PZT optical memristors

Abstract

Optical memristors represent a monumental leap in the fusion of photonics and electronics for neuromorphic computing and artificial intelligence. Here, we reveal the first lead zirconate titanate (PZT) optical memristor, working with a paradigm of functional duality: non-volatile setting and ultrafast volatile modulation via the Pockels effect. Fine-tuning and large modulation depth are achieved with an index change of 4.6 × 10^-3 when setting above a threshold voltage V_th and the switching energy is 12.3 pJ only. The non-volatility is highly stable even with >100,000 cycles. Sub-nanosecond volatile modulation (48 Gbps, 432 fJ/bit) is realized with high efficiency (V_πL ~ 0.5 V·cm) via the strong Pockels effect below V_th. Our wafer-scale manufacturing process shows great potential for mass production. The present PZT optical memristors bridge the gap between high-speed photonics and non-volatile memory, offering transformative potential for high-speed and energy-efficient optical interconnects, quantum computing, neural networks, in-memory computing, and brain-like architecture.

Fig. 1. PZT optical memristor.

a Wafer-scale sol-gel fabrication processes of PZT Optical Memristors. b Photograph of the spin-coating wafer. c Scanning electron microscopy (SEM) picture of the top view of the thin-film PZT (thickness~ 300 nm). d AFM image of the thin-film PZT. e Photograph of a 4-inch wafer patterned by using an ultraviolet stepper lithography system. f Colorized SEM of PZT MRR (blue). g SEM picture of the cross-section of a PZT ridge waveguide. h Simulated field distribution for the TE₀ mode. i PZT unit cell representation before and after poling. j Schematic diagram of the ridge waveguide cross-section and the poling process. k The dependence of the refractive index changes ∆n_eff (P_ν) against the net ferroelectric domain polarization, and the corresponding simplified top-view schematic of ferroelectric domains in PZT.

Fig. 2. MRR-based PZT optical memristor.

a Optical microscopy image of an MRR with a bending radius of 60 μm. b Single-cycle demonstration of one distinct non-volatile state. c Measured transmissions at the through port of the MRR-based PZT optical memristor with different setting voltages V_set. d Measured resonance-wavelength detuning of the MRR-based PZT optical memristor set with different setting voltages V_set as the driving voltage V_dr varies. e Eye diagrams for NRZ modulation at the data rate of 48 Gbps with a driving voltage V_dr of 3 V.

Fig. 3. The present MZI-based PZT optical memristors.

a Schematic of the MZI -based 2 × 2 PZT optical memristors, consisting of two imbalanced arms with 1-mm-long phase shifters and two 2 × 2 multimode interferometers with a splitting ratio of 50%: 50%; the purple arrow represents the direction of the poling electric field; the blue arrow represents the direction of the applied drive voltage V_dr. b Optical microscopy image of the fabricated 2 × 2 MZI optical memristors. c Non-volatile tuning of the wavelength notch with the setting voltage V_set varying from 8 V to 32 V. Before the setting voltage is applied to set any state, this optical memristor was re-initialized by applying a high poling voltage V_pol of 80 V. d Real-time optical modulation measurements. Normalized output intensity from the through port when the device operates with a drive voltage V_dr of 4 V_pp at the frequency of 10 MHz. The upper one represents the input electrical signals, and the bottom one represents the switched optical signals. e Measured transmissions at the cross port when operating with V_dr = 0 and 5.0 V, respectively.

Fig. 4. The present FP-cavity-based PZT optical memristor.

a Schematic configuration. MWGs, multimode waveguide gratings, Modemux, mode multiplexers. b Optical microscopy image of the fabricated device. c Measured transmission spectral responses at the through port with different setting voltages V_set. d Extracted resonance wavelength shifts (detuning with 1559.2 nm) versus different setting voltages V_set. e Five distinct non-volatile states correspond to the setting voltages V_set of 30, 21, 23, 25, and 36 V. For each non-volatile state, there are 20 cycles measured for the resonance-wavelength detuning. f Time domain measurements after 10 and 10000 cycles, showing that the optical memristor operates well without any degradation. s. Real-time optical modulation measurements: g–j Normalized output optical intensity at the through port when operating with rectangular driving-voltage signals (V_dr=4V_pp) at the frequency of 1 Hz (g), 100 kHz (h), and 5 MHz (i). j The rising/falling edge of the switched optical signals. k Measured spectral responses at the through port of the FP cavity when operating with different driving voltages V_dr of −1, 0, and +1 V, respectively.

Fig. 5. Examples of multifunctional photonic circuits enabled by scalable PZT optical memristors.

a Optical microscopy image of the 4 × 4 optical switch. b Chart of the measured transmissions from the input ports (I₁, I₂, I₃, and I₄) to the output ports (O₁, O₂, O₃, and O₄) at the initial state with random phase errors. c–e Chart of the measured transmissions from the input ports (I₁, I₂, I₃, and I₄) to the output ports (O₁, O₂, O₃, and O₄) when all the 2 × 2 MZI switches are reconfigured with the desired optical routines (as defined by configurations #1, #2 and #3) by applying the appropriate setting voltages. f Measured eye diagrams for the optical signals propagating along the routines (I₁-O₂, I₂-O₁, I₃-O₃, I₄-O₄) as defined by configuration #3. FP-cavity-based PZT optical memristors in cascade for adaptive memristive spectral manipulation: (g) Schematic configuration; (h) Optical microscopy image of the fabricated device; (i) Measured transmission spectra at the through ports for the initial state without any correction; (j) Measured transmission spectra at the through/cross ports when the resonance wavelength of each FP cavity was aligned carefully to have uniform channel spacing by applying the corresponding setting voltage V_set; (k) Measured transmission spectra at the through/cross ports when channels #1 and #3 are erased and retrospectively set so that their resonance wavelengths are respectively aligned with channels #2 and #4.

Fig. 6. Part of the potential applications in the Future.

PZT optical memristors enable compact wavelength-division-multiplexed optical interconnects, microwave photonics, neural networks, and LiDAR by combining volatile high-speed modulation (below V_th) and non-volatile trimming (V_set).