CC BY
3*
ALT'23
The 30th International Conference on Advanced Laser Technologies
N-O-2
Terahertz generation from a single-color filament: ponderomotive
force versus light pressure
I.A. Nikolaeva1'2, D.E. Shipilo1'2, N.A. Panov1'2, O.G. Kosareva1'2
1-Faculty of Physics, Lomonosov Moscow State University, Moscow, Russia 2-P.N. Lebedev Physical Institute of the RAS, Moscow, Russia nikolaevaia@lebedev. ru
Terahertz (THz) emission of a single-color femtosecond filament is presumed to origin from the free electrons motion driven either by ponderomotive force that pushes the electrons out of the filament core thereby providing a quadruple source [1] or by the light pressure force that pushes plasma electrons along the beam axis thereby producing a longitudinal dipole source [2]. Both models of the local plasma response under the interference integral are able to successfully reproduce the conical spatial shape of the far-field distribution of THz radiation [3-6]. However, none of these models reproduces the THz spatial pattern at ~1 THz that was observed in 2D measurements [5,6], i.e. the two lobes separated by the driving laser pulse polarization plane and having the polarization almost orthogonal to the laser one.
In this work, we analyze the models of the filament plasma oscillations [7,8] so as to elucidate the quantitative relation between the ponderomotive and the light pressure sources. We show that the destructive interference of the THz waves driven by these sources can provide the two-lobe pattern observed in [5,6] on the condition of the >n/2 phase shift between the magnetic dipole and quadruple contribution to the current. We note that the n/2 phase shift between the two sources corresponds to a ~250 fs delay for the frequency of ~1 THz. Since both sources develop within the laser pulse duration of <100 fs, such a phase shift should origin from the propagation effects including the plasma refraction. The simulations of such an effect require vectorial propagation equation in (t, x, y, z) geometry.
For the simulations we used Unidirectional Hertz vector propagation equation (UHPE, [9]) with the crossed domains for optical and terahertz radiation (XDOT, [10]) scheme. The latter allows us to simulate the propagation of the optical pulse on the moderate size numerical grids in the scalar axially-symmetric approximation using the Unidirectional pulse propagation equation (UPPE, [11]). The THz field is simulated with the account for all three vectorial components. Figure 1 shows the broken symmetry of the THz far field conical ring for the two selected phase shifts between the dipole and quadruple contributions to the free electron current. In Fig. 1(b) the THz radiation reveals the two lobes and is polarized orthogonally to the laser pulse polarization direction in agreement with [5,6].
Fig. 1. Angular distribution of ~1 THz radiation emitted from single-color filament simulated using interference model with both ponderomotive force and light pressure accounted as emission sources. The laser polarization is horizontal. The results of simulations without (a) and with (b) artificial phase factor. The latter one (b) closely represents the experiments [5,6].
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