Multiwavelength Optical Pulse-Induced Temporal State Mapping in an ITO/SnO₂/ITO Optoelectronic Synaptic Device

A vertical ITO/SnO₂/ITO optoelectronic synaptic device was fabricated by magnetron sputtering for optical sensing and temporal-state encoding. The ITO/SnO₂ stack exhibited 88.3% average transmittance over 400-800 nm. The device responded to 380, 550 and 660 nm illumination and showed paired-pulse facilitation (PPF) together with pulse-width, interval and number-dependent modulation, evidencing finite-timescale history dependence and pulse-dependent accumulation. These dynamics are consistent with rapid photocarrier excitation followed by slower localized-state trapping, detrapping, redistribution and relaxation, detrapping, redistribution and relaxation. All 16 possible 4-bit optical sequences were experimentally applied and identical pulse numbers arranged at different temporal positions produced different final current states. The measured state library was used for visual feature construction: a simple multilayer perceptron achieved 96.06 ± 0.19% test accuracy for MNIST over five independent runs, while a hybrid representation achieved 98.26 ± 0.28% over five independent runs for four-frame trajectory recognition; ablation showed that device-state features preserved temporal-order information. These results support temporal-order-sensitive physical encoding in a simple transparent oxide device.