空气中等离子光栅诱导探测光丝三次谐波辐射放大的实验研究

对探测光丝的不同位置与等离子光栅相互作用和探测光丝作用到等离子光栅不同位置引起三次谐波的增强进行了实验研究.研究发现,探测光丝的三次谐波信号强度对于探测光丝不同位置与等离子光栅相互作用和等离子光栅内部钳制的激光强度具有极强的依赖关系.与等离子光栅相互作用,三次谐波信号与等离子光栅基波信号的相位匹配与否是解释探测光丝三次谐波信号强度变化的关键.控制探测光丝以小角度与等离子光栅相互作用是实现探测光丝三次谐波信号有效放大的最佳途径.     

离散效应对高强度激光三次谐波转换的影响

 使用分步傅里叶变换和四阶龙格库塔法(R-K) ，对高强度激光以Ⅰ/Ⅱ类角度失谐方式，在KDP晶体中的谐波转换进行了研究，详细讨论了离散效应对三次谐波转换的影响。结果表明，离散效应不但降低了三次谐波转换效率，而且使谐波光束质量显著降低；初始入射基频光束腰半径较小时，离散效应是二次谐波转换和三次谐波转换效率降低的主要因素，失谐角对三次谐波转换的影响较小；随着束腰半径的增加，离散效应的影响逐渐减小，失谐角对三次谐波转换的影响逐渐增加。     

Analysis of broadband third harmonic generation with non-collinear angular dispersion in KDP crystals

A new scheme with non-collinear sum frequency generation (SFG) process of broadband third harmonic generation (THG) with two KDP crystals (one doubler and one mixer) using angular dispersion was proposed. The principle of broadband harmonic generation with non-collinear angular dispersion was presented. The comparison between the schemes of non-collinear and collinear SFG process was performed. The effects of the angular dispersion on the conversion efficiency, the pulse shape and the spectrum of the third harmonic field were discussed. The results show that, if proper angular dispersion is added to the fundamental and the second harmonic field outside the mixing KDP crystal, respectively, the broadband third harmonic conversion efficiency can be improved significantly. However, the difficulty of this scheme arises due to the requirement of two gratings with different angular dispersion outside the mixer. With the new scheme, it can be simpler that only one grating is needed to realize the broadband phase matching in the non-collinear SFG process. Although the fundamental and the second harmonic field inside the mixing crystal are non-collinear except the center wavelength, a blazing grating with proper angular dispersion for the fundamental field (twice as that for second harmonic field) can yield the well compensation for phase mismatching in the SFG process. Consequently, the conversion efficiency and the characteristics of the third harmonic field for the broadband third harmonic generation can be improved significantly.     

Enhanced third harmonic generation in periodic structures

We consider third harmonic generation in a periodic layered medium with alternate nonlinear media. We show enhanced third harmonic generation when the fundamental frequency matches one of the mode frequencies of the distributed feedback structure. The observed feature is explained in terms of large local field enhancement for the fundamental wave.     

Influence of third-order nonlinear and phase variations on third harmonic generation

A theoretical analysis is presented for third-harmonic generation in KDP for type I/type II angle-detuning scheme of high-intensity laser to produce third harmonic radiation near 0.35 μm. The effects of the third-order nonlinearity and the phase variations on the frequency conversion have been discussed. The results shown that the third-order nonlinear interactions decreases the tripling efficiency, and increases of the modulate strength of the output intensity of 3ω radiation. However, adjusting the angular detuning can compensate effectively the effects of third-order nonlinearity. Furthermore, 3ω conversion efficiency will drop with the increase of the degree of phase variations, and the increase 3ω conversion efficiency can decrease the influence of third-order nonlinearity on the conversion of third harmonics.     

Transverse and longitudinal phase-matching in third harmonic generation induced by the Bessel beams

Influence of transverse and longitudinal phase matching on the spectral and intensity properties of third harmonic generation induced by the Bessel pump beams has been investigated both theoretically and experimentally. Transverse phase matching, revealing itself as a ring pattern in the third harmonic intensity angular spectrum was demonstrated in both the normally and anomalously dispersive media. The conditions were found for the optimization of this conical third harmonic signal.     

Nonresonant third harmonic generation in thallium vapour

Feasibility of nonresonant third harmonic generation in thallium is investigated. The third order susceptibility, for driving frequencies in the visible region, is calculated and the phase matching with Ar as buffer gas is investigated. Other related quantities like coherence length, minimum pulse lengths required to get phase matching and power input requirements to achieve 50% conversion are also calculated.     

