Plasmonic nanoantenna designs are quickly evolving in the direction of practical molecular sensing applications hence their wavelength range is being extended from the visible towards the mid-infrared. The problem of obtaining, in the mid-infrared, the same degree of plasmonic confinement obtained with gold in the visible range is related to the perfect conductor behavior of metals at long wavelengths. Here we fabricated bow-tie nanoantennas made of bottom-up assembled “metallic germanium” with free electron density of the order of 1020 cm-3 and therefore short plasma wavelength of 4.5 μm. We demonstrate the existence in the antenna gaps of confined hotspots with radius of the order of 100 nm, which we imaged by near-field photoexpansion microscopy at a wavelength of 5.8 μm in order to provide a clear proof of strong field confinement in the mid-infrared.
Eugenio Calandrini, Tommaso Venanzi, Felice Appugliese, Michela Badioli, Valeria Giliberti, Leonetta Baldassarre, Paolo Biagioni, Francesco De Angelis, Wolfgang M. Klesse, Giordano Scappucci, and Michele Ortolani, "Electromagnetic field confinement in the gap of germanium nanoantennas with plasma wavelength of 4.5 micrometers," Proc. SPIE 10111, Quantum Sensing and Nano Electronics and Photonics XIV, 101110F (Presented at SPIE OPTO: January 29, 2017; Published: 27 January 2017); https://doi.org/10.1117/12.2253841.
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