Photoionization of an ambient gas by a tightly focused, femtosecond, weakly relativistic laser pulse leaves behind

the pulse a column of electron density (a “filament”). At the column surface, the density drops to zero within a thin (micronscale) boundary layer. Ponderomotive force of the pulse drives within the filament a cylindrical wave of charge separation (laser wake). If the pulse waist size is much smaller than the Langmuir wavelength, electron current in the wake is mostly transverse. In the filament surface area, this current rapidly decays (electrons, crossing the sharp density gradient, phase out of wake within a few Langmuir oscillation cycles.) Coupling electron wake velocity to the sharp radial density gradient generates at the filament surface a short-lived, almost aperiodic rotational current. This current serves as a source for a broadband THz electromagnetic pulse co-moving with the wake. As long as the background gas is uniform, the wake phase velocity is slightly subluminal, and the

THz pulse is evanescent in the radial direction. The evanescence is, however, slow, occurring on a millimeter to centimeter length scale. Properties of the evanescent THz pulse contain information on the wake currents, and may thus serve as optical diagnostics.