超短脉冲强激光场中氪的高次谐波研究

报道了在 10 5fs 0 .6TW激光装置上以氪气为介质产生高次谐波的实验结果。实验中主要研究了不同气压、不同激光能量、不同偏振特性对谐波辐射的影响 ,并从理论上进行了分析。结果表明 ,要想得到较强的谐波辐射信号 ,必须有适当的气体密度和激光能量 ;要想得到较高次的谐波辐射信号 ,除了有较高的激光功率密度外 ,还必须具备灵敏度高的探测器。     

高次谐波辐射的实验研究

报道了在本实验室建立的45fs-2 TW级超短超强钛宝石激光装置上.以Ar为非线性工作介质.进行的高次谐波实验的初步研究结果.初步研究了激光能量和气体密度对高次谐波的影响,目前得到的最高次谐波为第25次谐波.相应波长为31.4nm.还检验了激光器、压电气阀、光谱仪以及信号处理和记录系统的工作性能.实现了相互间的协调.     

利用非均匀抽运探测激光增强阿秒脉冲强度

为了增强高次谐波光谱及阿秒脉冲的强度，采用数值求解薛定谔方程的方法，理论研究了H₂+在抽运探测激光驱动下高次谐波辐射特点。结果表明，在抽运激光驱动下，H₂+首先被激发到多光子共振电离区间，进而增大电离几率; 随后在探测激光驱动下，谐波辐射强度得到增强; 当采用不对称非均匀激光场时，谐波截止频率可以进一步延伸，并且谐波平台区只由单一谐波辐射能量峰贡献; 最后通过叠加傅里叶变换后的谐波可获得脉宽在32as的脉冲。该研究对单个阿秒脉冲的产生是有帮助的。     

中红外双色场量子轨道调控下宽谱带阿秒脉冲的产生

采用由2000 nm中红外驱动场和1000 nm弱倍频调制场组成的双色场作为驱动源,提出了产生水窗波段宽带高次谐波谱的新方法。弱倍频场对原子中电子的电离阶段进行调制,不仅扩大了连续谱的带宽,而且大大提高了连续谱的产生效率。通过求解一维氦原子模型下含时薛定谔方程,发现该双色场驱动下能够得到谱宽为190 eV的连续谱,并且较2000 nm中红外单色驱动下,连续谱谐波的效率提高了两个数量级。此外,计算中还发现调整双色场的相对载波包络相位,能够增强一个轨道电子的贡献同时抑制另一个电子轨道的贡献,利用这种方法能够得到脉宽为70 as的单阿秒脉冲。     

Generation of high-order rotational lines in hydrogen by four-waveRaman mixing in the femtosecond regime

More than 40 rotational Raman lines are generated using an 800-fs Ti:sapphire laser. The spectral region extends from the near-infrared to the far-ultraviolet with a considerably flat intensity distribution. The effects of laser polarization, pulsewidth, hydrogen pressure, and focusing conditions on the efficiencies of stimulated Raman scattering, four-wave Raman mixing, self-phase modulation, self-focusing, and harmonic generation are investigated. A white light continuum, generated by self-phase modulation, acts as a seed beam for the generation of high-order rotational lines through four-wave Raman mixing. Strong self-phase modulation, however, suppresses the generation of the Raman emission, due to a line broadening of the pump beam. Thus, optimization of experimental conditions is necessary for the efficient generation of high-order rotational lines     

静电场对强激光场中氢原子产生高次谐波的影响

利用分裂算符方法数值求解了真实氢原子在强激光场和静电场作用下的含时薛定谔方程,研究了静电场对强激光场中氢原子产生高次谐波的影响.研究表明,静电场加入后,奇次谐波的强度有所降低,并且谐波谱中出现偶次谐波和双平台结构.利用小波变换,观察了高次谐波在不同时刻的发射情况.借助于半经典理论,解释了在静电场作用下高次谐波发生变化的机制.     

半周期波形调控产生超宽谐波光谱平台区

为了获得高强度超宽谐波光谱平台区,采用数值求解含时薛定谔方程的方法,提出利用多色组合场调控半周期波形来获得最佳半周期谐波辐射条件。结果表明,通过调节双色场啁啾参量可以获得最佳的负向半周期波形;通过调节啁啾延迟可以获得最佳的正向半周期波形;在上述波形下,谐波截止能量得到延伸;引入紫外激光场后,在紫外共振增强电离的影响下,可以使谐波强度得到增强;当紫外光能量满足单、双光子共振增强电离时,谐波强度增强明显;随着紫外光子能量减小,谐波强度增强效果减弱;通过叠加谐波平台区谐波还可获得脉宽在50as以下的单个脉冲。这一结果对高次谐波调控以及阿秒脉冲产生是有帮助的。     

Experiments concerning the nature of trapped-plasma mode harmonic extraction from P+-N-N+avalanche diodes

