A silicon waveguide photodiode, designed by a team at the Hong Kong University of Science and Technology (HKUST), addresses many of the challenges that affect existing on-chip power monitors for programmable photonics. The researchers implanted their photodiode with germanium, a CMOS-compatible, group IV element. Programmable photonic devices provide faster processing speeds, higher bandwidths, and greater energy efficiency than conventional electronic devices. These advantages make programmable photonics suitable for challenging tasks like real-time deep learning and data-intensive computing. However, power monitoring, a critical enabler for programmable photonics, is difficult to implement successfully in on-chip applications. Effective power monitoring solutions must simultaneously address demands for low optical absorption loss, adequate responsivity, minimal dark current, and low power consumption. Moreover, existing on-chip photodetectors are faced with a trade-off. To obtain a strong reading, they must absorb much of the optical signal, which will degrade the signal’s quality. Otherwise, they must forego the sensitivity necessary for them to operate at low power levels without additional amplifiers. Implanting ions into on-chip silicon photodetectors can enhance signal conversion by introducing controlled defects into the detector’s silicon structure. These defects can absorb photons with energies too low for pure Si, enabling the photodiode to detect light across a broader range of wavelengths. Previous attempts to improve photodetector performance by implementing this approach used either boron, phosphorus, or argon ions. When these ions were introduced into a silicon structure, many free carriers were brought into the silicon lattice, degrading optical performance. Germanium, an element from the same group as silicon, can replace silicon atoms in the photodiode’s crystal structure without introducing significant numbers of free carriers. By using germanium as a substitute at the lattice site of their photodiode, the HKUST researchers demonstrated extension of the sensitivity of the photodiode without compromising signal quality. The researchers conducted comparative experiments to test the silicon waveguide photodiode under relevant conditions. The germanium-implanted photodiode showed high responsivity at both 1310 nm (O-band) and 1550 nm (C-band), two telecommunications wavelengths. The device also demonstrated an extremely low dark current and extremely low optical absorption loss. The experimental results suggested that the silicon waveguide photodiode could be integrated into photonic circuits without disturbing the primary signal flow. The newly developed waveguide photodiode can be incorporated into various types of programmable photonic devices to measure the intensity of light traveling in the waveguide. MZIs: Mach-Zehnder interferometers. Courtesy of Niu and Poon. “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,” professor Andrew Poon said. By leveraging a CMOS-compatible ion implantation process, the researchers achieved a one-order-of-magnitude enhancement in responsivity with their photodetectors, compared with surface-state absorption (SSA)-assisted, all-silicon photodetectors. The germanium-implanted silicon photodetectors showed a ten-orders-of-magnitude reduction in absorption loss compared with III-V-on-silicon photodetectors. The low optical absorption loss exhibited by the new photodetectors could facilitate their integration into a Mach-Zehnder interferometer (MZI) configuration, or micro-ring networks, without the need for optical gain compensation modules. The development could represent a step toward practical, large-scale programmable photonic systems: By building a photodetector that can meet the stringent demands of on-chip monitoring, the researchers further strengthened the potential of light-based computing. And, beyond their role in enabling programmable photonics, the germanium-implanted silicon waveguide photodiodes could also support future applications in biosensing and lab-on-chip technologies. “The combination of an extremely low dark current upon a low bias voltage positions our device as an ideal candidate for energy-efficient, ultrasensitive biosensing platforms, where low-noise detection of weak optical signals is paramount,” Poon said. “This would enable direct integration with microfluidics for lab-on-chip systems.” The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.7.6.066005).