In order to know more about the mechanism of laser propulsion, the temporal evolvement and spatial distribution of the air plasma produced by a Transversely Excited Atmospheric (TEA) pressure CO 2 laser were investigated with a image intensified charged coupled device (ICCD) camera. The energy of the TEA CO 2 laser was 5 ~ 6 J, with pulse FWHM of 100 ns at 10.6 μm, trailing approximately 2 μs. The experimental results indicated that the radial evolution was almost symmetrical but the axial evolution was not. During the development stage of the breakdown, the laser-absorption region was propagated along the beam axis in the direction opposite beam incidence. The axial length of plasma and the maximal intensity of the laser-induced plasma were investigated, which could vary with the laser power. The propagation speed of the laser-induced plasma was about 10 4 m/s at the initial stage of breakdown, and then the propagation speed of the laser-induced plasma decreased gradually.

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A simple terahertz (THz) cavity and a TEA CO 2 laser for the optically pumped THz emission is studied experimentally. To obtain high peak power of pump laser, pressure ratios of gas mixture in the cavity of the TEA CO 2 laser are discussed. When CH 3 OH are pumped by the 9P(16) and 9P(36) CO 2 laser lines, the generation of terahertz radiation with energy as high as 353 µJ and 307 µJ are obtained, respectively. The corresponding photon conversion efficiencies are 0.705% and 0.29%. Meanwhile, higher peak power of pump laser effectively improves the photon conversion efficiency. And the optimum THz laser pressure increases with narrower pulse width of pump laser.