Sub-mW low-latency silicon thermo-optic phase shifter using a foundry substrate undercut heater

We demonstrate a compact, foundry-compatible silicon thermo-optic phase shifter (TOPS) based on a substrate-undercut doped-silicon heater, achieving a π-phase-shift power of P _π =0.74 mW at a drive voltage of only V _π =0.91 V. Prior low-power phase shifters reach comparable or lower P _π only by extending the heated length, enlarging the footprint, and none report CMOS-compatible drive voltages. Here, both the heater and the 2×2 multimode interference (MMI) couplers are optimized using particle swarm optimization (PSO), the former to jointly minimize P _π and optical loss at a fixed 100 µm length in a compact 240×73 µm² footprint. Integrated into a Mach-Zehnder switch, the device shows an insertion loss of 0.90 dB, an extinction ratio of 23 dB, and rise/fall times of 1.38/1.02 ms. To our knowledge, this is the first foundry-fabricated TOPS to combine sub-milliwatt P _π , a compact low-latency arm, and a sub-1-V CMOS-compatible V _π , offering a scalable route to low-energy programmable silicon photonic meshes.