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12THz-O-1
THz emission spectra produced by filamentation of single-color IR and UV laser pulses
A. Ionin1, O. Kosareva1,2, Y. Mityagin3, D. Mokrousova1, N. Panov1,2, G. Rizaev1, S. Savinov3, L. Seleznev1, D. Shipilo1,2
1P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Division of Quantum Radiophysics, Moscow, Russian Federation
2Lomonosov Moscow State University, Faculty of Physics and International Laser Center, Moscow, Russian Federation
3P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Division of Solid State Physics, Moscow, Russian Federation
We report on the observation of THz radiation spectra emitted by a single-color infrared (IR) and ultraviolet (UV) filaments. We use Ti: sapphire laser system pulse at the central wavelength of 740 nm and the third harmonic at 248 nm. The pulse duration is about 90 fs. The laser pulse is focused by the lens with the focal length of 7 cm. The THz radiation is directed to the Fourier interferometer and registered by superconducting NbN hot-electron bolometer. For both wavelengths the terahertz radiation is emitted into the cones. The maximum terahertz signal for the IR filament is stronger than for the UV one by more than an order of magnitude. Observed terahertz radiation spectra are shown in Figs.1a and 1b for the IR and UV filaments, respectively.
In both IR and UV pulse cases, the spectra on the optical axis (angle is equal to zero) have their longer wavelength components enhanced as compared with the spectra of the pulses propagating in the off-axis direction. These off-axis spectra are similar to each other. At all the directions studied the terahertz radiation spectra for IR pulses are spread towards 5-6 THz in contrast to the spectra of the UV pulse which extend towards ~2.5 THz only.
Possible explanation to this phenomenon is that the IR filament is characterized by the higher intensity and plasma density as well as the smaller transverse size as compared to the UV filament. This results in the higher transverse gradient of intensity and plasma density in the IR filament. Therefore, the ponderomotive electron acceleration and photo current in the IR filament are higher than in the UV one. Consequently, the IR filament should deliver higher THz signal and larger spectral broadening than the UV filament. This is in agreement with our experimental observation.
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Fig. 1. Terahertz radiation spectra formed in the IR (a) and UV (b) filament observed under different angles.
