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Tao Wang1 6 Peining Li1 5 Dmitry Chigrin1 7 Alexander Giles2 Francesco Bezares3 Joshua Caldwell2 4 Thomas Taubner1

1, RWTH Aachen Univ, Aachen, , Germany
6, Institute or Materials Research and Engineering, Singapore, , Singapore
5, CIC Nanogune, San Sebastian, , Spain
7, DWI - Leibniz-Institute for interactive Materials, Aachen, , Germany
2, US Naval Research Laboratory, Washington, District of Columbia, United States
3, Universidad de Puerto Rico, Cayey, Puerto Rico, United States
4, Vanderbilt University, Nashville, Tennessee, United States

A conventional thermal emitter exhibits a broad emission spectrum with a peak wavelength depending upon the operation temperature. Recently, narrowband thermal emission was realized with periodic gratings or single microstructures of polar crystals such as SiC [1, 2]. These polar crystals support Surface Phonon-Polaritons (SPhPs) [3], which offer lower losses and higher resonance quality factors due to longer lifetime than the commonly used Surface Plasmon Polaritons (SPPs). Due to the strong confinement of SPhPs, subwavelength resonators can host different, spectrally narrow modes depending on geometry and period.
Here, we go one step further and investigate the coupling of adjacent phonon-polaritonic nanostructures, specifically deeply sub-diffractional bowtie-shaped silicon carbide nanoantennas. We experimentally demonstrate that the nanometer-scale-gaps can control the thermal emission frequency while retaining emission linewidths as narrow as 10 cm-1[4].
To prove that the thermal emission originates from of nanoantenna structures and for an unambiguous assignment of the strongly localized SPhP resonant modes, we employ infrared far-field reflectance spectroscopy and compare it with full-wave electromagnetic simulations and near-field optical nanoimaging. The latter is based on scattering-type scanning near-field optical microscopy (s-SNOM) and enables us to directly visualize the rather complex modes of our 3-dimensional nanostructures. We also observe slight differences between individual bowties in our array, again indicating the strong influence of the nanoscale gaps on some of the narrow emission lines.
We believe that the observed narrow emission linewidths and exceptionally small modal volumes will provide new opportunities for the user-design of near- and far-field radiation patterns for advancements in infrared spectroscopy, sensing, signaling, communications, coherent thermal emission, and infrared photo-detection.

[1] J. J. Greffet et al, Nature 416, 61 (2002).
[2] J. A. Schuller et al, Nature Photon. 3, 658 (2009).
[3] J. D. Caldwell, et al., Nanophotonics 4, 44 (2015).
[4] T. Wang et al., ACS Photonics 4, 1753 (2017).

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