Abstract
We investigate emission rate and ellipticity of high-order harmonics generated exposing a homonuclear diatomic molecule, aligned in the laser-field polarization plane, to a strong orthogonally polarized two-color (OTC) laser field. The linearly polarized OTC-field components have frequencies rω and sω, where r and s are integers. Using the molecular strong-field approximation with dressed initial state and undressed final state, we calculate the harmonic emission rate and harmonic ellipticity for frequency ratios 1:2 and 1:3. The obtained quantities depend strongly on the relative phase between the laser-field components. We show that with the OTC field it is possible to generate elliptically polarized high-energy harmonics with high emission rate. To estimate the relative phase for which the emission rate is maximal we use the simple man’s model. In the harmonic spectra as a function of the molecular orientation there are two types of minima, one connected with the symmetry of the molecular orbital and the other one due to destructive interference between different contributions to the recombination matrix element, where we take into account that the electron can be ionized and recombine at the same or different atomic centers. We derive a condition for the interference minima. These minima are blurred in the OTC field except in the cases where the highest occupied molecular orbital is modeled using only s or only p orbitals in the linear combination of the atomic orbitals. This allows us to use the interference minima to assess which atomic orbitals are dominant in a particular molecular orbital. Finally, we show that the harmonic ellipticity, presented in false colors in the molecular-orientation angle vs. harmonic-order plane, can be large in particular regions of this plane. These regions are bounded by the curves determined by the condition that the harmonic ellipticity is approximately zero, which is determined by the minima of the T-matrix contributions parallel and perpendicular to the fundamental component of the OTC field.
Highlights
When atomic or molecular targets are exposed to a strong laser field many nonlinear processes can happen
The high-order harmonic emission rate and ellipticity strongly depend on the relative phase φ between the orthogonally polarized two-color (OTC) laser field components
To illustrate how the simple man’s model estimates the maximal harmonic order and the maximal emission rate, in Figure 5, we present the logarithm of the harmonic emission rate of the N2 molecule, obtained using the ω–3ω OTC laser field, as a function of the harmonic order and the relative phase
Summary
When atomic or molecular targets are exposed to a strong laser field many nonlinear processes can happen. For the orientation-averaged molecular medium exposed to the ω–2ω OTC laser field, the harmonics are linearly polarized due to the symmetry considerations This symmetry-based selection rule was analytically rigorously derived by Neufeld et al [24] using a group theory approach in a general manner that includes the full symmetry of the laser-matter system. Using aligned molecular targets and a strong ω–2ω OTC laser field, it is possible to generate elliptically polarized high harmonics. Polarized harmonics can be generated using atomic (or unaligned molecular) targets and other frequency ratios r:s, but it should be taken into account that it is difficult to obtain experimentally high laser intensity of the s ≥ 3 component. This paper is devoted to the HHG process by neutral homonuclear diatomic molecules driven by an ω–2ω or ω–3ω OTC field In this case, the emitted harmonics are elliptically polarized.
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