Abstract
The hyperbolic phonon polaritons supported in hexagonal boron nitride (hBN) with long scattering lifetimes are advantageous for applications such as super-resolution imaging via hyperlensing. Yet, hyperlens imaging is challenging for distinguishing individual and closely spaced objects and for correlating the complicated hyperlens fields with the structure of an unknown object underneath. Here, we make significant strides to overcome each of these challenges. First, we demonstrate that monoisotopic h11BN provides significant improvements in spatial resolution, experimentally resolving structures as small as 44 nm and those with sub 25 nm spacings at 6.76 μm free-space wavelength. We also present an image reconstruction algorithm that provides a structurally accurate, visual representation of the embedded objects from the complex hyperlens field. Further, we offer additional insights into optimizing hyperlens performance on the basis of material properties, with an eye toward realizing far-field imaging modalities. Thus, our results significantly advance label-free, high-resolution, spectrally selective hyperlens imaging and image reconstruction methodologies.
Highlights
Sub-diffractional imaging in conventional optical microscopy is not possible due to Abbe’s diffraction limit, as light scattered from deeply sub-wavelength objects rapidly decay from the surface, resulting in evanescent fields that do not propagate into the far-field
Sub-wavelength modes cannot propagate into free space, this reflection at the top surface results in evanescent fields that extend just above the surface of the hyperbolic slab that can be probed via external means sensitive to near-fields, such as s-SNOM5,6
For frequencies close to that of the longitudinal optic (LO) phonon of hexagonal boron nitride (hBN), the propagation angle will be near normal (Fig. 1e), resulting in hyperlens fields probed on the hBN top surface that approximates a slightly magnified replica of the underlying object
Summary
Sub-diffractional imaging in conventional optical microscopy is not possible due to Abbe’s diffraction limit, as light scattered from deeply sub-wavelength objects rapidly decay from the surface, resulting in evanescent fields that do not propagate into the far-field. For a curved hyperbolic material[7,8], these hyperbolic “rays” propagating along the radial directions are expanded, resulting in magnified hyperlens fields that can become resolvable in the far-field when the features expand beyond the diffraction limit. This is possible through (flat) metalens designs[9,10], or in the near-field via probes such as scattering-type scanning near-field optical microscopy (sSNOM)[5,6], which we have employed here
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