Colloidal Quantum Dot Vertical-Cavity Surface-Emitting Laser (CQD-VCSEL)
The QDs used in this work were made from a spherical core of CdSe, covered by a thin layer of Zn0.5Cd0.5S. The core sphere was made in three different dimensions – 2.5, 3.2 and 4.2 nm – while the external layer was always 1 nm thick. These nanoparticles were prepared by suspension in a viscous fluid using an appropriate organic molecule, or a colloidal form (Colloidal Quantum Dot, CQD).
To test the CQD performance, researchers adopted the Vertical-Cavity Surface-Emitting Laser (VCSEL) configuration. In this configuration, the suspension is placed between two layers of a special glass with very high reflectivity, called a distributed Bragg reflector (DBR). Then the liquid is evaporated; the evaporation leads to an aggregation of the QDs in a form of a very dense layer.
Watch the video below, provided by Dr. Dang as supplemental materials to the research published in Nature Nanotechnology, to see how the device is prepared, and the laser emission process.
The researchers stimulated QDs prepared in this way with an appropriate power source, and a laser emission was observed. The results showed how QDs with different diameters emitted lasers of different colors; more specifically, blue, green and red lasers corresponded to QDs with diameter of 2.5, 3.2 and 4.2 nm respectively.
Furthermore, the emission took place after pumping with energy densities lower than previously reported for similar devices. Energy threshold values of 90, 145 and 800 mJ cm-2 were observed for red, green and blue laser emissions respectively.
Lasers and Quantum Dots: An Important Step Forward
The QD preparation was a multi-step process, and all parts of this process were essential and carefully considered, according to Dr. Dang, who tells Decoded Science that “the synthesis process starting from designing QD structure, synthesizing QD solution, to making densely packed film.” are all important to the overall results.
This research is significant to many industries. Dr. Dang explained that the results obtained represent an important step towards full-color single-material lasers. He says, “This is achieved using much less power, therefore enabling the most practical laser emission from QDs.” More results with less power – that’s always good news for new research.
C. Dang, et al. Red, green and blue lasing enabled by single-exciton gain in colloidal quantum dots films. (2012). Nature Nanotechnology, doi:10.1038/nnano.2012.61. Accessed May 2, 2012.
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