【作者】 徐智君；

【导师】 朱晓农；

【作者基本信息】 南开大学 ， 光学工程， 2013， 博士

【摘要】 脉冲宽度在皮秒(10-12秒)至飞秒(10-15秒)之间、脉冲能量在毫焦耳量级的超短脉冲激光具有极短的持续时间和极高的峰值功率。这样的超短脉冲激光在与物质相互作用过程中会产生很多的非线性现象。其中,飞秒激光诱导空气电离便是一个基本而重要的物理现象,并且也是超快激光光学诸多应用的基础。等离子体与靶材的相互作用能够导致一系列的应用,微观上飞秒激光微纳制备及光伏材料中非晶硅薄膜的晶化等有广阔的应用前景,宏观上超短脉冲激光烧蚀推进技术作为一种新型的光推进技术,有望成为未来空间推进的一种重要方式。飞秒激光诱导的等离子体韧致辐射还有可能存在受激辐射窗口,提供光增益,形成光振荡。等离子体的产生及时空演变、等离子体辐射的特性、飞秒激光与等离子体非线性相互作用是本论文的研究主线。本论文主要内容和结果如下：1.在实验中首次发现了一种从相对紧聚焦的飞秒激光脉冲诱导空气等离子体中产生的超亮喷射状相干光束,并对这种特殊光束的光谱特性和产生机制进行了分析研究。通过适当控制具有特定F数(f-number)的汇聚透镜的倾角或水平偏移量,我们可以在被明显扰动的锥形辐射中观察到单个或双个喷射状超亮光束。这种超亮喷射状相干光束与通常的光丝(Optical filaments)不仅在空间分布和光谱特性上存在很大的区别,而且它们产生的机制不同,非线性相移的构成也不一样。实验结果表明,超亮喷射状相干光束的产生主要源于在锥形辐射锥面上的四波混频过程。相位匹配计算获得的反斯托克斯和斯托克斯波长分别为547nm和1064nm,与光谱测量结果相吻合。该反斯托克斯波长很好地解释了实验中观察到的超亮光束所呈现出来的耀眼的亮黄色。这种独特的超亮喷射状光束经过准直后,可以在空间中传输很长的距离,因而有可能在未来的遥感探测中找到应用。2.针对实验所用参数,借助电子密度速率方程模拟、分析了100mm透镜聚焦条件下50fs超短脉冲激光诱导空气电离时电离区内电子密度的时空演化过程。模拟结果显示,在所用飞秒脉冲的峰值点到达焦点之前,空气已被完全电离。根据模拟得到的电子密度分布,我们求得了等离子体的折射率时空分布,并发现在电离中心区存在一个折射率变化的饱和区域(饱和等离子体的折射率值为0.9923)。基于此,我们可以借助空气-等离子体界面折射模型来直观地解释随着激光功率的增加,正常电离情况下锥形辐射的色序分布及远场衍射光斑张角先增加而后趋于不变的现象。3.搭建了超短脉冲激光瞬态光谱测量系统,记录了单脉冲激光诱导空气等离子体辐射谱,并利用时间分辨光谱系统研究了不同实验条件下超短脉冲激光诱导空气等离子体的动态演化特性。通过对空气等离子体辐射进行普朗克黑体辐射模拟,得出了在50fs脉冲诱导空气电离后2ns等离子体的温度为5500K。而由时间分辨光谱测量获得的微区等离子体的典型寿命约为5.5ns(f=100mm)。同时我们还比较了不同聚焦条件下不同脉冲能量和宽度的脉冲激光诱导空气等离子体的动态辐射光谱中连续谱和线谱演变的不同。在将4倍物镜电离空气形成的微区等离子体成像耦合进光谱仪竖直狭缝后进行空间分辨光谱测量的实验中,我们发现远离透镜端的等离子体具有更强的氧离子和氮离子线谱。4.实验研究了不同聚焦条件下,同向传输双飞秒脉冲激光诱导空气等离子体的时域特性和光谱特性。在利用焦距为100mm透镜相对浅聚焦飞秒脉冲激光电离空气的实验中,我们发现在双脉冲间隔为0-8ns之间时,双脉冲激光诱导空气电离产生的等离子体辐射比单一先到脉冲电离辐射有明显的增强。而相比之下,在10倍物镜紧聚焦电离空气的实验中,我们只在双脉冲时间间隔小于0.5ns的情况下才观察到双脉冲电离产生的等离子体辐射比先到脉冲电离辐射增强；在双脉冲时间间隔达到8ns时,后续脉冲会直接穿过由前脉冲电离空气形成的等离子体膨胀所造成瞬态真空通道,而没有产生进一步的空气电离。利用这种穿孔效应有可能实现超短脉冲的动态空间滤波,而无需借助实体的滤波小孔和复杂的真空设备。5.研究了光学“成丝”中光强钳制效应对激光烧蚀率的影响。研究发现当入射光的功率大于自聚焦临界功率时,由于光强钳制效应的存在,不同气体氛围中的光学“成丝”烧蚀铝箔的烧蚀率并不随着入射激光功率的增加而增加,而是接近一稳定值。这一特性不仅可以帮助确定不同气体介质中飞秒脉冲激光自聚焦临界功率,同时也提供了一种确定气体介质非线性折射率系数n12的方法。应用此方法,我们测得一个大气压下空气中50fs脉冲激光的自聚焦功率为7.2GW,非线性折射率约为1.3×10-19cm2/W;而一个大气压的氩气所对应的50fs脉冲激光的自聚焦临界功率为3.8GW,非线性折射率为2.6×10-19cm2/W。6.在国内率先开展了利用“光丝”进行烧蚀推进的实验。在进行光学“成丝”烧蚀推进玻璃小球的实验中,我们使用焦距为111cm的透镜,我们发现光学“成丝”等离子体通道上最强荧光的产生位置并不对应最大的“成丝”烧蚀率和最远的小球推进距离。通过数值模拟,我们计算了光学“成丝”丝上不同位置的横向激光能流分布,并通过“双温”模型估算烧蚀深度,我们证实了烧蚀量在光学“成丝”推进中扮演了决定性的因素。即使是在飞秒激光“成丝”这种存在较长空气电离通道的情况下,烧蚀过程中的物质去除所导致的反冲力仍是主要的推力来源。光学“成丝”推进依然是一种依赖于激光烧蚀的推进。7.提出并验证了一种确定激光烧蚀竖直推进微型小球的冲量耦合系数的简易测量方法一阴影法。该方法能够直观、准确地测出被烧蚀小球的运动飞行时间,并具有较高的灵敏度和稳定性。在利用脉冲能量为毫焦量级的飞秒激光单脉冲推进质量为1.4mg小铁球的实验中,采用此阴影法测得的冲量耦合系数约为5dyne/W。利用这种方法,我们研究了不同脉冲能量和脉宽的超短激光脉冲对烧蚀推进所获得的冲量耦合系数的影响。实验结果显示,在50fs、800nm波段的激光脉冲作用下,随着入射脉冲能量的增加,冲量耦合系数呈现单调下降的趋势；而当入射脉冲能量固定、脉冲宽度改变时,冲量耦合系数先增加,在500fs左右达到最大,而后减小并最终趋于平稳。50fs的最短脉宽并未导致最大的冲量耦合系数,这主要是因为烧蚀机制由50fs时的原子化变为长脉冲下的相位爆炸所致。更多还原

【Abstract】 Ultrashort pulses with duration from ps (10-12s) to fs (10-15s) and millijoule pulse energy can possess extremely high peak intensity and introduce many nonlinear phenomena during the interaction with all kinds of materials. Among them, femtosecond laser-induced air ionization is a topic of fundamental importance and is inevitably involved in many ultrafast-optics-related applications. The generation of plasma and its spatial-temporal evolution, the characteristics of plasma radiation and the nonlinear interaction of plasma with femtosecond laser pulses are the central issues of this thesis. It is speculated that Bremsstrahlung radiation from femtosecong laser-induced air plama may involve a stimulation process and thus provide optical gain for light amplification.In the meantime, the interaction of laser-induced plasma with solid target has found a broad range of applications. On the microscopic scale, the micro-nano-fabrication and crystallization of amorphous silicon membrane with femtosecond lasers have technically attractive advances. On the macroscopic scale, ultrashort pulse laser ablative propulsion has the potential to be an important alternative in space