Number 277 (Story #1), July 1, 1996 by Phillip F. Schewe and Ben Stein AN EXCITON ANALOG OF A LASER may be possible. In a semiconductor crystal, an electron can be excited from a valence state to a conduction state. The "hole" left behind can be filled by another nearby electron, but this only moves the hole to a new location. In effect, the hole acts like a positively charged object which can move about and bind itself to a negatively-charged electron. This tiny "atom" is called an exciton. Under special conditions excitons can condense into clouds. Insofar as excitons are bosons (particles with a spin of 0 or 1), they can even coalesce into a single quantum state, namely a Bose-Einstein condensate. Physicists at the University of Ottawa (Emery Fortin, fortin@physics.uottawa.ca) and the Ecole Polytechnique in Palaiseau, France have shown that it is possible to amplify a coherent beam of energy based not on light waves, as in a laser, but on excitons. In their experiment a laser creates an ongoing bath of excitons in a small semiconductor sample. A second laser establishes a small moving cloud of excitons (350-400 microns in size). Through a process of stimulated scattering the moving cloud gains new excitons as it proceeds; it grows not in size but in density. In other words, the directed beam of excitons is undergoing amplification. Because "exciton mirrors" do not yet exist, this amplification process occurs only over a single pass through the sample, and consequently there is not the repeated buildup of signal one gets in a laser. Furthermore, because excitons subsist of electrons and holes in the crystal, a beam of excitons could never be extracted from the sample and exist as an external beam as laser beams do. Still, the exciton beam in this present experiment may offer new ways of studying the coherent transfer of energy and charge. (A. Mysyrowicz et al., upcoming article in Physical Review Letters.)