Opportunities and Challenges for Large-Scale Phase-Change Material Integrated Electro-Photonics

Abstract

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.

A Survey on Optical Phase-Change Memory: The Promise and Challenges

“…The same comparison can be made for crystalline state using FOM 2 . We can see from [21] that for GSST, FOM 2 = 4.17, which again is higher than GST with FOM 2 = 2.52 at 1550 nm. Therefore, GSST applications to construct OPCMs can be expanded to obtain photonic memory arrays and computing units to alleviate the thermal instability and high material loss penalty in GST.…”

“…However, a large extinction coefficient change (κ) in the amorphous and crystalline state imposes excessive insertion loss to the OPCM cell due to material absorption (see II-D). The work in [21] showed that for GST, FOM 1 = 109.72, which is lower than that for GSST with FOM 1 = 116.73 at 1550 nm. This comparison can be interpreted as a higher refractive index and lower material loss that GSST in amorphous state offers compared to GST in the same state at 1550 nm.…”

“…One important material parameter related to PCMs is the melting temperature (T l ). The regions of the material after exposure to the heat source that has a temperature above T l will be melted and quenches (cools down with the rates higher than 10 9 K.S −1 [21]) in about 700 picoseconds [22]. The quenched region will have an amorphous structure regardless of the initial state of the material.…”

“…This process is called set (see Fig. 1(a)) and can take hundreds of nanoseconds or higher [21]. High T g is desirable to improve the phase stability of the material, which is crucial to enable reliable data retention in nonvolatile memories [23].…”

“…GSST, which has been studied for PCM-based photonic applications, shows superior performance in terms of thermal stability-through doping Se atoms and excess Ge atoms, which leads to an increase of the T g [42], [122], [123]-and lower material loss contrast compared to GST (because of lower κ) [34], [122], [123]. The work in [21] suggested that different figures of merits (FOMs) of FOM 1 = n/ κ a and FOM 2 = n/ κ c can be used to quantify the performance of OPCMs. For memory purposes, a drastic change in the refractive index ( n) upon the phase change in the OPCM cell is required.…”