External focusing effect on terahertz emission from a two‐color femtosecond laser‐induced filament in air

A systematic investigation of external focusing effect on terahertz (THz) emission from a two-color femtosecond laser-induced filament in air is demonstrated. By collecting the THz emission precisely under different external focusing conditions, an optimum external focal length around 50 cm was observed to generate stronger THz emission. The optimum dependence on the filament volume and the filament's core intensity with external focusing could be responsible for this stronger THz emission.