When a powerful infrared laser pulse hits a plasma of ionized hydrogen, it generates a tightly focused ring of coherent ultraviolet light. A similar process could lead to a compact source of coherent x-rays. In a step toward that goal, a research team at the University of Michigan (Ann Arbor, MI) has detected high-frequency coherent light generated by a new technique. The scientists sent two 400-fs-long, 1053-nm pulses (with intensity 1017 W/cm2) through a gas of hydrogen or helium--the first pulse to ionize the atoms, the second to generate the harmonics. They detected a phase-matched third harmonic at 351 nm, which was emitted with high efficiency in a tight cone of coherent light. Their theory proposes that the third-harmonic light propagates slightly slower through the gas than the incident light, so with an angle of 6° between them, the two light waves are in phase, and the emission from each electron in the plasma is synchronized. The scientists believe much higher frequencies should be possible in the future. Contact Donald Umstadter at [email protected].