Self-Powered Wavelength-Switchable Bipolar FTO/CdSe/Bi ₂ Te ₃ Photodetector for Decoding-Free Encrypted Optical Communication

/Au heterostructure that leverages dual built-in electric fields to achieve wavelength-dependent photocurrent polarity reversal. The device generates negative photocurrent under 460 nm illumination and positive photocurrent under 850 nm light, operating with microsecond-scale response times (47-136 μs) and a 30 kHz bandwidth without external bias. Crucially, we demonstrate a novel, decoding-free three-beam encryption scheme. By utilizing 460 and 850 nm as interfering beams that produce canceling photocurrents ("0"), and a 685 nm data beam that transmits valid signals ("1") directly, the scheme eliminates decoding latency. This architecture uniquely integrates device physics with encryption logic, overcoming the reliance on software decryption in conventional systems. The BPD distinctively combines self-powered operation, broadband response, high speed, and a facile sputtering fabrication process, positioning it as a promising platform for next-generation secure optoelectronics.