Precision Measurement of Phonon-Polaritonic Near-Field Energy Transfer between Macroscale Planar Structures Under Large Thermal Gradients

Despite its strong potentials in emerging energy applications, near-field thermal radiation between large planar structures has not been fully explored in experiments. Particularly, it is extremely challenging to control a subwavelength gap distance with good parallelism under large thermal gradients. This article reports the precision measurement of near-field radiative energy transfer between two macroscale single-crystalline quartz plates that support surface phonon polaritons. Our measurement scheme allows the precise control of a gap distance down to 200 nm in a highly reproducible manner for a surface area of $5\times 5{\mathrm{mm}}^2$. We have measured near-field thermal radiation as a function of the gap distance for a broad range of thermal gradients up to $\sim 156\mathrm{K}$, observing more than 40 times enhancement of thermal radiation compared to the blackbody limit. By comparing with theoretical prediction based on fluctuational electrodynamics, we demonstrate that such remarkable enhancement is owing to phonon-polaritonic energy transfer across a nanoscale vacuum gap.

Figure 1

(a) Schematic illustration of the near-field experimental setup, disassembled to clearly show components mounting on its two stages. (b) Schematic of prealigned top and bottom samples with contact sensors.

Figure 2

[(a)–(d)] Sample engagement steps for parallel gap spacing: (a) bottom sample is initially tilted and approaches to make the first corner contact, (b) bottom sample rotates to make a two-corner contact. [(c) and (d)] The same procedure is repeated in the other direction until four corners can make contact. (e) Real-time monitoring of four contact signals during the sample engagement and retraction. Note that the slight inclination of the lines is just due to a small sampling rate for plotting interface, not the actual electrical response.

Figure 3

(a) Measurement principle of near-field thermal radiation, where $Q_R$ is measured at different gap distances while $T_{\text{Heater}}$ and $T_{\mathrm{TEC}}$ are maintained at set points. (b) Near-field contribution in thermal radiation ($\mathrm{\Delta }{Q}_R$) between quartz plates as a function of gap distance when $\mathrm{\Delta }T$ is $48\pm 2\mathrm{K}$ (i.e., $T_H=349\pm 1\mathrm{K}$ and $T_C=301\pm 1\mathrm{K}$). The red-colored band is the theoretical calculation obtained from fluctuational electrodynamics. The inset clearly shows the $1/d^2$ gap dependence of near-field thermal radiation, which is a strong evidence of surface-mode dominant energy transfer.

Figure 4

Gap dependence of near-field thermal radiation for different thermal gradients. The symbols show the experimental near-field thermal radiative power, where the colored bands are fluctuational electrodynamics predictions. For better presentation of the results, the obtained $\mathrm{\Delta }{Q}_R$ curves are slightly offset by 0.05 W. The inset shows the maximum enhancement factor (${\eta }_{e,\max }$) at the gap distance of 200 nm.

Figure 5

Effect of parallelism on near-field thermal radiation, $\mathrm{\Delta }{Q}_R$, as a function of ${\theta }_x$ when ${\theta }_y=0$ deg. The thermal gradient is set to $\mathrm{\Delta }T=19\pm 1\mathrm{K}$ (i.e., $T_H=319.5\pm 0.5\mathrm{K}$ and $T_C=300.5\pm 0.5\mathrm{K}$) at the gap distance of 400 nm.