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9ALT'22
THz-I-3
NONLINEAR AND TERAHERTS PHOTONICS
Directional diagram of THz radiation from femtosecond filament in DC-biased and transition regimes
O.G. Kosareva12, N.A. Panov12, D.E. Shipilo12, I.A. Nikolaeva1-2, D.V. Pushkarev1,2, G.E. Rizaev2, D.V.Mokrousova12, A.V. Koribut2, Y.V. Grudtsynu, L.V. Seleznev12, A.A. Ionin2
1-Faculty of Physics, Lomonosov Moscow State University, 119991, Moscow, Russia 2-P.N. Lebedev Physical Institute of the RAS, 119991, Moscow, Russia kosareva@physics.msu.ru
The most efficient way to generate broadband THz radiation in air is a two-color (©-2©) filamentation [1]. At the same time, the THz radiation source based on the plasma channel biased by external electrostatic field (i.e. a single-color DC-biased filament) is free of the tedious ©-2© pulse delay adjustment [2]. Besides, the directional diagram of THz radiation from the DC-biased plasma channel is characterized by the on-axis maximum [3]. The decrease of the conversion efficiency in a single-color filament as compared with the two-color one is compensated by the unlimited energy scalability when using terawatt peak power femtosecond laser sources producing multiple plasma channels [4].
In this work, we study THz generation during filamentation in the external electrostatic field (DC-biased single-color filamentation) [5, 6]. Directional diagram, frequency content, threshold of the external electrostatic field, providing for the transition from the conical to the on-axis THz emission, are studied experimentally using 744 nm single-color plasma channel and simulated based on the unidirectional pulse propagation equation ensuring frequency content from 0.01 THz to 3 PHz and angular divergence till 60°. Special consideration is given to the transition from the light pressure dipole radiation source at sub-THz frequencies to quadrupole source at higher frequencies in the unbiased plasma channel (filament) regime. This transition is observed in the measured 2D frequency-resolved far-field distributions of THz emission, which evolves from the conical one at 0.1-0.5 THz to the two-lobe one at higher frequencies [6].
In the experiment [5] with the DC field of EDC = 10 kV/cm we focused the laser pulse (744 nm, 0.5 mJ, 90 fs) into the air gap between the plane electrodes. At the selected by the THz narrowband filters frequencies v below 1 THz, a
wide flat-top angular distribution was measured by a bolometer (Fig. 1). The simulations showed the transition of THz directional diagram from the flat-top to the conical one for v > 8 THz due to the destructive interference of THz waves from the ionization front propagating with the superluminal velocity. At lower or the absence of the DC field, the transition from conical to on-axis emission was observed [6,7]. Fig. 1. Angular distributions of THz fluence obtained for three frequencies in the experiment (symbols) and simulations based on unidirectional pulse propagation equation (UPPE) [8] (lines).
Exp. Simul.
□ 0.3 THz -
A 0.5 THz -
* 1.0 THz -
•10 0 10 Angle, deg
We acknowledge the support from Russian Science Foundation (21-49-00023) and National Natural Science Foundation of China (12061131010).
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