Although the terahertz frequency range has long remained undeveloped because of a lack of compact coherent radiation sources, this may soon change. Researchers at Massachusetts Institute of Technology in Cambridge have developed a 3.4-THz quantum-cascade laser based on longitudinal-optical phonon scattering. They report pulsed laser action in the GaAs/AlGaAs structure at temperatures up to 65 K (29 K with a 50 percent duty cycle). Operating at 5 K in pulsed mode, the device had a threshold density of 840 A/cm2 and an approximate peak power of 2.5 mW.