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Excitation of the isomeric nuclear state $^{229\mathrm{m}}\mathrm{Th}$ in the process of thorium atom irradiation by a two-color femtosecond pulse of a Ti:Sa laser at the fundamental wavelength and second harmonic ($\ensuremath{\omega}+2\ensuremath{\omega}$) is analyzed. It is shown that the rate of isomeric state excitation can be enhanced significantly with respect to other nucleus excitation processes in laser plasma or by an external coherent source at the resonance wavelength. This enhancement is due to the process of ``nonlinear laser nuclear excitation,'' which consists in excitation of a nuclear transition by e.m. radiation, generated by an electronic shell of the atom interacting with an intense laser field, with the photon energy coinciding with the nuclear transition energy. The process is based on the following. First, the atomic current associated with the motion of valence electrons in a strong $\ensuremath{\omega}+2\ensuremath{\omega}$ field is a nonlinear function of the field amplitude. Second, the energy of photons at the frequency of the Ti:Sa laser fifth harmonic coincides with the energy of the $^{229\mathrm{m}}\mathrm{Th}$ state. The yield of this harmonic is much higher if the two-color pulse (first and second harmonics of the fundamental) is used. Third, the intensity of the field produced by valence electrons at the atomic nucleus reaches the near-atomic value, which exceeds significantly the field strength associated with any other mechanism of laser plasma emission.

International audience; We demonstrate experimentally the control of the polarization (ellipticity and polarization axis) of femtosecond high-order harmonics. The method relies on a two-color (omega - 2 omega) configuration, where both omega and 2 omega generating fields have linear polarizations with a variable crossing angle. We correlate our measurements with the conservation rules of energy and linear momentum accounting for harmonic generation in a two-color field. We evidence that the generation process, and especially the number of omega and 2 omega photons absorbed for generating a given harmonic, strongly depends on the crossing angle. Finally, relying on a simple formalism, we derive analytical formulae for calculating both polarization axis and ellipticity of the separate harmonics. The model corroborates our results and represents a base for future investigations.

International audience; Recently several techniques demonstrated the production of elliptically polarized high harmonics. One of these techniques consists of the interaction in a noble gas of two-color laser beams having orthogonal linear polarizations. Here we present the theoretical explanation of such a result observed in Lambert et al. [Nat. Commun. 6, 6167 (2015)]. Numerical calculations based on the nonperturbative light-atom interaction theory reproduce well the experimental data. The degree of polarization is analyzed for different harmonic orders and found to be high. With the help of a simplified theoretical model it is shown that the degree of harmonic ellipticity depends mainly on the population of atomic state sublevels with different angular momentum projections.

We present an interpretation of the terahertz (THz) generation in atomic gases interacting with two-color laser fields. Within the presented interpretation the process of the THz generation is associated with purely intra-atomic nonlinearity and the energy of emission is determined by the work (executed by the laser field) on the atomic electron motion in the nonharmonic intra-atomic field. To illustrate the specific features of this generation mechanism we present the results of numerical simulations on an argon atom interaction with a two-color laser field formed by the arbitrary polarized pulses at fundamental frequency and second harmonic of Ti:sapphire laser. The intensity of the laser field has been limited by ${I}_{0}l6.77\ifmmode\times\else\texttimes\fi{}{10}^{12}\frac{W}{{\mathrm{cm}}^{2}}$ (for $\ensuremath{\lambda}=800$ nm) that enables us to model the ionization-free regime of the light-atom interaction. It is demonstrated that the spectrum of the THz emission, integral efficiency of generation, and polarization state of spectral components strongly depend on the angle between the polarization vectors of laser field components. So, the variation of this angle provides the effective method of the THz generation control.