Laboratory St. Elmo's fire produces VUV light
Lamps emitting at vacuum-UV wavelengths (VUV; the region below approximately 190 nm, where air starts to strongly absorb light) could find use in ozone production, cleaning and surface modification of semiconductors, photochemistry, and water purification, among other applications.
Lamps emitting at vacuum-UV wavelengths (VUV; the region below approximately 190 nm, where air starts to strongly absorb light) could find use in ozone production, cleaning and surface modification of semiconductors, photochemistry, and water purification, among other applications. Because such uses do not require coherent light, large and expensive excimer lasers are overkill in these cases. But small and practical VUV lamps are only now being developed. For example, a light-bulb-like lamp from TuiLaser (Germering, Germany) excites excimer gases with an electron beam to produce up to 300 mW at discrete VUV wavelengths at an electrical-to-optical, or wall-plug, efficiency of 10% (see Laser Focus World, May 2003, p. 20).
A VUV light source contains 21 needles that generate St. Elmo's fire (blue). Here, the emitted VUV light causes a UV-sensitive glass disc to fluoresce (green).
Like any form of lighting, however, increased efficiency begets reduced costs. Researchers at Rutgers University (Newark, NJ) have now developed an excimer-based light source—based on the phenomenon known as St. Elmo's fire (technically known as corona discharge)—that emits 172-nm light at wall-plug efficiencies of up to 55%.1 In the most well-known form of St. Elmo's fire, a point isolated from ground—a ship's mast, for example—creates a glow during a thunderstorm; the point locally increases the electric field in the air, causing a plasma to form. The Rutgers researchers have scaled the phenomenon down for practical use, placing one or more conductive needles at a high negative voltage in a dense rare gas contained within a cell.
In the simplest configuration, a single sharp tungsten needle and a grid ground plane are placed in a cell filled with xenon gas to a 1.3-bar pressure. A 4.5-kV potential and a 30-µA current produce VUV light from both the corona discharge and a drift region between the needle and grid. The light exits through a calcium fluoride window beyond the grid. Measurements of the optical output showed a wall-plug efficiency of 55%.
A second configuration contains 21 needles 6 mm apart in a square array; the cell is filled with xenon to a pressure of 2 bars and has a window of UV-enhanced fluorine-dope fused silica. The device has emitted 35 mW/cm2 at the center of the window at a wall-plug efficiency of 20%. The researchers have proposed an improved 172-nm source with 130-mW/cm2 output and a possible wall-plug efficiency of close to 48%.2
The device is not restricted to a 172-nm emission. "Other inert gases can be used, yielding for example 129 nm for argon and 147 nm for krypton," says Daniel Murnick, one of the researchers. "A monochromatic source at 121.6 nm is possible using a neon-hydrogen mixture. Other gas mixtures are also possible; but the physics of the system may change, resulting in lower efficiency."
Because nitrogen is unaffected by 172-nm radiation, the emitter would be especially well suited to produce ozone from air (ozone is used for water purification), with each photon yielding two ozone molecules.
- M. Salvermoser and D. E. Murnick, Appl. Phys. Lett., 1932 (Sept. 8, 2003).
- M. Salvermoser and D. E. Murnick, J. Appl. Phys., 3722 (Sept. 15, 2003).