Non-volatile programmable photonics based on phase-change materials

Abstract

Programmable photonics can enable a plethora of exciting applications from next-generation optical interconnects to quantum information technologies. Conventionally, photonic systems are tuned by mechanisms such as thermos-optic effect, free carrier dispersion, electro-optic effect, or mechanical movement. Although these physical effects allow either fast (> 100GHz) or large contrast (> 60dB) switching, they are not optimal for programmability which does not require frequent switching. Phase-change materials (PCMs) can offer an ideal solution thanks to their reversible switching, large index contrast, and non-volatile behavior, enabling a truly ‘set-and-forget’ switch element with no static power consumption. Recent years, we have indeed witnessed the fast adoption of PCMs in programmable photonic systems, from photonic integrated circuits (PICs) to meta-optics. Despite the tremendous progress in the field, a few remaining challenges must be addressed before the technology can be scalable and ultimately commercialized. For example, the high optical loss of the traditional PCMs, such as Ge2Sb2Te5 (GST), is prohibitive for large-scale PICs. Secondly, the energy required to electrically switch PCMs remains to be high (~tens of nano-joules), and the device footprint is still large (> 64µm). Lastly, so far there has not been an ideal solution towards non-volatile phase-only control in the free-space due to the high loss of the PCMs and microheaters. In this dissertation, we aim to circumvent these limitations. First, we demonstrated non-volatile phase modulation (\Delta\phi~0.17\pi) with near zero insertion loss in both Si and SiN integrated photonics using a low-loss PCMs Sb2S3. Through device engineering, an ultra-compact (33µm coupling length) directional coupler switch was realized based on Sb2Se3. Individual control of the phase and coupling in racetrack resonator was achieved. We further showed that ultra-low switching energy down to 8.7±1.4aJ/nm3 can be achieved using graphene microheaters for tuning the PCMs with excellent endurance over 1,000 cycles. Finally, leveraging a high-Q metasurface, we demonstrated non-volatile phase-only modulation of free-space light with ~0.2π phase shift and near zero change in intensity. Our work represents a crucial step in the development of disruptive non-volatile photonic technologies based on PCMs.