Coherent synthesis of terahertz radiation from femtosecond laser filaments in air

We report on the coherent synthesis of pulsed terahertz radiation from femtosecond laser filaments organized in an array. The terahertz intensity is proportional to the square of the number of the filaments, which provides a simple method for scaling up of the terahertz energy with a powerful femtosecond laser. Moreover, directional off-axis terahertz radiation can be achieved. This paves the way for applications of this terahertz source for remote sensing.