Fig. 4: Polaritonic lenses with gradient effective refractive indices.
From: A lithography-free approach to polaritonic Luneburg lenses
a Near-field image (real part) of a polaritonic Lunrburg lens, Re(σ2), at the frequency (ω) of 1500 cm−1, overlapped on its three-dimensional topography, where the thickness of the hBN slab (dhBN) is 89 nm. b Line scan signals along the focal spot and corresponding Gauss fitting. c Simulated field distribution in the x−y plane, denoted by the real part of the electric field in the z direction, Re(Ez). d Corresponding magnitude of the simulated electric field (|E | ). e Ray-tracing result. Experimental (f) and simulated results (g–i) for another polaritonic Luneburg lens with a larger size. j–m Same as (f–i) but for a polaritonic lens with a gentler index distribution than that of a Luneburg lens. Green and grey curves indicate the concave interference patterns resulting from edge-launched and edge-reflected tip-launched polaritons, respectively.