travel as a new type of light propulsion.The main contents and results in this thesis are listed as follows:1. For the first time, we report the observation of a novel phenomenon of the generation of super-luminescent jet light beams emanating from the micro air plasma induced by a relatively tight focused near-infrared femtosecond laser pulses. A systematic study of the key characteristics of such jet beams and its generation machanism are conducted. Single or double jet-like optical beams, being slightly divergent and coexisting with severely distorted conical emission of colored speckles, are obtainable only when the focal lens of proper f-number is slightly tilted or shifted. In addition to the apparent difference between SJBs and the conventional optical filaments (OFs) in both spatial and spectral characteristics, the two also differ substantially in the underlying generation mechanisms.(In particular, the different constituents of the nonlinear phase shift in the two cases also set them apart.) It is analyzed and confirmed that the four-wave mixing (FWM) process contributes most to the generation of such jet-like beams. The Stokes and anti-Stokes waves with distinct1064nm and547nm peak wavelengths obtained from the phase matching calculation are in good agreement with the experimentally recorded spectrum. The anti-Stokes wavelength at547nm also well explains the observed brilliant yellow color of the jet beams. As a new type of coherent optical beams from disrupted conical emission, these unique super-luminescent jet-like beams can propagate over a long distance in air after proper collimation, and very likely they may find some applications in remote sensing in future.2. By using electron density rate equation, we have analyzed the time evolution and spatial distribution of electron density of the air plasma induced by tightly-focused50fs laser pulses with100mm focal length lens. It reveals that for the chosen laser parameters the air can be totally ionized well before the pulse peak comes to the focusing point. The corresponding distribution of plasma related refractive index has been then obtained from the electrons density calculation, and it is found that there exists a saturation region in the central focal area where the refractive index of plasma becomes constant (～0.9923). Moreover, a simple plasma-air interface refraction model is proposed to explain the different spectral distribution and optical power dependent of the divergence of the "nonlinearly diffracted beam", namely, the divergence angle first increases fast and then stabilizes at a certain value more than twice of the ordinary diffraction angle.3. We have built a transient spectrum measurement system including a fast gated spectrometer. By this system we are able to record the dynamic spectra of a single pulse induced air plasma over a selectable time window. Plasma temperature of5500 K is determined by fitting the continuum spectra of plasma emission with the well-known Planck’s blackbody emission formula at2ns after air ionization. Typical lifetime of the micro air plasma is determined to be～5.5ns based on the spectrally integrated and time-resolved spectral measurements. Evolution characteristics of both continuum and line spectra of the plasma are examined for various laser pulse energies associated with different pulse durations. Spatially resolved spectroscopic measurements have shown that compared with the spectra obtained from the plasma section closer to the focal lens (4X objective), the spectra from the other part of the plasma channel, namely away from the focal lens, has more pronounced oxygen and argon ionic lines.4. Temporal and spectral characteristics of air plasma generated by dual collinearly-propagating50femtosecond laser pulses are experimentally studied under different