Conical Forward THz Emission from Femtosecond-Laser-Beam Filamentation in Air

We attribute a strong forward directed THz emission from femtosecond laser filaments in air to a transition-Cherenkov emission from the plasma space charge moving behind the ionization front at light velocity. Distant targets can be easily irradiated by this new source of THz radiation.

Figure 1

Comparison between the radial (left side) and forward (right side) THz emission from a filament. Detection of the radial emission requires a reduction by 1 order of magnitude of the distance between filament and detector.

Figure 2

Experimental setup (schematic).

Figure 3

Polarization of the THz forward emission measured at $\theta =10°$ with a focal lens of 75 cm, obtained by rotating a grid linear polarizer in front of the heterodyne detector. The heterodyne detector itself acts also as a linear polarizer. The THz emission yields a squared Malus Law: $I\propto {sin}^4(\theta )$ (dotted line), indicating that the polarization of the THz signal is perpendicular to the emission cone surface. (b) Recapitulates the polarization diagram of the THz emission.

Figure 4

Emission diagrams $I(\theta )$ for the THz signal obtained with different focal lengths for the laser. Upper curves: experimental results. Lower curves: calculations. The filament plasma length $L$ ($f=2000$ and $f=750\mathrm{mm}$) is obtained from direct measurement. Simulations yield similar values. Relative THz intensities are indicated for each diagram.