Near-field radiative heat transfer between nanoscale gaps, particularly those below 100 nm, represents a significant departure from classical far-field thermal radiation. At such small separations, evanescent waves dominate, enabling heat transfer rates that exceed the blackbody limit by orders of magnitude. This phenomenon is governed by the coupling of electromagnetic modes, such as surface plasmon polaritons (SPPs), surface phonon polaritons (SPhPs), and hyperbolic modes, which arise due to the unique optical properties of materials at these scales.
Plasmonic enhancement plays a critical role in near-field heat transfer. Metals like gold and silver support SPPs, which are collective oscillations of free electrons at the interface between a metal and a dielectric. These modes confine electromagnetic energy to subwavelength dimensions, dramatically increasing the local density of states (LDOS). For instance, at a 10 nm gap between two gold surfaces, the heat flux can reach up to 10^4 times the blackbody limit. The spectral overlap of SPP resonances between the two surfaces determines the efficiency of energy transfer, with narrower gaps leading to stronger coupling.
Hyperbolic materials, such as hexagonal boron nitride (hBN) and vanadium dioxide (VO2), further enhance near-field thermal radiation due to their anisotropic permittivity. These materials exhibit hyperbolic dispersion, where the permittivity tensor components have opposite signs along different crystal axes. This property allows for the propagation of high-momentum waves, known as hyperbolic phonon polaritons (HPhPs), which significantly boost the photonic LDOS. Experimental studies have demonstrated that hBN can achieve heat flux enhancements of 10^3 at 50 nm gaps, with spectral tuning possible via material composition and temperature.
Phonon-polariton coupling is another key mechanism, particularly in polar dielectrics like silicon carbide (SiC) and alumina (Al2O3). These materials support SPhPs, where infrared photons couple with optical phonons, creating resonant modes that enhance thermal radiation. The Reststrahlen band, a frequency range where the permittivity is negative, is critical for SPhP-mediated heat transfer. For example, SiC exhibits strong SPhP resonances around 10–12 µm, leading to near-field heat fluxes that are orders of magnitude higher than far-field predictions. The gap distance and temperature gradient directly influence the spectral matching of these modes, with optimal transfer occurring when the surfaces’ SPhP resonances align.
The theoretical framework for near-field radiative heat transfer is primarily based on fluctuational electrodynamics, which combines Maxwell’s equations with the fluctuation-dissipation theorem. This approach models thermal emission as stochastic currents arising from thermal agitation, with the Green’s function formalism used to compute the radiative heat flux. Numerical methods, such as the scattering matrix approach and finite-difference time-domain (FDTD) simulations, are employed to solve these equations for complex geometries. Comparisons between theory and experiments show strong agreement for simple planar systems, though discrepancies arise in nanostructured or disordered interfaces due to fabrication imperfections and surface roughness.
Experimental validation of near-field heat transfer has advanced significantly with the development of precision gap control techniques, such as piezoelectric actuators and atomic force microscopy (AFM) probes. Early experiments using parallel plates of silica and SiC confirmed the enhancement of heat transfer at sub-100 nm gaps, with measured fluxes matching theoretical predictions within 20%. More recent studies have explored dynamic modulation of near-field radiation using phase-change materials like VO2, which switches between insulating and metallic states, enabling tunable thermal rectification.
Applications of near-field radiative heat transfer are particularly promising in thermophotovoltaics (TPV), where a hot emitter radiates photons to a photovoltaic cell at close proximity. By operating in the near field, TPV systems can achieve higher power densities and efficiencies compared to conventional far-field designs. For instance, simulations predict that a SiC emitter paired with a germanium PV cell at a 10 nm gap could reach power densities exceeding 10^4 W/m², with efficiencies approaching 40% when spectral filtering is optimized.
Nanoscale thermal rectifiers, which allow heat to flow preferentially in one direction, are another application enabled by near-field effects. Asymmetric materials or gap distances create a directional bias in photon tunneling, leading to rectification ratios as high as 10:1 in theoretical models. Experimental realizations using phase-change materials or graded nanostructures demonstrate the feasibility of such devices for thermal logic circuits and energy management systems.
Challenges remain in scaling near-field thermal devices for practical use, including maintaining nanoscale gaps over large areas and minimizing parasitic conduction. Advances in material synthesis, such as the development of ultra-smooth interfaces and engineered metamaterials, are critical for overcoming these limitations. Future research directions include exploring quantum effects in extreme near-field regimes (<1 nm) and integrating near-field thermal components with electronic and photonic systems for multifunctional devices.
In summary, near-field radiative heat transfer at sub-100 nm gaps is a rich field driven by plasmonic, hyperbolic, and phonon-polariton interactions. Theoretical models based on fluctuational electrodynamics provide a robust foundation, while experimental techniques continue to validate and refine these predictions. Applications in thermophotovoltaics and thermal rectification highlight the potential for transformative advances in energy conversion and thermal management technologies.