High-performance colloidal quantum-dot surface-emitting laser array

Colloidal quantum-dots (CQDs) have emerged as promising gain media for lasers, offering low lasing thresholds, compatibility with low-cost solution-based synthesis and processing methods, and tunable emission across a wide spectral range. These advantages make CQD lasers potential game-changer for non-epitaxial semiconductor lasers. Surface-emitting lasers (SELs), characterized by narrow beam divergence, high efficiency, and compatibility with two-dimensional array integration, have found widespread applications in display, sensing, and communication. Thus, developing CQD-based SEL arrays offers a pathway to low-cost, highly integrable, and full-color coherent light sources.

 

However, optically pumped CQD lasers still face several challenges, including:

• High lasing threshold, often requiring femtosecond pumping source. To enhance their practical utility, it is necessary to reduce the lasing threshold and improve compatibility with more cost-effective picosecond, nanosecond, or quasi-continuous-wave pumping sources.
• Poor operational stability. The high lasing threshold imposes significant challenges on the stability of CQDs under intense laser pumping, and no CQD laser has been reported to operate continuously for more than 10 hours to date.
• Limited integration density. Constrained by the limited optical field confinement in conventional vertical-cavity surface-emitting laser (VCSEL) and the resulting large mode volume (V), reported CQD SEL arrays have achieved integration densities of only 100 to 300 pixels-per-inch (PPI).

 

To address these challenges, a collaborative team from Southern University of Science and Technology, led by Professors Kai Wang and Xiao Wei Sun, in conjunction 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, has developed a CQD-based SEL array with low threshold, high stability (continuous operation for 1000 hours), and high integration density (2100 PPI). This was achieved by simultaneously improving both the CQD material and the laser cavity design. Specifically, they developed CQD materials with alloyed graded core-shell structure and integrated them with circular Bragg resonator (CBR) that supports strong optical field confinement.

 

6. Main innovation

A low lasing threshold and high optical gain stability in CQD are prerequisites for achieving high-performance CQD lasing. To this end, the researchers have developed a new type of CQD material with a graded alloyed core-shell structure of CdZnSe/ZnSe/Zn_xCd_1-xS (Figure 1a). By smoothing the exciton confinement potential within the CQDs, they effectively suppressed the Auger recombination in the multi-exciton regime, contributing to a lower lasing threshold and enhanced stability. Analysis of the second-order differential absorption spectrum of the CQDs (Figure 1b) revealed a large light-heavy hole splitting energy of 147 meV, significantly exceeding the thermal energy at room temperature (k_BT ≈ 26 meV). This indicates that thermally induced intra-band transition in the CQDs can be effectively suppressed, further enhancing the stability of the optical gain. Under sub-nanosecond pulsed laser pumping, the amplified spontaneous emission (ASE) threshold of the CQDs was as low as 10 μJ/cm² (Figures 1c and 1d), laying a solid foundation for achieving low-threshold lasing.

 

To realize high-performance and compactly integrated CQD SEL arrays, efficient manipulation of the optical field distribution within the CQD laser cavity is essential. This requires: (i) effective coupling between the optical field and the CQD gain medium (evaluated by the optical confinement factor Γ); (ii) strong optical field confinement (evaluated by the mode volume V); and (iii) a strong Purcell effect that matches the CQD gain spectrum (evaluated by the Purcell factor F_P). However, CQD VCSEL, as a type of microcavity system based on one-dimensional photonic crystal structure, only provides effective optical confinement along the Z-axis, leaving significant room for improvement in the above three aspects. Therefore, the researchers have developed a new type of CQD CBR laser, which utilizes a circular Bragg grating structure in the XY plane to achieve a dimensional upgrade of the optical field confinement from one-dimensional (Z-axis) to two-dimensional (XY plane) (see Figure 2). In this device, CQDs not only serve as the gain medium but also, together with the relatively low-refractive-index silicon dioxide, form a complete CBR resonant cavity. Numerical simulations based on the finite-difference time-domain (FDTD) method show that, thanks to its strong optical confinement, the mode volume V in the CBR cavity is reduced by an order of magnitude compared to that in CQD VCSEL, and both the optical confinement factor Γ and the Purcell factor F_P are significantly improved (see Figure 2).

 

Thanks to the significantly enhanced optical confinement factor Γ and Purcell factor F_P in the CBR cavity, the lasing threshold of the CQD CBR laser (17 μJ/cm²) is significantly lower than that of the CQD VCSEL (56 μJ/cm²) under 0.3-ns pulsed excitation, as shown in Figure 3. Meanwhile, the small mode volume V resulting from strong optical confinement enables high-density array integration of CQD CBR lasers, with an integration density of up to 2100 PPI, which is the highest level among current CQD SEL arrays.

 

Moreover, benefiting from high-quality CQD material and the CBR cavity, the CQD CBR laser exhibits excellent operational stability. It has achieved a continuous operating lifetime of 1000 hours at room temperature, corresponding to 360 million stable pulsed lasing, both of which represent the best values reported for solution-processed nanocrystal lasers(see Figure 4).

 

7. Conclusion

This work elucidates the mechanism by which efficient optical field manipulation within the microcavity improves the lasing properties of CQDs. By combining high-quality CQD material with a CBR cavity featuring strong optical field confinement, the researchers have developed a CQD SEL array with low threshold, high integration density, and high stability. This breakthrough overcomes the current technical bottlenecks of CQD lasers in terms of integration density and operational stability, laying a solid foundation for the realization of diode-pumped and even electrically pumped CQD lasers.

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