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SUVOS Pushes UV LEDs And Lasers To Shorter Wavelengths

In just two years, DARPA's Semiconductor Ultraviolet Optical Sources (SUVOS) program has made 280 nm LEDs a reality, demonstrated prototype UV biosensor and communication systems, and yielded lasers operating below 350 nm. Tim Whitaker reports.
Semiconductor light sources operating in the ultraviolet (UV) region are required for a number of applications, including solid-state lighting, biological agent detection, covert communication and sterilization. Various groups around the world are attempting to achieve shorter-wavelength operation, while at the same time improving the output power and extraction efficiency of their devices.

In the US, this effort is being coordinated by DARPA s Semiconductor Ultraviolet Optical Sources (SUVOS) program, which currently consists of 17 projects with a total of more than 20 participants. The program is about halfway through its four-year cycle, and by the time it finishes in spring 2006, DARPA will have invested a total of around $50 million.

The original goal of SUVOS was to develop device structures operating down to 340 nm during Phase I of the program, which lasted for 18 months and has now finished. In Phase II, the participants were due to investigate shorter-wavelength devices operating down to 280 nm, as well as integrating the sources into system test beds and prototypes.

In fact, SUVOS has exceeded expectations, according to program manager John Carrano. "The program has already demonstrated UV LEDs operating down to 275 nm, with output powers in the milliwatt range under DC conditions," he said. "Accelerated lifetime testing is under way, and we hope to reach 1000 h lifetimes very shortly."

Sterilization and decontaminationCarrano says that the "wildly successful" results achieved for LEDs during Phase I of SUVOS have opened the door to further progress. "We are now intending to expand the goals of the program to get all the way down to 265 nm LEDs," he said.

Crucially, 265 nm represents the sweet spot for water and air purification as well as surface decontamination. While there is a military requirement for such functionality, there is also a large commercial market; water treatment facilities around the world use UV lamps to kill bacteria. They currently use mercury vapor lamps, which are inefficient and offer little directional control. However, semiconductor emitters operating at 265 nm should be able to provide considerable improvement.

Another application is the sterilization of water in remote areas. Hydro-Photon, Inc of Blue Hill, ME, sells a product called Steri-Pen, a pocket-sized device containing a mercury vapor tube emitting at 253.7 nm, which can be used to sterilize a beaker of water in around one minute (see picture). The company is investigating the use of solid-state sources as an alternative, and owns a patent entitled "Hand-held water purification system using solid-state devices" (US patent number 6,579,495). The company has several other patents pending in this area.

"At the moment we are working under contract with the Office of Naval Research and with the help of DARPA to develop a portable UV LED-based water purification system for troops in the field," said Hydro-Photon s Miles Maiden. "Our chief subcontractor on this project is the materials group headed by Shuji Nakamura and Steve DenBaars at the University of California, Santa Barbara."

BiosensorsDetecting biological warfare agents is of particular concern to the US military. One technique for discriminating between these agents and harmless particles or organisms is to excite the sample with a UV source and measure the resulting fluorescence. Tryptophan, a common constituent of biological organisms, has an absorption peak at 280 nm, while the biochemical compound NADH has an absorption peak at 340 nm.

Carrano says that three years ago, when the SUVOS program was being put together, the prospect of a viable 280 nm LED seemed very remote. "Now we have devices that are good enough to use in real applications," he said. One such application is the world s first and only dual-wavelength LED-based biosensor. Using 340 and 280 nm LEDs, the system, which is currently about the size of a large notebook PC, can detect single particles of a species that simulates anthrax. It can also, to a certain extent, discriminate between potential hazards and other particles such as dust, dandruff, coffee creamer and smoke particles. Discrimination is enabled by the combination of two excitation sources and several detectors tuned to different fluorescence bands.

Laser-based systems have also been demonstrated. While these will ultimately provide higher performance, LED-based systems will be sufficient for certain biosensor applications. "In future, we will need a suite of sensors - there won t be just one that will fit all customers and requirements," said Carrano. "The LED-based systems will be deployed sooner, and will also be smaller and considerably cheaper than the laser-based systems."

SUVOS participants have made good progress developing short-wavelength lasers as well as LEDs. Cree recently announced that it had made a 348 nm CW laser operating under electrical injection at room temperature. This is the shortest wavelength reported for a semiconductor laser diode. Carrano says that similar devices operating at a slightly longer wavelength had a lifetime in excess of 100 h and produced many milliwatts of optical power.

Covert communicationsAnother application for UV LEDs that is of particular interest to DARPA is non-line-of-sight (NLOS) communications. For example, UV sources will enable ultra-low-power, covert systems that can be used in urban terrain or to provide a network of unattended sensors. Systems operating at around 280 nm are able to exploit the solar-blind region of the spectrum, a gap in which the terrestrial component of the sun s radiative flux is essentially nil. This low background enables excellent signal-to-noise, so low-power optical sources can be used, but the high extinction coefficient in the UV means that the range is limited to around 250 m, and hence there is a low probability of jamming or interception.

A prototype NLOS communication system using deep-UV LEDs has been demonstrated in broad daylight. UV radiation from a cluster of six LEDs operating at 275-280 nm was scattered by the atmosphere and detected by a NLOS receiver about 15 m away. The NLOS digital link had a data rate of 200 bit/s, so there is obviously room for improvement. However, at its current rate of progress the SUVOS program seems set to yield high-performance NLOS communication systems, as well as biosensors and sterilization systems, before it finishes in two years time.

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