Broadband lateral leakage suppression for TM polarization empowered by slab-photonic-crystal-modified ridge waveguide

Ridge waveguides are key components in passive integrated circuits, silicon modulators, and hybrid lasers. However, TM polarization modes in shallow-etched ridge waveguides suffer from unexpected leakage loss due to the TM-TE polarization conversion. The leakage loss of TM modes can be suppressed to minimums only in specific ridge widths, which limits the diverse components design, as known as accidental bound states in the continuum. In this paper, we put forward and experimentally demonstrated an effective strategy for a universal high TM-mode transmission ridge waveguide. By introducing the hexagonal lattice photonic crystal into the slab region, we reduced the equivalent-medium index of the local slab and suppressed the TE-TM polarization conversion strength. For TM₀ mode, the maximum transmission improvement is over 16 dB in theory, and we also verified the feasibility of high-order TM mode. In the wavelength range of 1350-1450 nm, the experimental result shows the distinct advantages in low-loss TM₀ mode transmission and resonance suppression. The minimum loss is below 1 dB at the wavelength of 1397 nm, and the max transmission improvement above 20 dB is realized near the wavelength of 1427 nm. That method is meaningful for overcoming the limitation of bound states in the continuum and expanding diverse ridge waveguide devices for TM polarization modes.