光学相干控制中光束相位畸变的诊断与优化

阐述了超短激光光束位相差对相干控制光电流注入的控制作用,发展了一种诊断光束波阵面位相畸变的方案.通过控制基频光、倍频光在全光孔径干涉光场内的等相面保持一致的实验处理方法,制作了一套相位差稳定自动控制系统,并应用于光学相干控制光电流实验中,获得较好的实时控制效果.模块化封装后的相位差稳定自动控制系统能使相干控制实验中控制激光脉冲的相位差在全光口径内保持一致.     

二次谐波产生中光场振幅K次方压缩

本文研究了二次谐波产生过程中产生光场振幅K次方压缩的条件,结果表明,若初始的二次谐波为相干态(包括真空态),那么基波的振幅K次方压缩就可能存在。     

超短脉冲高次谐波谱的连续性

研究超短脉冲（3个光学周期）高次谐波发射功率谱的性状及其成因. 结果表明, 短脉冲谐波谱与长脉冲（10个光学周期）相比, 其平台区末端的谐波谱变得伪连续. 这是由于3个原因导致该现象: (1) 在短脉冲情形下, 谐波的生成仅经历1~2次辐射, 在长脉冲情形下, 谐波的生成经历多次辐射的累加; (2) 对于相同的谐波级次, 短脉冲比长脉冲的谐波线宽宽; (3) 短脉冲比长脉冲的激光频谱宽.      

利用啁啾场与单极场的组合场驱动He+发射高次谐波

数值研究了He~+在啁啾场与单极场下发射高次谐波及阿秒脉冲的特点.计算结果表明,当He~+的初始波函数布局在基态与激发态的叠加态时,其谐波强度比单基态时增强7个数量级.随后在啁啾场及单极控制场的作用下,谐波发射的截止能量明显增强,谐波的干涉结构也明显减小.引入空间非均匀效应,谐波截止能量得到进一步延伸,形成一个320eV的平台区.通过叠加谐波,可获得3个持续时间在45~48as的X射线脉冲.其强度比单基态输出的脉冲增强5~6个数量级.     

激光波形优化产生水窗区单阶谐波

通过激光波形调控,理论提出一种获得水窗区间单阶谐波的方法.研究表明,在双色激光场中适当引入啁啾调制可获得强度增强一个数量级的单阶谐波.随后,在啁啾波形处适当引入半周期激光场可使单阶谐波进一步向高能区延伸,进而覆盖整个水窗区间.最后,在不同啁啾场以及半周期场的组合下可获得波长可调的水窗区间单阶谐波.     

A frequency comb in the extreme ultraviolet

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.     

Ways to produce and measure atto- and femtosecond soft X-ray pulses

The ways to produce and measure atto-and femtosecond soft X-ray pulses are reported. The laser phase relation of high-order harmonic generation(HHG) shows two different radiation energy distribu-tions in time(or laser phase) domain. These energy-phase relations are helpful for realizing the dy-namic processes of HHG. Two presented parameterized formulas can be used to calculate the durations of the energy distributions with a bandwidth of the pulse. These formulas are useful in calculating and simulating pulses transports and interactions with mediums. The time structures of atto-and femto-second soft X-ray pulses can be directly measured with photoelectron spectrum transfer equations and the related laser phase determination methods without any previous pulse shape and the instantane-ous frequency assumptions. These equations and methods can be used to evaluate and improve the technical parameters of the ultra-short X-ray sources. They have wide measurement ranges and high time resolutions,which may enable ultra-fast measurements to reach metrological precisions,and lead to a new tide of scientific researches in physics,chemistry,biochemistry,etc. The application of atto-and femtosecond X-rays as well as the theoretical and technical problems in measurements are briefly discussed.     

激光强度对单阶谐波增强的影响

通过调控激光波形, 理论上可以实现单阶谐波的增强. 随后, 当基频场强度增强时, 单阶谐波增强现象消失. 当控制场强度增强时, 单阶谐波增强倍率增大, 并且在某一特定光强下具有最大的增强值. 理论分析表明, 单阶谐波的增强现象不仅来源于谐波辐射的折叠区域, 并且与多重谐波辐射能量峰的干涉有关.     

表面等离激元波导中表面等离激元二次谐波的产生机制

在表面等离激元波导的二次谐波产生过程中, 波导较大的色散导致的相位失配阻碍了其实际应用, 能够消除相位失配的波导尚未见实验报道。通过将单层二硫化钼与平面表面等离激元波导相结合, 在波导中实现从基频表面等离激元到倍频表面等离激元的转化, 并研究单层二硫化钼-银结构的二次谐波产生特性, 为表面等离激元波导中二次谐波产生的实际应用打下基础。     

不同啁啾调制对光谱连续区的影响

利用啁啾场调控激光波形,理论研究了不同啁啾场对高次谐波光谱的影响. 结果表明：当采用对称中间啁啾调控时,谐波截止能量的延伸及光谱连续区来自于激光中间区域. 当采用不对称负向啁啾调控时,谐波截止能量的延伸及光谱连续区来自于激光下降区域. 虽然,谐波截止能量在不同啁啾调控下都可以得到延伸,但是,不对称负向啁啾场下光谱连续区的强度要比对称中间啁啾场下光谱连续区强度高2个数量级. 最后,通过叠加光谱连续区上的谐波可以获得2个脉宽在38 as的单个阿秒脉冲. 并且,负向啁啾场下获得脉冲强度要比对称中间啁啾场下获得脉冲强度高2个数量级.     

