Directionality of terahertz radiation emitted from an array of femtosecond filaments in gases

An array of filaments in air is numerically shown to be an efficient tool to direct the energy of terahertz (THz) radiation into a narrow and controllable cone. By increasing the number of filaments in an N × N array, the THz energy growth per unit angle of the cone exceeds the classical gain in N². The backwardly directed radiation from the filament array appears when the transverse size of the array reaches the filament length. The optimum spacing between the filaments in the array is the THz wavelength, which ensures constructive interference and independent development of filaments.