Design of optical fiber devices based on multi-functional metasurfaces

Abstract

In recent years, metasurface technology has transformed optics and photonics by enabling precise control of optical wavefronts using nanostructures at a subwavelength scale. Simultaneously, inspired by the success of the "lab-on-fiber" concept, integrating metasurfaces with optical fibers has garnered significant attention. Over the last decade, this integration has introduced a novel platform for the creation of "all-in-fiber" metasurface-­based devices. This work aims to put forward novel high-performance optical fiber meta-devices and find their potential applications. Firstly, we propose a dual-wavelength achromatic metalens that generates polarization-dependent bifocal spots using the spatial segment method and holographic principle. This approach achieves bifocal generation and achromatic focusing together. By selecting Silicon nano bricks unit cells, we ensure efficient transmission and polarization conversion under circularly polarized light at discrete wavelengths. Dual-wavelength achromatic focusing is attained with holographic principle-enabled polarization-dependent bifocal spots. Simulation results validate this design, applicable in multi-wavelength achromatic imaging systems, biomedical imaging, and VR/AR. Secondly, we've successfully demonstrated an all-dielectric bifunctional metalens on the LMA-PCF end facet, integrating high-index a-Si nano bricks at 1310 nm wavelength. This metalens enables simultaneous center focusing and ellipticity detection through propagation and geometric phase modulation. Its broadband center focusing spans up to 100 nm and achieves enhanced optical intensity peaking at 8.3 times across wavelengths. The ellipticity detection function is confirmed by relating the relative intensity of off-axis foci to incident ellipticity while maintaining similar focusing performance. Our proposal establishes the groundwork for compact optical meta-devices integrated with fiber technology, with potential in multifunctional miniaturized optical devices for communication and biological imaging. Thirdly, we developed a compact non-interleaved metasurface based on a large-mode­-area photonic crystal fiber (LMA-PCF) to detect full-Stokes parameters. This metasurface manipulates dynamic and Pancharatnam-Berry (PB) phases concurrently, assigning unique helical phases to orthogonal circular polarization bases. This enables the representation of amplitude contrast and relative phase difference using non-overlapping foci and an interference ring pattern. Single-shot measurements become possible, precisely determining the relative phase between orthogonal polarization states. Simulation shows full-Stokes parameters calculated with a 2.84% average detection deviation for 20 samples. Our fiber-tip metasurface achieves full-Stokes parameters detection with a low deviation rate, overcoming small pattern area limitations. This study establishes a quantitative framework for fiber-compatible polarization detection, promising for miniaturized research. The metasurface-based approach holds potential for practical polarization detection systems, offering insight for further exploration. At last, Next, we explore vortex beam generation and AM detection. We begin with creating perfect vortex beams by combining the axicon phase, spiral phase, and Fourier transformation. The metadevice shapes light from an LMA-PCF tip into these beams. Far-field optical intensity of beams generated (at a longitudinal position of 35 μm) with TC of 2 and 4 shows identical ring radii, verifying the TC-independent property of PVB. We propose and verify a twelve-channel OAM and SAM detection scheme, where propagation and geometric phase modulations are jointly controlled for SAM detection. Different solid focal spot positions reveal TC carried by the OAM beam, enabling simultaneous determination of OAM and SAM through the metadevice.