Destructive interference during high harmonic generation in mixed gases

We report on the first experimental evidence of the destructive and constructive interference of high harmonics generated in a mixed gas of He and Ne, which facilitates the coherent control of high harmonic generation. Theoretically, we develop an analytical model of high harmonic generation in mixed gases and succeed in reproducing the experimental results and deriving the optimization conditions for the process. The observed interference modulation is attributed to the difference between the phases of the intrinsically chirped harmonic pulses from He and Ne, which leads to a novel method for broadband measurement of the harmonic phases and for observing the underlying attosecond electron dynamics.     

加深光学速调管自发辐射谱调制的研究

相干谐波储存环自由电子激光的相干增强因子正比于光学速调管自发辐射谱调制率的平方,加深光学速调管自发辐射谱的调制是相干谐波储存环自由电子激光研究的重要一环.介绍在中国科学技术大学800MeV电子储存环上所进行的加深光学速调管自发辐射谱调制的研究工作,调制率已从0.236提高到了0.80,为相干谐波产生实验奠定了基础.     

Optical tubular structures produced by diffraction of circular apertures

The coherent optical field diffracted through a circular aperture can produce optical tubular structures single-closed or twice-closed, by choosing the suitable experimental conditions. Both structures can be generated on the same optical set-up only by changing the experimental parameters involved on it, i.e. the aperture radius, the on-axis observation point, the wavelength and the on-axis source position. Such structures, generated in this way, have strong intensity gradients at their edges and their dimensions can be controlled in order to make them appropriate structures for optical trapping of particles, for instance.     

相干谐波储存环自由电子激光

相干谐波自由电子激光不用光学谐振腔及反射镜,可望工作在紫外和真空紫外光波段,是第四代同步辐射光源的可能途径之一。由于对束流品质要求很高,相干谐波自由电子激光首先是以储存环作为驱动器而发展起来的。由于光阴极微波电子枪的发展和直线加速器技术的进步,目前,已开始有直线加速器驱动的高增益相干谐波自由电子激光的建议。文章介绍了相干谐波储存环自由电子激光的原理、现状及展望。     

Evidence of high harmonics from echo-enabled harmonic generation for seeding x-ray free electron lasers

Echo-enabled harmonic generation free electron lasers hold great promise for the generation of fully coherent radiation in x-ray wavelengths. Here we report the first evidence of high harmonics from the echo-enabled harmonic generation technique in the realistic scenario where the laser energy modulation is comparable to the beam slice energy spread. In this experiment, coherent radiation at the seventh harmonic of the second seed laser is generated when the energy modulation amplitude is about 2-3 times the slice energy spread. The experiment confirms the underlying physics of echo-enabled harmonic generation and may have a strong impact on emerging seeded x-ray free electron lasers that are capable of generating laserlike x rays which will advance many areas of science.     

多色部分相干偏心光束在non-Kolmogorov湍流中的传输

推导了多色部分相干偏心光束在non-Kolmogorov 湍流中传输的总光强、轴上光谱、相干度的解析表达式, 研究了光束偏心参数β 、湍流广义指数α和源光谱带宽Ω对激光传输特性的影响. 研究表明: β越大, 则光束重心偏离传输轴越远, 相干度的不对称性越明显, 但是, β对轴上光谱几乎没有影响; 湍流广义指数α对总光强、 轴上光谱和相干长度的影响是非单调的, 当α=3.1时, 湍流对光束传输特性的影响最大. 值得指出的是: 在某些传输距离处, 不同α对应的轴上光谱位移量相同; 在某些传输距离处, 轴上光谱位移量为零, 且该传输距离与Ω无关, 但湍流使得该传输距离缩短. 所得结论对多色部分相干偏心光束在 湍流大气中传输的相关应用具有重要意义. 关键词： non-Kolmogorov湍流 多色部分相干偏心光束 光谱强度 相干度     

LANDAU LEVELS IN A TWO-DIMENSIONAL NONCOMMUTATIVE SPACE: MATRIX AND QUATERNIONIC VECTOR COHERENT STATES

The behavior of an electron in an external uniform electromagnetic background coupled to an harmonic potential, with noncommuting space coordinates, is considered in this work. The thermodynamics of the system is studied. Matrix vector coherent states (MVCS) as well as quaternionic vector coherent states (QVCS), satisfying required properties, are also constructed and discussed.     

