Thermal radiation spectra of individual subwavelength microheaters

Polarization resolved spectra of infrared radiation from individual electrically driven platinum microheaters have been measured by Fourier-transform infrared spectrometry as a function of heaters’ width. When the heater width approaches zero, the signal with polarization parallel to the heater long axis converges to a finite value, while its perpendicularly polarized counterpart drops below our detection limit. As a result this leads to strongly polarized radiation for very narrow heaters. Further, while the parallel polarized radiation spectra appear to be insensitive to heater width variation (at least within the sensitive range of our light detector), the perpendicular polarized spectra were heavily affected. We observed a $\lambda /2$-like resonance that we attribute to correlation of charge oscillations across the heater’s width, which are possibly mediated by surface plasmons. These findings provide implications for fabrication of nanoscale electrically driven thermal antennas.

Figure 1

(Color online) (a) SEM image of an $8\times 0.8\mu {\text{m}}^2$ microheater. (b) Schematics for the experiment. Radiation emitted by the nanoheater is analyzed with the FTIR system.

Figure 2

(Color online) FTIR spectra obtained from heaters at a temperature of 750 K. (a) For polarization parallel to heater long axis. (b) For polarization perpendicular to heater long axis. Numbers next to each spectrum on the right side indicate the corresponding heater width in microns. For each width, spectra for both polarization directions are divided by the same normalization factor in order to align peaks for different widths in the parallel polarization case to the same height.

Figure 3

Integrated intensity $\left(I\right)$ from $2000$ to $3500{\text{cm}}^{-1}$ of the FTIR spectra in Fig. 2 against heaters width $\left(w\right)$. The polarization direction is parallel (circles) and perpendicular (crosses) to the heater long axis. The solid line shows the best linear fit to the parallel polarization data. Inset: illustrations of radiating dipoles and surface plasmons across the heater’s width. The two rectangular boxes represent the same microheater, viewed from above the sample surface. The horizontal and vertical boundaries of the boxes represent the width and length of the heater, respectively. Short solid arrows within the two boxes represent oscillating dipoles for parallel (left box) and perpendicular (right box) polarizations, respectively. Long dashed arrows represent the propagation direction of corresponding surface plasmons.

Figure 4

(Color online) Reduced intensity ratio (data normalized with the value at $8\mu \text{m}$) versus heater width for averaged value in the windows of 2000–2500 (red squares), 2500–3000 (green diamonds), and 3000–3500 (blue circles) ${\text{cm}}^{-1}$. The solid curves are best fits based on our model (see text) for wave numbers 2250 (red), 2750 (green), and 3250 (blue) ${\text{cm}}^{-1}$.