Two-Photon-Pumped Lasing Form Semiconductor Core-Shell Quantum Dots

Semiconductor quantum dot（QD）exhibit potential laser properties such as low laser threshold,tunable emission wavelength with size,and temperature-insensitive emission,making lasers with semiconductor QDs as optical gain media widely used in fields such as communication,medicine,and military.Two-photon absorption（2PA）,as a nonlinear physical process,achieves frequency upconverted laser emission without the need to satisfy phase matching conditions.Currently,the main problem in realizing low-threshold two-photon pumped QD lasers is that the small size of QDs leads to a limited 2PA interface and fast Auger recombination（AR）.Therefore,the research on low threshold two-photon pumped quantum dot lasers is of great significance.To reduce the threshold of two-photon pumped QD lasers,we advocate thick-shell pure-phase wurtzite（WZ）CdSe/CdS core-shell QDs as gain media.The thick-shell pure-phase WZ CdSe/CdS core-shell QDs prepared by a high-temperature method significantly reduce the lattice mismatch between the core and shell,releasing stress from three CdS monolayers to eleven CdS monolayers.The 2PA cross-section₂（000 nm）of QD was measured as 1.5×10⁵ GM by Z-scan and nonlinear transmission,which is nearly 30 times higher than that of unshelled CdSe QDs and approximately1.3 times higher than that of the same volume of mixed-phase core-shell QDs.This demonstrates that the thick pure-phase shell can increase the QD volume and effectively passivate the defect states on the QD surface,thereby increasing the 2PA cross-section,significantly suppressing Auger recombination（AR）,and achieving ultra-low-threshold laser emission.Using polymer-doped thick-shell pure-phase WZ CdSe/CdS core-shell QDs,we prepared optical limiters and stabilizers,and the actual working light intensity was measured to be 10 GW/cm².The signal amplitude fluctuation of the stabilizer was reduced by nearly 66%,and the average output power of the limiter decreased by nearly 40%.We successfully achieved two-photon pumped amplified spontaneous emission（ASE）with a threshold of 2.7 m J/cm² by thick-shell（11 CdS monolayers）pure-phase WZ CdSe/CdS core-shell QDs,which is significantly lower than that of lasers with other QDs.ASE of both 1P and 1D states was observed,indicating that the AR of the QD was significantly suppressed.Based on the whispering gallery mode（WGM）microcavity resonant principle,an optical microcavity suitable for the QD was designed.The optical microcavity was simulated and analyzed using COMSOL Multiphysics,and then the microcavity was fabricated,perfectly coupled with the QD,and completed laser emission with a threshold of approximately 2.9 m J/cm².This work not only promotes the practical application of two-photon pumped QD lasers,but also has guiding significance for the study of enhanced 2PA.