Optically Tunable Synaptic Plasticity and Memory Emulation in Au-Nanoparticle Enhanced HfTiO_x/Al₂O₃-Based Photonic Memristors

The optically stimulated synaptic device incorporates gold nanoparticles (Au-NPs) embedded within an atomic-layer-deposited HfTiO_x/Al₂O₃ bilayer, which enables the photonic modulation of synaptic plasticity. The HfTiO_x/Au-NP interface enhances visible light absorption and utilization efficiency by generating ionized oxygen vacancies through the neutral oxygen vacancy sites, thereby modulating the device conductance states. This architecture effectively mimics key biological synaptic functions, including paired-pulse facilitation (PPF), memory transitions from short-term to long-term memory (STM to LTM), and spike-rate-dependent plasticity (SRDP). The device further demonstrates optical logic operations and Pavlovian associative learning under dual-wavelength light stimulation (405 and 450 nm). The intensity-dependent generation of photocarriers and their nonlinear interaction with oxygen vacancies enable robust synaptic behavior, facilitating the emulation of human visual perception in a 4 × 4 optoelectronic synapse array with short-term memory capabilities. Moreover, the wavelength- and sequence-dependent synaptic responses can be finely controlled for the design of light-programmable reservoir computing systems. These results demonstrate the potential of the HfTiO_x/Au-NP/Al₂O₃ switching layer as a promising platform for efficient neuromorphic computing and vision-based information processing using integrated optoelectronic synapses.