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±1 GHz. We describe the characteristics of this emission and show evidence of constructive interference between two separate strings.