Objective Featuring wide bandgap tunability, high quantum efficiency, and cost-efficient solution processible fabrication methods, colloidal quantum dots (QDs) have been studied and applied in various optoelectronic devices including photo detectors, light-emitting diodes (LEDs), and solar cells. In addition to applications based on the absorption and spontaneous emission of colloidal QDs, their stimulated emission potential has attracted extensive research interests, aiming toward a landmark target: the realization of the colloidal QD laser diodes. In the study of colloidal QD lasers, different laser architectures have been demonstrated, including Fabry-Perot cavity, distributed feedback laser cavity, whispering gallery mode cavity, and photonic crystal microcavity. The optical gain has been successfully realized in colloidal QDs under direct current pumping, demonstrating a major progress toward electrically pumped colloidal QD lasers. Furthermore, a dual function device based on specially engineered QDs that can function as an optically pumped laser and an LED is fabricated and characterized, revealing a promising pathway for realizing colloidal QD laser diodes. Different from edge-emitting lasers, vertical-cavity surface-emitting lasers exhibiting surface-emitting properties, wafer-level fabrication & characterization capability, and array integration ability have been widely used in optical fiber communication, laser printers, computer mouse, and three-dimensional facial recognition fields, etc. Here, we propose and design a colloidal quantum dots vertical cavity surface emitting laser, combining with a quantum dots light-emitting diode like current injection structure to realize the electroluminescence ability.

Methods As shown in Fig. 1, the QLED-like structure containing the QD gain medium is sandwiched by two high-reflective distributed feedback reflectors to form a vertical-cavity surface-emitting laser(VCSEL)-like device. The device is designed to work under optical or electrical pumping. The DBR parameters and cavity lengths, are determined by numerical simulations with optimal performance. A DBR mirror is formed by periodically arranging two materials with different refractive indices. The reflectance spectrum is determined by both the DBR materials and DBR periods. Herein, we designed and calculated two types of DBRs with different periods (Fig. 3): (a) SiN_x/SiO₂ DBR and (b) TiO₂/SiO₂ DBR. It is found that 10 periods of the designed dielectric DBR can realize a peak reflectance of greater than 99%. The cavity length is a crucial parameter of the VCSEL device. After determining the DBR parameters, the permitted longitude modes inside the cavity can be tuned using the cavity length. Here, we use the FDTD method to build the designed QD-VCSEL device model and sweep the cavity length parameter. The current injection structure along the vertical direction includes the QD gain materials, electron and hole transmission layers, and electrodes. To tune the effective cavity length while retaining the optimized current injection capability, transparent ITO electrodes are selected and designed according to a suitable thickness. By theory, the smallest cavity length of a VCSEL device is λ/2. Thus, based on this length, the current injection structure and the thickness of the gain medium are fixed, while the thickness of the transparent ITO electrode is used to change the cavity length and then tune the resonant mode (Fig. 5). In addition to the λ/2 cavity length device, a 3λ/2 cavity length device is designed and simulated to theoretically optimize the optical parameters.

Results and Discussions Under optical excitation, the designed λ/2 cavity length QD-VCSEL device can support single-mode lasing at 629.5 nm with a cavity length of 172 nm. The calculated quality factor is 259632. Alternatively, the 3λ/2 cavity length device can be optimized with a 520-nm cavity length. The lasing mode is realized at 632 nm, and the quality factor is 148291. Compared to the cavity with the smallest cavity length, a longer cavity suffers further optical loss while facilitating a thicker gain region. However, a considerably longer cavity length is not favored because of the difficulty in the formation of a very thick QD layer with a high concentration. The simulated far-field pattern reveals that the designed devices achieve a low output beam divergence, comparable to conventional VCSEL devices, which is an intrinsic advantage of this type of semiconductor laser. This work proposes a new scheme for realizing QD laser diodes, providing a theoretical basis and a parameter reference for future experimental verification.

Conclusions In this work, a CdSe QD vertical-cavity surface-emitting laser is designed. The QD-VCSEL device is simulated with a QLED-like structure sandwiched by two dielectric DBR mirrors. The DBR parameters and cavity lengths are determined by numerical simulations with optimal performance. Single-longitude mode lasing can be supported by two designed cavities with different lengths with a maximum quality factor Q over 250000. The new solution toward electrically pumped colloidal QD lasers is revealed with our design, along with the theoretical model and key factors, which can be helpful in subsequent experimental work.