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0Frequency-angular properties of terahertz emission during single-color filamentation
L. Seleznev1'2*, G. Rizaev1'2, D. Pushkarev1'2
1-P.N.Lebedev Physical institute of RAS, Russia 119991 Moscow Leninskii pr. 53
2- Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia
* seleznev@lebedev.ru
Propagation of a femtosecond laser pulse with overcritical power through transparent medium results in self-focusing and plasma channels formation (process of filamentation [1]). Plasma formed during filamentation of laser pulses is one of terahertz radiation sources [2]. The simplest scheme for terahertz generation is single-color filamentation in air. However the mechanisms and properties of the terahertz emission from single-color filament have been poorly studied due to relatively low efficiency.
In our experiments we used laser pulses with 740 nm wavelength, 90 fs duration and energy up to 5 mJ. To create filament plasma focusing elements with different numerical apertures were used. We detected terahertz emission with a bolometer sensitive in the range of 0.1-12 THz. To distinguish various frequencies we set narrowband terahertz filters in front of the bolometer input window. By moving the bolometer around the plasma channel, we measured two-dimensional angular distributions of terahertz radiation.
-30 -20 -10 0 10 20 30 -24 -16 -8 0 8 16 24 -12 -8 -4 0 4 8 12
Horizontal angle (deg.)
Fig. 1. Normalized two-dimensional angular distributions of terahertz radiation at frequency of 0.3 THz (a), 1 THz (b) and 10 THz (c).
The pulse energy is 3 mJ, the numerical aperture is 0.02.
For example, Fig. 1 shows normalized two-dimensional angular distributions of terahertz radiation at different frequencies. At a frequency of 0.3 THz (Fig. 1a), terahertz radiation propagates into a cone with a minimum on the axis. At a frequency of 1 THz (Fig. 1b), the pattern structure takes the form of two maxima located on the axis perpendicular to laser polarization. At higher frequencies the cone-shaped structure is restored again. Figure 1c shows the distribution at 10 THz.
It should be noted that the propagation angles of terahertz radiation at different frequencies differ significantly and for low frequencies can reach more than 30 degrees. In addition to information about the propagation structure, energy characteristics of terahertz radiation can be obtained from two-dimensional patterns by integrating signals over the distribution. This approach allows to take into account terahertz radiation propagating at wide angles. Thus, we investigated terahertz patterns, spectral and energy characteristics of terahertz emission under various laser parameters such as wavelength, pulse duration, numerical aperture and energy.
The work is supported by Russian Science Foundation grant #24-19-00461.
[1] A. Couairon, A. Myzyrowicz, Phys. Reports 441, 47 (2007).
[2] W. Sun, X. Wang, Y. Zhang, Opto-Electronic Science 1, 220003 (2022).