An experimental investigation has clarified the nature of both fundamental and harmonic extraction from p⁺-n-n⁺avalanche diodes operated in the trapped-plasma mode. A standard coaxial cavity was found to be suitable for low-order (nleq5) harmonic generation. Calculations revealed that the circuit behaved as a bandpass filter which allowed power to be extracted at a particular output harmonic while "reactively" terminating the diode at other harmonics. Measurements were made of external current anddV/dtwaveforms for illustrative cases of fundamental and low-order harmonic extraction. IntegrateddV/dtwaveforms exhibited overvoltages which increased with increasing peak displacement currents. In general, the duration of the plasma-removal period of high current increased with increasing overvoltage. Carroll's [16] suggested dichotomy of voltage waveforms into independent reactive and resistive components was found to be appropriate for optimum first and second harmonic extraction. For third, fourth, and fifth harmonic extraction, however, it was observed that optimum efficiency was associated with significant changes in the physical states of the diode. Fourier analysis of the integrateddV/dtwaveforms for low-order harmonic extraction revealed that maximum voltage amplitudes occurred at the harmonic being extracted. Combination ridge waveguide coaxial cavities were used for high-order harmonic extraction inXandKband. A far-infrared interferometer detected components in the power spectrum from theK-band cavity at frequencies up to 40 GHz. It was found empirically that, to first order, the optimum observed extraction efficiency was equal to 50 percent/ (harmonic number).     

Second-harmonic generation with the GaAs laser

The second harmonic of pulsed GaAs injection lasers was generated with the nonlinear crystal α-iodic acid (HIO3) phase matched by angular tuning. The injection laser was operated with an external cavity, and the HIO3crystal was placed inside the cavity to increase the conversion efficiency. The maximum harmonic power observed was 15 μW with a fundamental power of 4.5 watts inside the cavity. Since the lasing bandwidth was typically 30 Å and the calculated bandwidth for collinear harmonic generation was only 6.3 Å, a grating was used to narrow the lasing bandwidth to 3 Å. The expected large increase in harmonic power did not occur. It is shown that this is due to sum-frequency generation by laser modes lying outside the harmonic bandwidth. It is also found that the spectral width of the harmonic becomes bigger than the collinear bandwidth when a divergent laser beam is used.     

Focus conditioning effects on molecular field-free alignment observed with high-order harmonic generation

We investigate the focus conditioning effects on molecular field-free alignment observed with high-order harmonic generation(HHG) from CO2 molecules.We also experimentally demonstrate that both the spectral shape and alignment signal of HHG significantly vary with changing focus position.A maximal alignment signal is achieved at a given focus position because of the optimal intensity of the driving laser.This intensity is related to the ionization potential of the molecules.These results indicate that a unique focus position provides an optimal alignment signal for practical applications.     

Millimeter wave semiconductor diode detectors, mixers, and frequency multipliers

The devices described include video detectors, mixers, harmonic mixers, and harmonic generators. A general discussion of the theoretical potential and limitations of these diodes is given, based on the equivalent circuit representation. This model is shown by experimental results to be valid up through 600 GHz. Diode materials, structures and forming techniques are described which have resulted in significantly improved electrical performance, as well as improved reliability and repeatability. Experimental results are quoted for detector sensitivities from 70 through 420 GHz, fundamental mixer conversion losses from 35 through 300 GHz, harmonic mixer conversion losses from 93 through 600 GHz, and harmonic generation efficiencies from fundamental frequencies of 60 through 140 GHz. The repeatability of mixer performance indicates that construction of balanced mixers is now feasible throughout the millimeter wavelength region. The improved harmonic mixer conversion losses permit construction of practical superheterodyne receivers at frequencies where local oscillator sources are impractical or unavailable. The harmonic generation capability at high frequencies (for example, at 140 GHz, one mW with diodes in the variable resistance mode and 15 mW with diodes in the varactor mode) is seen to permit the extension of frequencies for solid-state oscillators. If full advantage is taken of the capabilities of these devices, a large variety of millimeter wave systems can be assembled, including high resolution raders, high thermal sensitivity radiometers, broad bandwidth communication systems, as well as laboratory systems for mearsurement use or spectroscopic studies.     

基于基片集成波导的W波段基波注入锁定谐波振荡器

研制了一种基于基片集成波导的W波段平面注入锁定谐波振荡器.为了获得大的注入功率,注入锁定谐波振荡器采用基波端口强耦合结构,利用谐波提取技术的频率倍频作用,自由振荡输出频率在90.2 GHz附近.当基波注入信号在45.08 GHz附近时,锁定带宽大干120 MHz,输出功率大于6.5 dBm.将该平面集成的注入锁定谐波振荡器与低频参考信号同步,能够产生稳定的W波段低相噪信号.     

