A capacitive photodiode for analogue and digital processing

An optical wireless communication system requires signal preprocessing and computation, which typically rely on distinct analogue and digital modules. However, this module separation inevitably increases system footprint and fails to meet the space and power constraints of edge applications. Here, we report a capacitive photodiode that embeds both analogue filtering and digital processing functions. This device uses an antimony oxide dielectric layer formed in situ on an antimony sulfide light absorber. The resulting capacitor-photodiode structure exhibits enhanced charge accumulation at high frequencies, thereby realizing a high-pass filtering function that prevents eavesdropping. Meanwhile, the in-situ formation of the dielectric layer introduces anionic vacancies. Under high-energy photon illumination, these vacancies become ionized and dope the semiconductor. This wavelength-dependent photodoping inverts the built-in electric field, thereby reversing the diode polarity and enabling the XOR optoelectronic logic function. Finally, we demonstrate an optical wireless communication system using this capacitive photodiode as both an analogue sensing front-end to extract high-frequency signals from low-frequency noise, and a digital processing back-end to execute XOR logic-based decryption for signal recovery and secured actuator control.