Generation of ultrashort coherent vacuum ultraviolet pulses using electron storage rings: a new bright light source for experiments

We demonstrate for the first time that seeded harmonic generation on electron storage rings can produce coherent optical pulses in the vacuum ultraviolet spectral range. The experiment is performed at Elettra, where coherent pulses are generated at 132 nm, with a duration of about 100 fs. The light source has a repetition rate of 1 kHz and adjustable polarization; it is very bright, with a peak power several orders of magnitude above that of spontaneous synchrotron radiation. Owing to high stability, the source is used in a test photoemission electron microscopy experiment. We anticipate that seeded harmonic generation on storage rings can lead to unprecedented developments in time-resolved femtosecond spectroscopy and microscopy.     

Two-Photon Ionization of He through a Superposition of Higher Harmonics

We present experimental results and theoretical analysis of two-photon ionization of He by a superposition of the 7th to the 13th harmonic of a Ti:sapphire laser. Solving the time-dependent Schr?dinger equation for He in a coherent polychromatic field, the He+ yield is calculated. From this yield the number of He+ ions produced has been estimated and found in reasonable agreement with its measured value. The present results establish the feasibility of a second-order autocorrelation measurement of superposition of harmonics, and thus they represent the precursor towards the direct temporal characterization of attosecond pulse trains.     

Gradient enhanced third harmonic generation in a femtosecond filament

The third harmonic generated during femtosecond filamentation in air is studied. By establishing a gradient from atmospheric pressure to vacuum conditions, we truncate the filament abruptly at defined positions. The introduction of the pressure gradient leads to an enhancement of the generated third harmonic radiation by 3 orders of magnitude. This effect is attributed to an improved on-axis phase-matching condition. We investigate the spectral shape and the conversion efficiency of the third harmonic during the propagation in the filament.     

Spectral shift of partially coherent flattened multi-Gaussian beams with a hole passing through ABCD optical systems

Based on flattened multi-Gaussian beam model and partially coherent theory, the expression for cross-spectral density of partially coherent flattened multi-Gaussian beam with a hole was given. The analytical expression for on-axis spectrum of partially coherent flattened multi-Gaussian beam with a hole passing through a paraxial ABCD optical system was derived. The spectral shifts of the partially coherent flattened multi-Gaussian beam with a hole propagating in free space and passing through the lens have been analyzed. The effects of the beam profile, spatial coherence parameter and the system parameters on the relative spectral shift have been discussed. Our results show that the inner radius of flattened multi-Gaussian beam with a hole determines its relative spectral shift in near field and its outer radius determines that in far field. For the flattened multi-Gaussian beam with a hole passing through a lens, the on-axis relative spectral shift changes sharply near the focal plane with the increase in Fresnel number and the spatial coherence parameter.     

相干谐波辐射的测量方法和系统

 研究了光学速调管自发辐射谱,其调制率已达0.80,即将进行相干谐波辐射实验。介绍了相干谐波辐射实验的主要考虑因素、相干增强因子的测量方法和测量系统以及种子激光与电子束的准直和同步。     

Quantum polarization fluctuations of partially coherent dark hollow beams in non-Kolmogorov turbulence atmosphere

Non-classical polarization properties of dark hollow beams propagating through non-Kolmogorov turbulence are studied. The analytic equation for the polarization degree of the quantization partially coherent dark hollow beams is obtained.It is found that the polarization fluctuations of the quantization partially coherent dark hollow beams are dependent on the turbulence factors and beam parameters with the detection photon numbers. Furthermore, an investigation of the changes in the on-axis propagation point and off-axis propagation point shows that the polarization degree of the quantization partially coherent dark hollow beams presents oscillation for a short propagation distance and gradually returns to zero for a sufficiently long distance.     

储存环相干谐波自由电子激光器新光学速调管工作状态分析

 合肥国家同步辐射实验室（NSRL）正在开展储存环相干谐波自由电子激光研究，为了在较高电子束能量下进行实验、且具有较高的相干辐射强度，光学速调管从原来的对称结构改造为非对称结构。分析了相干谐波实验中各参数之间的关系，及对相干谐波辐射的影响，并给出了两种工作状态下的最佳工作参数。     

Preparation of multicomponent motional coherent and squeezed coherent states of a trapped ion

In this paper, we present a scheme for preparation of multicomponent motional coherent and squeezed coherent states of the quantized centre of mass of an ion trapped in a one-dimensional harmonic potential and driven by two travelling-wave laser beams tuned to the nth red and blue vibrational sidebands, respectively. In addition, our scheme also provides experimental possibility for quantum state engineering.     

