Near-field and far-field studies of single and double sub-λ sized infrared plasmonic nano-antennas

The electromagnetic modes of a sub-wavelength sized antenna can be excited by thermal fluctuations. This thermal radiation is extremely weak, requiring in general the fabrication of a huge number of identical antennas to be detectable. Here, we will first demonstrate experimentally emission spectrum measurement and superresolved mapping of a single nanoantenna, sub-λ in the infrared spectral domain, based on the detection of its thermal radiation despite an overwhelming background thermal radiation [1]. To achieve a background free detection of the thermal radiation from single or a few sub-λ sized resonators, we have developed an infrared spatial modulation spectroscopy (IR-SMS) technique using a lateral modulation of the sample heated at ~150 °C combined with lock-in detection of the infrared signal detected through a Fourier transform infrared spectrometer. We have applied it in combination with thermal radiation scanning tunnelling microscopy [2] to study the thermal radiation from single plasmonic metal-insulator-metal (MIM) antennas both in the near field and in the far field. Our studies performed on single MIMs have revealed the surprising result that when silica is used as insulating material, its strong dispersion in the mid-infrared domain is such that the fundamental spatial mode of the antenna can be thermally excited at various wavelengths. This causes multiple resonances in the thermal radiation spectrum to which correspond the same spatial distribution of near-field thermal radiation [1].