空气激光:强场新效应和远程探测新技术 下载: 2979次内封面文章特邀综述
1 引 言
随着激光的出现[1],光与物质相互作用的研究进入了一个崭新的时代。光不仅可以作为信号的载体来探测物质的内部结构,而且能够改变甚至操纵物质的结构,如激光冷却原子[2]、激光操纵病毒和细菌[3]、激光焊接[4]等。近30年来,超强超短激光技术的迅速发展,进一步将光与物质相互作用的研究推进到了非微扰的强场范畴,极大地拓展了科学研究的参数区间,开启了激光科学的新篇章[5-6]。超强超短激光不仅会产生阈上电离[7]、非序列双电离[8]、高次谐波产生[9-10]等丰富的物理现象,而且在阿秒科学[11]、高能电子和离子加速[12-13]、微纳加工[14]、飞秒激光眼科手术[15]、大气遥感[16,17]等领域具有重要应用,推动了物理、化学、生物医学、环境科学等多个学科领域的发展。
近几年,空气激光作为一个新颖的强场超快现象,引起了国内外研究人员的关注。空气激光是指以空气的主要物质成分或其衍生物为增益介质,通过无腔放大的方式产生的具有高准直度、高相干性及高强度的辐射。与荧光辐射相比,空气激光沿特定方向传输,具有良好的空间指向性(即高准直度),并具有很高的空间相干性以及皮秒到纳秒量级的时间相干性,其强度比荧光信号高若干数量级。这些特征使其成为远程遥感技术的理想光源。空气激光的最早研究可追溯到1988年,当时,Vaulin等[18]利用高功率微波脉冲泵浦空气分子产生了激光辐射。然而,微波脉冲很容易发散,难以在空气中实现远距离传输。超强超短激光技术的发展,特别是啁啾脉冲放大技术的发明[19],使得激光强度有了前所未有的提升。峰值功率超过几个GW(1 GW=109 W)的飞秒激光在大气中非线性传输时可以克服自然衍射,产生一个内核直径为100 μm、峰值光强高达50~100 TW/cm2(1 TW=1012 W)的自导引通道,即飞秒光丝[20]。飞秒激光成丝现象的发现为远程空气激光研究提供了新机遇。2003年,Luo等[21]研究了飞秒激光在大气中成丝诱导的背向荧光辐射,发现背向氮气荧光辐射随着光丝长度的增加呈指数增长的趋势。他们将该现象归因于放大的自发辐射(ASE),并首次提出了空气激光的概念。2011年,Dogariu等[22]和Yao等[23]分别报道了紫外皮秒激光驱动的氧原子激光和中红外飞秒激光泵浦的氮分子离子激光,并从实验上证实了它的高强度和高准直度等特点。自此之后,空气激光的研究迅速升温,各类空气激光相继被报道[24-26]。
在强场超快条件下开展远程空气激光研究不仅对强场分子物理、超快非线性光学、量子光学等基础研究具有重要意义,而且为光学遥感提供了一条全新的技术途径。因此,空气激光迅速发展成为强场超快光学领域中的一个独立分支,国内外有近20个研究小组在开展相关研究。目前,人们不仅可以产生以氮气和氧气为增益介质的激光[21-23,27-32],而且探索了在稀有气体[33]、二氧化碳[34]、水[35,36]等空气中含量较低的介质中产生激光辐射的可能性。一方面,空气激光具有高准直度、高相干性和高强度等优点,在大气痕量污染物遥感、爆炸物远程探测、核泄漏预警等环境科学和**安全领域具有广阔的应用前景。因此,自远程空气激光被报道之后,国内外多个研究小组就对其遥感应用进行了探索[37-42]。另一方面,远程空气激光涉及强激光与空气分子多种物质形态(原子、分子、离子)的相互作用,涵盖电离、解离、光激发、多轨道作用、分子核波包运动等多个时间尺度的动力学过程,物理内涵丰富而新颖。因此,在空气激光的研究中,人们发现了很多新奇的强场物理、非线性光学和量子光学效应,为探索强激光与物质相互作用提供了新的视角[43-57]。
为了纪念激光器诞生60周年,本文综述了近几年来空气激光的主要研究进展。首先介绍了三类空气激光的产生方法和基本特征,然后重点阐述了氮气离子(
2 空气激光的产生途径
按照增益介质的类型,空气激光可以分为原子类、分子类、离子类三种。原子类空气激光主要以空气中含量较多的氮气或氧气分子解离产生的氮、氧原子以及空气中含量甚微的稀有气体为增益介质。这类激光通常采用紫外光泵浦,增益寿命较长,并具有双向传输特性。分子类激光主要是以空气中的氮气分子为增益介质,通过激发态氩原子或者热电子碰撞激发实现粒子数反转。由于氧气的淬灭作用,在空气中产生的氮分子激光信号通常较弱。离子类空气激光主要以光电离产生的氮分子离子作为增益介质,增益寿命较短,主要沿前向传输。这类激光的物理机制备受争议,已成为研究者关注的焦点。本章将介绍三类空气激光的产生方法以及各自的特点。
2.1 原子类空气激光
2011年,Dogariu等[22]报道了以氧原子为增益介质的空气激光现象。他们采用中心波长为226 nm的紫外皮秒激光脉冲激发空气分子,在前向和背向均观测到了波长为845 nm的激光信号,其产生机制如
