Superresolution Imaging Using a Tapered Bundle of High-Refractive-Index Optical Fibers

The spatial resolution of an optical fiber bundle is limited by the size of a mode propagating in an individual fiber, which can be as small as a single wavelength approximately $\ensuremath{\lambda}$ for conventional low-to-medium refractive-index optical fibers. High-refractive-index optical fiber bundles have a potential for the subwavelength-resolution imaging due to strong mode confinement in a fiber core, but they suffer from inconvenient image readout from the bundle output end, which is done by energy-inefficient near-field probes. To address this issue, this work introduces a tapered high-refractive-index terahertz (THz) optical fiber bundle. It comprises the subwavelength-diameter sapphire optical fibers, that are stacked tightly in the object plane to sample the near field with a subwavelength resolution. The fibers diverge from the object plane and, thus, stretch the captured near field for its further read out from the output bundle end using conventional diffraction-limited lens. Such a tapered fiber bundle was investigated numerically and then implemented experimentally at $0.33$ THz. The subwavelength resolution of this bundle was confirmed, and varies over its aperture and can be as high as $0.35\ensuremath{\lambda}$. The developed imaging principle allows the $\ensuremath{\simeq}0.5\ensuremath{\lambda}$ Abbe resolution limit of a free-space focusing to be overcome and almost any common diffraction-limited optics for the near-field applications to be adapted.