【作者】 张鹤；

【导师】 金明星；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2021， 博士

【摘要】 飞秒激光脉冲在透明介质中传播时,会受到克尔自聚焦效应的影响,光束发生汇聚,造成脉冲强度升高,当脉冲到达一定的强度之后,会使介质中的分子或原子电离,产生等离子体,等离子体又会使光束发散,两种效应会相互竞争,形成动态平衡,使介质中形成一条高亮的等离子体通道,此过程被称为飞秒激光成丝。飞秒激光成丝过程会伴有很多的非线性光学现象,如等离子体的产生,超连续谱(白光)的产生,太赫兹辐射,高次谐波辐射,锥角辐射和荧光发射等。其中飞秒激光成丝诱导产生的超连续谱是非常重要的研究课题,因为其具有很高的潜在应用价值,如远程探测大气中污染物成分。虽然,现在已经有很多人做了飞秒激光成丝产生的超连续谱的研究,但是对于整形飞秒激光脉冲成丝产生的超连续谱的研究还很少。同时,对飞秒涡旋激光在透明介质中成丝产生的超连续谱背后的物理机制还有待深入研究。所以,本文将对飞秒激光成丝诱导产生的超连续谱进行系统地研究,主要研究内容如下:1.在强飞秒激光脉冲与空气相互作用时,光谱展宽是一种非常重要的非线性效应。本文通过实验研究了光谱展宽与气压和脉冲能量的关系。实验发现增加气压和脉冲能量都可以增强白光光谱强度,但只有增加气压会导致光谱出现明显的短波长方向展宽,而改变脉冲能量对光谱展宽几乎没有影响。为了更好地解释光谱展宽的机制,本文模拟了瞬时频率与脉冲强度随时间的变化率和等离子体密度随时间的变化率之间的关系。结果表明,等离子体的自相位调制效应产生的光谱展宽与气压有关,同时,在光谱的蓝移中起重要作用。此外,本文还测量了激光脉冲的能量透过率,透过率随气压的升高而降低。实验结果证明了高气压有助于激光脉冲能量的转移。这项研究将有助于更好地理解飞秒激光成丝诱导空气中超连续谱产生的物理机制。这对应本文第三章的内容。2.本文通过实验研究了拉盖尔-高斯飞秒激光在水中成丝产生的超连续谱的性质。结果表明,拉盖尔-高斯光束的拓扑电荷数对成丝诱导的光谱展宽影响不大,但是其对光谱强度影响很大。尽管拉盖尔-高斯飞秒激光在空气中成丝产生的超连续谱携带轨道角动量,但其在水和白宝石中成丝产生的超连续谱失去了轨道角动量。尽管如此,拉盖尔-高斯光束成丝产生白光后,其中心波长的光依然携带轨道角动量。该研究有助于理解拉盖尔-高斯光束在水中成丝产生超连续谱的物理机制。此部分对应文章第四章的内容。3.本文通过实验研究了贝塞尔-高斯光束和拉盖尔-高斯光束成丝产生白光的特点。实验发现贝塞尔-高斯光束和拉盖尔-高斯光束成丝后产生的光谱展宽不会随着拓扑电荷数改变。同时,贝塞尔-高斯光束成丝造成的光谱展宽达到最大值的距离随着拓扑电荷数的增加而增加。此外,实验还探究了贝塞尔-高斯光束和拉盖尔-高斯光束成丝后产生白光的锥角辐射情况。实验结果表明,贝塞尔-高斯光束和拉盖尔-高斯光束产生的白光在白屏上呈现完全不同的光斑样式。同时,我们还研究了贝塞尔-高斯和拉盖尔-高斯光束成丝后的锥角辐射发散角的情况,实验结果表明拉盖尔-高斯和贝塞尔-高斯涡旋光的锥角辐射发散角都随着波长的增加而减小,而其拓扑电荷数对发散角没有明显影响。这种现象可以根据自相位调制效应理论来解释。以上这些研究工作有助于我们更好地应用飞秒激光成丝产生的白光,同时帮助我们深层次地理解白光产生过程中的物理机制。更多还原

【Abstract】 When femtosecond laser pulse propagates in the transparent media,it is affected by the self-focusing effect resulting in the convergence of the laser beam and the increase of the intensity of the pulse.When the intensity of the pulse increases to a certain value,the molecules or atoms in the transparent media are ionized resulting in the plasma generation,which causes divergence of the pulse.The two effects compete with each other and reach a dynamic equilibrium generating an illuminating plasma channel,which is called filament.Femtosecond filamentation is accompanied by lots of optical nonlinear phenomena,such as plasma generation,supercontinuum generation,terahertz generation,high-order harmonic generation,conical emission and fluorescence emission,etc.The supercontinuum generation/white light is a hot topic due to the potential application,such as light detection and ranging(LIDAR).Although,there are lots of the researches on the supercontinuum generated by femtosecond laser filaments,the research on filamentation induced by spatially shaped femtosecond laser pulses is still insufficient.At the same time,it is also necessary to discuss the physical mechanism of the supercontinuum generated by femtosecond optical vortex filament.In this article,we research the filament-induced supercontinuum generation using spatially shaped femtosecond laser pulses.The main contents are as follows:1.Spectral broadening is an important nonlinear effect during the interaction of intense femtosecond laser pulses with air.In this paper,we experimentally study the dependence of spectral broadening on pressure and pulse energy.It is found that increasing both pressure and pulse energy can enhance the spectral intensity,while only increasing pressure leads to obvious blue shift of spectra,and the pulse energy has little effect on it.To understand the mechanism of spectral broadening deeply,the numerical simulation relating the instantaneous frequency to the time dependent pulse intensity and time dependent plasma density in air is carried out.The results indicate that the plasma generation induced self-phase modulation which is related to the pressure plays an important role in blue shift of spectrum.Besides the transmission of the laser pulse is measured and decreases with pressure.It proves that the energy transfer of the laser pulse is promoted by pressure.This study will be helpful to understand physical mechanism of femtosecond filament induced supercontinnum generation in air deeply.This part is discussed in Chapter 3.2.We experimentally study the femtosecond supercontinuum generation by an Laguerre-Gaussian vortex beam in water.It is demonstrated that the topological charge of the optical vortex beam has little influence on spectral broadening induced by filament,while it has strong influence on spectral intensity.Despite the supercontinuum induced by filamentation of optical vortices in air possesses orbital angular momentums,the supercontinuum generated in water and sapphire loses orbital angular momentum.However,the light at central wavelength possesses orbital angular momentums after filamentation in transparent media.This study is helpful to the understanding of the physical mechanism of femtosecond supercontinuum generation by Laguerre-Gaussian vortices in water.This part is discussed in Chapter 4.3.We study the characteristic of the supercontinuum generated by Laguerre-Gaussian vortex beam and Bessel-Gaussian vortex beam.As the topological charge increases,the spectral broadening induced by both Laguerre-Gaussian vortex beam and BesselGaussian vortex beam stays constant.And the distance for the spectral broadening induced by Bessel-Gaussian vortex beam reaching its maximum increases with the topological charge.Besides,the beam profiles of Laguerre-Gaussian vortex beam and Bessel-Gaussian vortex beam shown on the board are totally different.It is also found that the angels of the conical emission for Laguerre-Gaussian vortex beam and BesselGaussian vortex beam both decrease with increasing wavelength of the supercontinuum,and have no relationship with the topological charge.It is known that the conical emission can be described by self-phase modulation(SPM)effect.The content is discussed in Chapter 5.The work in this article can make the supercontinuum generation be better used and make us understand the mechanism of the supercontinuum generation deeply.更多还原