图 1. 氧原子激光的产生机制与基本特性[22]。(a) 226 nm紫外皮秒激光驱动产生氧原子激光的示意图;(b)背向采集的氧原子激光光谱,插图为背向氧原子激光的远场光斑;(c)背向相干辐射与4π立体角积分的侧向非相干辐射的比较
Fig. 1. Generation mechanism and basic properties of oxygen atomic lasing[22]. (a) Schematic of oxygen atomic lasing pumped by a 226 nm ultraviolet picosecond laser; (b) the spectrum of oxygen atomic lasing measured from backward direction along the pump laser propagation path, and the inset shows the far-field profile of backward oxygen atomic lasing; (c) the backward coherent emission versus the side incoherent emission integrated over the solid angle
空气中的氮气和氧气以分子形式存在,因此要产生氮原子激光或氧原子激光,必须先将空气分子解离为原子,这无疑对泵浦激光的强度提出了很高的要求。在单光束紫外光泵浦方案中[22],氧原子激光产生的阈值高达100 GW/cm2,而紫外光能量有限,必须采用紧聚焦方式才能达到如此高的光强,从而大大限制了激光辐射的强度和传输距离。2014年,Laurain等[28]利用一束高能量纳秒激光脉冲预解离氮气和氧气分子,随后利用紫外光进行双光子激发,极大地降低了氮原子激光和氧原子激光的阈值,获得了单脉冲能量高达1 μJ的背向激光输出。虽然目前已经获得了较强的背向氮、氧原子激光,但是深紫外泵浦光在大气中的强吸收限制了其在远程探测中的应用。
稀有气体是空气中天然存在的原子气体,也是产生空气激光的备选增益介质。氩气由于是空气中含量最高(接近1%)的稀有气体而引起了研究人员的关注。最近,Dogariu等[33]采用波长为262 nm左右的飞秒或皮秒激光共振激发氩原子,观测到了波长为1327 nm、双向传输的氩原子激光。相对于氮、氧和氢原子激光,氩原子激光不需要预解离,并且262 nm波段的泵浦激光在大气中的吸收显著降低,为实际应用提供了可能。然而,他们的研究表明,只有当空气中氩气的摩尔分数达到10%以上时才能观测到氩原子激光[33],这与真实的大气环境还有一定差距。采用更高能量的飞秒激光泵浦来提高氩原子激光的增益,有望解决这一问题。
在人们探索各种原子空气激光产生方法的同时,原子空气激光的增益机制也得到了关注[44,58]。2012年,Traverso等[44]利用高能量紫外纳秒激光泵浦产生了氧原子激光,发现激光辐射在时域上出现了不规则的振荡结构,由此认为原子间的相干性对激光增益具有重要贡献。2013年,Yuan等[58]通过系统的理论研究,揭示了量子相干性是导致氧原子激光特殊时域结构的重要原因。
2.2 分子类空气激光
氮气在空气中的含量最高,是产生空气激光的首选增益介质。氮分子的发光主要来自第二正带系统,对应三重态C3Πu(以下简称“C”)到B3Πg(以下简称“B”)的跃迁[59]。由于N2的基态X1Σg到这两个三重态的光跃迁是禁戒的,因此氮分子激光主要靠碰撞激发实现粒子数反转。目前,产生氮分子激光的泵浦机制可分为两类:一类是通过激发态氩原子(Ar*)与氮分子碰撞建立粒子数反转[29],另一类则是靠热电子碰撞激发[30-31,60-61]。下面将对这两种机制逐一介绍。
利用激发态原子碰撞激发产生氮分子激光的物理过程如
图 2. 激发态氩原子碰撞激发产生的氮分子激光[29]。(a)激发态氩原子碰撞激发产生氮分子激光的能级图;(b) 3.9 μm激光脉冲在0.1 MPa氮气中产生的荧光光谱,这些辐射对应氮分子从C态到B态不同振动能级的跃迁;(c) 3.9 μm激光脉冲在0.1 MPa氮气和0.5 MPa氩气的混合气体中产生的背向氮分子激光光谱
Fig. 2. N2 lasing produced by collisional excitation of excited argon atoms[29]. (a) Energy-level diagram of N2 lasing produced by the collisional excitation of excited argon atoms (Ar*); (b) spectrum of fluorescence from the 0.1 MPa nitrogen gas induced by 3.9 μm laser pulses, these radiations correspond to the electronic transition from C state to B state of nitrogen molecules, and the corresponding vibrational levels a
