V. A. Andreeva, O. G. Kosareva, N. A. Panov, D. E. Shipilo, P. M. Solyankin, M. N. Esaulkov, P. González de Alaiza Martinez, A. P. Shkurinov, V. A. Makarov, L. Bergé, and S. L. Chin
https://journals.aps.org/prl/accepted/4507aYb8Wf411d4d98f601634b2a9b1dd8b3ddd05
We have solved the long standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal that contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt downshift towards near the plasma frequency. Ringshaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.
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