Photoionization of an ambient gas by a weakly relativistic, femtosecond laser pulse leaves behind the pulse a flat-top plasma column. The uniform-density core of the column is surrounded by a micron-thin shell, within which the density of plasma species falls down to zero. As the laser pulse ponderomotive force drives the wave of charge separation (the laser wake), electron fluid velocity oscillates along this boundary layer. Coupling velocity oscillations to the transverse density gradient forms an azimuthally polarized rotational current confined within the boundary layer. At each radial offset, this current oscillates with a local (THz-scale) Langmuir frequency, transforming the cylindrical electrostatic plasma wave into a radially evanescent Fourier component of an electromagnetic signal. Summing up these Fourier components yields the time-domain THz signal which is (a) non-local (i.e. not phase-locked to the Langmuir oscillation at any given location), (b) contains the entire frequency band from zero to the Langmuir frequency of the density plateau, and (c) is detectable at a distance orders of magnitude larger than the plasma column radius. A few millimeters away from the column, rapid evanescence of the higher-frequency components transforms the signal into a radially polarized, single-cycle picosecond pulse with a kV m −1 -scale electric field. Detecting this pulse with existing (e.g. electro-optical) techniques may bring information on the efficiency of laser coupling to the plasma, the amplitude of wake fields, and the structure of the column boundary.

Photoionization by a femtosecond, terawatt laser pulse generates a plasma column in a neutral ambient gas. Velocity of electrons, pushed by the laser ponderomotive force along the column surface, couples to the the radial density gradient at the column boundary, generating an azimuthally polarized THz rotational current (RC). The same mechanism produces the low-frequency RC in a leaky plasma channel. Applying external voltage to the channel induces a radially non-uniform electron flow (direct current) and a constant, azimuthally polarized magnetic field. Coupling them to the electron density perturbations adds two more terms to the RC. The surface RC in the plasma column supports a broadband, evanescent THz signal accompanying the wake. A few millimeters away from the column, rapid evanescence of high-frequency components turns this THz signal into a radially polarized, single-cycle pulse.

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We demonstrate that is it possible to optimize the yield of microwave radiation from plasmas generated by laser filamentation in atmosphere through manipulation of the laser wavefront. A genetic algorithm controls a deformable mirror that reconfigures the wavefront using the microwave waveform amplitude as feedback. Optimization runs performed as a function of air pressure show that the genetic algorithm can double the microwave field strength relative to when the mirror surface is flat. An increase in the volume and brightness of the plasma fluorescence accompanies the increase in microwave radiation, implying an improvement in the laser beam intensity profile through the filamentation region due to the optimized wavefront.

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