Nature Nanotech. 7, 335–339 (2012)

Credit: © 2012 NPG

CdSe-based colloidal quantum dots (CQDs) are used in applications ranging from fluorescent biolabels to prototype light-emitting diodes. However, losses due to non-radiative Auger recombination and requirements for high packing densities have so far hindered the development of lasers based on CDQs. Cuong Dang and co-workers in the USA have now demonstrated a low-threshold vertical-cavity surface-emitting laser (VCSEL) based on core–shell CdSe/Zn0.5Cd0.5S CQDs. First, they used high-temperature organometallic synthesis to prepare CQDs spin-coated on fused silica substrates. The CQDs had diameters of 4.2 nm, 3.2 nm and 2.5 nm for red, green and blue emission, respectively, and had a packing density of around 50%. The researchers used absorption and photoluminescence measurements to confirm single-exciton gain. A blue-shift of the lowest excitation absorption peaks with respect to the photoluminescence reduced the self-absorption of emitted photons, thereby lowering the threshold for amplified spontaneous emission. Using the variable stripe length method, the researchers measured gains of 95 cm−1 and 60 cm−1 for red- and green-emitting CQD films, respectively. The red-emitting CQD-VCSEL exhibited an optical pumping threshold as small as 60 μJ cm−2 at the 400 nm pump wavelength, which indicates that the device's low-loss resonator design allows a stimulated emission threshold close to the fundamental single-exciton limit.