We investigated the broadband terahertz (THz) emission from sub millimeter laser-induced plasmas. The angular dependent emission from the source was characterized with coherent detection, showing that the radiation pattern peak is located 80 degrees away the propagation direction of the excitation laser. Our findings support the laser ponderomotive force as the main driver for the generation of THz transients. We report the measurement of THz waveforms generated in ambient air with laser excitation energies as low as 660 nJ. The experimental results suggest the laser-induced microplasma as a promising approach to reduce the laser threshold typical of THz air photonics systems of many orders of magnitude, with the result of scaling down the cost of broadband terahertz spectroscopy.
THz wave air photonics involves the interaction of intense femtosecond laser pulses with air or selected gases. The very
air that we breath is capable of generating and detecting THz waves with field strength greater than 1 MV/cm and useful
spectral coverage from 0.1 THz to 60 THz. Broadband THz wave remote sensing is feasible.