Number 240 (Story #1), September 18, 1995 by Phillip F. Schewe and Ben Stein|
LASING WITHOUT INVERSION (LWI) , a fundamentally new technique for producing laser light, has been achieved in gases, opening new possibilities for cleverly sidestepping traditional difficulties of producing ultraviolet and x-ray laser light. In a gas of atoms, laser light buildup begins when a single photon, emitted by an atom in a high-energy (excited) state, stimulates other excited atoms to emit photons with identical attributes. Ordinary lasers normally require the energy-intensive process of "population inversion," in which a majority of the atoms must be excited into a high-energy state. Promoting atoms into excited states prepares them for participating in the laser process, but it also serves to prevent them from soaking up the light and thereby sabotaging the laser process. However, maintaining a population inversion in ultraviolet and x-ray lasers is extremely difficult because the high-lying excited states necessary to produce such light are so short-lived. A US-German-Russian group (A.S. Zibrov et al, Phys. Rev. Lett, 21 August 1995; contact Marlan Scully, 409-862-2333) has recently achieved LWI in a gas of rubidium atoms in a vapor cell, yielding 795-nm infrared light. In another paper submitted to Physical Review Letters, 18 December, a US-German team (including Scully and Edward Fry, 409-845-1910) reports LWI in a sodium atomic beam, producing yellow-orange light (590 nm). Previous experiments had produced nanosecond bursts of light without population inversion (Update 121), but the new papers are the first to report a sustained laser beam through LWI. In these experiments, an external laser beam essentially creates two pathways for the atoms to get from the ground state (state 1) to the excited state (state 2). In the rubidium experiment, for example, the probability of getting atoms from state 1 into state 2 becomes the overlap of the likelihood of getting from state 1 to state 2 directly (1-->2) and going from state 1 to an even higher excited state (state 3) then dropping to state 2 (1-->3-->2). Under the proper conditions, the overlapping likelihoods can interfere so as to cancel each other out, preventing absorption. Ironically, by creating more ways of getting into state 2, one can reduce the number of atoms that get there. Future goals are to achieve LWI in inexpensive diode lasers (like those in CD players) and to produce x-ray and UV light through LWI.