Reduced threshold

Nat. Nanotech. http://doi.org/cfxr (2017)

The threshold for optical gain in colloidal semiconductor quantum dots (QDs) can be dramatically reduced by the use of interface engineering and charge doping. Kaifeng Wu and co-workers from Los Alamos National Laboratory in New Mexico, USA fabricated QDs consisting of a core of CdSe, an alloy of CdSe _x S_1–x and a shell of CdS. The QDs have an energy bandgap of ~1.9 eV (which corresponds to a red emission wavelength of 650 nm) and extra electrons are introduced through the use of a photochemical reduction, where a hole from the QD is transferred to lithium triethylborohydride (LiEt₃BH) that acts a ‘hole scavenger’. The degree of charge doping can be controlled by the amount of LiEt₃BH used. Heavily charged QDs with ~6 electrons were found to have dramatically reduced optical gain thresholds that were about 50 times smaller than neutral QDs. The researchers say that the approach indicates the viability for zero-threshold optical gain. Analysis also shows the laser threshold of optically excited, doubly charged QDs is around 0.5 kW cm^–2, which is more than one order of magnitude smaller than neutral or singly charged QDs and potentially compatible with the use of commercial LEDs as a pump source.