电子碰撞激发是产生氮分子激光的另一个重要机制,主要通过热碰撞将电子能量传递给周围的氮分子从而实现粒子数反转。电子碰撞激发截面的大小与电子能量密切相关[64],如
图 3. 电子碰撞激发产生的氮分子激光。(a)氮分子B态和C态的电子碰撞激发截面,阴影区表示可以产生粒子数反转的电子能量区域[64];(b) 9.3 mJ、800 nm飞秒激光脉冲驱动产生的背向337 nm辐射的强度随四分之一波片角度的变化,0°、90°、180°、270°对应线偏振光,45°、135°、225°、315°对应圆偏振光[30];(c) 10 J、10 ps、 1053 nm激光脉冲驱动产生的前向337 nm辐射,插图为泵浦光三次谐波的远场光斑[31];(d) 8 mJ、3.9 μm飞秒激光脉冲驱动产生的前向337 nm辐射,插图为337 nm辐射的远场光斑[60]
Fig. 3. N2 lasing produced by electron impact excitation. (a) Electron impact excitation cross sections for B state and C state of N2 molecules, electrons residing in the shaded region are capable of producing population inversion[64]; (b) backward emission at 337 nm induced by the 9.3 mJ and 800 nm femtosecond laser pulses as a function of the angle of the quarter wave plate, angles 0°, 90°, 180°, 270° correspond to the linearly po
由此可见,通过调控泵浦激光的偏振、脉宽和波长,能够将电子加速到实现氮分子粒子数反转所需的能量,从而打开了远程氮分子激光的可能性。但是,由于超快激光在大气中传输时具有光强钳制效应,电子浓度通常较低,直接影响了碰撞激发的效率。更为重要的是,电子在超快激光场中获得的能量难以长时间维持,致使粒子数反转寿命只有几十皮秒[66-67]。这样,沿着泵浦激光相反方向传输的背向辐射不能被有效地放大。因此,与激发态氩原子碰撞激发相比,电子碰撞激发产生的背向氮分子激光信号要弱很多。此外,与传统氮分子激光器类似,氧分子对于氮分子激光具有很强的淬灭作用[30,66-67]。就遥感应用而言,延长增益寿命、抑制氧气的不利影响,是目前氮分子激光研究的重点。
2.3 离子类空气激光
空气分子不仅可以解离产生原子激光,而且经过多光子或隧穿电离之后能够产生离子,为激光辐射提供新的增益介质。离子类空气激光的研究以氮分子离子为主,其产生过程涉及的物理效应更为丰富,产生机制备受争议,是人们研究得最多的一类空气激光[23,32,41-43,46-57,70-111]。
2011年,Yao等[23]利用中红外可调谐飞秒激光激发空气分子,发现了氮分子离子激光现象。当1.9 μm、0.5 mJ的飞秒激光脉冲聚焦到空气中时,五倍频光谱上出现一个波长约为391 nm的细锐尖峰。该波长对应氮分子离子B2
2013年,Yao等[32]利用钛宝石激光器开展了
图 4. 中红外飞秒激光脉冲驱动产生的 激光。(a)中红外飞秒激光脉冲激发空气分子产生的不同波长的
图 5. 800 nm(泵浦)和400 nm(探测)飞秒激光脉冲诱导的N 2 + 激光。(a) Fig. 5. N 2 + lasing induced by 800 nm (pump) and 400 nm (probe) femtosecond laser pulses. (a) Schematic for generating
Fig. 5. N 2 + lasing induced by 800 nm (pump) and 400 nm (probe) femtosecond laser pulses. (a) Schematic for generating
不同于其他两类空气激光,
3
与原子类和分子类空气激光相比,离子类空气激光的产生同时包含中性分子和分子离子两个体系与强激光场的作用,涉及强场电离、内层分子轨道激发、多电子态耦合、核波包的振动和转动等过程,因此它的物理效应更为丰富,增益机制更为复杂。本章将从
