Optical fiber meta-tips

We present the first experimental demonstration of an optical-fiber “meta-tip” (OFMT). OFMTs are the result of the integration of a metasurface on the tip of an optical fiber (Figure 1a). Metasurfaces (MSs) are 2D... [ view full abstract ]

We present the first experimental demonstration of an optical-fiber “meta-tip” (OFMT). OFMTs are the result of the integration of a metasurface on the tip of an optical fiber (Figure 1a). Metasurfaces (MSs) are 2D implementations of metamaterials and are able to impress abrupt phase and amplitude variations in an impinging light wavefront so to allow local light manipulation for a large variety of aplications. MSs are much easier to fabricate with respect to their bulk (3D) counterpart and compatible with on-chip photonics. We use plasmonic metasurfaces, based on an array of rotated rectangular aperture (Figure 1c) nanoantennas milled in a thin gold film deposited on the tip of an optical fiber. The antennas present a suitable spatial-modulation of the element sizes so as to implement a linear phase (or a costant phase gradient) trend (Figure 2). Such a MS will impress abrupt phase variations in the impinging field wavefront. We fabricated (Figure 3) and characterized in the far-field (Figure 4) several near-infrared prototypes implementing generalized forms of the Snell’s transmission/reflection laws, in terms of beam steering, with various angles θ_t (Figure 1b). The measured steering angles are in fairly good agreement with the theory for all fabricated samples. As limit case we realized an OFMT exciting surface waves in transmission, i.e. with steering angle exceeding 90°, and explore its capability to work as a sensor. OFMT's sensitivity to refractive index variations has been tested and compared to a gradient-free plasmonic benchmark array: the sensitivity of the OFMT overwhelms that of the benchmark by a factor of 1.7. Our results set the stage for the integration of metasurfaces, and their exceptional light-manipulation capabilities, in fiber-optics technological platforms, within the emerging “lab-on-fiber” paradigm. By leveraging the versatility and ubiquity of fiber-optics technology, this may constitute a key step towards the widespread use of optical metasurfaces in real-world applications to communications, signal processing, imaging and sensing.