A Southern University of Science and Technology collaborative team developed a CQD-based SEL array with a low threshold, high stability (continuous operation for 1000 hours), and high integration density (2100 PPI). The study was published in Light: Science & Applications.
(a) A schematic illustrating the graded alloyed core-shell structure of CdZnSe/ZnSe/ZnxCd1-xS CQDs. (b) Spectra of the CQDs, including PL, linear absorption, and second-order differential absorption. (c) Evolution of PL spectra of the CQDs with increasing pump intensity under sub-nanosecond pulsed laser excitation. (d) The dependence of spontaneous emission and ASE intensity of the CQDs on pump intensity. Image Credit: Yangzhi Tan, Yitong Huang, et al.
The new amplifier will significantly impact future data centers, AI processors, and high-performance computing systems—all of which stand to gain from quicker, more effective data transfer. Furthermore, the uses go beyond data transfer to include metrology, optical sensing, and even LiDAR systems for self-driving cars.
Due to these benefits, CQD lasers have the potential to revolutionize non-epitaxial semiconductor lasers. Display, sensing, and communication have extensively used surface-emitting lasers (SELs), which are distinguished by their tiny beam divergence, high efficiency, and compatibility with two-dimensional array integration.
Thus, creating CQD-based SEL arrays provides a route to full-color coherent light sources that are inexpensive and highly integrable.
Nevertheless, there are still several difficulties with optically pumped CQD lasers, such as:
- High lasing threshold: This frequently requires a pumping source that operates at femtoseconds. To increase their practical value, reducing the lasing threshold and improving compatibility with more affordable picosecond, nanosecond, or quasi-continuous-wave pumping sources are essential.
- Unstable operations. The high lasing threshold severely hampers the stability of CQDs under intensive laser pumping, and to date, no CQD laser has been known to run continuously for longer than 10 hours.
- Restricted density of integration. According to reports, CQD SEL arrays have only been able to attain integration densities of 100 to 300 pixels-per-inch (PPI), which is hampered by the low optical field confinement in traditional vertical-cavity surface-emitting lasers (VCSEL) and the associated large mode volume (V).
Professors Kai Wang and Xiao Wei Sun, along with Professor Hoi Wai Choi from the University of Hong Kong, Associate Professor Dan Wu from Shenzhen Technology University, and Dr. Yunjun Wang from Suzhou Mesolight were involved in the study.
This was accomplished by concurrently enhancing the design of the laser cavity and the CQD material. They specifically created alloyed graded core-shell CQD materials and combined them with a circular Bragg resonator (CBR) to provide robust optical field confinement.
Main Innovation
High-performance CQD lasing requires a low lasing threshold and good optical gain stability. The researchers created a novel CQD material with a CdZnSe/ZnSe/ZnxCd1-xS graded alloyed core-shell structure. By smoothing the exciton confinement potential within the CQDs, the researchers successfully inhibited the Auger recombination in the multi-exciton regime, which helped to improve stability and lower the lasing threshold.
The second-order differential absorption spectra of the CQDs found a considerable light-heavy hole-splitting energy of 147 meV, which is much higher than the thermal energy at room temperature (kBT = 26 meV). This suggests that the CQDs' thermally generated intra-band transition can be successfully inhibited, improving the optical gain's stability even further.
The CQDs' amplified spontaneous emission (ASE) threshold under sub-nanosecond pulsed laser pumping was as low as 10 μJ/cm2, providing a strong basis for low-threshold lasing.
Effective control of the optical field distribution within the CQD laser cavity is crucial for the realization of high-performance and compactly integrated CQD SEL arrays. This necessitates: (i) strong optical field confinement (measured by the mode volume V); (ii) an effective coupling between the optical field and the CQD gain medium (measured by the optical confinement factor Γ); and (iii) a strong Purcell effect that corresponds to the CQD gain spectrum (measured by the Purcell factor FP).
However, there is still much space for development in the aforementioned three areas because CQD VCSEL, a form of microcavity system based on one-dimensional photonic crystal structure, only offers effective optical confinement along the Z-axis.
As a result, the researchers created a novel kind of CQD CBR laser that upgrades the optical field confinement from one-dimensional (Z-axis) to two-dimensional (XY plane) by using a circular Bragg grating structure in the XY plane. Along with the comparatively low-refractive-index silicon dioxide, CQDs create a complete CBR resonant cavity in this device, in addition to acting as the gain medium.
The mode volume V in the CBR cavity is reduced by an order of magnitude compared to that in CQD VCSEL due to its strong optical confinement, as demonstrated by numerical simulations based on the finite-difference time-domain (FDTD) method. Additionally, the optical confinement factor Γ and the Purcell factor FP are both markedly improved.
The lasing threshold of the CQD CBR laser (17 μJ/cm2) under 0.3-ns pulsed illumination is substantially lower than that of the CQD VCSEL (56 μJ/cm2), due to the significantly improved optical confinement factor Γ and Purcell factor FP in the CBR cavity.
Meanwhile, CQD CBR lasers can be integrated into high-density arrays with a maximum integration density of 2100 PPI, the greatest level among existing CQD SEL arrays, thanks to the tiny mode volume V brought about by strong optical confinement.
Furthermore, the CQD CBR laser has outstanding operational stability thanks to the CBR cavity and premium CQD material. It has attained the highest recorded results for solution-processed nanocrystal lasers, with a continuous operational lifespan of 1000 hours at room temperature and 360 million steady pulsed lasing.
Conclusion
This study clarifies the process via which effective microcavity optical field modification enhances CQDs' lasing capabilities. The researchers have created a CQD SEL array with a low threshold, high integration density, and good stability by fusing premium CQD material with a CBR cavity that has strong optical field confinement.
This innovation provides a strong basis for the development of diode-pumped and even electrically pumped CQD lasers by overcoming the present technical limitations of CQD lasers in terms of integration density and operational stability.
Journal Reference
Tan, Y., et al. (2025) Low-threshold surface-emitting colloidal quantum-dot circular Bragg laser array. Light: Science & Applications. doi.org/10.1038/s41377-024-01714-9