Researchers at the Thomas Jefferson National Accelerator Facility in Newport News, Va., have demonstrated second-harmonic lasing in the infrared with a superconducting free-electron laser (FEL). In a report that appeared in the Aug. 20 issue of Physical Review Letters, they described how they produced an average of 4.5 W of 2.925-µm light with the system set to lase at 6 µm.Typically, gain exists in a free-electron laser only at odd harmonics because of the symmetry in the system, so the researchers used an antisymmetric resonator mode and misalignment of the injected electrons with the optical axis. The preferred lasing mode of the system at the second harmonic was TEM01, which they attributed to the favoring of modes with no on-axis gain.