The explosive growth of big data, the Internet of Things, and cloud computing has led to an unprecedented surge in optical communication traffic, making information security a critical concern. Two-dimensional (2D) van der Waals (vdW) materials, with their strong light–matter interaction, electrical tunability, and compatibility with heterogeneous integration, hold great promise for next-generation optoelectronic devices. Yet, reconfigurable phototransistors for secure optical communication still face three major hurdles: achieving simultaneously high sensitivity, low dark current, fast response, and large photoconductive gain in a single device; generating multiple stable and distinguishable output states beyond conventional binary responses; and avoiding complex control schemes, such as gate‑voltage tuning, polarization modulation, or multi‑wavelength excitation that increase power consumption, system complexity, and hinder on‑chip integration.
In a new paper published in Light: Science & Applications , a research team led by Professor Li Gao at Nanjing University of Posts and Telecommunications, in collaboration with colleagues from Southeast University and Nanjing University, reports a dual‑mode reconfigurable phototransistor based on a PtTe₂/WS₂ van der Waals heterostructure. By simply adjusting the source‑drain bias, the device’s dominant photoresponse mechanism can be switched between photovoltaic (PV) and photoconductive (PC) modes. Under positive bias, the depletion region in WS₂ widens, effectively suppressing dark current and promoting rapid separation and transport of photogenerated carriers, yielding PV‑mode operation. Under negative bias, the depletion region narrows, and carrier trapping at defect states becomes more pronounced; the prolonged carrier lifetime enhances photoconductive gain, switching the device to PC mode.
These two modes offer complementary performance. In PV mode, the device achieves an outstanding specific detectivity of 9.42×10¹⁴ Jones and fast response times of 26.3 μs (rise) and 22.6 μs (fall), along with an ultra‑low dark current of ~10⁻¹⁴ A. In PC mode, it delivers a high responsivity of 1.37 A/W due to photoconductive gain, and the photocurrent is significantly larger, making it suitable for weak‑light detection. The team also demonstrated high‑resolution imaging under both modes, showcasing the device’s versatility for reconfigurable optoelectronic sensing.
Leveraging the dual‑mode response, the same phototransistor enables multi‑state information processing. By using light on/off states and bias polarity as two independent binary inputs, the device produces four stable and easily distinguishable current levels, corresponding to dark and illuminated conditions in PV and PC modes, respectively. These four states were encoded as 00, 01, 10, and 11 to implement quaternary image encryption. In a proof‑of‑concept demonstration, a predefined binary key sequence controlled the optical input, while the image to be transmitted was converted into a bias‑control sequence; their combined action generated a photocurrent stream that carried the encrypted information. The average adjacent‑pixel correlation coefficient of the encrypted images was reduced to about 0.03, which is substantially lower than that of the original image, indicating strong encryption effectiveness. Additionally, the device was configured to perform XNOR, XOR, and NOR optical logic gates, further expanding its potential for on‑chip data processing.
By integrating switchable photodetection, multi‑level current output, optoelectronic logic, and image encryption within a single phototransistor, this work introduces a physical‑layer obfuscation primitive that can complement conventional cryptographic protocols. The approach offers a promising route toward compact, low‑power, and multifunctional vdW optoelectronics. Future developments may extend this strategy to device arrays and CMOS‑compatible integration, as well as to color‑image encryption and near‑infrared communication bands, paving the way for advanced secure optical communication systems.
Light: Science & Applications
Dual-mode switchable and reconfigurable Van der Waals phototransistor for multi-state image encryption