High-Gain PMMA-Modified Graphene Photodetectors for Dual-Wavelength Secure Communication Utilizing Distinct Temporal Photoresponses.

Photodetectors exhibiting oppositive polar responses at two different wavelengths have been used extensively for secure optical communication. However, current technologies are susceptible to signal cancellation and cross-talk, due to the time-independent responses of equal strengths. In this study, a different encryption system based on a PMMA-modified graphene (PMG) photodetector is demonstrated that shows remarkable wavelength-dependent temporal responses: the PMG detector shows a largely constant response for wavelengths longer than 580 nm, while a linear time dependence for shorter wavelengths. It is found that the qualitatively different temporal responses are attributed to the photogating effects and the PMMA-mediated oxygen desorption process, respectively. Moreover, the PMG device achieves exceptionally large normalized-gain values of 2.18 × 10 mV and 2.97 × 10 ‍mV for the two cases, respectively, both surpassing previously reported values by an order of magnitude for the bipolar responsive photodetectors in ambient conditions. By harnessing these distinct temporal photoresponses as encrypted and key signals at different wavelengths, an effective encryption system for optical communication is demonstrated that enhances signal recovery and avoids signal cancellation. The research introduces a straightforward yet efficient structural design for photodetectors with temporally distinct photoresponses at specific wavelengths and represents a significant advancement in the field of secure communication.