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On‐chip mode exchangers play very important roles for multimode photonics, which has drawn much attention for many applications. However, it is still very challenging to achieve compact mode exchangers with high performances in an ultra‐broad bandwidth. Here a universal approach of on‐chip mode manipulations is proposed by manipulating the multimode excitations/interferences, and ultra‐compact mode exchangers are realized with low excess losses (ELs) and high extinction ratios (ERs) using special metamaterial structures. The presented TEi–j mode exchangers enable the exchange between the TEi and TEj modes in an ultra‐broad bandwidth. The designed TE0–1 and TE0–2 mode exchangers have footprints of 1.3 × 2.7 µm2 and 1.9 × 2.9 µm2, respectively. Theoretically, they have ELs ≈ 0.2 dB and ERs = 19–32 dB at 1550 nm, while their bandwidths for achieving ER > 10 dB reach 340–400 nm. The fabricated devices have measured ELs of 0.2–0.3 dB and ERs above 12.0–13.2 dB in the setup‐limited wavelength range of 1520–1607 nm. The proposed approach is flexibly extendable to enable on‐chip mode exchanges, which are promising for multimode silicon photonics in many applications.

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A broadband dual-mode 2 × 2 3 d B power splitter on silicon is proposed by utilizing multimode interference (MMI) couplers with a shallowly etched multimode region. The present MMI coupler is designed optimally to be with reduced phase errors for all the excited higher-order modes in the MMI region by engineering the refractive-index profile with the assistance of a shallowly etched multimode region. Simulation results show the device works well for the T E 0 and T E 1 input modes over the C-band. For the T E 0 input mode, the excess losses (ELs) and the imbalances (IBs) are, respectively, less than 0.4 and 0.12 dB in the wavelength range of 1.5–1.6 µm, while the ELs and the IBs for the T E 1 input mode are, respectively, less than 0.5 and 0.2 dB in the wavelength range of 1.53–1.58 µm. Besides, the intermode cross-talk is less than − 22 d B for T E 0 and T E 1 modes in the wavelength range of 1.5–1.6 µm. The proposed dual-mode MMI coupler can be extended for more than two input modes.

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