Impact of the dipole contribution on the terahertz emission of air-based plasma induced by tightly focused femtosecond laser pulses

The present paper studies the generation mechanism of terahertz (THz) radiation from tightly focused femtosecond laser pulses in a gas medium. We measured the angular radiation pattern under different focusing conditions and observed that, with the deepening of focus, the angular radiation pattern changes and optical-to-THz conversion efficiency increases. The analysis of the observed phenomena led to the assumption that the dipole radiation prevails in most cases despite the existing conception regarding the dominating role of the quadrupole mechanism of radiation. Based on these assumptions, the transient photocurrent theory of the phenomenon presented in this paper was developed by us and used for the numerical fit of the experimental data.

Figure 1

Schematic presentation of the experimental setup used to study the angular radiation pattern (top view): a laser beam is focused with a lens into ambient air. A pair of off-axis parabolic mirrors (PM1, PM2) positioned at angle $\theta$ from the optical beam axis collimates and refocuses the THz radiation into the window of the Si bolometer. The angle $\theta$ can be continuously varied in the range $[-{90}^{\circ },{90}^{\circ }]$.

Figure 2

Side view CCD images of photo-induced plasma fluorescence taken under different focusing conditions. Measured length $L$ and radii $a$ of the filaments are indicated in Table 1.

Figure 3

Measurements of angular distribution $dW/d\mathrm{\Omega }$ of THz radiation for different focusing conditions (green dots), theoretical fit [red (gray) line]), and theoretical fit corrected by instrumental function (black line); focal lengths $F$ are indicated on the plots (see Table 1 for the parameters of the fit): (a) $F=100$ mm, (b) $F=50$ mm, (c) $F=40$ mm, (d) $F=25$ mm, (e) $F=7$ mm, and (f) $F=4.6$ mm.

Figure 4

Angle ${\theta }_{max}$ for which the maximum of the THz intensity is observed vs focal length.

Figure 5

Black circles: Efficiency of the optical-to-THz energy conversion versus focal length. Red crosses: Contribution of the dipole current to the THz signal. Red boxes: Contribution of the quadrupole current to the THz signal.

Figure 6

Angular patterns of $dW/d\mathrm{\Omega }$ normalized over their maximums for $a=c\tau$ and various ratios $L/c\tau$: (a) $0.4$, (b) 1, (c) 2, and (d) 8. Solid blue line: dipole radiation due to $I_1$. Black dashed line: quadrupole radiation due to $I_2$.

Figure 7

Comparison of experimentally measured total energy $dW$ radiated in solid angle $d\mathrm{\Omega }$ with the numerical fits: No. 1 (solid line) and No. 2 (dashed line) for focal lengths $F=7$ mm (a) and $F=4.6$ mm (b).

Figure 8

(a) Scheme showing the decrease in efficiency of THz radiation collection for wide emission cones. (b) Values of the correction coefficient for the experimental parameters used.