Enhanced extreme-ultraviolet-radiation generation via controllable spontaneous emission induced by two-color orthogonally polarized laser fields

We theoretically investigate the enhancement of extreme-ultraviolet-radiation (XUV) generation by orthogonally polarized laser-controlled spontaneous emission of Ar atoms. The three-dimensional time-dependent Schr"odinger equation is solved numerically using the generalized pseudospectral method. Applying an intense third-harmonic field of 1030-nm laser results in a series of spectral lines appearing in the harmonic spectrum between 13.5 and 15.8 eV. These lines correspond to field-free transitions between the ground state and $3{s}^{2}3{p}^{5}nd$ ($n=3,4,5,...$) excited states of argon. Introducing a weak multicycle fundamental laser pulse perpendicular to the third-harmonic field leads to a narrowband XUV emission centered at 14.15 eV, corresponding to the transition from the $3{s}^{2}3{p}^{5}3d$ excited state to the ground state. Combining population analysis with the synchrosqueezed transform time-frequency spectra, we attribute those emissions to free induction decay from the coherently excited states. The perpendicular fundamental field will cause the ionization of excited states, leading to the depletion of those states and the generation of excited states' high-order harmonics, which suppress the free induction decay emission. For higher excited states, the fundamental field alters the excitation pathways, significantly reducing state populations. The use of orthogonally polarized laser fields enables selective excitation of specific electronic states, enhancing the spectral purity. Our findings demonstrate the potential of orthogonal two-color laser pulses in generating bright, narrowband XUV radiation. This research provides new insights for developing XUV and soft x-ray light sources, offering promising applications in various fields requiring precise and intense XUV radiation.