Surface-emitting colloidal quantum-dot laser

Abstract

Colloidal quantum dots (CQDs) offer compelling advantages as gain media for next-generation lasers, including broad wavelength tunability, high photoluminescence efficiency, and compatibility with low-cost solution processing. However, realizing practical, high-performance CQD lasers, particularly surface-emitting ones suitable for displays, sensing, and optical interconnects, remains challenging due to issues such as high lasing thresholds dominated by Auger recombination, limited operational stability, difficulties in achieving high integration densities, and suboptimal light-matter interaction in conventional microcavity designs. This dissertation addresses these critical challenges through synergistic approaches combining materials considerations, advanced microcavity engineering, optimized fabrication techniques, and investigation into fundamental emission physics. The research leverages detailed experimental methodologies established for thin-film deposition (spin coating, thermal evaporation, e-beam evaporation) and comprehensive characterization (AFM, profilometry, ellipsometry, spectrophotometry, laser-pumped photoluminescence spectroscopy), as detailed in Chapter 2. Building upon this foundation, Chapter 3 explores the complex photophysics beyond standard band-edge emission, demonstrating low-threshold anisotropic polychromatic emission derived from multi-excitonic states in monodisperse CQDs integrated within a specifically designed microcavity. This work provides insights into controlling novel emission pathways and realizing unique light source characteristics. Chapter 4 tackles the critical fabrication challenge in realizing high-performance CQD vertical-cavity surface-emitting lasers (VCSELs). By developing and implementing an optimized process involving gentle in-situ deposition of the top distributed Bragg reflector that preserves the optical integrity of the embedded CQD layer, VCSELs exhibiting a record high quality factor exceeding 2000 were successfully demonstrated, alongside a low lasing threshold under optical pumping. Furthermore, Chapter 5 investigates the CQD-integrated circular Bragg resonator (CBR) as an advanced surface-emitting architecture designed to overcome inherent limitations in conventional VCSELs regarding confinement, threshold and stability. Leveraging the strong two-dimensional optical confinement provided by the CBR design, which results in significantly enhanced mode confinement, reduced mode volume, along with increased Purcell factor, record-breaking device performance was achieved. This includes a substantial reduction in the lasing threshold (from 56 to 17 µJ/cm2), unprecedented operational stability exceeding 1000 hours under pulsed excitation – the highest reported for solution-processed nanocrystal lasers, and the realization of laser arrays with exceptionally high integration densities surpassing 2100 pixels-per-inch. Collectively, this dissertation demonstrates significant advancements in CQD-based surface-emitting light sources by implementing effective cavity engineering strategies and exploring novel emission physics. The achieved improvements bring solution-processed CQD lasers substantially closer to practical viability for diverse technological applications.