  Scalable programmable photonic integrated circuits (PICs) can potentially
transform the current state of classical and quantum optical information
processing. However, traditional means of programming, including thermo-optic,
free carrier dispersion, and Pockels effect result in either large device
footprints or high static energy consumptions, significantly limiting their
scalability. While chalcogenide-based non-volatile phase-change materials
(PCMs) could mitigate these problems thanks to their strong index modulation
and zero static power consumption, they often suffer from large absorptive
loss, low cyclability, and lack of multilevel operation. Here, we report a
wide-bandgap PCM antimony sulfide (Sb2S3)-clad silicon photonic platform
simultaneously achieving low loss, high cyclability, and 5-bit operation. We
switch Sb2S3 via an on-chip silicon PIN diode heater and demonstrate components
with low insertion loss (<1.0 dB), high extinction ratio (>10 dB), and high
endurance (>1,600 switching events). Remarkably, we find that Sb2S3 can be
programmed into fine intermediate states by applying identical and thermally
isolated pulses, providing a unique approach to controllable multilevel
operation. Through dynamic pulse control, we achieve on-demand and accurate
5-bit (32 levels) operations, rendering 0.50 +- 0.16 dB contrast per step.
Using this multilevel behavior, we further trim random phase error in a
balanced Mach-Zehnder interferometer. Our work opens an attractive pathway
toward non-volatile large-scale programmable PICs with low-loss and on-demand
multi-bit operations.
