On‐chip multimode transmission has drawn tremendous interest for its ability to expand the link capacity with a single wavelength carrier. The waveguide crossing is a basic component in the on‐chip photonic circuit allowing the routing of a complex network. The situation has arisen where the multimode waveguide crossing is required to achieve the high‐density multimode networks. However, the conventional designs of the waveguide crossing only work for the single‐mode. Some works have been reported which realize the multimode crossing, but all suffer from significant losses. Here a multimode waveguide star‐crossing is proposed using the metamaterial‐based Maxwell's fisheye lens, where four multimode waveguides converge at a single junction. The index‐engineered silicon metamaterial is utilized to obtain the Maxwell's fisheye lens on the silicon‐on‐insulator platform. By combining the Maxwell's fisheye lens and tapered waveguides, both TM0 and TM1 modes propagate through the star‐crossing with low losses and cross talk. The fabricated multimode waveguide star‐crossing shows low losses of <0.3 dB and cross talk of <‐20 dB.

The polarization beam splitter (PBS) is an essential component in the polarization diversity circuit to overcome the strong polarization dependence of silicon nanophotonic devices. Among various PBS structures, the asymmetrical directional couplers (ADCs) are most commonly used due to the best overall performances. However, it is still challenging to realize an on‐chip silicon PBS with a small footprint, low loss, high extinction ratio, and, especially, broad bandwidth, simultaneously. The working bandwidth is limited for ADCs, since the wavelength difference will introduce deviations to both the effective indices and the coupling strength, which would distort the phase‐matching property and the optimal coupling length. In this paper, an on‐chip silicon PBS is proposed and demonstrated using “effective medium anisotropy” instead of “configuration asymmetry” to break the bandwidth bottleneck. The metamaterials with engineered anisotropy and dispersion are utilized to form a hetero‐anisotropic slab. For TM polarization, the slab performs as a multi‐mode interference coupler, while for TE polarization, the slab performs as two isolated waveguides. The fabricated PBS shows low loss <1 dB, high extinction ratio >20 dB, and a record working bandwidth >200 nm, with a footprint as small as 12.25 × 1.9 µm2.

On-chip multi-mode communication has attracted much attention for its potential to greatly expand the bandwidth of data transmission. However, the application of multi-mode transmission is still restricted since the multi-mode waveguide can't be compactly routed, especially in the sharp bending region. The challenge lies in decreasing the significant inter-mode coupling in the multi-mode bending with a small radius. Few works have been reported to achieve multi-mode bending, but all suffer from the complex fabrication. In this paper, an ultra-sharp multi-mode waveguide is proposed, which is assisted with metamaterial-based mode converters. The sub-wavelength structures are directly written onto the polymer cladding. By engineering the effective medium index of the metamaterial cladding in the mode converters, the inter-mode coupling is dramatically inhibited. The fabricated sharp bending with a 30-μm radius shows that the crosstalk is <−20 dB and the loss is <1 dB over a >80 nm wavelength band.

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A dual-mode waveguide crossing is proposed and realized for the application of on-chip multimode interconnection. The present structure is realized by two 90° crossed multimode-interference (MMI) couplers. By properly choosing the width and length of the MMI section, the self-images for both the incident TM₀ mode and the TM₁ mode could be at the center of the crossing. The characterization results for the fabricated device show that low insertion loss below ∼1.5  dB and low crosstalk below ∼-18  dB can be achieved over a large bandwidth more than 80 nm for both the TM₀ and TM₁ modes.