A Rainbow of Lasers

A new material makes it possible to use a single manufacturing process to build lasers that emit different colors under low-energy excitation. Current technology requires different materials and manufacturing methods for lasers of different colors, and producing the full visible spectrum has not been possible. The material extends the range of achievable colors for lighting and other electronics applications.

The material consists of densely packed films of quantum dots (semiconducting nanocrystals) that lase when triggered with low excitation energies. It was developed by engineering professor Arto Nurmikko, senior research associate Cuong H. Dang, and coworkers at Brown University and the company QD Vision, in Watertown, Mass. (Nat. Nanotechnol., DOI: 10.1038/nnano.2012.61). The films emit different colors depending on the size of the quantum dots. Making a laser with a different color requires nothing more than using a different-size quantum dot.

The researchers made the films with quantum dots comprising a CdSe core surrounded by a ZnCdS shell coated with aromatic ligands. The inorganic shell and organic layer shield the quantum dots so they can form a dense network without interfering with one another.

These quantum dots are type I, in which an electron-hole pair, or exciton, is confined inside the core. In type II quantum dots, the electron and hole are separately confined in the core and the shell. When excitons recombine upon excitation of the quantum dots, they release energy in the form of light or heat.

In the past, type I quantum dots have typically been excited in such a way that multiple excitons per crystal must form for lasing to occur. The excitons can interact with one another, causing the quantum dot to lose as much as 99% of its energy as heat, Dang says.

Other researchers previously demonstrated single-exciton lasing with type II quantum dots, but type II quantum dots are inefficient light emitters. The Brown team now shows that single-exciton lasing is also possible with the more efficient type I quantum dots. They incorporated the quantum dot films into a device called a vertical-cavity surface-emitting laser and achieved red, green, and blue lasing. Instead of producing multiple excitons per dot, the low-energy excitation produced on average 0.8 excitons per dot.

Lasing with colloidal quantum dots was first demonstrated in 2000, but “serious problems have remained which prevent their use in practical devices,” says Patanjali Kambhampati, a chemistry professor at McGill University, in Montreal. The current work is a “significant step forward for lasing using colloidal quantum dots, as it reveals that the standard type I dots are in fact an efficient and size-universal” lasing material.

To achieve lasing with “only one exciton per dot is very exciting,” says Christopher B. Murray, a chemistry professor at the University of Pennsylvania. “Solid-state lighting applications are really gaining traction, and this new work expands the possibilities significantly with lasing at lower intensities than have been seen before.”

Such lasers could be used in a variety of devices, Dang says—displays, televisions, and even fancy pens. “Right now you have green laser pointers everywhere,” he says. “You don’t see yellow; you don’t see purple. With this, we can do any color.”