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. 2019 Nov 29;5(11):eaaw2687.
doi: 10.1126/sciadv.aaw2687. eCollection 2019 Nov.

Plasmonic nanogap enhanced phase-change devices with dual electrical-optical functionality

Nikolaos Farmakidis  1 , Nathan Youngblood  1 , Xuan Li  1 , James Tan  1 , Jacob L Swett  1 , Zengguang Cheng  1 , C David Wright  2 , Wolfram H P Pernice  3 , Harish Bhaskaran  1
Affiliations

Plasmonic nanogap enhanced phase-change devices with dual electrical-optical functionality

Nikolaos Farmakidis et al. Sci Adv. .

Abstract

Modern-day computers rely on electrical signaling for the processing and storage of data, which is bandwidth-limited and power hungry. This fact has long been realized in the communications field, where optical signaling is the norm. However, exploiting optical signaling in computing will require new on-chip devices that work seamlessly in both electrical and optical domains, without the need for repeated electrical-to-optical conversion. Phase-change devices can, in principle, provide such dual electrical-optical operation, but assimilating both functionalities into a single device has so far proved elusive owing to conflicting requirements of size-limited electrical switching and diffraction-limited optical response. Here, we combine plasmonics, photonics, and electronics to deliver an integrated phase-change memory cell that can be electrically or optically switched between binary or multilevel states. Crucially, this device can also be simultaneously read out both optically and electrically, offering a new strategy for merging computing and communications technologies.

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Figures

Fig. 1
Fig. 1. Mixed-mode plasmonic memory cell integrated in a photonic waveguide.
(A) 3D illustration of device concept. Light is delivered to the nanoscale device via a photonic waveguide, while the Au contacts serve as both device electrodes and plasmonic nanogap to focus incoming light. (B) Optical and (C and D) SEM images of device after fabrication {scale bar [inset of (C)], 100 nm}. The width of the nanogap was measured to be approximately 50 nm for the devices used. (E) Eigenmode simulations of field enhancement inside the plasmonic nanogap when the GST is in the amorphous (top) or crystalline state (region between Au electrodes, bottom). The field enhancement is much stronger when GST is in the amorphous state owing to the significantly lower optical loss. (F) FDTD simulation of the transmission of device before and after crystallization. The significant change in the refractive index changes the coupling between the nanogap and waveguide, which reduces reflection from the input waveguide, thereby increasing overall transmission of the device in the crystalline state. (G) Experimental measurement of total energy in the waveguide required to achieve a nonvolatile phase transition. The switching threshold was measured to be 16 ± 2 pJ according to a linear fit to the data (black dashed line).
Fig. 2
Fig. 2. Nonvolatile optical programming of the plasmonic memory cell with electro-optical readout.
(A) Top: Illustration of device programmed using an optical signal with both electrical and optical readout of the device state. Bottom: Schematic of the experimental setup used for electrical programming of the device. Optical write (piecewise pulse, 7.5 mW for 8 ns + 3 mW for 400 ns) and erase (7.5 mW for 8 ns) pulses are used to switch the GST between crystalline and amorphous states, respectively. A continuous wave (CW) optical probe signal and constant voltage source are used to monitor the optical transmission and electrical resistance of the GST simultaneously. Electro-Optic Modulator (EOM), Erbium Doped Fiber Amplifier (EDFA). (B) Real-time trace of the device’s optical transmission during multiple write and erase cycles. (C) Simultaneous readout of the device’s electrical resistance showing nonvolatile switching of the GST between the amorphous and crystalline states.
Fig. 3
Fig. 3. Nonvolatile electrical programming of the plasmonic memory with electro-optical readout.
(A) Top: Illustration of programming the memory cell using an electrical signal with simultaneous electrical and optical readout. Bottom: Schematic of the experimental setup used for electrically programming the device. Electrical write (rectangular: 350 mV for 10 ns) and erase (triangular: 350 mV with 5-ns/500-ns rise-fall time) pulses are used to switch the state of the GST between the amorphous and crystalline states. Again, a CW optical probe signal and constant voltage source are used to monitor the transmission and resistance simultaneously. (B) Real-time trace of the device’s optical transmission during multiple write and erase cycles. (C) Simultaneous readout of the device’s electrical resistance showing nonvolatile switching of the GST between the amorphous and crystalline states.
Fig. 4
Fig. 4. Multilevel operation and cyclability of the mixed-mode device.
(A) Five consecutive cycles of multilevel operation with a fixed write pulse and a linearly increasing erase pulse energy (8-ns pulse width, 80 erase pulses per cycle). The variation in resistance is much greater than that in optical transmission because of the stochastic nature of crystal domain growth within the nanogap. Cumulative plots of change in (B) resistance and (C) optical transmission for multilevel traces shown in (A). Cyclability plots of both the electrical resistance and optical transmission during multiple (D) optical and (E) electrical write and erase cycles.
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