Helicity of harmonic generation and attosecond polarization with bichromatic circularly polarized laser fields

We theoretically investigate the high-order harmonic generation(HHG) of helium atom driven by bichromatic counterrotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive-to negativevalue around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.     

The study of high-order harmonic generation using ellipticity-modulated laser pulse

The high-order harmonic generation using ellipticity-modulated laser field is investigated experimentally and theoretically. The laser field with time-varying ellipticity is obtained using two quarter wave plates. The harmonic spectral intensity peaks appear periodic for the localization induced by the ellipticity modulation of the laser pulse. We calculate the single-atom response to the driving laser field using the Lewenstein model and compare the calculated and measured harmonic spectra generated with ellipticity-modulated driving laser pulses. The theoretical simulation and the experiment results agree well with each other.     

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation(HHG) of neon gas with 5-fs and 25-fs driving laser pulses.It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHG spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs;whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs.The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse.The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.     

Single Ultrashort Attosecond Pulse Generation via Combination of Chirped Fundamental Laser and an Ultraviolet Controlling Pulse

We theoretically study the high-order harmonic generation (HHG) from a hydrogen atom in an intense few-cycle chirped fundamental laser in combination with an ultraviolet (uv) controlling pulse.The high-order harmonic spectrum is calculated by solving the time-dependent Schr¨odinger equation using the split-operator method.In our calculation,we present the difference of the high-order harmonic spectrum from one-dimensional (1D) model hydrogen atom and three-dimensional (3D) real hydrogen atom.We found that the plateau of the high-order harmonic generation from the 1D case and 3D case are all extended effectively to I p + 35U p due to the presence of the chirped laser pulse and the HHG supercontinuum spectrum is generated by adding an ultraviolet controlling pulse at a proper time,but the efficiency of the HHG for 3D case is more higher at the near cut-off region than the 1D case.Therefore,the generation of the attosecond pulse by synthesizing the harmonics near cut-off region have some slight differences between 1D and 3D simulations.As a real 3D case study,we show that an isolated 18 as pulse with a bandwidth of 232.5 eV is generated directly by optmizing the combination laser fields.     

Ultra-broadband supercontinuum generation with a nonlinear chirped pulse for controlling quantum path

We theoretically investigate the high-order harmonic generation in helium atom driven by a nonlinear chirped laser pulse with few-cycle duration. By employing appropriate chirp to the driving pulse, an efficient electric field waveform of controlling quantum path for ultra-broadband supercontinuous harmonics is realized, and then an isolated sub-50 as pulse with bandwidth of 739 eV can be significantly obtained.     

Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed.     

使用多色合成激光场拓展高次谐波谱平台区域的宽度

本文提出了一种多色合成激光方案用来有效的提高高次谐波谱平台区域的宽度. 我们首先通过在800nm基本激光脉冲上添加一束或者两束控制激光脉冲在理论上得到双色以及三色合成激光场;其次,通过求解一维含时薛定谔方程,计算得到了在多色合成激光场条件下的高次谐波谱,计算结果表明,多色合成激光场方案可以有效的拓展高次谐波谱的平台区域;最后,我们结合电子的经典回碰动能分布以及时频分布,分析了在多色场条件下高次谐波谱平台区域拓宽的机理和微观机制.     

Modulation of multiphoton resonant high-order harmonic spectra driven by two frequency-comb fields

The frequency-comb structure in the extreme ultraviolet(XUV) and vacuum ultraviolet(VUV) regions can be realized by the high-order harmonic generation(HHG) process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental-and third-harmonics, respectively.The many-mode Floquet theorem(MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamentaland third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts(CEPs) fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields.We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation(HHG) spectra,driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths.It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.     

使用多色合成激光场拓展高次谐波谱平台区域的宽度

本文提出了一种多色合成激光方案用来有效的提高高次谐波谱平台区域的宽度. 我们首先通过在800nm基本激光脉冲上添加一束或者两束控制激光脉冲在理论上得到双色以及三色合成激光场;其次,通过求解一维含时薛定谔方程,计算得到了在多色合成激光场条件下的高次谐波谱,计算结果表明,多色合成激光场方案可以有效的拓展高次谐波谱的平台区域;最后,我们结合电子的经典回碰动能分布以及时频分布,分析了在多色场条件下高次谐波谱平台区域拓宽的机理和微观机制.     

Supercontinuous high harmonic generation from asymmetric molecules in the presence of a terahertz field

We have investigated high-order harmonic generation from asymmetric molecules. It is found that supercontinuous high harmonics,which are produced from asymmetric molecules by significantly steering the ionization,are broken down when the electric field of the 5-fs driving laser pulse is increased to 0.16 a.u. The high harmonic generation from asymmetric molecules with the presence of a terahertz field is also investigated. This reveals that the terahertz controlled laser pulse significantly increases the energy difference between photons,emitted from the ejected electrons,in the first and second halves of the optical cycle at the centre of the driving laser pulse. In this way,a 200-eV broadband supercontinuum can be produced in the plateau,from which a 60-as pulse with a bandwidth of 60 eV can be directly obtained with a minor post-pulse.     

Generation of isolated attosecond pulses with a specific waveform two-color laser field

We theoretically propose a new method for generating intense isolated attosecond pulses during high-order harmonic generation (HHG) process by accurately controlling electron motion with a two-color laser field,which consists of an 800-nm,4-fs elliptically polarized laser field and a 1400-nm,～43-fs linearly polarized laser field.With this method,the supercontinua with a spectral width above 200 eV are obtained,which can support a ～15-as isolated pulse after phase compensation.Classical and quantum analyses explain the controlling effects well.In particular,when the pulse duration of the 800-nm laser field increases to 20-fs,sub-100-as isolated pulses can be obtained even without any phase compensation.     

