Crumb Trail
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Sunday, February 19, 2006

Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light.

The fact that the Kerr effect can transform a high-power infrared laser into a remote source of white light opens the door to a number of exciting applications. For example, the tendency for some of the light to be reflected backward suggests that we could create an artificial "guide star" for use in adjusting astronomical telescopes equipped with adaptive optics. But there are other nonlinear optical effects of the Teramobile laser that can be exploited as well. One is something called multiphoton fluorescence.

In normal fluorescence, a substance, say the phosphor powder that coats the inside of a fluorescent lamp, absorbs high-energy photons (typically in the ultraviolet) and releases lower-energy photons (having, usually, visible-light wavelengths). In multiphoton fluorescence, two or more low-energy photons are absorbed simultaneously, raising an electron's energy level enough to allow a single high-energy photon to be given off when the electron returns to its original state. But because the chance of an atom absorbing two photons at once is quite low, light of very high intensity (that is, containing a very large number of photons) is needed. The pulsed Teramobile laser provides just such light, which proves a great boon for remotely sensing certain compounds using the phenomenon of multiphoton fluorescence.

In a 2002 experiment, my colleagues and I showed that the Teramobile beam and detection apparatus could sense biological aerosols at a distance. The motivation was to be able to map a cloud, say, of bacteria (perhaps given off during some industrial mishap or even a biological attack) and to identify potentially pathogenic agents among the various background atmospheric aerosols, among which may be more mundane organic particles such as soot or pollen.

Our test used water droplets sized to mimic bacteria and laced with the compound riboflavin, which fluoresces at visible wavelengths when it absorbs two infrared photons, producing a characteristic spectrum in the backscattered light. The experiment, carried out on a cloud located about 45 meters from the Teramobile laser, showed that it was easy to distinguish such a plume from a cloud of pure water droplets. With refinement, this technique could, potentially, be quite sensitive. We calculated that a laser tuned to excite two-photon fluorescence in the amino acid tryptophan would boost sensitivity by a factor of 10, allowing concentrations of as little as 10 bacteria per cubic centimeter to be detected 4 kilometers away. Although lidar systems based on normal fluorescence could also be used to probe for biological agents, the laser employed would have to operate at a shorter wavelength and thus be more prone to attenuation, limiting the distance over which it could function effectively.

The ability of laser filaments to deliver high-intensity light at substantial distances also opens the door to other very interesting applications. For example, it becomes possible to conduct elemental analyses of the surfaces of metals, plastics, minerals or liquids from an appreciable distance, using a variation of a technique called laser-induced breakdown spectroscopy. For that, a powerful laser is focused on the material of interest, causing some of it to be transformed into plasma. The emission spectrum of the glowing plasma can then be analyzed, revealing the nature of the substrate, with a detection limit that can be as little as a few parts per million for some elements. This method is currently used for such applications as the identification of highly radioactive nuclear waste and for monitoring the composition of molten alloys, because the tests can be performed without having to touch the sample. Imagine being able to do such probing from a large distance away! Normally, diffraction limits the intensity of light that can be focused on a remote target. But laser filaments can deliver intensities that are higher than the ablation threshold of many types of materials, at distances of hundreds of meters or even kilometers.

Another application under investigation may prove more spectacular yet—the control of lightning strikes. Lightning has always fascinated people, in part because of its unpredictable nature and destructive power—qualities that make these electrical discharges very difficult to study. Investigators from Electricité de France and CEA partially overcame those obstacles in the 1970s, when they developed a technique to trigger lightning on command using small rockets trailing thin wires. If shot upward at the right moment, the rockets and the wires they unspooled behind them served to initiate and channel the flow of electric current.

This is the death ray to go with the cloaking device mentioned below.

posted by back40 | 2/19/2006 11:48:00 PM


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