Metasurfaces integrated with a single-mode waveguide array for off-chip wavefront shaping

Abstract

The integration of metasurfaces and Silicon-on-Insulator (SOI) chips strategically combines the benefits of both metamaterials and silicon photonics. This integration not only facilitates innovative light-shaping functionalities within planar configurations but also ensures compatibility with the CMOS (complementary metal-oxide-semiconductor) production standards. To extract light from a two-dimensional metasurface into the free space vertically, the existing approach is to utilise wide slab waveguides. However, the inherent multi-mode characteristics of such wide slab waveguides make the device susceptible to modal distortion. To address this challenge, I have proposed a different approach that seamlessly integrates metasurfaces with single-mode waveguides, leveraging the advantages of single-mode waveguides to enhance the mode robustness against perturbations introduced by nano-scatters. Si nanopillars with different dimensions are judiciously placed on these waveguides, achieving the required 2π phase coverage for comprehensive wavefront shaping. In the first part of my PhD work, two example types of devices are designed and numerically studied in this thesis: a beam deflector and a metalens. The beam deflector demonstrates the capability to deflect light at a consistent angle of approximately 38° for both input directions. The metalens exhibits precise light focusing capabilities, effectively focusing light at a distance of 46.5 µm above the metasurface, forming a focal point that closely aligns with the ideal Fraunhofer diffraction size. This study presents a straightforward method for integrating metasurface onto the SOI chip, offering potential benefits for emerging applications such as metalens arrays and neural probes that require off-chip light shaping from relatively compact metasurfaces. The second part of my PhD work further develops this integration of single-mode waveguides and nano-scatters, the continuous 2D beam steering is realised at a fixed wavelength by applying coherent control over the nanopillar-on-waveguide structure. The shadowing effect, is effectively exploited through coherence control, enabling the mode manipulation within the waveguide, resulting in the nanopillars radiating the wavefront with the desired phase gradient. Two aperture expansion strategies are explored and demonstrated: Port-selecting structures and the aperiodic pitch algorithm. The resulting coherent control-based Optical Phased Array (OPA) exhibits a competitive beam steering range (13° in the x-direction, 62° in the y-direction) and a relatively small beam width (1.2° × 0.33°). Compared to existing fixed wavelength 2D beam steering ii approaches, the beam steering range is significantly broader. Moreover, in comparison to the conventional two-dimensional beam control methods that require phase and wavelength tuning, this novel approach not only poses lower requirements on the laser source but also enables faster beam steering speed. In addition, a comprehensive demonstration of the fabrication process and outcomes for essential optical devices is presented in this thesis, including single-mode waveguides, grating couplers, and ring resonators. These fabrication outcomes are supported by measurement results and theoretical analyses, providing a thorough understanding of the fabricated optical devices. A series of large-scale grating couplers were systematically designed and fabricated to simplify alignment with fibres, showcasing a competitive coupling efficiency of -5.96 dB per grating coupler accompanied by a relatively wide bandwidth of 46 nm. Leveraging these advantages of this grating coupler design, the ring resonators exhibit relatively good performance, offering an FSR of 6.48 nm for a micro ring with a radius of 15 µm, an extinction ratio of -29.2 dB and a Q-factor of 681.52. Moreover, efforts were made to fabricate the nanopillar-on-waveguide structure, the binary splitter tree was fabricated initially, aiming to validate the feasibility of the device fabrication

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Identifiers

ORCID for Siyu Chen: orcid.org/0000-0002-9125-7850

ORCID for Xu Fang: orcid.org/0000-0003-1735-2654

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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