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10 Feb 2005
German scientists make a pulsed ultraviolet laser using the same type of materials found in OLED displays.
Researchers in Germany claim to have made the first organic semiconductor laser that emits pulsed UV light. The team from the Technical University of Braunschweig says that the development could ultimately lead to a new excitation source for cost-effective bio-fluorescent monitoring (Advanced Materials 17 31). The news comes just six months after the team reported an organic laser that operates in the blue region (see Tunable organic laser emits blue light).
The UV laser is optically-pumped and consists of a distributed feedback (DFB) structure containing a light-emitting layer of spiro molecules, a class of materials found in OLED displays. It emits pulses of light with a power of up to 6.8 W and operates at a wavelength as short as 377.7 nm. The organic material was supplied by Covion organic semiconductors of Frankfurt, a leading supplier of OLED materials which has just been purchased by Merck in a EURO 50 million deal (see Merck acquires Covion).
The laser is made by depositing a 350 nm-thick layer of the spiro material on top of a silicon substrate that features a series of etched parallel grooves (a grating). By varying the pitch of the grating between 200 and 225 nm it is possible to make lasers with an emission anywhere between 377.7 and 395 nm. And the team believe that even shorter wavelengths could be achievable in the future.
"Emission wavelengths down to 300 nm should be possible, provided that there is a suitable stable material," said Thomas Riedl, head of the Braunschweig research team. "The problem with higher photon energies [shorter wavelengths] will be the onset of photochemical reactions in the organic material."
According to Riedl, the wide gain-bandwidth of an organic laser gives a big advantage over gas or diode lasers which usually have a fixed wavelength of operation or very limited tuning capabilities. "Our organic UV laser provides wavelength tunability over 18 nm and is perfectly suited to selectively excite a sample. Such a large tuning range in this spectral region is not possible with UV laser diodes."
Although in initial experiments the laser has been optically pumped by pulses from a nitrogen laser at 337 nm, the team is now striving to construct an electrically-driven version. "We now have a project, funded by the German government (BMBF), with the aim to demonstrate electrically pumped organic lasers within the next two years," Riedl told Optics.org "The prerequisites are there in principle. All the organic materials used to build our lasers bring the ability to transport charges and emit light upon charge injection. It works for OLEDs, so why not for lasers?"
Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
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