $\delta=0,60,240,800\,$nm (see legend).
• Figure 4: (a) Spectrum of the Poynting vector at the origin (below the particle) for different values of film thickness and in the absence of film (see legend). (b) Dispersion relation of the surface modes for the vacuum--film--substrate system, along with the one for a SiC-vacuum interface (black dashed line).
• Figure 5: HWHM (main part) and Poynting vector amplification (inset) for (a) $R=50\,$nm and $d=150\,$nm, (b) $R=100\,$nm and $d=1\,\mu$m.