Broadband Mode Converter With Customizable Modal Ratio Leveraging Subwavelength Engineering for Advanced Multimode Silicon Photonics

ABSTRACT Mode conversion is a critical process in multimode silicon photonics, enabling power exchange between fundamental and higher‐order modes. This operation supports a broad spectrum of applications, from high‐capacity data transmission to optical signal processing and sensing. Despite recent progress, no reported solution offers customizable modal conversion ratios. This work presents, for the first time, a mode converter that provides control of the conversion ratio between the fundamental and second‐order transverse‐electric modes. The device consists of two back‐to‐back Y‐junctions interconnected by subwavelength gratings, with a central region incorporating graded‐index metamaterials. By adjusting the number of graded‐index segments, the device offers multiple conversion ratios across the 1350‒1700 nm broad wavelength range. Experimental measurements validate 15 discrete conversion levels, each exhibiting a consistent conversion factor of 7.7 ± 0.4% per added graded‐index segment. The device enables customizable conversion ratios from a maximum between 94% and 98.9% to a minimum between 0.4% and 1.2%, maintained over the 1500‒1600 nm bandwidth. With a compact size of 9.3 × 2.6 µm, the proposed device is well‐suited for applications in quantum photonics and programmable photonics. Moreover, we demonstrate the scalability of the design methodology through a compact TE 0 –TE 4 converter that achieves 14 design‐defined conversion levels with broadband response.