双色光场驱动产生单个阿秒脉冲过程中的宏观效应

通过数值模拟三维传播方程研究了双色场产生高次谐波过程中宏观效应的影响,重点分析了阿秒脉冲的时域形状.在6 fs/800 nm和21.3 fs/400 nm组成的双色场中,通过调节双色场间的相对延时,双色场各焦点相对于气体盒子入口的间距和双色场束腰半径和气体压强等参数,可以在平台区得到超宽带连续谱.对该连续谱中420—540 eV的谐波直接进行滤波后可以得到时间同步性很好的单个阿秒脉冲.这种性质对实验上通过双色场方案产生超短的阿秒脉冲非常有利. 关键词： 传播效应 阿秒脉冲 时间同步性     

High-order harmonic generation of the N2 molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N₂ molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 eV is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.     

Restraint of spatial distribution in high-order harmonic generation from a model of hydrogen molecular ion

The spatial distribution in high-order harmonic generation(HHG) is theoretically investigated by using a few-cycle laser pulse from a two-dimensional model of a hydrogen molecular ion. The spatial distribution in HHG demonstrates that the harmonic spectra are sensitive to the carrier envelope phase and the duration of the laser pulse. The HHG can be restrained by a pulse with the duration of 5 fs in the region from the 90 th to 320th order. This characteristic is illustrated by the probability density of electron wave packet distribution. The electron is mainly located near the nucleus along the positive-x direction from 3.0 o.c. to 3.2 o.c., which is an important time to generate the HHG in the plateau area. We also demonstrate the time-frequency distribution in the region of the positive-and negative-x direction to explain the physical mechanism.     

The control of the electron quantum paths for the generation of isolated attosecond pulse using a three-color laser scheme

We theoretically propose a three-color laser scheme to enhance the high-order harmonic intensity and generate an isolated attosecond pulse. By adding a 3 fs, 1600 nm laser pulse to a synthesized two-color laser field (5 fs, 800 nm and 10 fs, 1200 nm), the harmonic intensity is effectively enhanced and an isolated attosecond pulse with duration 41 as is generated. In this scheme, the short trajectory is suppressed, the selection of the long quantum path can be achieved. We also investigate emission time of harmonics in terms of the time-frequency analysis and the semi-classical three-step model to illustrate the physical mechanism of high-order harmonic generation.     

Probing subcycle spectral structures and dynamics of high-order harmonic generation in crystals

Subcycle spectral structures and dynamics of high-order harmonic generation (HHG) processes of atoms and molecules driven by intense laser fields on the attosecond time scale have been originally studied theoretically and experimentally. However, the time scale of HHG dynamics in crystals is in the order of sub-femtosecond, and the carrier dynamics of HHG in crystals driven by subcycle laser pulses are largely unexplored. Here we perform a theoretical study of subcycle structures, spectra, and dynamics of HHG of crystals in mid-infrared laser fields subject to excitation by a subcycle laser pulse with a time delay. The HHG spectra as a function of time delay between two laser fields are calculated by using a single-band model for the intra-band carrier dynamics in crystal momentum space and by solving the time-dependent Schrödinger equation in velocity gauge for the treatment of multi-band crystal systems. The results exhibit a complex time-delay-dependent oscillatory pattern, and the enhancement and suppression of the HHG related to subcycle pulse are observed at the given time delay in either single-band or multi-band crystal systems. To understand oscillation structures with respect to the dependence for the subcycle laser fields, the time-frequency characteristics of the HHG as well as the probability density distribution of the radiation are analyzed in detail.     

Generation of an isolated sub-30 attosecond pulse in a two-color laser field and a static electric field

We theoretically investigate high-order harmonic generation(HHG) from a helium ion model in a two-color laser field,which is synthesized by a fundamental pulse and its second harmonic pulse.It is shown that a supercontinuum spectrum can be generated in the two-color field.However,the spectral intensity is very low,limiting the application of the generated attosecond(as) pulse.By adding a static electric field to the synthesized two-color field,not only is the ionization yield of electrons contributing to the harmonic emission remarkably increased,but also the quantum paths of the HHG can be significantly modulated.As a result,the extension and enhancement of the supercontinuum spectrum are achieved,producing an intense isolated 26-as pulse with a bandwidth of about 170.5 eV.In particular,we also analyse the influence of the laser parameters on the ultrabroad supercontinuum spectrum and isolated sub-30-as pulse generation.     

Unified microscopic-macroscopic formulation of high-order difference-frequency mixing in plasmas

We investigate high-order difference-frequency mixing in plasmas, taking into account the microscopic rescattering physics and propagation effects for the first time. We show that phase matching can occur over a broad frequency range, up to very high photon energies, and that it is confined to specific temporal and spatial windows. This gated phase matching mechanism is driven by the continuous phase slip between two driving fields and can be employed for manipulating the temporal, spatial, and spectral properties of high harmonic emission.     

双色高频激光作用下原子低阶次谐波的理论研究

通过数值求解含时薛定谔方程,研究了高频双色激光脉冲与原子相互作用产生的光辐射.研究表明,光辐射谱中既有基频光的谐波,又可观测到谐波能量附近的多个频率的光辐射产生,且辐射的峰值强度随着入射激光强度的提高呈指数增强,相邻辐射频率差值为入射的两束激光脉冲频率差.     

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.