Facile Mode Selection in Colloidal Quantum Well Lasers for Ultralow-Threshold Reconfigurable Single-/Multi- Mode Operation

Whispering-gallery-mode (WGM) lasers and vertical-cavity surface-emitting lasers (VCSELs) are pivotal for optical communications and optoelectronic sensing. However, their further development has been hindered by challenging fabrication processes and the high thresholds typically required for single-mode operation. Here we demonstrate a viable route to address these limitations by integrating CdSe/CdSeS core/alloy crown colloidal quantum wells (CQWs) with a hollow-core silica microsphere-based mode-selection strategy. The solution-processability of CQWs substantially simplifies cavity fabrication and eliminates the need for stringent high-temperature vacuum conditions. Using the proposed microsphere-mode-selection approach, we achieve green single-mode lasing under 355 nm nanosecond pulsed excitation (7 ns, 20 Hz) with an ultralow threshold of 510 nJ cm–2, which is, to the best of our knowledge, the lowest reported value to date for CQWs-based laser. Moreover, without altering the cavity architecture, we realize flexible multichannel switching between single-mode and multimode lasing in VCSELs. This work offers a practical solution to key challenges faced by conventional laser technologies and is expected to enable broad applications in optoelectronics, particularly in optical communications and high-precision sensing.