Enhanced Coupling of Light From Subwavelength Sources Into a Hyperbolic Metamaterial Fiber

A hyperbolic metamaterial has one or two negative components of the dielectric permittivity tensor while the others are positive. In an optical fiber inhomogenously filled with hyperbolic in homogeneous metamaterials, modes with large angular momentum are localized near the centre of the fiber. Such a hyperbolic metamaterial fiber with radial anisotropy has no cutoff frequency for the transverse electric or the transverse magnetic modes simultaneously. This makes it possible to couple the near-field evanescent modes, associated with large transverse wavevectors of small subwavelength sized sources to these high angular momentum fiber modes well below the conventional cutoff frequencies in the butt-coupling geometry. Such hyperbolic and inhomogeneous fibers can out-couple power from a suhwavelength sized source very efficiently and the coupling can be more than seven orders of magnitude larger compared to a conventional step index waveguides over lengthscales of few wavelengths. Specifically, we show that radial hyperbolicity (epsilon(r) < 0, epsilon(z) > 0, epsilon(phi) > 0) is more advantageous than axial hyperbolicity ( epsilon(z) < 0, epsilon(r) > 0, epsilon(phi )> 0) for such near-field coupling effects. These inhomogeneous hyperbolic fibers can be synthesized from nanoporous alumina microtubes and have immense potential to be used as near-field couplers for nanophotonic devices and near-field imaging probes.