振幅调制和位相扰动对三倍频参数优化的影响

 详细讨论了入射基频光的振幅调制和位相扰动对三倍频转换效率和动态范围的影响，提出了提高三倍频转换效率和增大动态范围的方法。采用I/II类角度失谐的KDP晶体三倍频方案，对倍频晶体的参数进行了优化设计。     

High frequency SAW devices based on third harmonic generation

We demonstrate the third harmonic generation in a ZnO/Si layered structure to obtain high frequency SAW devices. This configuration eliminates the need of high lithography resolution and allows easy integration of such devices and electronics on the same wafer. A theoretical study was carried out for the determination of the phase velocity and the electromechanical coupling coefficient (K²) dispersion curves of the surface acoustic waves. These results are also in agreement with those measured on a SAW filter designed for the third harmonic generation and the operating frequency is up to 2468 MHz.     

High-order harmonic generation in sub-one-cycle regime

We theoretically investigate the high-order harmonic generation from the hydrogen atom driven by the laser pulses with the durations less than the optical cycle.It is found that the switching term of the laser field may have an obvious influence on the cutoff,intensity or plateau structure of the high-order harmonic spectrum.Generally speaking,the switching term can shorten the cutoff of the high-order harmonic spectrum for a relatively longer pulse and extend the cutoff for a relatively shorter pulse.     

High-order harmonic generation with a two-color laser pulse

We theoretically investigate the electron dynamics of the high-order harmonics generation process by combining a near-infrared 800 nm driving pulse with a mid-infrared 2000 nm control field.We also investigate the emission time of harmonics using time-frequency analysis to illustrate the physical mechanisms of high-order harmonic generation.We calculate the ionization rate using the Ammosov-Delone-Krainov model and interpret the variations in harmonic intensity for different control field strengths and delays.We find that the width of the harmonic plateau can be extended when the control electric field is added,and a supercontinuum from 198 to 435 eV is generated,from which an isolated 61-as pulse can be directly obtained.     

High-order harmonic generation with a two-color laser pulse

We theoretically investigate the electron dynamics of the high-order harmonics generation process by combining a near-infrared 800 nm driving pulse with a mid-infrared 2000 nm control field. We also investigate the emission time of harmonics using time-frequency analysis to illustrate the physical mechanisms of high-order harmonic generation. We calculate the ionization rate using the Ammosov-Delone-Krainov model and interpret the variations in harmonic intensity for different control field strengths and delays. We find that the width of the harmonic plateau can be extended when the control electric field is added, and a supercontinuum from 198 to 435 eV is generated, from which an isolated 61-as pulse can be directly obtained.     

Pulse shape analysis of second harmonic and difference-frequency generation

A computer model has been used to investigate the pulse shape evolution of interacting waves during second harmonic generation and difference-frequency generation processes. Using general assumption and coupled equations theory we describe in detail the back and forth energy transfer in connection with pulse shape. Two type of phase-match (modal phase-match and quasi-phase-match) with experimental relevance are discussed. In the case of quasi-phase-match uniform and chirped grating are discussed. The connection of the model with material properties and non-linear optical (NLO) strength is presented. The proposed model have allowed us to derive the essential feature of pulse shaping and compressing that rely on NLO process only.     

高次谐波辐射发射特性研究

报告由不同脉冲宽度(半高宽,FWHM)和不同载波-包络相位(CEP,Φ)的激光产生的高次谐波辐射能量输出时间特性即发射特性的研究结果. 计算表明,由宽度为几个周期的激光产生的高次谐波辐射的截止能量明显低于由无限长脉冲宽度激光产生的截止能量ω_max=3.17U_p+I_p(其中ω_max为光子角频率,U_p和I_p分别为激光有质动力势和原子的电离能). 例如,由两周期(FWHM),Φ＝15°的激光产生的高次谐波辐射的截止能量为ω_max=2.90U_p+I_p,此时发射特性单脉冲(即分布单脉冲)具有最大的能量带宽0.86U_p. 脉冲中心位置的载波相位和时间宽度分别为0.94rad(弧度)和1.29rad. 而该激光脉冲在Φ＝-75°时能产生截止能量为ω_max=2.70U_p+I_p,最大能量带宽为0.70U_p的双分布脉冲,其中心位置分别为-0.58rad和2.43rad,宽度分别为1.22rad和1.33rad. 随着激光脉冲宽度的增加,分布单脉冲的能量带宽比时间宽度下降得更快. 对于一定宽度的激光脉冲,所产生的分布单脉冲的能量带宽和时间宽度的CEP依赖性显示出180°的周期结构. 利用这个有趣的特点,在实验上可以通过调节CEP来选择分布脉冲的能量参数,也可用来定位和控制阿秒脉冲的时间参数. 理论分析指出,只要选择合适的阿秒X射线能量带宽,CEP不稳定性对于光电子谱和测量结果的影响将大为降低,甚至在最大程度上消除这种影响. 这些研究结果不仅有助于在物理上深入了解高次谐波辐射的动力学过程,而且对于进一步在实验上优化和选择阿秒单脉冲和双脉冲具有重要的参考和指导意义. 关键词： 高次谐波产生 鞍点方法 谐波发射特性 分布脉冲     

