CC BY
9THz-I-5
Directional Diagram of THz radiation from DC-Biased Filament
O. Kosareva12*, N. Panov12, I. Nikolaeva12, D. Shipilo12, D. Pushkarev2, G. Rizaev2, D. Mokrousova1,2, A. Koribut2, YA. Grudtsin1,2, L. Seleznev1,2, W. Liu3, A. Shkurinov14, A.
Ionin2
1 Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia 2P. N. Lebedev Physical Institute of the Russian Academy of Sciences, 53 Leninskiy prospect, Moscow 119991,
Russia
3Institute of Modern Optics, Tianjin Key Laboratory of Micro-Scale Optical Information Science and Technology, Nankai University, Tianjin 300350, China 4ILITRAS—Branch of the FSRC "Crystallography and Photonics" RAS, Svyatoozerskaya 1, 140700, Shatura,
Moscow Region, Russia
kosareva@physics. msu.ru
Terahertz generation from an air-plasma source is valuable for its broadband spectrum and scalable energy [1]. Remote generation of THz radiation is possible by frequency doubling locally and propagating the two optical beams in free space where they focus and mix at a distance to generate THz radiation. In this two-color plasma channel we have up to 50 THz spectrum with the ring-shaped directional diagram or conical emission [2]. The far-field THz beam profile in the low-frequency (< 3 THz) spectral range, which is important for explosives detection [3], has been suggested to have Gaussian shape instead of a conical one [4]. The directional diagram with on-axis maximum was observed from a DC-biased filament at 0.1 THz by Houard et al [5] and at 0.5 THz by Fukuda et al [6].
These observations raised the question if the far-field distribution of THz radiation from DC-biased filament preserves the on-axis maximum at the frequencies of ~1 THz and higher. Previous measurements showed angular distribution at a single frequency [5,6]. In our experiment THz angular distribution from a DC-biased (744 nm, 0.5 mJ, 90 fs) filament has a flat-top shape at all the selected frequencies v = 0.3, 0.5, 1 THz in the same conditions. Although a THz generation from a DC-biased filament has been studied since 2000 [7], no one has ever shown that the plasma refraction almost does not influence the THz low-frequency angular distribution. At higher frequencies the destructive interference of THz waves from the ionization front propagating with the superluminal velocity ensures the on-axis minima. Transition from the flat-top to conical angular distribution is explicitly shown in our manuscript due to 3D+time simulations based on Unidirectional Pulse Propagation Equation with unprecedently fine 0.01 THz resolution.
Funding. Russian Science Foundation (grant 21-49-00023), and National Natural Science Foundation of China (grant 12061131010).
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