Coherent Subterahertz Radiation from Femtosecond Infrared Filaments in Air

We report on what is believed to be the first observation of coherent subterahertz (sub-THz) emission from a 1-m string in the atmosphere. The sub-THz pulse emitted by the filamentary structure from an intense IR femtosecond laser pulse is detected perpendicularly to the laser propagation axis by use of two heterodyne detectors at 94+/-1 and 118+/-1GHz . We describe the characteristics of this emission and show evidence of constructive interference between two separate strings.