3.1
自
电子再碰撞激发机制是Liu等[47]首次提出的。其基本思想是:在线偏振强激光场作用下,隧穿电离的电子波包被激光场加速后能够再返回到母离子,并与母离子碰撞,将
多态耦合是理解
图 6. 在800 nm激光场作用下,N 2 + 三个电子态(X态、A态、B态)的布居随时间的演化[51]。 (a)考虑X-A耦合得到的结果;(b)不考虑X-A耦合得到的结果
Fig. 6. Dynamic evolution of the populations in three electronic states of N 2 + (i.e., X, A and B) driven by the 800 nm laser pulses[51]. (a) With X-A coup
图 7. 800 nm飞秒激光脉冲在氮分子离子中建立粒子数反转的示意图[43]
Fig. 7. Schematic for establishing population inversion in molecular nitrogen ions with 800 nm femtosecond laser pulses[43]
三态耦合模型通过考虑中间态以及激光场与电子态的共振相互作用,解决了
最近,Zhang等[52]建立了中性分子及其单电离产生的分子离子在激光场中同时进行动力学演化的物理模型,即瞬时电离耦合模型。该模型同时包含整个激光脉冲内的电离与耦合过程,弥补了之前三态耦合模型的不足,并揭示了阿秒电离门对
图 8. 基于瞬时电离耦合模型计算的N 2 + 布居演化[52]。 (a)强激光场中氮分子和氮分子离子的超快动力学过程;(b)只考虑电离与同时考虑电离和耦合两种情况下,N 2 + ions calculated with the transient ionization-coupling model[52]. (a) Schematic for study
3.2
氮分子的瞬时电离以及泵浦激光与氮分子离子的共振相互作用,不仅导致粒子数在各电子态重新布居,而且建立了电子态、振动态和转动态的相干性。这些量子相干性的建立增加了
对
图 9. 近红外(800 nm)和中红外(1580 nm)激光共同泵浦产生的N 2 + 激光[53]。(a)近红外和中红外激光共同泵浦产生的Fig. 9. N 2 + lasing produced by near-infrared (800 nm) and mid-infrared (1580 nm) pump lasers[53]. (a) 下载图片 查看所有图片
Fig. 9. N 2 + lasing produced by near-infrared (800 nm) and mid-infrared (1580 nm) pump lasers[53]. (a) 下载图片 查看所有图片
在基于钛宝石激光器的泵浦-探测机制中,800 nm飞秒泵浦光构建了
图 10. N 2 + 激光的相干控制。(a) 800 nm和400 nm飞秒激光与
Fig. 10. Coherent control of N 2 + lasing. (a) Schematic of a V-type three-level quantum system created in 下载图片 查看所有图片
此外,振动相干性在
除了电子态和振动态的相干性,转动相干性也是
图 11. 氮分子离子中的共振非线性光学效应。(a) 1580 nm泵浦激光在氮气中产生的紫外辐射谱随气压的变化[56];(b)在相同的条件下,2 kPa氮气和2 kPa氩气产生的紫外辐射谱的对比[56];(c)在1580 nm泵浦激光激发的N 2 + <
Fig. 11. Resonant nonlinear optical effects in nitrogen molecular ions. (a) Ultraviolet spectra in the nitrogen gas excited by the 1580 nm pump laser as a function of gas pressures[56]; (b) the comparison of ultraviolet spectra from the 2-kPa nitrogen gas and 20-kPa argon gas in the same conditions[56]; (c) measured probe spectra as a function of the pump-probe delay when injecting an ultraviolet probe las
