Researchers from the Hong Kong University of Science and Technology developed a germanium-ion-implanted silicon waveguide photodiode. The device is intended for programmable photonics systems, where precise, low-loss on-chip optical power monitoring is required for adaptive control. Current programmable photonics systems are constrained by on-chip power monitors that struggle to balance high responsivity with low optical absorption loss. In addition to programmable photonics, the photodetector is suitable for energy-efficient, ultra-sensitive biosensing systems requiring low-noise optical detection. Programmable photonics uses reconfigurable photonic circuits to perform computation and signal processing with high bandwidth and low energy consumption. Their advantages over electronic technologies make programmable photonics particularly well-suited for demanding tasks such as real-time deep learning and data-intensive computing. Andrew Poon (left), Head and Professor of the Department of Electronic and Computer Engineering at HKUST, and Ph.D. student Niu Yue at the Photonic Device Laboratory. Courtesy of HKUST. On-chip monitors are core components for building programmable photonic networks, as their performance directly determines the system's adaptive adjustment accuracy, stability, and effectiveness. Existing photodetectors maintain extremely low optical absorption loss to prevent significant attenuation of transmitted optical signals, and require high responsivity to detect weak optical power, along with low dark current and minimal power consumption. The developed germanium-ion-implanted silicon waveguide photodiode overcomes the challenges faced by existing on-chip power monitors, which struggle to balance responsivity and loss. It is a small light detector that can be directly integrated into an optical waveguide. Its purpose is to convert a small portion of the light traveling through the waveguide into an electrical signal that can be measured using conventional electronics. Responsivity is enhanced through ion implantation, which introduces controlled defect states into the silicon waveguide. Germanium ion implantation enables sub-bandgap absorption without introducing significant free-carrier loss, extending detection into telecom wavelengths. The photodiode demonstrated high responsivity at both 1310 and 1550 nm, along with ultra-low dark current and optical absorption loss low enough to avoid measurable signal degradation in the waveguide. This combination of features makes the device well-suited for integration into photonic circuits without disturbing the primary signal flow. The device could also be used within energy-efficient, ultra-sensitive biosensing platforms, where low-noise detection of weak optical signals is paramount. These characteristics support integration with microfluidic lab-on-chip platforms for low-noise biosensing applications. This research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.7.6.066005).
