強雷射脈衝導致非線性原子激發之Fourier-Floquet分析

Abstract

近年來由強飛秒雷射所產生的現象已受到理論上和實驗上的關注。當一個分子受到強飛秒雷射的激發時，所發射出來的螢光訊號已被證實是來自於中性的分子碎片，此被稱為"中性分裂"的現象被視為可以發展雷射應用的重要關鍵。造成此現象的機制或許可被解釋成以下：經由雷射的非線性激發，分子先處於高激發態，然後再經由解離變成許多電子激發態的中性分子碎片，最後散發出螢光訊號。儘管我們了解此現象的重要性，但是對於其中的高非線性激發的理論機制還未被完整的了解。 在此論文中，我們選取氫原子作為模擬系統並應用Fourier-Floquet光譜去檢視高非線性激發的理論機制，經由以上方法我們成功了提出相同奇偶的角動量最終激發態擁有相同的激發機制。另外，一般被視為只能適用於弱微擾場下的"Ramsey fringe"也被我們證實了在高非線性激發過程也是適用的。

The atomic and molecular processes induced by intense (~1014 W/cm2) femto-second (10-15 s) lasers have attracted significant attention, both theoretical and experimental. When an intense femto-second laser is irradiated to a molecule, fluorescence signals are emitted from neutral fragments of the molecule. This phenomenon, called the neutral fragmentation, is expected to open new possibilities in laser technologies such as remote sensing and remote lasing. The mechanism may be understood as: (1) highly nonlinear excitation of the molecule, (2) the dissociation of the excited state molecule into electronically excited neutral fragments, (3) fluorescence from the neutral fragments. In spite of its importance, the mechanism of the highly nonlinear excitation has not been well understood. In this thesis, we take hydrogen atom as an example to examine the mechanism of the highly nonlinear excitation using the Fourier-Floquet spectra. We succeeded in proposing the detailed mechanism, which we find different depending on the parity of the angular momentum of the final state. Furthermore we have shown that the Ramsey fringe, which is believed only applicable in the perturbatively weak field, works nicely in highly nonlinear processes.