4 空气激光在远程探测中的应用
空气激光具有高准直度、高相干性、高强度以及自由空间传输等优点,因此在远程遥感领域具有广阔的应用前景。遥感技术通常采用线性光学的方法进行测量,地面观测站接收的是待测物的背向散射等非相干信号[113]。由于散射信号没有特定的方向性,因此随着传输距离增加,收集到的信号表现为平方衰减规律,严重影响了远程探测的信噪比和灵敏度。利用空气激光指向性好、强度高等优点,有望将远程遥感的灵敏度提高若干个数量级。因此,自空气激光现象报道之后,人们便对其在远程探测中的应用进行了探索[37-42]。
2011年,Hemmer等[37]指出:将背向空气激光与双光子吸收或受激拉曼散射技术结合,可以进行远程分子探测。2012年,Malevich等[38]利用背向传输的激光脉冲模拟背向空气激光,对背向受激拉曼散射方案进行了原理性验证,证实了背向空气激光用于远程探测的可能性。2015年,Malevich等[39]利用氮气和氩气的混合气体产生的背向氮分子激光演示了背向空气激光在气体分子检测方面的应用,他们利用背向氮分子激光和另一束可调谐激光的受激拉曼效应,成功地对CH4气体进行了检测。然而,这一技术的探测灵敏度较低,目前只能探测纯净的、气压较高的分子气体。另外,该方案使用的背向氮分子激光是在氮气和氩气的混合气体中产生的,而混合气体必须用空气代替才能真正用于远程探测。
基于空气激光的转动拉曼散射研究也取得了一些进展。2014年,Ni等[41]利用高能量的800 nm飞秒激光脉冲驱动产生了
图 12. 利用N 2 + 激光产生的高阶拉曼散射[42]。(a)利用N 2 + lasing[42]. (a) Schematic of the experimental setup for high-order Raman scattering produced with
利用超短激光脉冲作为泵浦源,人们还研究了有机分子燃烧过程中产生的激光行为,为高灵敏度远程燃烧诊断提供了有效途径[114-117]。此外,利用空气激光特有的光学特性,也有望拓展出其它方面的应用。例如,利用空气激光与太赫兹辐射的关系可能发展出新的太赫兹测量技术[88]。
5 结束语
纵观空气激光的发展历程不难发现,空气激光不仅对强场物理、非线性光学、量子光学等基础研究具有重要意义,而且在远程探测等技术领域具有显著优势。
从基础研究角度看,空气激光是强激光场与空气中多种物质形态相互作用的结果,物理内涵新颖而丰富。首先,空气激光来源于激发态原子、离子、分子的相干辐射,强调了激发态以及共振效应在强场物理中的贡献,为研究强场条件下激发态的行为以及强场共振相互作用提供了光学探针。其次,
从应用层面看,空气激光具有高准直度、高相干性和高强度等优点,在远程探测中具有显著优势。首先,空气激光具有良好的方向性,从而有效克服了传统光学遥感的平方衰减定律。同时,空气激光的强度比荧光信号强若干个数量级,并具有良好的相干性,可以激发污染物或爆炸物分子产生非线性指纹谱,实现多种污染物的同时测量。因此,空气激光与非线性指纹谱技术相结合能够发展出一种普适的、高灵敏度的新型遥感技术,用于环境科学以及****领域。此外,空气激光一般以飞秒激光作为泵浦源。利用飞秒激光成丝效应,车载TW级泵浦激光脉冲可以实现长距离无衍射传输,为发展基于空气激光的远程探测新技术奠定了重要基础。同时,利用飞秒激光成丝独特的光强钳制效应,还可以有效避免激光抖动、大气中颗粒散射、气流不稳、复杂云雾环境等因素对信号采集的影响,提高远程探测的稳定性。
目前,各类空气激光已相继被发现,
在基础研究层面,下述问题值得进一步探讨。第一,虽然
在应用技术方面,空气激光的研究刚刚起步,仍存在一些关键问题需要攻克:第一,在真实大气环境中产生高亮度背向运转的空气激光是将其用于远程探测的关键,目前仍是一个重要的技术挑战。现有的背向空气激光只在原子或中性氮分子中产生。然而,原子类激光主要以紫外光为泵浦源,大气在紫外波段的强吸收以及紫外激光器较低的能量限制了其在遥感中的应用。由于电子碰撞激发难以维持长时间的增益,并且氧气具有很强的淬灭效应,背向氮分子激光难以在大气中运转。因此,提高增益寿命、抑制氧气的负面影响,是实现背向氮分子激光必须解决的问题。第二,将空气激光用于远程分子检测时必须将待测分子置于大气环境中,因此如何抑制大气的背景信号,进而实现痕量气体分子的高灵敏度检测,也是需要解决的关键问题。第三,空气激光在非线性超快光谱、燃烧诊断等领域也具有广阔的应用前景,值得进一步研究。
综上所述,在强场超快条件下开展远程空气激光研究有望在强场分子物理、超快非线性光学、量子光学等基础研究领域取得重要突破,并将推动远程探测、非线性光谱、燃烧诊断等应用技术的发展。
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