Polarization-Resolved Near-Field Characterization of Nanoscale Infrared Modes in Transmission Lines Fabricated by Gallium and Helium Ion Beam Milling

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Abstract

We demonstrate that functional infrared transmission lines (TLs) with gap widths of 25 and 5 nm can be fabricated by gallium and helium ion beam milling of gold films thermally evaporated on insulating substrates. Interferometric and polarization-resolved scattering-type scanning near-field microscopy at λ0 = 9.3 μm is applied to image in real space the propagation and confinement of the TL modes. Mapping the p-polarized scattered field, we obtain the distribution of the out-of-plane near-field component of the mode. When s-polarized scattered field is detected, we map the strongly confined in-plane fields propagating inside the gap. With decreasing gap width, we experimentally confirm the predicted reduction of mode diameter Dm, wavelength λm, and propagation length Lm. For TLs with 25 nm gap width we experimentally verify λm = 5.1 μm mode wavelength and Dm = 42 nm mode diameter (λ0/220). The propagation length is about Lm = 8.3 μm, which is more than 2 orders of magnitude larger than the mode diameter; Lm/Dm = 200. For the TLs with a 5 nm gap we find a mode wavelength of λm = 3.6 μm, propagation length of Lm = 3.9 μm, and mode diameter of Dm = 30 nm. Infrared transmission lines are thus interesting candidates for the development of ultracompact infrared circuits or biochemical waveguide sensors or circuits.

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