Negative Differential Photoresistance for Optically Tunable Resonator in a 2D Nb₃I₈ Tunnel Junction

Negative differential resistance (NDR) offers a compact route to sustain and condition radio-frequency resonators, and is also exploited in logic and neuromorphic circuits. However, electrically tunable NDR commonly trades tunability for circuit complexity, area, and power, constraining practical integration and fast reconfigurability. Here, we demonstrate optical control of negative differential photoresistance (NDPR) in a vertical two-dimensional tunnel junction based on Nb₃I₈ tunnel barrier. Under illumination, the photocurrent rises with bias and then falls beyond a threshold, forming a clear NDPR region. The effective negative photoresistance is continuously tuned by light power and wavelength, spanning ~ -1 to -17 MΩ within our measurement window, while the device, as a photodetector, retains high sensitivity (responsivity up to ~0.49 AW⁻¹). Wavelength-dependent measurements indicate multiple photocurrent generation pathways and suggest that the NDPR onset correlates with the Nb₃I₈'s band structure. We further carry out simulations by integrating a lumped NDPR element into an LC oscillator circuit, which yields self-oscillation and effectively enhances the resonator quality factors. These results establish adjustable NDPR in a 2D van der Waals tunnelling platform and enable light-programmable impedance for reconfigurable optoelectronics, suggesting promising potential for all-optical communication systems.