Physics of Natural Laser in Air and Other Gases

Fig. (left) Forward spectrum of air induced by femtosecond laser filamentation with the pump laser at 1906 nm, and (right) spatial profile of the laser emission at 391 nm.

In environmental science, it is desirable to generate coherent light sources with different frequencies at a designated position in air, especially for measuring atmospheric trace species and monitoring global warming and stratospheric ozone depletion as well as early-warning of biological and nuclear power plant leakages, which are threats to public and defense security. A collaborative study among professors See-Leang Chin of Laval University, Canada, Huai-Liang Xu of Jilin University, China, Zhi-Zhan Xu and Ya Cheng of SIOM, China, and Kaoru Yamanouchi of the University of Tokyo, Japan, revealed the physical mechanism responsible for the remote laser in air. They proposed a new universal scheme responsible for the population inversion of molecular nitrogen ions and also in other similar gaseous molecular ions. This study was published in Chinese Optics Letters, issue 1, 2013 (COL2013, 11(1),013201). The first realization of remote tunable multi-wavelength laser in air, principally through the excitation of molecular nitrogen ions inside a filament in air by using mid-infrared femtosecond laser pulses, was reported in 2011; however, the physical mechanism was still ambiguous.

Remote laser in air is mainly based on the light amplification with air molecules serving as the gain medium. In the reported method, amplification of the seed in the air (N₂⁺) medium was readily achieved by using the self-generated harmonics of the infrared femtosecond pump laser as the seed during filamentation of the pump. However, such a process requires population inversion in the molecular nitrogen ions to be built up within a femtosecond time scale because the self-generated harmonic has an ultrashort pulse duration comparable to the pump laser.

In the above mentioned experiments in air, the starting neutral gas molecules are all in the ground state. Generally speaking, population inversion in the nitrogen molecular ion could not be built up by directly exciting the ground neutral nitrogen molecules through highly nonlinear processes, such as multiphoton or tunnel ionization. That is to say, the ground state of molecular nitrogen ions should always be more populated than the excited state. However, inside an air filament of the pump pulse, the intensity is very high, of the order of 1014 W/cm². This clamped intensity is reached within the ultrafast pulse duration while filamentation and ionization take place. Thus, the ground molecular ions would experience the high intensity inside the filament once it is created. Since the energy difference between the electronically excited states of N₂⁺ and the ground ionic state is a few times smaller than the ionization potential of the neutral molecule, N₂⁺ prepared in the ground state would be ↑ immediately |- further pumped into the excited ionic states through lower order multiphoton processes with very high efficiency. This would immediately result in population inversion in the molecular ions.

In principle, any molecular species having similar characteristics would exhibit population inversion. Indeed, this was demonstrated by the SIOM and Jilin University groups in carbon dioxide gas. It is thus proposed that the formation of a naturally population-inverted system of many gaseous molecular ions would be a universal process in a femtosecond intense laser filament. This study provides a new strategy for the generation of remote laser emissions, opening up potential applications of this technique in different research fields.

空气和其他气体中的自然激光发射

1906 nm抽运光通过飞秒光丝诱导的空气前向光谱(左)和391 nm激射的空间分布图(右)。

在环境科学中，发展长距离遥感技术以满足测量大气微痕粒子浓度，监测全球变暖、臭氧层耗尽以及早期生物和核工厂的核泄漏污染的预警等环境方面的要求，通常需要可在远程不同距离产生不同频率的可控相干光源。

最近，加拿大拉瓦尔大学陈瑞良教授、吉林大学徐淮良教授、中科院上海光机所徐至展院士与程亚研究员、日本东京大学山内薰教授研究了产生远程空气激光的物理机理，他们认为存在一个可实现N₂⁺以及其他分子离子粒子数反转的普遍规律。相关研究成果发表在Chinese Optics Letters 2013年第1期上(COL2013, 11(1),013201)。虽然研究人员之前已经利用中红外飞秒强激光在光丝中激发空气氮分子离子（N₂⁺），实现了远程多波长可调谐空气激光，但相关的物理机制仍然不是很清楚。

远程空气激光主要是利用空气分子作为增益介质实现光放大。实验过程中，他们利用红外飞秒激光光丝化过程中在空气(氮分子)介质中产生飞秒抽运光的奇次谐波作为种子源，实现光放大。但这种过程因为自产谐波的超快特性要求氮分子离子的粒子数反转建立的时间必须在飞秒量级。

在上述有关空气激光的实验研究中，气体分子的起始态都处在基态。通常来说，N₂⁺的粒子数布局反转很难通过非线性效应(如多光子或隧穿电离)直接从中性氮分子基态获得，这是由于在这些离化过程中N₂⁺基态粒子数总是高于激发态。然而，在飞秒光丝中，激光光丝钳制光强在很短的激光脉冲时间内达到1014W/cm²数量级，并伴随着分子的离化过程。因此，基态的分子离子一旦产生就处于激光光丝的强场中。从能量角度来说，N₂⁺的基态和激发态的能态差远小于中性分子离化势能；因而，处于飞秒光丝强场中的离子基态分子会立刻通过一个高效的低阶多光子过程被抽运到离子的激发态，实现粒子数反转。

他们认为，原则上任何其他分子离子的激发态与基态能级具有相似于N₂⁺能态结构均可产生粒子数反转。事实上，上述结论已被最近中科院上海光机所和吉林大学的一篇有关二氧化碳气体中的远程激光实验的论文加以证实。因此，在飞秒强激光光丝中，多气体分子离子的粒子数反转系统的自然形成应是一个普遍规律。

该研究提供了产生远程空气激光的新方法，并且该激光技术具有非常重要的潜在应用价值。