image: Figure | Tandem colloidal quantum dot photodiode for spectral-selective 1550 nm LiDAR. a, An illustrative scenario for 1550 nm LiDAR detection in a realistic outdoor environment, where the desired laser echo signal and the ambient interference light are simultaneously collected by the optical module and detected by the PIN device. The laser pulse sequence and the received echo waveform are synchronized for time-of-flight ranging. b, The schematic configuration of the tandem colloidal quantum dot photodiode by electrically cascading two matched photodiodes and optically stacking two absorption junctions. The electrical cascading reduces the effective RC time for faster pulse response, while the optical stacking provides built-in spectral selectivity for suppressing visible-light interference. c, The measured performance of the tandem photodiode, including faster response speed, enhanced spectral selectivity, and stable pulse response under intrusive background illumination. These results demonstrate the potential of tandem colloidal quantum dot photodiodes for low-cost, fast, and interference-resistant 1550 nm LiDAR systems.
Credit: Junrui Yang et al.
Light detection and ranging, or LiDAR, is becoming a key sensing technology for autonomous driving, robotics, mixed reality, industrial monitoring, and other emerging intelligent systems. By measuring the travel time of short laser pulses, LiDAR systems can reconstruct three-dimensional information about a target scene.
The operating wavelength of LiDAR is now moving from 905 nm toward 1550 nm. The 1550 nm band offers advantages in eye safety and coherent sensing, and is especially attractive for next-generation high-performance LiDAR systems. However, practical deployment of 1550 nm LiDAR still faces a major detector challenge. InGaAs photodiodes can operate in this spectral region, but their high cost and complicated fabrication processes make them less suitable for large-area, low-cost detector arrays in consumer electronics and autonomous vehicles.
Colloidal quantum dots provide a promising alternative. These solution-processed semiconductor nanocrystals have tunable bandgaps, strong infrared absorption, and compatibility with silicon-based readout circuits. They have already shown potential for infrared imaging. Yet, when moving from two-dimensional imaging to three-dimensional LiDAR, colloidal quantum dot photodiodes encounter two critical obstacles: insufficient spectral selectivity and slow response speed.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Liang Gao from Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, China, and co-workers have developed tandem PbSe colloidal quantum dot photodiodes designed for 1550 nm LiDAR. The researchers introduce a device architecture that functions as an integrated photonic-electronic link. Instead of relying only on material optimization, the tandem structure uses device-level design to simultaneously process incoming light and accelerate electrical response.
The key idea is to stack two matched quantum dot photodiodes in series. When light enters the device, it first passes through the front junction and then reaches the rear junction. This geometry creates a built-in spectral filtering effect. Visible light is absorbed more strongly and mainly generates carriers in the front junction, causing a photocurrent mismatch between the two junctions. Because the two sub-junctions are connected in series, this mismatch suppresses the visible-light response. In contrast, 1550 nm light penetrates more deeply and generates more balanced photocurrents in the two junctions, so the target LiDAR signal is preserved.
At the same time, the tandem configuration reduces the effective junction capacitance through series connection. This lowers the RC time constant, which is a major speed bottleneck in high-mobility quantum dot photodiodes. Compared with simply increasing the absorber thickness or shrinking the device area, the tandem approach reduces capacitance while maintaining a similar carrier transit path, providing a structural route to faster response without sacrificing the basic light-collection mechanism.
The optimized tandem PbSe quantum dot photodiodes achieved a response time of 1.01 ns, a linear dynamic range above 99.6 dB, and a specific detectivity of 2.29 × 10^11 Jones at 1550 nm. The tandem architecture also reduced the visible-range external quantum efficiency by about 13-fold compared with single-junction devices, while maintaining comparable response at the target 1550 nm wavelength.
The team further demonstrated a 1550 nm LiDAR system based on the tandem quantum dot photodiodes. In the ranging experiment, the device enabled centimeter-level spatial resolution. Under strong broadband background illumination, the tandem photodiodes maintained 97.9% of their pulse signal intensity, outperforming single-junction quantum dot photodiodes under the same interference condition.
The researchers summarize the significance of their work as follows:
“Our tandem quantum dot photodiode provides a structural solution to two long-standing challenges in colloidal quantum dot LiDAR detectors: background-light interference and response-speed limitation. By integrating optical pre-processing and electronic post-processing in a single device, the tandem architecture enables fast and spectrally selective detection at 1550 nm.”
This work opens a new pathway toward low-cost, integrated 1550 nm LiDAR detectors. With further advances in quantum dot surface passivation, thin-film uniformity, array fabrication, and silicon readout integration, tandem colloidal quantum dot photodiodes may find applications in autonomous driving, robotic vision, consumer depth sensing, mixed reality, and infrared imaging.
Journal
Light: Science & Applications
Article Title
Spectral-selective light detection and ranging at 1550 nm using tandem colloidal quantum dot photodiodes