Number 121 (Story #1), March 31, 1993 by Phillip F. Schewe and Ben Stein|
OPTICAL AMPLIFICATION WITHOUT POPULATION INVERSION has been achieved in a vapor of samarium, a rare-earth element. Optical amplification is a first step in the laser process in which a single photon, emitted by an excited atom, stimulates other atoms to emit photons having identical attributes, resulting in a buildup of light intensity. Optical amplification normally requires a population inversion whereby a majority of atoms must be excited into a higher energy state; otherwise the ground state atoms, which readily absorb photons, would sabotage this process. W. Lange and colleagues at the University of Munster in Germany use lasers to prepare a sample of samarium atoms in a superposition of two ground states which interfere in such a way as to prevent the atoms from absorbing light. As a result, only a small fraction of atoms needs to be in an excited state to induce optical amplification. This technique may one day be exploited in a working laser, particularly in an x-ray or gamma-ray laser, where population inversion is difficult to achieve because excited states are so short-lived. (A. Nottelman et al., 22 March 1993 Phys. Rev. Lett.; contact Lorenzo Narducci, Drexel University, 215-895-2711, for background information.) Amplification without inversion in sodium vapor was reported from China several months ago, but certain critical verification tests were not performed in that instance.