Programmable photonic integrated circuits (PICs) have recently gained significant interest because of their potential in creating next-generation technologies ranging from artificial neural networks and microwave photonics to quantum information processing. The fundamental building block of such programmable PICs is a 2 × 2 programmable unit, traditionally controlled by the thermo-optic or free-carrier dispersion. However, these implementations are power-hungry and volatile and have a large footprint (typically >100 μm). Therefore, a truly “set-and-forget”-type 2 × 2 programmable unit with zero static power consumption is highly desirable for large-scale PICs. Here, we report a broadband nonvolatile electrically controlled 2 × 2 programmable unit in silicon photonics based on the phase-change material Ge2Sb2Te5. The directional coupler-type programmable unit exhibits a compact coupling length (64 μm), small insertion loss (∼2 dB), and minimal crosstalk (<−8 dB) across the entire telecommunication C-band while maintaining a record-high endurance of over 2800 switching cycles without significant performance degradation. This nonvolatile programmable unit constitutes a critical component for realizing future generic programmable silicon photonic systems.

Programmable photonics have the potential to completely transform a range of emerging applications, including optical computing, optical signal processing, light detecting and ranging, and quantum applications. However, implementing energy-efficient and large-scale systems remains elusive because commonly used programmable photonic approaches are volatile and energy-hungry. Recent results on nonvolatile phase-change material (PCM) integrated photonics present a promising opportunity to create truly programmable photonics. The ability to drastically change the refractive index of the PCMs in a nonvolatile fashion allows creating programmable units with zero-static energy. By taking advantage of the electrical control, nonvolatile reconfiguration, and zero crosstalk between each unit, PCMs can enable extra large-scale integrated (ELSI) photonics. In this Perspective, we briefly review the recent progress in PCM photonics and discuss the challenges and limitations of this emerging technology. We argue that energy efficiency is a more critical parameter than the operating speed for programmable photonics, making PCMs an ideal candidate. This has the potential for a disruptive paradigm shift in the reconfigurable photonics research philosophy, as slow but energy-efficient and large index modulation can provide a better solution for ELSI photonics than fast but power-hungry, small index tuning methods. We also highlight the exciting opportunities to leverage wide bandgap PCMs for visible-wavelength applications, such as quantum photonics and optogenetics, and for rewritable photonic integrated circuits (PICs) using nanosecond pulsed lasers. The latter can dramatically reduce the fabrication cost of PICs and democratize the PIC manufacturing process for rapid prototyping.

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 (<1.0 dB), high extinction ratio (>10 dB), high cyclability (>1600 switching events), and 5-bit operation. These Sb2S3-based devices are programmed via on-chip silicon PIN diode heaters within sub-ms timescale, with a programming energy density of $$\sim 10\,{fJ}/n{m}^{3}$$ ~ 10 f J / n m 3 . Remarkably, Sb2S3 is programmed into fine intermediate states by applying multiple identical pulses, providing controllable multilevel operations. Through dynamic pulse control, we achieve 5-bit (32 levels) operations, rendering 0.50 ± 0.16 dB per step. Using this multilevel behavior, we further trim random phase error in a balanced Mach-Zehnder interferometer.

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 (< 1.0 dB), high extinction ratio (> 10 dB), high cyclability (> 1,600 switching events), and 5-bit operation. These Sb2S3-based devices are programmed via on-chip silicon PIN diode heaters within sub-ms timescale, with a programming energy density of ∼10fJ/nm3. Remarkably, Sb2S3 is programmed into fine intermediate states by applying multiple identical pulses, providing controllable multilevel operations. Through dynamic pulse control, we achieve 5-bit (32 levels) operations, rendering 0.50 ± 0.16 dB 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 large-scale energy-efficient programmable PICs with low-loss and multi-bit operations.