基于基片集成波导的W波段基波注入锁定谐波振荡器

研制了一种基于基片集成波导的W波段平面注入锁定谐波振荡器.为了获得大的注入功率, 注入锁定谐波振荡器采用基波端口强耦合结构, 利用谐波提取技术的频率倍频作用, 自由振荡输出频率在90.2 GHz附近.当基波注入信号在45.08 GHz附近时, 锁定带宽大于120 MHz, 输出功率大于6.5 dBm.将该平面集成的注入锁定谐波振荡器与低频参考信号同步, 能够产生稳定的W波段低相噪信号.     

宽带激光二倍频特性的理论分析

在激光核聚变(ICF)物理实验中,采用相干度较小的宽带倍频激光,能有效抑制高阶非线性效应和提高靶的激光辐照均匀性。根据不同频率成分和频耦合原理,建立了宽带二倍频转换的物理模型,作了理论计算分析,发现二倍频光的光谱变窄,结果对宽带三倍频研究具有一定应用价值。     

利用晶体串接实现宽带三次谐波转换

从耦合波方程出发,分别在小信号、高功率密度(1.5 GW/cm2)条件下研究了晶体串接三次谐波(THG)转换方案,并利用实验验证了该方案。实验观察到这种方案可以有效提高宽带激光三次谐波转换效率,但混频晶体之间的距离对宽带三次谐波转换效率影响明显。在1.5 GW/cm2平均功率密度下,实验中最佳距离25 mm处,对带宽3.5 nm的啁啾脉冲取得了19.75%的三倍频效率,这对于高功率激光宽带三次谐波转换的解决很有意义。     

光子晶体中二阶非线性过程产生三次谐波的研究

运用非线性耦合波理论,分析了在非线性光子晶体中,通过二阶过程产生三次谐波的准相位匹配条件。数值模拟结果表明,在入射光中引入倍频光信号,能够极大地改变三次谐波的转换效率。当入射光中倍频光与基频光强度比以及两者复振幅的初相角满足一定的条件时,可在宽的入射光强范围内获得很高的三次谐波转换效率。     

高强度飞秒脉冲单块BBO晶体三倍频实验

针对脉冲宽度100 fs,带宽25 nm,能量6 mJ左右的超短脉冲基频光(经过透镜缩束后峰值光强为200~900 GW/cm2),采用单块厚度为1.5 mm的BBO晶体进行了三倍频实验研究。在入射基频光强度约300 GW/cm2时,得到的三倍频转换效率约0.8%。采用分步傅里叶变换及四阶龙格-库塔算法,对描述飞秒脉冲单块晶体三倍频的耦合波方程组进行了数值计算。研究结果表明,三倍频光主要是由三阶非线性效应产生的;基频光带宽较大是限制三倍频转换效率的主要因素之一;对基频光的入射角度及方位角进行优化,可较好地补偿非线性相位失配,提高单块晶体三倍频转换效率。     

双色飞秒强激光作用下的CO2分子高次谐波

实验研究了双色场条件下CO2 分子高次谐波辐射, 通过改变充气气压获得偶次谐波最大为28th。此外,通过变化气体盒内激光焦点位置从而显著改变24th 谐波光子能量。实验发现在气体盒內谐波强度存在两个极大值,这是因为不同区域内相位匹配出现长短轨道选择的结果。实验结果表明,双色场驱动的分子高次谐波可成为具有发展前景的多波长可调谐相干XUV 光源。     

基于级联外部调制器产生宽光梳系统的研究

本文研究了基于双边带调制方式采用级联两个强度 调制器(MZM)或级联MZM和相位调 制器(PM)分别生成平坦宽光梳的两种方案。通过理论分析得到合适的调制器关联变量-调 制电压和偏置电压的关系。在此基础上,我们分别讨论了高阶边带以及相位偏转对两种方案 产生宽光梳平坦度的影响。两种方案如果要达到相同的平坦度,级联相同MZM所输出的光梳 频带宽度可高达500 GHz,比级联MZM和PM的输出频带更宽;如果两种 方案在相同的输出光梳 频带宽度(300 GHz)下,级联MZM的平坦度可以达到0. 2 dB,而级联MZM和PM系统的平坦度只 有2.1 dB。因此级联两个强度调制器产生的宽光梳在光梳平坦度方面 性能更为良好。这为产生平坦光梳及调制器的灵活运用提供了一种解决方案。     

红外激光与远紫外激光场驱动H2+辐射谐波

为了了解H₂+谐波辐射的过程,采用数值求解非玻恩-奥本海默近似薛定谔方程的方法,理论研究了H₂+在10fs/800nm红外激光与6fs/30nm远紫外激光驱动下谐波辐射的特点。结果表明,谐波辐射的贡献主要来源于拉比振荡、多光子共振电离、电荷共振增强电离以及离解态电离;随着远紫外光的加入,谐波光谱呈现能量间隔为远紫外光子能量的多重谐波截止结构;当远紫外光与红外激光的延迟时间大于零或小于零时,谐波光谱呈现红移和蓝移的现象。该研究对理解分子谐波辐射过程是有帮助的。