Photonic non-volatile memories using phase change materials

Abstract

We propose an all-photonic, non-volatile memory and processing element based on phase-change thin-films deposited onto nanophotonic waveguides. Using photonic microring resonators partially covered with Ge 2 Sb 2 Te 5 (GST) multi-level memory operation in integrated photonic circuits can be achieved. GST provides a dramatic change in refractive index upon transition from the amorphous to crystalline state, which is exploited to reversibly control both the extinction ratio and resonance wavelength of the microcavity with an additional gating port in analogy to optical transistors. Our analysis shows excellent sensitivity to the degree of crystallization inside the GST, thus providing the basis for non-von Neuman neuromorphic computing.The ability to write, store and retrieve data is at the very heart of information processing. Various techniques are employed to efficiently cope with the vast spread of speed and long term storage needs. In particular Phase Change Memories (PCMs), promise to revolutionize the field of information processing by bridging the gap between the short-term, but very quick operation of on-chip memories and the long-term, but relatively slow storage systems such as solid-state devices and hard-drives [1][2][3]. Not only can phase change materials switch in a matter of picoseconds [4-6], they are also able to retain information for very long periods of time [2,7]. In addition, they scale extremely well to the nanoscale, with present-day demonstrations of 6nm cells employing electrical switching [7,8]. Specifically Ge 2 Sb 2 Te 5 (GST) is the most commonly used alloy for such applications. By reversibly transforming the crystalline structure between amorphous and crystalline states using electrical pulses, the resistive properties of the thin film can be varied by several orders of magnitude [9]. PCMs also demonstrate a large difference in reflectivity upon phase-transition, an effect that has led to their commercial use in optical storage discs, such as DVDs and Blue-Ray discs [10].Herein, we propose a chalcogenide-based integrated photonic memory element, with the ability for sub-nanosecond reading and writing, while still retaining data for several years. We analyze the photonic architecture as illustrated in Fig.1(a), which comprises a microring resonator coupled to nanophotonic waveguides. In contrast to photonic memories and mechanical resonators [11], the photonic circuit allows for static tunability which is maintained when the control light has been switched off. We base our analysis on silicon nitride-on-insulator substrates for broadband optical applications. Silicon nitride can be used to fabricate high-quality nanophotonic components for both telecoms and visible applications [12,13]. The feeding waveguide is optimized for single mode operation at 1550nm input wavelength. Inside the ring resonator a small region of the waveguide is suspended similar to waveguides used for nanomechanical sensing [14] and opto-mechanical operation [15]. A thin film of phase change material (GST) is...

Integrated all-photonic non-volatile multi-level memory

“…3.4pJ write energy and 20 ns write pulses in 14 ), the performance of our photonic memories can also be further improved by operating them with shorter pulses and by moving to devices with smaller footprint, as well as through the development of new materials with faster and lower temperature switching. Higher signal to noise ratio to improve the read-out contrast could also be obtained with the use of optical cavities, which would also reduce switching energies as discussed theoretically by us previously 26 . To reduce the device footprint, alternative architectures as for -11-example plasmonic antennas could be explored.…”

“…These beneficial properties of PCMs have already led to prominent commercial applications in optical data storage such as rewritable optical discs (in DVD and Blu-ray formats) 21 and more recently for use in optoelectronic modulation and display applications in the visible spectrum 22 . -3-Many PCMs show significant change in refractive index in the visible and even larger changes in the near-infrared wavelength regime, which is the spectral region of choice for telecommunication applications [23][24][25][26] . Here, by using such nanoscale PCMs embedded in nanophotonic circuits, we demonstrate that fast and repeatable all-optical, multi-level, multi-bit, nonvolatile memory operations, with wavelength division multiplexed (WDM) access, can be achieved on a chip at telecommunications wavelengths compatible with on-chip optical interconnects.…”

Simulations of Silicon-on-Insulator Channel-Waveguide Electrooptical 2 × 2 Switches and 1 × 1 Modulators Using a ${\bf Ge_2}{\bf Sb_2}{\bf Te_5}$ Self-Holding Layer

“…optically by an external 700-nm light beam [2]- [4]. We, however, consider electrical control to be more suitable for practical applications than the optical actuation.…”

Optical switching at 1.55 μm in silicon racetrack resonators using phase change materials

“…Indeed, a reprogrammable optical circuit that can hold its configuration without an external continuous source is extremely desirable for a multitude of applications ranging from photonic routers to optical cognitive networks. Recently, new solutions for non-volatile photonic memories have been proposed, involving the use of phase-change materials (PCM) and microring resonators 8,9 .…”