Tunable reflection/transmission switching using multimode interference with phase-change material

phase transitions facilitated by total internal reflection. Subwavelength tooth structures are incorporated on the reflecting facets to engineer the optical field distribution via the gradient-index effect. The device achieves an insertion loss of 0.61 dB and exhibits low crosstalk below -21 dB over a 300-nm bandwidth. Fabrication tolerances are evaluated through structural variations, indicating stable operation. In addition, multi-level tuning of the power splitting ratio is achieved, suggesting potential applications in reconfigurable photonic signal processing.