【作者】 毕洁；

【导师】 陆培祥；

【作者基本信息】 华中科技大学 ， 光电信息工程， 2011， 博士

【摘要】 圆锥辐射是超短激光脉冲与介质相互作用时产生的一种奇特的非线性现象,本论文主要围绕飞秒激光脉冲在二阶非线性介质—BaB2O4(简称BBO)晶体中产生的不同类型的圆锥辐射的特性进行了理论和实验研究,主要研究内容包括：研究了在飞秒激光二次谐波产生过程中由于时空耦合调制不稳定性作用下出现的蓝绿色圆锥辐射现象。实验观测了时空耦合调制不稳定性的特征,当入射光强增大、脉冲宽度减小以及晶体厚度增大时,蓝绿色圆锥辐射的光谱都会出现中心波长蓝移的趋势。建立了蓝绿色圆锥辐射产生的物理模型,模拟计算了辐射锥角与波长的变化关系,与实验结果完全一致,说明了蓝绿色圆锥辐射正是源于二次谐波与基波之间的作用。研究了飞秒激光二次谐波在BBO晶体中由于时空耦合调制不稳定性作用下产生的彩色圆锥辐射现象。建立了彩色圆锥辐射产生的物理模型,模拟计算了辐射锥角与波长的变化关系,解释了泵浦光的入射角对彩色圆环半径的影响,说明彩色圆锥辐射是由于二次谐波经过参量下转换过程转化而来。归纳阐述了近年来解释超连续圆锥辐射现象的多种物理理论模型：四波混频、Cerenkov辐射、非线性X波和自相位调制,并利用前三种物理模型分别模拟计算了二阶非线性晶体和等方性非晶体介质中产生的辐射锥角与波长的变化关系。开展了不同偏振方向的飞秒激光在BBO晶体和BK-7玻璃中产生的圆锥辐射的实验研究。实验发现,当飞秒激光的线偏振方向发生改变时,在BBO晶体中伴随二次谐波的蓝绿色圆锥辐射和超连续圆锥辐射会交替出现,而在BK-7玻璃中只出现超连续圆锥辐射。较为精确的测量了超连续圆锥辐射的光谱及其对应的锥角,实验测量结果与非线性X波理论符合。更多还原

【Abstract】 Conical emission is a kind of interesting phenomenon, which occurs as ultrashort laser pulses interact with media. In this dissertation, theoretical and experimental studies on the properties of different kinds of conical emissions as femtosecond laser pulses propagate in a quadratic nonlinear medium-BBO crystal, the main contents are classified as follows.Blue-green conical emissions caused by spatiotemporal modulational instability by means of femtosecond laser pulses second harmonic generation have been investigated. With the larger pumped intensity, the shorter pulse width and the thicker crystal, the central wavelength of the blue-green conical emission spectrum had a blue shift, which was a characteristic of spatiotemporal modulational instability. A theoretical model based on the emergence of blue-green conical emission was numerically calculated conical angles on different wavelengths, which was accord with the experimental results. The observed conical emission was originated from spatiotemporal coupling the fundamental and second harmonic pulses.Green and red conical emissions caused by spatiotemporal modulational instability have been investigated pumped by second harmonic femtosecond laser pulses. A theoretical model based on the emergence of green and red conical emissions was numerically calculated conical angles on different wavelengths, which explained the observed phenomenon of green and red conical emission overlapping and inversing. Green and red conical emission was originated from parametric down conversion process, in which second harmonic photons decay into photon pairs.Several interpretations on supercontinuum conical emissions have been introduced: four-wave mixing, Cerenkov radiation, nonlinear X-wave and self-phase modulation. The conical angles on different wavelengths were numerically calculated by the former three theoretical models on the quadratic nonlinear medium and isotropic amorphous medium.Supercontinuum conical emissions have been experimentally investigated in quadratic nonlinear medium and isotropic amorphous medium by femtosecond laser pulses with different polarization. As the polarization of pumped laser pulses changed, blue-green conical emissions associated with second harmonic pulses and supercontinuum conical emissions occurred alternately in a quadratic nonlinear medium, while only supercontinuum conical emissions occurred alternately in an isotropic amorphous medium. The spectrum and according conical angles of supercontinuum conical emissions was accurately measured experimentally, which can be explained well by nonlinear X-wave model other than FWM model.更多还原