Chirp control on molecular harmonic generation and distribution in H2+

Chirp control on the molecular harmonic emission and distribution in H₂⁺ has been theoretically investigated. The results show that (i) with the introduction of the up-chirped pulse, the intensities of the higher harmonics are decreased; while with the introduction of the down-chirped pulse, the intensities of the higher harmonics can be enhanced. When E(t) < 0.0, the harmonic emission events (HEEs) from the positive-H are higher than those from the negative-H; while when E(t) > 0.0, the HEEs from the negative-H are higher than those from the positive-H. As a result, when the higher harmonics are produced from E(t) < 0.0 or E(t) > 0.0, the positive-H or the negative-H plays the main role in the harmonic spectra. (ii) The distribution of the higher harmonics from the two-H nuclei can be controlled with the introduction of the chirps and the carrier-envelope-phases of the laser field. (iii) Some minima on the harmonic spectra can be obtained, which is attributed to the coupling of the two-center interference and the electron-nuclear dynamics. (iv) By properly superposing the harmonics from the down-chirped pulse, an isolated attosecond pulse (IAP) with the duration of 45 as can be obtained, which is nearly 1 order of magnitude enhancement in pulse intensity compared with the chirp- free case.     

Strong-interaction approximation in the theory of second harmonic generation of intense short laser pulses

The method of strong interaction of nonlinear waves has been developed to analyse the second harmonic generation of intense laser radiation in the transient regime. Analytic solution for frequency conversion efficiency – taking into account the pump radiation depletion, influence of phase mismatch, dispersion of group velocities and higher nonlinearities – has been first obtained. A comparison of theoretically derived results and known experimental data has been conducted, and a general agreement of these results has been obtained. The optimal conditions of the second harmonic generation of neodymium laser radiation into femtosecond pulse duration range have been determined.     

局域表面等离激元诱导的三次谐波增强效应

本文研究了二维金三角形纳米颗粒阵列的三次谐波非线性光学效应. 金三 角纳米颗粒阵列是基于微球刻印技术制成. 采用反射式光学系统, 观察到该纳米结构在光谱分辨的飞秒激光照射下产生明显的三次谐波. 研究表明, 当激光抽运频率接近金纳米颗粒的局域表面等离激元共振位置时, 三次谐波信号得到增强; 三次谐波辐射方向满足动量匹配条件. 关键词： 微球刻印 三次谐波产生 表面等离激元     

应用级联倍频方法提高倍频系统输出稳定性研究

提出应用级联倍频方法提高倍频系统输出稳定性,并就该方法的有效性进行了理论分析和模拟计算.分析和计算结果不但证明级联倍频方法能实现倍频系统稳定输出,而且还表明可以通过仔细调节第一块倍频晶体中波矢方向 k 与光轴间夹角、两块倍频晶体间的间隔,能调节改变实现倍频系统最稳输出时所需第二块晶体的理论计算长度,使之与第二块倍频晶体的实际加工长度一致,最终实现系统稳定倍频输出.级联倍频方法在实现高输出稳定性的同时能实现高的倍频转换效率,对应用于光参量啁啾脉冲放大系统的高稳定抽运源系统的设计建造具有重要参考意义. 关键词： 级联倍频 稳定倍频输出 光参量啁啾脉冲放大     

采用双脉冲提高谐波谱的谱线密度

通过计算线偏振双脉冲激光场与一维氢原子模型相互作用产生的高次谐波辐射谱的含时Schrdinger方程. 研究发现,选用适宜的双脉冲激光组合可以增加谐波谱的密度,这样的谐波谱特别适合作为产生孤立阿秒脉冲的光源. 详细地分析了谐波谱线密度增加的物理机理,给出不同脉宽与谐波谱线密度增加的关系,从而可以有针对性地控制谐波谱线的疏密程度. 关键词： 高次谐波 阿秒脉冲 双脉冲     

Asymmetric molecular harmonic assisted generation of the attosecond X-ray pulse

In this paper, we theoretically investigate the asymmetric molecular high-order harmonic emission and the attosecond X-ray pulse generation when the asymmetric molecular HeH²⁺ ion is exposed to an intense chirped pulse. It is found that the molecular harmonics are strongly dependent on the internuclear distance R as well as the chirp parameter, showing a smooth and intense harmonic plateau at R = 8.0 a.u. and β = 0.4. Further, by adding a subharmonic controlling pulse with the optimised conditions, a supercontinuum with a bandwidth of 602 eV can be obtained. Finally, by properly superposing the selected harmonics in the supercontinuum region, a series of isolated X-ray pulses with durations of sub-80 as are directly generated.