So, basically they're creating a lasing region of space, in mid-air, at a distance of 30 meters (~100 feet): The beam is transmitted through a lens that focuse

So, basically they're creating a lasing region of space, in mid-air, at a distance of 30 meters (~100 feet): The beam is transmitted through a lens that focuses it on a spot 30 or so meters away; there it converges and then diverges, giving this region an hourglass shape. Because the laser pulse is very brief—on the order of nanoseconds—the intensity is high enough to break molecules of oxygen into their constituent atoms over a few millimeters in the thin midsection of the hourglass. This happens so rapidly during the pulse that the same pulse subsequently excites many of the oxygen atoms into a high-energy state, creating what's called a population inversion. This unstable condition then leads to the familiar laser chain reaction in which an excited atom drops to a lower energy state, emitting a photon of a particular wavelength, and that photon in turn stimulates another excited atom to emit a photon of the same wavelength and phase. The thus excited region of space emits two laser beams, one in the direction of the original "trigger" beams, and one in the opposite direction i.e. back at the source. The intensity is said to be around 1,000,000 times that of the ambient radiation in that wavelength, so it's "easy" to detect. And the beam carries information about the molecular mix in the generating volume of space, which is of course the core feature here. That is ... pretty close to being magic, in that "naah, that can't be possible!" way that is so fantastically neat. Too bad the triggering impulse (heh) to develop this is something as nasty as land mines and IEDs, the technology itself is just beautiful.