Dual-mode switchable and reconfigurable Van der Waals phototransistor for multi-state image encryption

Switchable phototransistors equipped with high-sensitivity, dynamic encryption, low-power consumption, and CMOS processing compatibility are key components for optoelectronic encryption chips. Two-dimensional van der Waals (vdWs) heterostructures provide a solution toward this goal, despite continued efforts, excessive dependence on gate voltage modulation, multi-wavelength excitation and polarization modulation causes unsolved issues of high-power operation, channel interference and limited integration level, respectively. Here, we demonstrate a dual-mode vdWs phototransistor based on PtTe₂/WS₂ heterostructure. Through bias-modulated band alignment and carrier dynamics, the operation mechanism can be switched between photoconductive and photovoltaic modes. In photoconductive mode, the prolonged carrier lifetime donates a large photogain, yielding a high responsivity of 1.37 A W^-1. In photovoltaic mode, a wide unilateral depletion region effectively suppresses the dark current, contributing outstanding specific detectivity of 9.42 × 10¹⁴ Jones, weak light detection capability (~pW level), and high-speed response (rise time ~26.3 μs, fall time ~22.6 μs). More importantly, high on/off ratio of ~10⁵ and four distinct current states have been achieved via dual-mode operation in the phototransistor, enabling the realization of multiple optical logic gates (XNOR, NOR and XOR) and multi-state quaternary image encryption with superior average correlation coefficient of adjacent pixels of 0.03. This reconfigurable device provides a versatile platform for constructing multi-functional photoelectronic chips and advancing secure optical communication technologies.