Highly Efficient High-Harmonic Generation in an Orthogonally Polarized Two-Color Laser Field

Highly efficient high-harmonic generation was achieved in helium using a two-color laser field that consisted of the fundamental and the second harmonic fields of a femtosecond Ti:sapphire laser. By applying a high intensity second harmonic, the harmonics generated in the orthogonally polarized two-color field were stronger than those obtained in the fundamental field by more than 2 orders of magnitude, and even stronger than those of the parallel polarization case. A conversion efficiency as high as $5\times {10}^{-5}$ was obtained for the 38th harmonic at 21.6 nm. The physical origin of this enhancement was deduced by analyzing the electron behavior in the two-color field.

Figure 1

Experimental setup for high-harmonic generation driven by a two-color laser field. The polarizations of fundamental ($\omega$) and second harmonic ($2\omega$) fields were controlled using the wave plate. For control of the relative phase between the fundamental and the SH fields, a $150\mu \mathrm{m}$ thick glass plate was inserted.

Figure 2

Harmonic spectra from He generated in the fundamental ($\omega$), second harmonic ($2\omega$), and two-color ($\omega +2\omega$) laser fields. In the two-color field, the cases of parallel and orthogonal polarizations are given.

Figure 3

(a) Position expectation value of the electron under orthogonally polarized two-color laser field. The inset shows the Lissajous diagram of the electric field ($I_{\omega }=5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, $I_{2\omega }=8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$) A, B, C, and D correspond to 0.00, 0.05, 0.16, and 0.25 optical cycles, respectively. The points B and C correspond to the ionization times for the highest-order harmonics generation in the fundamental and the two-color fields, respectively. (b) Tunneling ionization rate and traveling time of the electron in the fundamental field and orthogonally polarized two-color field.

Figure 4

Modulation of $2(2n+1)$th order harmonics accompanied by controlling the relative phase in a two-color laser field. This relative phase between fundamental and SH fields was changed by rotating the glass plate shown in Fig. 1. (a) Orthogonal polarization case, and (b) parallel polarization case.