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Semiconductors and electronics

Semiconductors and electronics

Single laser makes many colours

30 Apr 2012 Hamish Johnston
A red CQD-VCSEL in action at Brown University

A new type of semiconductor laser that can be easily modified to create light over a range of different colours has been created by researchers in the US. The device is based on tiny particles called colloidal quantum dots (CQDs) that emit different colours of light according to their size – rather than their chemical composition.

Semiconductor lasers are found in a wide range of technologies, from DVD players to optical communications networks. While they are efficient and inexpensive to produce, their colour is defined by the electronic band gap of the semiconductor, which means that for every colour a different set of materials and structures is required. Integrating lasers of different colours into the same electronic device can therefore be very difficult.

What Arto Nurmikko and colleagues at Brown University in the US have managed to do is create a new type of semiconductor laser that, in principle, can produce light of different colours while being made of the same materials and design. The device is called a CQD vertical-cavity surface-emitting laser (CQD-VCSEL). The device is a variant of the VCSEL, which is a commercially available type of laser that uses a thin layer of compound semiconductor as its active optical medium.

Red, yellow and green

In Nurmikko’s prototype, the active material is a thin film of CQDs, which are nanometre-diameter spheres of the semiconductor cadmium selenide. In its studies, the team used CQDs with 4.2 nm diameters to create a red laser, 3.2 nm for green and 2.5 nm to produce blue light.

The CQDs are made using a wet-chemistry process, which creates a colloidal suspension of the spheres in a liquid. A tiny drop of this paint-like mixture is placed on the surface of a distributed Bragg reflector (DBR) – which is a special type of mirror used in VCSELs. A second DBR is then placed on top of the first and the drop is squeezed down to create a micron-thick active layer.

The team studied the devices by firing ultrashort pulses of light through the sandwich structures. This “pumps” the quantum dots into an excited energy state characterized by the presence of electron–hole pairs called excitons. An exciton can decay by emitting a photon, which can bounce back and forth between the mirrors and stimulate the emission of identical photons. As a result, the system will operate as a laser.

Too many excitons

But to excite enough of the quantum dots for this process to occur, a lot of power has to be delivered to the laser. This causes more than one exciton to occur in a dot and these are more likely to decay by emitting an electron (an Auger process) rather than a photon. This reduces the performance of CQD lasers, making them impractical.

To get around this problem, Nurmikko and colleagues coated cadmium-selenide quantum dots with an alloy of zinc, cadmium and sulphur. The team found that these latest quantum dots act as a laser when, on average, each dot has one or less excitons. This means that the laser requires a factor of 1000 less power to operate than previous devices based on uncoated cadmium-selenide quantum dots. This allowed the team to make the first working CQD-VCSEL.

“We have managed to show that it’s possible to create not only light, but laser light,” Nurmikko said. “In principle, we now have some benefits: using the same chemistry for all colours, producing lasers in a very inexpensive way, relatively speaking, and the ability to apply them to all kinds of surfaces regardless of shape. That makes possible all kinds of device configurations for the future.”

“Significant step”

Yury Rakovich of the University of the Basque Country in Spain described the work as “very well done and convincing”. He told that Nurmikko and colleagues’ device is a significant step towards full-colour, single-material lasers. He said the next step in the development of practical devices is to determine whether the CQD-VCSELs will operate in continuous mode, rather than the pulsed mode studied by Nurmikko’s team. He also believes that researchers will have to gain a better understanding of the lasing processes that occur in such tightly packed films of CQDs – and in particular whether interactions between individual dots play an important role in the laser.

The lasers are described in Nature Nanotechnology 10.1038/nnano.2012.61 .

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