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LASER COOLING BY
COHERENT SCATTERING
(AIP)
The American Institute of Physics Bulletin of
Physics News May 18, 2000 by Phillip F. Schewe and Ben Stein
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In early laser-cooling experiments inside atom traps, atoms were cooled when they encountered a laser beam coming at them with an energy that was slightly less than what is needed to be absorbed by an unmoving atom (promoting an electron from a lower to a higher energy level). But absorption can occur anyway if the atom's energy of motion equals the energy by which the laser beam is de- tuned. Thus the detuning compensates for the Doppler effect of the moving atom. Now a pair of Stanford physicists, Vladan Vuletic (650-725-2356, [email protected]) and Steven Chu (who won a Nobel prize for his work on laser cooling) are announcing a new cooling scheme, one in which the laser light is not absorbed but scattered. And scattered coherently in such a way that the atom loses a bit of energy in the encounter. The coherence helps to cool large samples because the scattered light circulates in an optical cavity and the scattering from one atom promotes scattering of light from other atoms. The detuning relationship in the new scheme is not between the incident laser beam and the atom but between the scattered light and the cavity. The cavity permits some light modes to propagate but not others; the incident laser light can be slightly detuned below what the cavity will accept, thus encouraging scattering events in which the scattered light has just the resonant energy. Furthermore, because the incident laser beam is not related to any particular transition inside the target atom, the target can actually be any number of atoms in different states or even a molecule with a diversity of ground states (owing to internal degrees of motion arising from rotation or vibration of the molecule). Vuletic believes that this cooling method will be realized in the lab in the next 6 to 12 months. (Physical Review Letters, 24 April 2000; Select Article.) |
MICRON-SIZED LASERS
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MICRON-SIZED LASERS can be made from chemicals, solvents, a hot plate and glass beakers, without the need for huge nano- fabrication facilities. Hui Cao and her colleagues at Northwestern University ( 847-467-5452) last year built a laser whose active medium consisted of a disordered powder of ZnO articles (Update 423). Now they have shrunk the size of the powder laser (see figure at www.aip.org/physnews/graphics) down to one micron in size and operate the device at room temperature. The lasing wavelength is 380 nm and the device operates at room temperature (Cao et al., Applied Physics Letters, 22 May 2000; Select Article.) |
AMPLIFYING AN ATOM WAVE
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AMPLIFYING AN ATOM WAVE while maintaining its original phase has been demonstrated for the first time, bringing about an atom laser that is the closest equivalent yet to an optical laser. The first atom lasers were passive devices: researchers simply prepared a Bose- Einstein condensate of atoms, and then extracted some of the BEC atoms to form a beam. In the latest round of demonstrations, two research groups (one at MIT and one at the University of Tokyo) have independently demonstrated an atom laser that amplifies its initial beam, in a way that's remarkably similar to how optical lasers augment an initial light wave. Unlike light, however, atoms cannot be created from the vacuum, so researchers must rely on a pre-existing supply of atoms to serve as the initial beam to be amplified. In the MIT demonstration, researchers shine a pair of laser pulses on a sodium BEC. First, some of the BEC atoms absorb a photon from a high-frequency beam and emit a photon towards a lower-frequency beam. These atoms recoil in the same direction, forming a weak atom wave. Then the lower-frequency beam is shut off, and some of the other BEC atoms absorb light from an intensified pulse coming from the high-frequency laser. The presence of the initial atom wave stimulates these atoms to emit a photon in the direction of the lower-frequency beam. This resulted in a phase-coherent amplified beam about 4 times as strong as the initial atom wave. The Tokyo group demonstrated similar results with a rubidium-87 BEC. In both demonstrations, the amplification is limited by the size of the BEC, which is depleted in the process. However, an atom-wave amplifier promises improvements in such applications as atom-wave gyroscopes and lithography. (Inouye et al., Nature, 9 December 1999; Kozuma et al., Science, 17 December.) |
COSMIC RAYS OBSERVED BY GRAVITY-WAVE DETECTOR
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The NAUTILUS detector at the Frascati Laboratory in Italy consists of a 2300-kg aluminum cylinder cooled to a temperature of 0.1 K. The plan is that a passing gravitational wave (broadcast, say, by the collision of two neutron stars) would excite a noticeable vibration in the cylinder. NAUTILUS has not yet recorded any gravitational waves, but scientists have now witnessed the cylinder vibrated by energetic particle showers initiated when cosmic rays strike the atmosphere. The signal generated by the rays is believable because conventional cosmic- ray detectors surrounding the bar also lit up when they were struck by the particles. In effect the detector is able to discern a mechanical vibration as small as 10^-18 meters, corresponding to an energy deposit as small as 10^-6 eV. (Astone et al., Physical Review Letters, 3 January 2000; Select Article. Contact Giuseppina Modestino, [email protected], 011-39-694-032- 756.) |
