Programmable photonics devices, which use light to perform complex computations, are emerging as a key area in integrated photonics research. Unlike conventional electronics that transmit signals with electrons, these systems use photons, offering faster processing speeds, higher bandwidths, and greater energy efficiency. These advantages make programmable photonics well-suited for demanding tasks like real-time deep learning and data-intensive computing.
A major challenge, however, lies in the use of power monitors. These sensors must constantly track the optical signal's strength and provide the necessary feedback for tuning the chip's components as required. However, existing on-chip photodetectors designed for this purpose face a fundamental trade-off. They either have to absorb a significant amount of the optical signal to achieve a strong reading, which degrades the signal's quality, or they lack the sensitivity to operate at the low power levels required without needing additional amplifiers.
As reported in Advanced Photonics , Yue Niu and Andrew W. Poon from The Hong Kong University of Science and Technology addressed this challenge by developing a germanium-implanted silicon waveguide photodiode. Their approach overcomes the trade-offs that have hindered existing on-chip power monitoring technologies.
A waveguide photodiode is a small light detector that can be integrated directly into an optical waveguide, which confines and transports light. Its purpose is to convert a small portion of the light traveling through the waveguide into an electrical signal that can be measured via more conventional electronics. One way to enhance this conversion is through ion implantation, a process that introduces controlled defects into the photodiode's silicon structure by bombarding it with ions. If executed properly, these defects can absorb photons with energies too low for pure silicon, enabling the photodiode to detect light across a broader range of wavelengths.
Previous attempts to build such detectors used boron, phosphorus, or argon ions. While these approaches improved performance in some respects, they also introduced many free carriers into the silicon lattice, which in turn degraded optical performance. In contrast, the team implanted germanium ions. Germanium, a Group IV element like silicon, can replace silicon atoms in the crystal structure without introducing significant numbers of free carriers. This substitution allows the device to extend its sensitivity without compromising signal quality.
The researchers conducted various comparative experiments to test the new device under relevant conditions. The germanium-implanted photodiode showed high responsivity at both 1310 nanometers (O-band) and 1550 nanometers (C-band), two critical wavelengths used in telecommunications. It also demonstrated an extremely low dark current, meaning little unwanted output when no light was present, as well as very low optical absorption loss. This combination makes the device suitable for integration into photonic circuits without disturbing the primary signal flow.
"We benchmarked our results with other reported on-chip linear photodetector platforms and showed that our devices are competitive across various performance metrics for power monitoring applications in self-calibrating programmable photonics," remarks Poon. Overall, this study represents a major step toward practical, large-scale programmable photonic systems. By providing a photodetector that can meet the stringent demands of on-chip monitoring, the researchers have brought the transformative potential of light-based computing closer to reality.
Beyond its immediate use in programmable photonics, the proposed device's unique characteristics also open doors to other promising applications. "The combination of an extremely low dark current upon a low bias voltage positions our device as an ideal candidate for energy-efficient, ultra-sensitive biosensing platforms, where low-noise detection of weak optical signals is paramount," explains Poon, "This would enable direct integration with microfluidics for lab-on-chip systems."
The germanium-implanted photodiode may help advance programmable photonics by improving on-chip light monitoring and could also support future applications in biosensing and lab-on-chip technologies.
For details, see the original Gold Open Access article by Y. Niu and A. W. Poon, " Broadband sub-bandgap linear photodetection in Ge+-implanted silicon waveguide photodiode monitors ," Adv. Photon. 7(6), 066005 (2025), doi: 10.1117/1.AP.7.6.066005 .
