Lasers produce nearly monochromatic light. However, not all applications demand pure, single-color light—digital displays and other devices require a wide range of colors. While it is possible to combine red, green, and blue (RGB) lasers to replicate the whole visible-light spectrum, current technology requires using three different types of lasers.

Researchers have now produced a single material capable of producing several wavelengths of laser light. Cuong Dang et al. constructed a full RGB laser using colloidal quantum dots (CQDs), thin films that produce light via quantum excitations. The size of a quantum dot determines the color of the light it emits, so by overlaying many small patches of CQDs on surface, the researchers observed broad-spectrum emission. While their device is not yet practical, it represents significant progress toward multi-wavelength, single-material lasers.

Quantum dots are small wafers of semiconducting material, and their tiny size means quantum excitations are highly controllable. For example, the light emitted by a quantum dot depends not only on the material it is made of, but also the physical size of the dot: larger dots emit longer wavelengths (redder light) than smaller dots. CQDs are thinner films, often based on a cadmium-selenium (CdSe) alloy.

Many modern digital devices use semiconductor lasers, since they are easily made small. However, producing RGB lasers on a single semiconductor is an elaborate process, since each color must be made of a different material, then patched together. Unfortunately, lasers based on CQDs have been failures up until now, because the energy input required to start light emission tends to create heat rather than laser light.

In constructing their CQDs, Dang et al. coated the CdSe core with a second alloy (Zn 0.5 Cd 0.5 S) and a proprietary bonding material with properties that were not fully described in the paper. The second alloy changed the electronic properties, directing the excitations within the material to make light rather than heat the primary output. They produced three different sizes of CQDs (corresponding to RGB colors) as a suspension in liquid, which they spread onto glass; after evaporation, many closely packed quantum dots remained as a film on the glass surface.

To stimulate light emission, the researchers directed short pulses from a laser onto the film. The monochromatic laser light stimulated the three different CQDs to re-emit light of red, green, and blue wavelengths. They applied a filter to remove most of the light from the original laser, leaving only the CQD-produced photons. A final test confirmed that the usual excitations that lead to heat were not present in the quantum dots. Dang et al. conducted the experiment in air (rather than vacuum or inert gas) at room temperature.

The new experiment is not by itself practical for making commercial multi-wavelength lasers, since the stimulating laser is laboratory grade—not something you will see incorporated into consumer devices. Nevertheless, this is a significant advance toward broad-spectrum lasers fabricated from a single material.

Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.61 (About DOIs).