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Plasmonic nanogap enhanced phase change devices with dual electrical-optical functionality
Nikolaos Farmakidis, Nathan Youngblood, Xuan Li, James Tan, Jacob L. Swett, Zengguang Cheng, David C Wright, Wolfram HP Pernice, Harish Bhaskaran
TL;DR
Existing phase-change devices support electrical or optical write/erase, but not both. This work combines plasmonics with phase-change materials in a nanogap device, demonstrating optical switching with 16 ± 2 pJ pulses and repeatable cycling.
Problem
Existing phase-change devices support write/erase operations either electrically or optically, but not both.
Method
The device reversibly switches GST in a nanogap while combining plasmonic and electrical design features for mixed-mode operation.
Results
16 ± 2 pJ using a 5 ns optical pulse switches the device, and the switching is repeatable for many cycles.
Takeaways & Limitations
The nanogap design improves both optical and electrical operation, resulting in an efficient mixed-mode device.
Takeaways & Limitations
Optical read-out is largely limited by its signal-to-noise ratio, which can be addressed by increasing the signal.
Abstract
from arXiv · showhide
Modern-day computers use electrical signaling for processing and storing data which is bandwidth limited and power-hungry. These limitations are bypassed in the field of communications, where optical signaling is the norm. To exploit optical signaling in computing, however, new on-chip devices that work seamlessly in both electrical and optical domains are needed. Phase change devices can in principle provide such functionality, but doing so in a single device has proved elusive due to conflicting requirements of size-limited electrical switching and diffraction-limited photonic devices. Here, we combine plasmonics, photonics and electronics to deliver a novel integrated phase-change memory and computing cell that can be electrically or optically switched between binary or multilevel states, and read-out in either mode, thus merging computing and communications technologies.
Introduction
Integrated photonic computing needs devices that can operate across electrical and optical domains, but phase-change devices face conflicting nanoscale electrical and optical requirements. The paper addresses this gap by combining plasmonic nanogaps, integrated photonics, and GST phase-change material.
- Motivation: Electrical switching requires metal contacts spaced tens of nanometres apart because phase-change materials have high conductive-state contrast.The required contact spacing constrains device dimensions.
- Motivation: The electrically switched conductive region is at most a few hundred nanometres in diameter, limiting the material volume available for light interaction.This creates a mismatch between electrically addressable switching regions and optical device requirements.
- Plasmonic approach: Plasmonic nanogaps can reduce device dimensions to tens of nanometres or below the diffraction limit while providing strong light–matter interaction.Their optical losses motivate combining nanoscale plasmonics with integrated photonics for low-loss light delivery.
- Contribution: The paper combines waveguide-integrated plasmonic nanogaps with Ge2Sb2Te5 to create an electro-optic memory cell addressable in both electrical and optical domains.This approach targets seamless conversion between electrical and optical signals on an integrated platform.
- Prior limitations: Earlier mixed-mode devices either used VO2 with significant retention power or supported write/erase operations in only one domain.These limitations motivate a phase-change cell supporting both electrical and optical switching.
- Contribution: Strong field confinement in the plasmonic nanogap enables electrical and optical non-volatile GST switching within the gap for mixed-mode PCM operation.The device exploits nanoscale dimensions and field confinement to address the electrical and optical requirements together.
Results and Discussion
The device combines a plasmonic nanogap with a photonic waveguide and GST phase-change material to support electrical and optical switching and read-out. Simulations and experiments show state-dependent optical transmission, low-energy optical switching, multilevel behavior, and repeatable operation.
- Device and optical design: The device integrates a plasmonic nanogap between metal electrodes with a photonic waveguide and GST phase-change material.The electrodes provide both electrical contacts and optical field concentration in the nanogap.
- Device and optical design: More than an order of magnitude of electric-field enhancement occurs in amorphous GST within the 50 nm nanogap, decreasing fivefold after crystallization.The reduction is attributed to the higher optical loss of crystalline GST.
- Mixed-mode operation: Optical switching changes electrical resistance and optical transmission in opposite directions: amorphization increases resistance and decreases transmission, while crystallization reverses both changes.The two observables therefore provide simultaneous electrical and optical state read-out.
- Mixed-mode operation: Electrical pulses also switch the GST state, with the resulting device state observable through both optical transmission and electrical resistance.Transmission increases when GST becomes crystalline and remains repeatable over many cycles.
- Multilevel operation and cyclability: The device supports multilevel operation and good cyclability for optically and electrically induced switching between amorphous and crystalline states.Resistance varies more than optical transmission because crystal-domain growth is stochastic, while optical read-out is limited by its signal-to-noise ratio.
- Comparison and significance: The authors report the first integrated device with full optical and electrical programming and read-out, while reducing active area and minimum switching energy relative to prior photonic memories.The comparison is presented as a literature-level assessment of the mixed-mode device.
Conclusion
The work demonstrates an integrated, reversible, non-volatile phase-change memory cell that supports optical and electrical programming and read-out in one device. Its plasmonic design bridges electro-optic mixed-mode operation while enabling compact hybrid computing and multilevel storage applications.
- The device provides electrical and optical programming and read-out through the combination of phase-change materials and nanoplasmonics.
- The integrated phase-change memory cell is reversible and non-volatile, bridging electro-optic mixed-mode operations.
- The plasmonic design reduces device footprint and contact separation while enhancing light–matter interaction.
- A single device enables direct comparison of optical and electrical read and write operations and their relative merits.
- Its non-volatile optical or electrical switching offers an outlook for switchable and reconfigurable metadevices and metamaterial-based optical components.
- The platform supports potential hybrid optoelectronic computing, in-memory computing, and multilevel data storage applications.