Terahertz emission from laser-induced microplasma in ambient air

The integration of ambient air plasmas as source and sensor in terahertz time-domain techniques allows spectral measurements covering the elusive terahertz gap (0.1-10 THz), further increasing the impact of those scientific tools in the study of the four states of matter. In this article we describe the experimental study of the terahertz emission from a laser-induced plasma of submillimeter size. The main direction of emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. We show that laser pulse energies lower than 1 mu J are sufficient to generate measurable terahertz pulses from ambient air. This significant decrease in the required laser energy will make plasma-based terahertz techniques more accessible to the scientific community. (C) 2015 Optical Society of America