Changes in the spectrum, polarization, and coherence of stochastic spatially and spectrally partially coherent electromagnetic J0-correlated Schell-model pulsed beams propagating in free space

Taking the stochastic electromagnetic J ₀-correlated Schell-model pulsed (JSMP) beam as a typical example of stochastic spatially and spectrally partially coherent electromagnetic pulsed beams, the analytical expressions for the cross-spectral density matrix, spectral density, spectral degree of polarization and spectral degree of coherence of stochastic electromagnetic JSMP beams propagating in free space are derived, and used to study the changes in the spectrum, polarization, and coherence of stochastic spatially and spectrally partially coherent electromagnetic JSMP beams. It is shown that the on-axis spectrum is blue-shifted in free-space propagation and is dependent on the pulse temporal coherence length and spatial correlation parameter. The distribution of the on-axis spectral degree of polarization depends on the frequency and spatial correlation parameter. The spectral degree of coherence increases with increasing pulse temporal coherence length. The results derived are interpreted physically.     

部分相干平顶光束在梯度折射率介质中的传输特性

利用广义惠更斯-菲涅尔衍射积分方法,推导出部分相干平顶光束在梯度折射率介质中传输的交叉谱密度函数解析表达式,对部分相干平顶光束在梯度折射率介质中的传输特性进行了分析,并进一步讨论了介质梯度折射率系数、光束阶数以及光束相干性对传输特性的影响.结果表明,部分相干平顶光束在梯度折射率介质中传输时的轴上光强分布呈现周期性变化了,且周期由梯度折射率系数决定,而与光束相干性无关.光束阶数越高、相干性越好、介质梯度折射率系数越大,则在介质中的轴上峰值光强越大.并且,在光强峰值位置附近的光强分布随传输距离的变化非常剧烈.     

A proposed method to characterize high harmonic microbunching in EEHG operation of SDUV-FEL

The echo-enabled harmonic generation(EEHG)scheme offers remarkable efficiency for generatinghigh harmonic microbunching with a relatively small energy modulation.A proof of principle experiment of the EEHG scheme has been proposed at the Shanghai deep ultraviolet(SDUV)free electron laser(FEL)facility,where the 4th harmonic of the seed laser is amplified in the 9 m long radiator.To explore the advantages of the EEHG scheme,in this paper,a method of measuring the coherent high harmonic radiation of the radiator is proposed to investigate the electron beam microbunching corresponding to the 10th-20th harmonics of the seed laser.The principle of the proposed method,comparisons with existing methods and the simulation results are presented and discussed.     

Beam properties of fully optimized,table-top,coherent source at 30 nm

We present results on development and experimental implementation of a 1-kHz, coherent extreme ultraviolet (XUV) radia- tion source based on high-order harmonic generation of the femtosecond, near-infrared laser pulses produced by the titanium-doped sapphire laser system (35 fs, 1.2 mJ, 810 nm) at the Institute of Physics AS CR / PALS Centre. The source comprises a low-density static gas cell filled with a conversion medium, typically argon. The comprehensive optimization of the XUV harmonic source has been performed with respect to major parameters such as gas pressure in the cell, cell length, position of the focus of the driving laser field with respect to the gas cell position, size of the driving near-infrared laser beam, chirp of the femtosecond pulse, and the focal length of the lens deployed in the experimental setup. Harmonic spectra were recorded using an XUV transmission grating spectrometer developed specifically for this purpose. Detailed characterization of the XUV source has been performed including measurement of the XUV beam profile, M² parameter of the beam, absolute energy, and spatial coherence.     

Effect of spatial coherence on the scintillation properties of a dark hollow beam in turbulent atmosphere

With the help of a tensor method, we derive an explicit expression for the on-axis scintillation index of a circular partially coherent dark hollow (DH) beam in weakly turbulent atmosphere. The derived formula can be applied to study the scintillation properties of a partially coherent Gaussian beam and a partially coherent flat-topped (FT) beam. The effect of spatial coherence on the scintillation properties of DH beam, FT beam and Gaussian beam is studied numerically and comparatively. Our results show that the advantage of a DH beam over a FT beam and a Gaussian beam for reducing turbulence-induced scintillation increases particularly at long propagation distances with the decrease of spatial coherence or the increase of the atmospheric turbulence, which will be useful for long-distance free-space optical communications.