Terahertz radiation in two-color laser fields: from single atom to extended gas response
EPJ Web of Conferences
One of the promising methods of the THz radiation generation is the use of the two-color laser field focused into the gas media [1, 2] . The polarization of the low frequency THz field generated in gas is linear  , although, in some conditions, it is elliptical due to the modulation of the laser phase and its polarization in the gas plasma  . At the same time, for some practical applications, it is critical to have the elliptically polarized THz radiation  . For its generation certain
... generation certain methods [2, 5] mostly based on the mixing of the elliptically polarized laser fundamental radiation (ω) with linear (or elliptically polarized) second harmonic radiation (2ω) exist, the generated elliptical THz radiation lies in low frequency region (<5THz). Here, we demonstrate the method of generation of the highly elliptical high-frequency THz radiation (>25 THz) by the extended gas media interacting with the two-color laser field having linearly polarized components but with a nonzero angle between them . The consistent quantum-mechanical description is given. The two spatial levels are introduced. Initially, the time-dependent Schrodinger equation for single atom "placed" in a finite number of positions in the extended gas (i.e. being under the action of the twocolor laser field having various intensities and relative phases between the pulses ) is solved. Then, we calculate the response of extended system by changing the spatial parameters of the gas (mainly, the gas length, l) and the laser beam (mainly, beam width, σ)). For the calculation of the properties of the THz radiation emitted by a single atom we have used the non-perturbative theory [8, 9] and the model of the Ar atom described in [7, 10]. In simulations we assume that the components of the two-color laser field have the fixed intensity (I ω = 6.77×10 12 W•cm -2 and I 2ω = 5.85×10 11 W•cm -2 ), fixed temporal width of the laser field components (τ 1,2 = 30 fs), zero delay time between the pulses (Δt 1,2 =0) and relative phase equal to φ 0 =π/2. At these parameters of laser field the ionization-free regime of interaction occurs . In this way we obtain a series of photoemission spectra for different angles between the polarizations of two-color laser field components (θ) and different delay times between the pulses. Single atom numerical calculations demonstrate that at frequences >25 THz there are some regions of high ellipticity (up to circular), the maximum value of the ellipticity and the widths of these regions decrease along with an increase in the THz frequency  . Basing on the single-atom response we have calculated the response of the extended gas media. Previously we have used the formula (4) from  for describing the linearly polarized THz laser field spa-tial distribution in an aria around the gas. In that calculations we have summarized two projections onto perpendicular axes of THz fields emitted from individual atoms and have extracted total intensity of the THz radiation at given points of the area. In current calculations of the elliptically polarized THz field response each atom emits elliptically polarized radiation which could be expanded on two projections on the perpendicular axes Fig. 1 ). Fig. 1 Schematic view of the gas (gray rectangle) with atoms (black circles) emitting the THz field (red arrows indicate two projections). By the black solid arrows and black dashed arrows indicated the two projections of the field formed by the sum of projections of the single color field on the one axes. y denotes the rotation angle. By using formulae (4) from  two projections of the summarized field could be found at given point of the area for each projections ( x and y ) of the THz field separately (see Fig.1 with shown projections only for y component of the THz field, the other two projections for x component of the THz field lies in the perpendicular plane). As a result, two projections of the single atom THz field response are transformed at four ones when the problem of extended gas media response is under consideration. But the turning of two perpendicular axes on the angle can decrease the number of projections. Indeed, if we turn yz axes on the angle α, the projections of the THz field on the new yz axes become cos sin , sin cos .