Infrared spatial modulation spectroscopy enables one to acquire background-free spectra of subwavelength sized objects. With this method we have investigated thermally excited single and double metal-insulator-metal plasmonic antennas (MIMs). On single MIMs with silica insulator, the same resonance condition is satisfied at different wavelengths due to the strong dispersion of silica. On double MIMs, the thermal radiation spectra bear the signature of hybridized electromagnetic modes which are simultaneously excited when the gap separation between the antennas is in the 100 nm range.

The study of hybrid modes in a single dimer of neighboring antennas is an essential step to optimize the far-field electromagnetic (EM) response of large-scale metasurfaces or any complex antenna structure made up of subwavelength building blocks. Here we present far-field infrared spatial modulation spectroscopy (IR-SMS) measurements of a single thermally excited asymmetric dimer of square metal-insulator-metal (MIM) antennas separated by a nanometric gap. Through thermal fluctuations, all the EM modes of the antennas are excited, and hybrid bonding and anti-bonding modes can be observed simultaneously. We study the latter within a plasmon hybridization model, and analyze their effect on the far-field response.

We propose a modulation method to record background-free far field FTIR spectra of single sub $-\lambda$ sized objects which we combine with near-field TRSTM measurements to characterize the thermal emission of plasmonic antennas and silica rods. The fundamental mode of MIM nanoantennas is excited at multiple wavelengths. © 2019 The Author(s)

The far-field spectral and near-field spatial responses of an individual metal-insulator-metal nanoantenna are reported, using thermal fluctuations as an internal source of the electromagnetic field. The far-field spectra, obtained by combining Fourier transform infrared spectroscopy with spatial modulation based on a light falloff effect in a confocal geometry, have revealed two distinct emission peaks attributed to the excitation of the fundamental mode of the nanoantenna at two distinct wavelengths. Superresolved near-field images of the thermally excited mode have been obtained by thermal radiation scanning tunneling microscopy. Experimental results are supported by numerical simulations showing that it is possible to excite the same mode at different wavelengths near a resonance of the insulating dielectric material forming the antenna.

Nanoantennas have the ability to spatially and spectrally manipulate light at the nanoscale. Arranged in arrays, they create metasurfaces with homogeneous optical properties but periodicity and coupling between nanoantennas can be detrimental to the study of their intrinsic optical response. In order to investigate the electromagnetic properties of a single nanoantenna, we use a set-up based on thermal radiation scanning tunneling microscopy (TRSTM) to characterize an isolated nanoantenna from the near field to the far field by achieving sub-wavelength imaging in the mid-infrared.