  Energy-efficient programmable photonic integrated circuits (PICs) are the
cornerstone of on-chip classical and quantum optical technologies. Optical
phase shifters constitute the fundamental building blocks which enable these
programmable PICs. Thus far, carrier modulation and thermo-optical effect are
the chosen phenomena for ultrafast and low-loss phase shifters, respectively;
however, the state and information they carry are lost once the power is turned
off-they are volatile. The volatility not only compromises energy efficiency
due to their demand for constant power supply, but also precludes them from
emerging applications such as in-memory computing. To circumvent this
limitation, we introduce a novel phase shifting mechanism that exploits the
nonvolatile refractive index modulation upon structural phase transition of
Sb$_{2}$Se$_{3}$, a bi-stable transparent phase change material. A zero-static
power and electrically-driven phase shifter was realized on a foundry-processed
silicon-on-insulator platform, featuring record phase modulation up to 0.09
$\pi$/$\mu$m and a low insertion loss of 0.3 dB/$\pi$, which can be further
improved upon streamlined design. We also pioneered a one-step partial
amorphization scheme to enhance the speed and energy efficiency of PCM devices.
A diverse cohort of programmable photonic devices were demonstrated based on
the ultra-compact PCM phase shifter.
