銫原子在中程紅外強場雷射中產生低閾值與近閾值高次諧波機制之探究

Abstract

在這篇論文中，我們從非微擾的量子力學觀點，研究銫原子在中程紅外強場雷射中產生高次諧波(high-order harmonic generation, HHG)的現象。我們建立了一個和角動量量子數有關的模型勢能，精準地描述了銫原子的電子結構。我們以時變廣義擬譜方法(time-dependent generalized pseudospectral method, TDGPS)有效率且準確地求解了三維的時變薛丁格方程式。 除了預期的奇數高次諧波外，由於銫原子6s與np能階間的強耦合，我們得到的高次諧波頻譜顯示了一些額外的結構。經由傅立業(Fourier)及小波(wavelet)分析，我們更進一步探索了高次諧波的時間與光譜特性。藉此我們可以瞭解在不同能量範圍之高次諧波產生的主要機制，特別是低閾值與近閾值(below- and near-threshold)高次諧波的產生。

In this thesis, we present a non-perturbative quantum study of high-order harmonic generation (HHG) of Cesium atoms in intense mid-infrared laser pulses. An accurate angular-momentum--dependent model potential is constructed for the high-precision description of the Cs atom electronic structure. The three-dimensional time-dependent Schr'{o}dinger equation is solved accurately and efficiently by means of the time-dependent generalized pseudospectral (TDGPS) method. Besides the expected odd harmonics, the calculated HHG spectra show additional structures due to the strong $6s-np$ couplings. The spectral and temporal characteristics of the HHG are further explored through Fourier and the wavelet transformation. As a result, we can investigate the prevailing mechanisms in different energy regimes, especially those contributing to the generation of below- and near-threshold harmonics.