平顶高斯光束谐波场的横向分布

对平顶高斯光束用于倍频和混频后的谐波场场强分布进行了理论计算,模拟了相同基波峰值强度下,不同阶数和不同偏振分量比的平顶高斯光束在长度为5mm的非线性晶体CsLiB6O10上的Ⅰ类、Ⅱ类倍频光场及Ⅱ类混频光场的横向分布曲线,并将其与基波场分布进行了比较,得到了谐波场仍为均匀平顶分布形式,但出现了横向光斑压缩现象,且基波光束的阶数愈高,横向压缩现象越弱;阶数相同,偏振分量不同的基波产生的谐波场光斑大小基本相同,但光场强度不同。     

分子取向对H2+谐波分布的影响

理论研究了H2+分子取向对谐波空间分布的影响. 结果表明：当激光偏振方向与分子轴方向一致时, 谐波辐射满足激光场正向时, 负向H核谐波辐射强度大于正向H核;激光场反向时, 正向H核谐波辐射强度大于负向H核. 随着分子取向角增大, 谐波辐射强度减弱, 尤其正向H核对谐波辐射的贡献明显减小, 因此导致正负向H核的谐波辐射强度差逐渐增大. 最后, 通过研究谐波辐射的时频分析以及电子波包随时间的演化给出了电子在双H核之间运动以及谐波空间分布的原因.     

非线性啁啾波形优化获得超短阿秒脉冲

本文通过调控非线性啁啾波形,提出了运用两种非线性啁啾组合波形延伸谐波截止能量以及增强谐波强度的方法. 研究结果表明,适当调节组合场的二阶和三阶啁啾参数、啁啾延迟以及其它激光参数后,激光波形得到最佳优化. 在最佳波形驱动下,谐波截止能量和辐射强度明显增大,进而我们可以获得脉宽在39 as的孤立脉冲.     

H2+核间距对单阶谐波增强的影响

本文理论研究了在特定激光波形下, H2+核间距对单阶谐波增强的影响. 研究结果表明, 当特定激光波形驱动原子时, 谐波光谱可以呈现单阶谐波强度增强; 但是, 当用相同激光波形驱动H2+时, 单阶谐波强度增强的现象与H2+核间距离有关. 具体来说, 当H2+在平衡位置时, H2+谐波光谱呈现单阶谐波增强现象; 当H2+核间距在3~7 a.u.变化时, 单阶谐波增强现象消失; 当H2+核间距大于8 a.u.时, 单阶谐波增强会再次出现. 理论分析表明, 多通道谐波干涉是导致H2+单阶谐波强度变化的原因.     

Nonlinear optics in the extreme ultraviolet

Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.     

空间非均匀激光场下He原子辐射谐波的量子路径调控

理论研究了He原子在空间非均匀激光场下辐射谐波的量子路径调控.计算结果表明,随着空间非均匀激光场引入位置由负向-r0到正向-r0移动,谐波截止能量呈单调递增趋势,而且只有单一的短量子路径对最大谐波辐射过程起作用.通过分析谐波辐射时频分析图和电子含时波包演化图,对谐波辐射的特点给出了合理解释.随后适当引入一束太赫兹激光场,谐波强度被增强2个数量级,并且形成一个1208eV的超长平台区.最后,通过叠加谐波,可获得一系列持续时间在34as的超短脉冲,其波段覆盖为10~1nm.     

激光波形调控实现单阶谐波的增强

通过对双色场激光波形的调控,理论上获得了单阶谐波的选择增强.理论分析表明,单阶谐波的选择增强来源于‘W’波形下谐波辐射的折叠区域.而且,第二束调控场的脉宽对‘W’波形下所形成的谐波辐射折叠区域有较大影响.该研究为获得单阶谐波脉冲提供了一种新的方案,对激光光源的发展有帮助.     

KTP和BBO晶体对1.06μm皮秒光脉冲的倍频效率

用５ｍｍ的ＫＴＰ（ＫＴｉＯＰＯ４）和ＢＢＯ（β－ＢａＢ２Ｏ４）非线性晶体，对皮秒的ＣＰＭ，Ｎｄ∶ＹＡＧ脉冲激光的腔外倍频效应进行研究，获得倍频能量转换效率分别为４２％和３２％，证明ＫＴＰ倍频效率高于ＢＢＯ，结果与理论分析相符．     

有机超薄膜中分子的光致重新取向及可逆性

用旋转样品二次谐波法研究了在皮秒红外偏振脉冲激光照射下，氐盐ＬＢ单层膜中分子的重新取向。发现，重新取向效应在光照时间较长时将达到饱和，并且具有一定程度的可逆性。     

调节X2+同位素分子电荷共振增强电离时刻获得高强度阿秒脉冲

利用X_2~+同位素分子(H_2~+、D_2~+、T_2~+)谐波辐射的特点,提出一种有效获得高强度谐波连续区和孤立阿秒脉冲的方法.研究表明,在不同脉宽激光作用下,H_2~+、D_2~+和T_2~+分子可分别进入电荷共振增强电离区域.当激光振幅区域的半个周期正好处于电荷共振增强电离区域时,具有最大辐射能量的谐波能量峰正好具有最佳的辐射强度.随后,在此区域引入半周期单极激光场,被选择出来的谐波能量峰可以继续延伸,进而获得一个仅由单一能量峰贡献而产生的高强度谐波连续区.通过叠加连续区上的谐波可以获得脉宽仅为42 as的孤立阿秒脉冲.