Nanoscopic Hyperlensing from Natural and Monoisotopic Hexagonal Boron Nitride Crystals*

Hyperbolic media, where the permittivity is opposite in sign along orthogonal axes, support highly directional propagation of volume-confined, hyperbolic polaritons (HPs) for use in super-resolution imaging via the hyperlens concept. Hexagonal boron nitride (hBN), a natural hyperbolic material, supports deeply subdiffractional, low-loss HPs in both planar slabs and nanoscale resonators within the mid- to long wavelength IR. These losses could be reduced even further by using monoisotopic (i.e. material with just a single boron isotope) hBN. Here we exploit these ultralow losses and natural hyperbolic response to realize unprecedented spatial resolution in hyperlensing with long-wavelength IR light. We provide a direct comparison of the imaging power of hyperlens designs using flat slabs of naturally abundant and monoisotopic hBN via scattering-type near field optical microscopy (s-SNOM). Our experimental (s-SNOM) and simulated results show the ability to resolve features as small as 50 nm with 6-7.1 µm free-space wavelength light, providing at least l/125 spatial resolution. We complement this with electromagnetic field simulations of the hyperlens response to demonstrate and quantify the improvements from the monoisotopic over the naturally abundant materials.