Coherent control of an electric dipole in air plasma created by femtosecond laser pulses

Summary form only given. Air plasma excited by a 800 nm femtosecond pulse with its second harmonic is capable of emitting intense terahertz (THz) radiation, which is of great interest due to its unique potential for stand-off THz applications. In this paper, we report the experimental observation of an electric dipole moment in such a plasma and its coherent control. In the experiment, a femtosecond pulse is focused by a convex lens of 40 cm into ambient air. A 100 mum thick type-I BBO crystal is inserted between the lens and the geometrical focus, see figure 1. A needle-shaped electric antenna is positioned 2 cm away from the air plasma. Typical signals recorded by the antenna are presented in fig. 2 (a). In fig. 2(b) the peak-to-peak amplitudes of these signals are plotted as dots in function of angle phi in the y-z plane containing the plasma. The solid line represents a theoretical fit of the results by function abs (cos (phi)). The good agreement between the experimental and theoretical results suggests the existence of an electric dipole oriented orthogonally to the laser propagation direction. Surprisingly, the antenna signal shows opposite polarities on the left and right sides of the air plasma. Moreover, by varying the BBO-to-plasma distance, the magnitude and polarity of this dipole can be controlled, as will be discussed in details in this presentation. We believe this is the first experimental demonstration of an unusual dipole-like emission displaying a p-like phase pattern.