Enhancing thermo-optic efficiency through inverse-designed mode-cycling topology

Reducing the power consumption of on-chip systems is of critical importance for large-scale photonic integration. Thermo-optic phase shifters, with advantages such as broadband response, CMOS compatibility, and fabrication simplicity, remain widely used in integrated photonics, yet their relatively high power consumption continues to be a major limitation for large-scale applications. Here, we introduce and experimentally demonstrate a mode-cycling topology that enhances thermo-optic modulation efficiency by sequentially converting the optical field among different guided modes, which forces the light to traverse the phase-shifting region multiple times under the same heater. This cycling in the mode domain effectively multiplies the light-matter interaction without enlarging the device footprint. The scheme employs an inverse-designed bi-directional (de)multiplexer and a mode exchanger, which provide compactness, low loss, and improved robustness compared to conventional asymmetric directional couplers. As a proof of concept, we fabricated a 2 × 2 mode-cycling thermo-optic enhanced switch. The measured results show the average insertion losses at about 2.5 dB with a dynamic extinction ratio over 32.7 dB. The device exhibits a half-wave power of approximately 11.9 mW, nearly halving the power requirement compared to a conventional reference switch of similar size. These results confirm the validity of the mode-cycling topology and highlight its potential as a scalable route toward energy-efficient and compact photonic integrated systems.