Enhanced Far-Field Thermal Radiation through a Polaritonic Waveguide

We experimentally demonstrate the enhancement of the far-field thermal radiation between two nonabsorbent Si microplates coated with energy-absorbent silicon dioxide (${\mathrm{SiO}}_2$) nanolayers supporting the propagation of surface phonon polaritons. By measuring the radiative thermal conductance between two coated Si plates, we find that its values are twice those obtained without the ${\mathrm{SiO}}_2$ coating. This twofold increase results from the hybridization of polaritons with guided modes inside Si and is well predicted by fluctuational electrodynamics and an analytical model based on a two-dimensional density of polariton states. These findings could be applied to thermal management in microelectronics, silicon photonics, energy conversion, atmospheric sciences, and astrophysics.

Figure 1

Experimental platform used to probe the radiative heat transfer. (a) Scheme of two Si microplates suspended by $706\text{−}\mathrm{\mu }\mathrm{m}$-long beams and mounted on a heating stage controlling their temperature. (b) Diagram of the hot and cold plates coated with ${\mathrm{SiO}}_2$ nanolayers and separated by a vacuum gap $g=10.7\mathrm{\mu }\mathrm{m}$. Scanning electron microscope image of the (c) device and (d) its close-up on the rectangular box in (c).

Figure 2

Experimental and theoretical values of the radiative thermal conductance through the gap between two (a) uncoated and (b) coated Si microplates. The blue zone in (a) represents the prediction of Planck’s theory for the far-field thermal conductance $G_{\mathrm{Planck},\mathrm{Si}}$ determined with a Si emissivity ${\epsilon }_{\mathrm{Si}}=0.7$ to 0.9. The green and red zones in (b) show the respective predictions of Planck’s theory ($G_{\text{Planck},\text{coated Si}}$) and Eq. (2) for the coated Si plate, while the corresponding blue and red lines are computed with fluctuational electrodynamics. The blackbody limit (${\epsilon }_{\mathrm{Si}}={ϵ}_{{\mathrm{SiO}}_2}=1$) is also shown via the black line. The error bars represent the standard deviations of multiple measurements with different input currents and modulation frequencies.

Figure 3

(a) Generalized flux spectrum for two coated and uncoated Si plates and (b) their corresponding thermal conductance calculated via Scuff-EM. (c) Density map of the in-plane Poynting vector for a coated Si plate showing the dominant in-plane emission of the electromagnetic flux. (d) Poynting vector components arising from the propagation of multiple guided modes inside the Si layer mainly. (e) Density map of the Poynting vector magnitude obtained for two facing coated Si plates. Calculations were done for $\omega /2\pi =28\mathrm{THz}$, for which SPhPs exhibit the resonant transmission shown in (a).