Vanishing Electrons

By Jeffrey Winters|Monday, July 01, 1996
Lasers are surprisingly inefficient devices. For every photon emitted in a laser beam, three or four are wasted. Some shoot out in useless directions, others are absorbed back into the lasing material itself. In fact, because under normal conditions just one electron in a million is excited enough to lase, all of a laser’s light would be swallowed up inside the laser were it not for constant agitation by another light source or an electric current. Now a team of physicists has developed a fundamentally new type of laser, one potentially far more efficient and powerful than conventional lasers. It’s a new chapter in laser physics, says Marlan Scully, a physicist at Texas A&M; University and the Max Planck Institute in Germany who helped develop the new laser. A German newspaper described it as ‘overthrowing a dogma in physics,’ and I think that’s true.

That dogma is known among physicists as population inversion, and it describes what happens to electrons and photons inside a laser. Electrons in a lasing material--and indeed in any material--tend to spend most of their time in low-energy states, although at any one time a few will be in higher energy levels. The high-energy electrons sooner or later emit radiation in the form of photons and drop down to a lower state; conversely, those at low levels may absorb radiation and jump to a higher level. (One of the tenets of quantum mechanics is that any electron that can absorb a photon of a certain wavelength can reemit a photon of the same wavelength.)

Most of the time, electrons emit their photons at random times and in random directions, and these get sopped up by low-energy electrons. Occasionally, though, a photon reaches an electron in the exact energy state of its own parent electron; when this happens, the photon coaxes the electron into emitting a duplicate photon in the same direction as the original. To make a useful laser, physicists need to get zillions of these photon clones marching in lockstep. And to do that they have to keep most of the electrons in the lasing material at the same high energy level--the unnatural state of affairs known as a population inversion. Otherwise any photons that are generated will tend to get swallowed up before they reach electrons that are ripe for lasing.

Population inversion works, of course, but it would be nice to do without it. For one thing, it isn’t always an easy task to accomplish. The higher the energy level of an electron, the less time it spends there before spontaneously emitting a photon. This isn’t much of a problem for reddish, low-energy lasers like the ones in grocery store scanners--those electrons don’t need much energy to get excited and emit red light. But it takes a drop from a very high state to a very low one to emit an extreme ultraviolet or X-ray photon, and an electron will wait a mere trillionth of a second before spontaneously unloading such a high-energy package. Physicists who want a powerful ultraviolet or X-ray laser must pump thousands of times more energy into the system than they get out in order to sustain a population inversion.

While other laser labs were rigging up ways to get more power from their conventional lasers, Scully and his colleagues at Texas A&M;, along with Stephen Harris of Stanford and Leo Hollberg of the National Institute of Standards, spent much of the last six years taking a more esoteric approach. They hoped to design a laser that would take advantage of a weird phenomenon called quantum interference. Over the last six months, their work has begun to pay off, although their prototype laser is still a bit cumbersome. For one thing, it requires a conventional laser to create the quantum interference.

According to a well-tested principle of quantum mechanics, electrons and all other particles have a dual nature--they sometimes behave like solid, discrete particles and at other times like waves. This gives particles some very unusual and counterintuitive properties. In quantum interference, for example, a single wavelike electron, prodded in just the right way by incoming laser light, is made to travel from a low-energy state to a slightly excited state along two distinct paths--exactly as a light wave does when it travels to a screen through two different slits in a grating. Like the light wave, the electron interferes with itself. Its crests overlap with its troughs, and the electron effectively vanishes.

What does this have to do with building better lasers? To make a laser, you don’t really need most of the electrons in the lasing material to be in the high-energy, ready-to-radiate-a-photon state; you just need them not to be in the low-energy, ready-to-absorb-a-photon state. Maintaining a population inversion--keeping most electrons in a high-energy state--is basically just an energetically expensive way of keeping many electrons out of the way of photons. A potentially cheaper way, Scully and his colleagues realized, would be to raise just a few of the electrons to the high-energy state--the ones you actually need to do the lasing--and make the rest interfere with themselves. Then the electrons that weren’t lasing would conveniently vanish instead of absorbing photons released by the other electrons.

Scully and his colleagues have built a prototype quantum- interference laser that does just that. In the prototype, a beam of rubidium or sodium atoms is illuminated with laser light of just the right frequency--infrared light for rubidium, visible for sodium--to cause many of the atoms’ low-energy electrons to interfere with themselves. With those electrons safely out of the way, a second light source knocks a few electrons to a high energy level. That touches off the lasing process.

Right now the output laser is still weak and extremely faint. With its multiple input light sources, the prototype is every bit as wasteful as conventional lasers. But Scully says that eventually a cheap, low-power laser could be used to create quantum interference, and the design will be perfected in order to take advantage of the efficient lasing effect. Without a single wasted photon, such lasers would be unimaginably efficient. (In population-inversion lasers, many photons still end up getting absorbed.) It may be possible, says Scully, to use low-power lasers to generate an intense ultraviolet or X-ray laser beam. He believes such an ultraviolet or X-ray laser wouldn’t use much more energy than the average optical lasers do today. That’s the holy grail, he says. How far we can go remains to be seen, but we’re very optimistic.
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