Fig. 4: Partial focusing of hybrid polaritons by substrate engineering.
From: Doping-driven topological polaritons in graphene/α-MoO3 heterostructures
a, Schematic of the design, where the heterostructure lies on top of a substrate composed of a Au–SiO2–Au in-plane sandwich structure. b, Isofrequency dispersion contours of hybrid polaritons for Au and SiO2 substrates at 910 cm−1 (λ0 = 10.99 μm). The shaded areas highlight the convex and concave dispersion contours in the region around the x axis on the gold and SiO2 substrates, respectively. With a wave vector inside the shaded area, negative refraction can happen at the Au–SiO2 interface when the polaritons on the gold substrate propagate towards that interface. The scheme for negative refraction is illustrated by further showing the incident wave vector ki and the Poynting vector Pi, together with the resulting transmitted kt and Pt. c, Experimentally measured near-field amplitude (S3) image of hybrid polaritons showing partial focusing in the system shown in a. The central SiO2 film was 1.5 µm wide and served as an in-plane flat lens. The antenna was located 1.0 µm away from the left Au–SiO2 interface. d, Experimentally measured hybrid polaritons on a Au substrate, as a control to c. Scale bar indicates 1.5 μm and also applies to c. e, Near-field profiles for the sections marked by the red (A) and blue (C) vertical dashed lines in c and d, respectively. The black dashed curves are Gaussian fittings. WA and WC indicate the full width at half maximum (FWHM) of profiles A and C, respectively. f, Near-field profiles of the sections marked by red (B) and blue (D) horizontal dashed lines in c and d, respectively. SB1, SB2, SD1 and SD2 represent the electric-field intensity at each fringe. The graphene was doped to EF = 0.6 eV and the α-MoO3 thickness was 320 nm.
