Ultra-compact mode-order converters with dielectric slots are demonstrated on a silicon-on-insulator platform. We propose a mode converter that converts the TE0 mode into the TE1 mode with an ultra-small footprint of only 0.8 × 1.2 µ m 2 . The measured insertion loss is less than 1.2 dB from 1520 nm to 1570 nm. To reduce the insertion loss, we further optimize the structure and design two mode converters that convert the TE0 mode into the TE1 mode and the TE2 mode with footprints of 0.88 × 2.3 µ m 2 and 1.4 × 2.4 µ m 2 , respectively. Their measured insertion losses are both less than 0.5 dB. Additionally, the proposed devices are cascadable and scalable for high-order mode conversion.

On-chip spatial mode operation, represented as mode-division multiplexing (MDM), can support high-capacity data communications and promise superior performance in various systems and numerous applications from optical sensing to nonlinear and quantum optics. However, the scalability of state-of-the-art mode manipulation techniques is significantly hindered not only by the particular mode-order-oriented design strategy but also by the inherent limitations of possibly achievable mode orders. Recently, metamaterials capable of providing subwavelength-scale control of optical wavefronts have emerged as an attractive alternative to manipulate guided modes with compact footprints and broadband functionalities. Herein, we propose a universal yet efficient design framework based on the topological metamaterial building block (BB), enabling the excitation of arbitrary high-order spatial modes in silicon waveguides. By simply programming the layout of multiple fully etched dielectric metamaterial perturbations with predefined mathematical formulas, arbitrary high-order mode conversion and mode exchange can be simultaneously realized with uniform and competitive performance. The extraordinary scalability of the metamaterial BB frame is experimentally benchmarked by a record high-order mode operator up to the twentieth. As a proof of conceptual application, an 8-mode MDM data transmission of 28-GBaud 16-QAM optical signals is also verified with an aggregate data rate of 813 Gb/s (7% FEC). This user-friendly metamaterial BB concept marks a quintessential breakthrough for comprehensive manipulation of spatial light on-chip by breaking the long-standing shackles on the scalability, which may open up fascinating opportunities for complex photonic functionalities previously inaccessible.

Wavelength‐division multiplexing (WDM) technology can offer considerable parallelism for large‐capacity data communications. While several configurations have been demonstrated to realize on‐chip WDM systems, their practical applications might be hindered by large footprints or compromised performances. Recently, metamaterial‐assisted silicon photonics is emerging for on‐chip light manipulation by subwavelength‐scale control of optical wavefronts. They can reach more compact footprints and broadband functionalities beyond the classical waveguide‐based architectures. Herein, wavelength (de)multiplexers are experimentally demonstrated in the subwavelength‐structured metamaterials regime with highly compact footprints. Two‐dimensional metamaterials composed of quasi‐periodic dielectric perturbation arrays patterned on a multimode waveguide are proposed to stimulate multiple high‐order waveguide modes at different wavelengths, which are then coupled to different WDM channels by cascading mode (de)multiplexers. The four‐channel wavelength (de)multiplexer is demonstrated in a box‐like spectrum with a compact footprint of 2.5 × 250 µm2, with the measured losses less than 2 dB and channel crosstalk less than –14.3 dB. By varying the patterned metamaterial structures, the proposed devices also have the merits of flexible operating wavelengths and bandwidths. The concept features large scalability, compactness, and competitive performance, which can offer versatile on‐chip light manipulation and significantly improve the integration density for various on‐chip WDM optical systems.

we present a four-channel flat-top coarse wavelength-division multiplexing (CWDM) (de)multiplexer employing contra-directional coupling between multiple modes simultaneously with shallow-etched dielectric etches in a single multimode waveguide.

On-chip integrated mode-division multiplexing (MDM) has been emerging as a promising technology to further improve the link capacity and satisfy the continuously increasing bandwidth demand in data communications. One of the most important components in MDM and multimode photonics is a mode converter. While several configurations have been developed to realize on-chip mode converters, it is still very challenging to achieve versatile high-order mode converters with high performance in a generic way to reduce the R&D and prototyping costs. Here we initiate a breakthrough utilizing a simple yet universal generic building block concept with metasurface structures to implement programmable arbitrary highorder mode converters with competitive performance, high reliability and compact footprints. The building block, i.e., the TE0-TE2 mode converter is first introduced to illustrate the generic concept, which exhibits low insertion loss of 0.3 dB, low crosstalk of -10 dB across broad wavelength band of 250 nm with a footprint of 2.7×1.3 µ 2 m . All even-order and odd-order mode converters can be realized by directly programming multiple parallel basic building blocks and coarsely engineering the waveguide widths simply in a universal approach. The proposed mode converter building blocks for high-order mode conversion highlight features of uniform performance with broad bandwidth, low insertion loss, compact footprints and good fabrication tolerance, plus the uniquely simple and scalable generic fashion, making them extremely attractive for on-chip multimode optical interconnections.