focusing conditions. In the case of relatively shallow focusing with a plano-convex lens of100mm focal length, enhancement of plasma emission is clearly observed for the whole inter-pulse delay range of0-8ns. In great contrast, air plasma generated with a10X objective (tightly focused) presents enhanced bremsstrahlung emission only when the inter-pulse delay is less than0.5ns, and for a longer inter-pulse delay of8ns, the delayed pulse passes through the transient vacuum channel due to the first pulse induced air plasma expansion without further inducing air ionization. This phenomenon may be utilized in dynamic spatial filtering of ultrashort and intense laser pulses without employing any hard apertures and vacuum hardwares.5. We have experimentally demonstrated that because of intensity clamping, when the laser peak power is higher than the critical power for self-focusing, further increase of the laser power cannot result in corresponding increase of the laser ablation rate of an aluminum foil placed in air. The ablation rate will approach a stabilized value once the incendent power is above a certain value. Also, the experimental technique implemented in our work may be used in measuring the self-focusing critical power and the nonlinear refractive index. The thus measured self-focusing critical power and the nonlinear refractive index are7.2GW and1.3×10-19cm2/W respectively under standard atmospheric pressure. While for the one atmosphere of argon gas, the corresponding values are3.8GW and2.6×10-19cm2/W respectively.6. We have accomplished for the first time the experiments of optical filament propulsion. In the propulsion of microbeads by femtosecond laser filament formed by111cm focal length lens, it is found that when placed at the position where the strongest filament fluorescence exists, the microbead does not obtain the maximal ablation rate as well as the maximal propelled distance. Moreover, the position where the maximal ablation rate exists is just consistent with that where the maximal propelled distance of the microbead occurs. Through numerical simulations of optical filamentation formation and adoping the two temperature model, it is confirmed that the actual ablation volumn plays a dominant role in the laser filamentation propulsion. Even in the optical filament scheme where there exists much longer air ionization channel, the thrust generated by the laser ablation of the target material remains the main origin of impulse force. In other words, optical filamentation propulsion is still a type of laser ablative propulsion.7. A shadowgraphic method is proposed to measure the impulse coupling coefficients of microbeads propelled vertically with femtosecond laser pulses. This method can directly record the flying time of beads with high sensitivity. In the experiments of single millijoule femtosecond laser propulsion of1.4milligram iron beads, impulse coupling coefficient of～5dyne/W is obtained. This shadowgraphic method is also employed to investigate the impulse coupling coefficients of iron beads with different pulse energies and pulse widths. For50fs laser pulses, when the pulse energy is changed from0.86mJ to1.62mJ, the generated impulse coupling coefficient decreases, which may be attributed to the dominate material removal mechanism of atomization occurred at laser fluence much higher than the optimum value. For laser pulses with fixed pulse energy of0.86mJ, as the pulse duration is increased from50fs to8ps, the generated impulse coupling coefficient doesn’t achieve maximum value at50fs, it first increases sharply, reaching a maximum around500fs, and then decreases at much slower pace, approximately maintaining at a relatively high value. This is caused by the change of the ablation mechanism involved from atomization to phase explosion.更多还原