We investigate near-field thermal radiation of gradient refractive index slabs (GRISs). It is demonstrated that internal polaritons (IPs) can be supported in the interior of GRISs. IPs provide more evanescent electromagnetic states especially in the lower frequency region and cause red-shift in the near-field heat flux spectrum, as well as enhance local absorption inside the medium. This work sheds light on the mechanism of near-field thermal radiation between GRISs and provides additional design freedom for near-field energy devices.

Near-field radiative heat transfer (NFRHT) between rough surfaces, due to its widespread presence in engineering practice of near-field energy utilization, requires indepth studies, especially from the perspective of physical mechanism. In this paper, an effective multilayer model is built to approach the NFRHT between random rough surfaces of silicon carbide (SiC). Using the effective medium theory (EMT), the effective dielectric function of each layer is obtained, which forms a gradient distribution of dielectric function along the depth of the medium. The influence of the effective dielectric function on surface phonon polaritons (SPhPs) is analyzed, showing that the effective layers with small filling fraction of SiC feature lower SPhP resonance frequencies than SiC bulk. The coupling of SPhPs from the gradient distribution of dielectric function produces new surface modes that dominates the NFRHT. Investigation on the effect of root mean square height (RMS height, σ) reveals that the peaks of local density of states (LDOS) and spectral heat flux are red-shifted as σ increases, while the spectral heat flux below the peak frequency gets larger. This can be attributed to the coupling of SPhPs inside the rough layer. We also found the total net heat flux between rough surfaces separated by an average distance exceeds that between smooth plates and increases with increasing σ, which offer a new way to enhance NFRHT. This work provides a reference for the simulation and understanding of the NFRHT between rough surfaces. (J. M. Zhao) R effective reflection coefficient D local density of states, LDOS p defined in section 3.2 ht defined in section 3.2 Greek Symbols  root mean square height, m τ correlation length, m  dielectric function  angular frequency, rad/s  damping factor, s -1  mean energy of a Planck oscillator, J  parallel wavevector component, m -1  vertical wavevector component, m -1 ħ Planck constant divided by 2π, 1.055×10 34 Js

We build a model to determine the dependency of near-field heat flux on bias voltage using the metal–oxide–semiconductor structures considering gradient distribution of dielectric function. Quantitative dependency of near-field heat flux exchanged by two biased metal–oxide–semiconductor structures on bias voltage is established. The distribution of carrier density and the resultant dielectric function in the semiconductor layer caused by the bias are determined. The corresponding near-field heat flux is calculated using an effective multilayer model. Significant tuning performance is demonstrated, which is due to the increase or decrease in high-frequency surface polariton states induced by the injection or extraction of major carriers. This work deepens the understanding of electrical control of near-field heat transfer with metal–oxide–semiconductor structures, promising for nanoscale thermal management devices and thermal circuits.

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