| 摘要: | 過去五十年來，非線性光學帶動了光通訊、雷射光譜學、材料處理、超快光學等領域的突破，但受到材料穿透率的限制，一般僅能用於可見光至紅外光。今天，高強度超快極紫外光(EUV)光源已有極大的進展，例如極紫外光自由電子雷射或是高階諧波產生，若能運用這些光源將非線性光學技術推進至極紫外光波段，將可帶動非線性光學的全新突破。我們計畫採用高游離態離子電漿作為EUV光之非線性混波介質。藉由將高強度近紅外光(NIR)脈衝聚焦於惰性氣體噴流，在其前端把惰性氣體原子游離至適當的游離態，而後即可透過這些離子與EUV光進行混波。若剩餘之束縛態電子的游離能高於EUV光子能量，則光子游離不會發生，EUV光的吸收就可以大幅抑制，而剩餘之束縛態電子則可以提供混波所需要的非線性響應。我們計畫實現兩種不同的非線性四波混頻，第一種是1顆EUV光子加上2顆NIR光子轉換至另1顆EUV光子，使用Ar2+離子為作用介質，預期轉換效率可達26%。第二種是2顆EUV光子加上1顆NIR光子轉換至第三個EUV光子，使用Ne2+離子為作用介質，預期轉換效率可達24%。我們相信使用離子為作用介質是探索EUV非線性光學的全新方向。 ;The development of nonlinear optics has made remarkable success in many laser-related fields in the past five decades, such as optical communication, laser spectroscopy, material processing, ultrafast optics, high-field physics, etc. Limited by the transmittance of materials and laser sources, the spectral range of nonlinear optics has been largely confined from near-infrared (NIR) to visible region. Today, several kinds of coherent extreme-ultraviolet (EUV) sources have been developed with ultrahigh intensities and ultrashort durations, such as EUV free electron laser and high-harmonic generation (HHG). Therefore, great advance may proceed if the EUV/x-ray can be implemented in the existing nonlinear optical technology.However, in EUV region, it is difficult to excite nonlinear interaction due to the fact that most materials absorb EUV waves strongly. In this proposal, we plan to use highly charged ion plasma produced in a gas jet as the interacting medium for EUV nonlinear mixing. The plasma is prepared by an intense NIR pulse focused onto a rare gas jet. The front edge of the intense NIR pulse ionizes the gas atoms into a highly charged ion plasma through strong field ionization. If the ionization potentials of the remaining electrons in the ion are larger than the EUV photon energy, photoionization will not occur and absorption of the EUV wave can be greatly reduced. Then the nonlinear response of the remaining bound electrons in the ions facilitates the nonlinear wave-mixing of EUV and NIR pulses.Since the third-order nonlinearity is the dominant nonlinear response for such kind of isotropic system, we focus on the four-wave-mixing (FWM) process. We plan to accomplish two kinds of FWM. The first one is the sum-frequency mixing of one EUV photon plus two NIR photons to a second EUV photon. The second one is the sum-frequency mixing of two EUV photons plus one NIR photon to a third EUV photon. For the first process, we propose to use Ar2+ ions as the interacting 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 the second process, we propose to use Ne2+ ions as the interacting medium. A conversion efficiency of 24% is expected by using one 1-μJ EUV pulse and one 21-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 EUV frequency conversion, but also can be used for EUV waveform characterization and waveform control. We believe using ion plasmas as the interacting media is a promising new direction to explore for the advancement of EUV nonlinear optics. |
