论文标题
飞秒强场离子激发和随后的激光的理论在n $ _2^+$中
Theory of femtosecond strong field ion excitation and subsequent lasing in N$_2^+$
论文作者
论文摘要
在最近的空气中，在空气中观察到了391和428 nm波长的延迟无腔向前激光，或在800〜nm的波长下用强烈的飞秒激光脉冲泵送的纯氮。负责激光的机制是高度争议的。在本文中，我们通过同时耦合的长寿命极化耦合x $^2σ_g^+$，向$^2π_U$和b $^2σ_U^+$单一离子化的氮分子n $ _2^+$。在强激光场中中性氮分子和随后的离子激发的电离由Bloch方程系统描述，该系统在激光脉冲末端提供了离子分布和激发状态A和B。 B-X跃迁波长处的延迟信号扩增由Maxwell-Bloch方程组的系统描述，具有较弱的激光后脉冲脉冲维持的极化耦合。确定了两个信号扩增的机制：低气压下的PS持续时间几个信号和高气压处的短（亚picsecond）信号。理论模型与实验结果相比有利。
Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800~nm. The mechanism responsible for the lasing is highly controversial. In this article we explain the delayed emission by the presence of long-lived polarizations coupling simultaneously ground state X$^2Σ_g^+$ to states A$^2Π_u$ and B$^2Σ_u^+$ of singly ionized nitrogen molecules N$_2^+$. Ionization of neutral nitrogen molecules in a strong laser field and subsequent ion excitation are described by a system of Bloch equations providing a distribution of ions in the ground and excited states A and B at the end of the laser pulse. The delayed signal amplification at the B-X transition wavelength is described by a system of Maxwell-Bloch equations with polarization coupling maintained by a weak laser post-pulse. Two regimes of signal amplification are identified: a signal of a few ps duration at low gas pressures and a short (sub-picosecond) signal at high gas pressures. The theoretical model compares favorably with experimental results.
