  Optical fiber is a well-established efficient way to guide and manipulate
light and allowing high bandwidth optical transmission for long distance
communication with low attenuation. Although a dielectric optical waveguide is
efficient for transmitting light, its functionality is limited by the
dielectric properties of the materials of core and cladding.The light coming
out from optical fiber is typically diverging, and the numerical aperture is
determined by the refractive index of the fiber materials. Thus, the light
intensity decreases significantly upon exiting the fiber. The use of
metasurfaces provides the opportunity to tailor light properties for advanced
light manipulation and to develop novel optical applications that are flat. By
producing a specific phase profile using spatially-varied nano-antenna
elements, metasurfaces can control the wavefront of the transmitted, reflected,
and scattered light, and enable novel ultrathin optical components such as flat
lenses. Here, we report an ultrathin optical metalens cascaded on the facet of
optical fiber that enables light focusing in the telecommunication regime.
In-fiber metalenses with focal lengths of 28 and 40 um and maximum efficiency
of ~16.4% at a wavelength of 1550 nm are demonstrated. The integration of an
ultrathin metalens and optical fiber will open the path to revolutionary
in-fiber optical devices for practical applications in optical imaging,
sensing, and fiber laser.
