Extreme-Ultraviolet Nonlinear Wave Mixing in Highly Charged Ion Plasmas( II )

The development of nonlinear optics has made remarkable success in manylaser-related fields in the past five decades, such as optical communication, laserspectroscopy, material processing, ultrafast optics, high-field physics, etc. Limitedby the transmittance of materials and laser sources, the spectral range ofnonlinear optics has been largely confined from near-infrared (NIR) to visibleregion. Today, several kinds of coherent extreme-ultraviolet (EUV) sources havebeen developed with ultrahigh intensities and ultrashort durations, such as EUVfree electron laser and high-harmonic generation (HHG). Therefore, greatadvance may proceed if the EUV/x-ray can be implemented in the existingnonlinear optical technology.However, in EUV region, it is difficult to excite nonlinear interaction due to the factthat most materials absorb EUV waves strongly. In this proposal, we plan to usehighly charged ion plasma produced in a gas jet as the interacting medium forEUV nonlinear mixing. The plasma is prepared by an intense NIR pulse focusedonto a rare gas jet. The front edge of the intense NIR pulse ionizes the gas atomsinto a highly charged ion plasma through strong field ionization. If the ionizationpotentials of the remaining electrons in the ion are larger than the EUV photonenergy, photoionization will not occur and absorption of the EUV wave can begreatly reduced. Then the nonlinear response of the remaining bound electrons inthe ions facilitates the nonlinear wave-mixing of EUV and NIR pulses.Since the third-order nonlinearity is the dominant nonlinear response for suchkind of isotropic system, we focus on the four-wave-mixing (FWM) process. Weplan to accomplish two kinds of FWM. The first one is the sum-frequency mixingof one EUV photon plus two NIR photons to a second EUV photon. The secondone is the sum-frequency mixing of two EUV photons plus one NIR photon to athird EUV photon. For the first process, we propose to use Ar2+ ions as theinteracting medium. A conversion efficiency of 26% is expected by using one 1-μJ EUV pulse and one 16-mJ NIR pulse mixed in a 2-mm Ar gas jet. For thesecond process, we propose to use Ne2+ ions as the interacting medium. Aconversion efficiency of 24% is expected by using one 1-μJ EUV pulse and one21-mJ NIR pulse mixed in a 1-mm Ne gas jet.For the prospect of such kind of interaction, it not only can be applied for EUVfrequency conversion, but also can be used for EUV waveform characterizationand waveform control. We believe using ion plasmas as the interacting media is apromising new direction to explore for the advancement of EUV nonlinear optics.