From smartphone cameras to fiber-optic communications, many modern technologies are made possible by sensors that detect photons of light and convert them into energy. Called photodetectors, these sensors have grown in sophistication over the years. Avalanche photodetectors (APDs), for instance, can rapidly multiply weak stimuli like a few photons of light into larger, detectable signals. By integrating APDs into optically-activated photonics circuits, many advances would be enabled by the faster speed, smaller device size and lower cost.
Still, photonics-integrated APDs remains restricted to capturing infrared wavelengths, likewise limiting their applications. Consequently, researchers are working to develop integrated APDs that can also detect visible light. With such devices, scientists could usher in the next generation of miniaturized bioimaging devices, high-speed communications tools, and improved remote sensors.
For the longest time, however, such capabilities remained out of reach. According to Victor Leong, a Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), the problem boiled down to fundamental design differences between integrated APDs for infrared and visible light.
“The technique typically used to deliver input light from the photonics circuit to integrated infrared APDs—known as an interlayer transition—cannot be used for visible-light devices without significantly compromising on noise and speed performance,” he explained.
In a breakthrough study, Leong and colleagues describe how they built the world’s first visible-light APD, integrated on a photonic chip. To achieve this feat, the team applied an out-of-the-box design approach, fabricating the photodetector and input waveguide on the same layer in an ‘end-fire’ configuration, instead of the conventional interlayer transition where the input waveguide is placed either above or below the APD.
“By placing both the detector and input waveguide on the same device layer, the novel end-fire configuration avoids the drawbacks of the interlayer transition,” Leong said. He adds that this architecture was novel and difficult to fabricate, spurring the team to undergo a rigorous fabrication optimization process.
To determine the optimal device design, the researchers varied different parameters, including the geometry and doping profile. While an interdigitated profile featuring alternating positively and negatively charged regions was more tolerant of fabrication errors, the trade-off was a loss of speed. Instead, the team found that a lateral doping profile with continuously aligned positive and negative regions was faster and more efficient.
Ultimately, the researchers’ final design demonstrated a strong, balanced performance relative to state-of-the-art integrated infrared APDs. Their device not only strongly amplified input light signals with low levels of noise but was able to transmit this data with a high bit rate of 56 Gbps.
According to Leong, the team plans to raise the bar even higher, building upon their design to pursue the ultimate limit of single-photon detection sensitivity. Achieving this would pave the way for even more possibilities in quantum communications, optical computing, and beyond.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and Institute of High Performance Computing (IHPC).