An Achromatic and Polarization-Insensitive Metafiber at the Entire Telecommunication Wavelengths

Optical fibers have been studied extensively due to their remarkable features such as flexible handling, long-distance light transportation, and strong light confinement. These features enable a variety of applications such as optical trapping, optical communications, sensing, and medical imaging. Particularly, the fiber-based endoscope allows <i>in vivo</i> imaging of biological samples. During the fiber imaging, the scattered light from the object is first collected by the lens, such as ball lens or gradient index lens (GRIN), then guided into the fiber core. However, these two bulky lenses suffer from chromatic aberration that hinders obtaining clear images over wavelengths of interest.<br/>Metasurface, a planar version of metamaterials, has shown a great capability to modulate its optical properties [1] such as amplitude, phase delay, polarization, chirality, and nonlinear coefficient. Especially, dispersion engineering using metasurface, modulation of phase delay over wavelengths of interest, enables diffraction-limited achromatic focusing and imaging, i.e. achromatic metalens [2, 3]. Interfacing fiber with the achromatic metalens could be the ultimate solution to deal with the aberration that exists in fiber-based imaging. Several efforts have been made to fabricate metasurface interfaced with fiber by using electron-beam lithography (EBL) [4], focused ion-beam milling (FIB) [5], and chemical etching techniques [6]. However, these fabrication methods limit the type of materials and geometry of meta-atom, thus prohibiting the design of broadband and large achromatic metalens on top of the fiber. Instead, laser nanoprinting based on two-photon polymerization may provide a platform for optically functionalizing a fiber facet with three-dimensional (3D) arbitrary microstructures [7].<br/>In this presentation, I will introduce an entirely new concept of an achromatic metafiber that focuses light coming out from the fiber facet over wavelengths of interest. The achromatic metafiber consists of achromatic metalens microprinted on a telecommunication single-mode fiber (SMF-28). The 3D meta-atoms of which height is a geometric degree of freedom unlike conventional meta-atoms can provide large variation of group delay, capable of realizing high time-bandwidth product. As a demonstration, direct scanning confocal imaging with our achromatic metafiber is facilitated over entire telecommunication wavelengths. We believe that our compact achromatic metafiber envisages many photonics applications such as hyperspectral imaging in endoscopy, in vivo deep-tissue imaging, and wavelength-multiplexed fiber communications.<br/><b>References</b><br/>[1] Jung, C. <i>et al.</i> Metasurface-Driven Optically Variable Devices. <i>Chem. Rev.</i> (2021) doi:10.1021/acs.chemrev.1c00294.<br/>[2] Khorasaninejad, M. <i>et al.</i> Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging. <i>Science (80-. ).</i> <b>352</b>, (2016).<br/>[3] Chen, W. T. <i>et al.</i> A broadband achromatic metalens for focusing and imaging in the visible. <i>Nat. Nanotechnol.</i> <b>13</b>, 220–226 (2018).<br/>[4] Wang, N., Zeisberger, M., Hübner, U. & Schmidt, M. A. Boosting Light Collection Efficiency of Optical Fibers Using Metallic Nanostructures. <i>ACS Photonics</i> <b>6</b>, 691–698 (2019).<br/>[5] Yuan, G. H., Rogers, E. T. & Zheludev, N. I. Achromatic super-oscillatory lenses with sub-wavelength focusing. <i>Light Sci. Appl.</i> <b>6</b>, e17036–e17036 (2017).<br/>[6] Kuchmizhak, A., Gurbatov, S., Nepomniaschii, A., Vitrik, O. & Kulchin, Y. High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams. <i>Appl. Opt.</i> <b>53</b>, 937 (2014).<br/>[7] Plidschun, M. <i>et al.</i> Ultrahigh numerical aperture meta-fibre for flexible optical trapping. <i>Light Sci. Appl.</i> <b>10</b>, 57 (2021).