紫外飞秒激光泵浦氮气离子相干辐射

氮气分子在不同波长（中红外、近红外、紫外）强场飞秒激光的泵浦下，其分子离子在传播前向能够发出具有良好相干性的可见光波段的窄带辐射。在400 nm紫外飞秒激光的激发下，波长为428 nm和423 nm的相干辐射受到的关注较少，物理性质尚不明确。本研究对该辐射的偏振性质、气压和泵浦激光能量依赖关系进行了系统的测量。实验发现，该辐射的偏振与线偏振泵浦激光的偏振态保持一致，辐射强度随着气压和泵浦激光能量呈现出非线性的增加。利用基于密度矩阵的强场电离和能级耦合模型，对氮气分子在强场中的电离和相关离子能级在强场作用下的耦合进行了数值模拟研究。结果表明，在较大的激光强度范围内，氮气离子上能级$ {\mathrm{B}}^{2}{\mathrm{\Sigma }}_{\mathrm{u}}^{+} $和其离子基态$ {\mathrm{X}}^{2}{\mathrm{\Sigma }}_{\mathrm{g}}^{+} $之间，对应428 nm和423 nm的振动态之间总是能够形成粒子数反转，而且该反转对于激光参数具有鲁棒性，与实验观测结果一致。

Coherent emission of nitrogen ions pumped by ultraviolet femtosecond laser pulses

Nitrogen ions emit narrowband coherent emission under the pump of intense femtosecond laser pulses of different wavelengths (mid-infrared, near-infrared, or ultraviolet). The 428 and 423 nm radiation of the nitrogen ions obtained by excitation with 400 nm femtosecond laser pulses has received less attention and their properties are unknown. In this study, the polarization of the 428 nm emission and its dependence on the nitrogen gas pressure and the pump laser energy are systematically measured. It is found that the polarization of the 428 nm emission is the same as the linearly polarized pump pulses. Moreover, the radiation signal presents nonlinear dependence on the gas pressure and the pump pulse energy. Based on the modeling of the strong-field ionization and the coupling of different energy levels of nitrogen ions in presence of the laser field, the population distribution of the nitrogen ions is simulated numerically. It is revealed that population inversion between the relevant energy levels of the upper level state $ {\mathrm{B}}^{2}{\mathrm{\Sigma }}_{\mathrm{u}}^{+} $and the lower $ {\mathrm{X}}^{2}{\mathrm{\Sigma }}_{\mathrm{g}}^{+} $level can be robustly established for a relatively large range of pump laser intensity, which agrees with the experimental observation.