Fig. 1
Schematics of the coaxial nanoapertures and its epsilon-near-zero (ENZ) optical property. a The illustration of coaxial nanoapertures shows geometrical parameters including an inner diameter (Din), gap size (G), period (P), and thickness of the gold film (T). Processing scheme: (1) Gold nanopillars are patterned via electron-beam lithography; (2) ALD coating of gap-filling insulators (Al2O3; thickness from 1 to 10 nm); (3) gold sputtering; (4) glancing-angle ion milling to planarize the top surface and expose the Al2O3-filled nanocoax array. b Scanning electron microscopy (SEM) images of coaxial nanoapertures with a 10 nm alumina gap, 250 nm diameter, 500 nm period, and 150 nm Au thickness. c Dispersion of a guided mode (real and imaginary parts) of a coaxial waveguide (250 nm diameter and 10 nm gap width) in a gold film illuminated with a linearly polarized beam at normal incidence
