David L. Shenkenberg, firstname.lastname@example.org
TAMPERE, Finland – Thin is in, at least for sleek mobile phones and laptops. Consumers are used to seeing mobile phones that are equipped with a plethora of gadgets, including touch screens and global positioning systems, but can you imagine a mobile phone that contains a digital projector? Yes, the very same digital projectors that you use to give presentations at meetings could fit into a mobile phone, researchers say.
Mircea Guina of Tampere University and his fellow researchers believe that the key to putting a projector into a mobile phone is the laser diode because it is compact, cheap and mass-producible. Laser diodes are already found in many familiar products such as CD players, bar-code readers and laser pointers.
A projector would contain laser diodes with red, green and blue beams because those colors can be mixed to generate other colors. However, the engineers have been stopped by what they say is a lack of commercially useful laser diodes with green beams. This so-called “green gap” is being addressed by Guina and colleagues.
Besides the “green gap,” another obstacle is that typical red laser diode beams have wavelengths from 640 to 650 nm, whereas the human eye is most sensitive to red wavelengths from 620 to 640 nm. The brightness of laser diodes is often insufficient for projection and, what’s more, they require a lot of electricity, and they break down if they get too hot.
Whereas standard laser diodes emit a laser beam through the edge of the semiconductor chip, the lasers developed by the researchers emit straight up from the chip. This type of configuration makes it easier for the laser to give off heat, so that it can operate at the high powers required for high brightness without breaking down. The engineers call the lasers optically pumped vertical external-cavity surface-emitting lasers, or VECSELs.
Pictured is a green VECSEL developed by European researchers. Courtesy of Antti Härkönen.
Research for the project has focused on a technique known as frequency doubling and on developing new gain media, such as quantum dots. The team, which included not only Tampere University but also the University of Strathclyde in Glasgow, Scotland, Osram Opto Semiconductors Inc., EpiCrystals Inc. and Toptica Photonics AG, developed high-power red lasers as well as green, amber-orange-red and UV lasers. Toptica has developed lasers for scientists doing spectroscopy, while Osram GmbH and EpiCrystals have made them for projection systems.
The lasers also could be used for cutting materials and for medical purposes, as well as for lithography in the case of the UV laser. The visible and UV lasers also could replace gas and ion types from a cost and maintenance perspective. The amber-orange-red lasers could help astronomers create artificial guide stars by exciting sodium in the atmosphere.
This work was born out of the NATAL project, which ended in July 2008 after having received €3.8 million in total funding from the European Union (EU) with about €2.8 million specifically allocated to research and development. The work on quantum-dot VECSELs is continuing via the Fast-Dot project, funded with €10.1 million from the EU. This project will end in 2012.