Enhancement of High-Order Harmonic Emission and Isolated 7-as Pulse Generation with Rydberg Atoms by Using a Two-Color Laser Field

An efficient method to enhance the harmonic efficiency and generate an isolated attosecond pulse is proposed by preparing the initial state as a coherent superposition of the ground state and a Rydberg state in a two-color laser field. By numerically solving the time-dependent Schrödinger equation, we found that for the superposition state with the optimal Rydberg state, not only is the harmonic efficiency enhanced significantly, but also the width of the continuous spectrum is extended. The ionization probability and the time-frequency distribution of the harmonic spectrum are also calculated to understand the physical origin of the enhancement of the harmonic spectrum. In addition, by adjusting the parameter of the laser pulse, a supercontinuum with both higher conversion efficiency and slighter modulation can be observed. As a result, an isolated 7-as pulse with a bandwidth of 495 eV can be obtained after phase compensation.     

迭加态原子高次谐波发射的选频提高

本文研究了由体系的基态和较高激发态迭加形成的原子在强激光场作用下的高次谐波发射.观察电离波包的演化过程,阐明了谐波发射效率提高的原因.在给定入射激光条件下,通过选择不同的激发态,可以实现对谐波发射的控制.     

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.     

在双色场中使用相干叠加态来提高高次谐波的发射

本文提出了一种在双色场中使用相干叠加态来提高高次谐波发射效率的方法.我们首先通过在800 nm基本激光脉冲上添加一束控制激光脉冲在理论上得到双色合成激光场;其次,通过求解一维含时薛定谔方程,计算得到了体系处于不同初始态时的高次谐波谱,并结合时频分布图时其微观机理进行分析.计算结果表明,使用相干叠加态作为体系的初始态,通过调节两束激光的相对相位可以有效实现提高高次谐波的发射效率的目的.     

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.     

在双色场中使用相干叠加态来提高高次谐波的发射

本文提出了一种在双色场中使用相干叠加态来提高高次谐波发射效率的方法. 我们首先通过在800nm基本激光脉冲上添加一束控制激光脉冲在理论上得到双色合成激光场;其次,通过求解一维含时薛定谔方程,计算得到了体系处于不同初始态时的高次谐波谱,并结合时频分布图对其微观机理进行分析. 计算结果表明,使用相干叠加态作为体系的初始态,通过调节两束激光的相对相位可以有效实现提高高次谐波的发射效率的目的.     

Relativistic laser plasmas for novel radiation sources

Relativistic laser-plasma interaction results in new sources of short-pulsed x-ray radiation. Here we consider two options. The first one is betatron radiation of electrons accelerated in underdense plasmas and oscillating in transverse fields of the laser wake. This radiation is incoherent and broadband, the pulse duration is comparable with that of the driving laser. The second option is the high harmonic generation (HHG) from overdense plasma surfaces. This radiation is coherent. The relativistic high harmonics are phase locked and emerge in the form of (sub-)attosecond pulses. One- and three-dimensional regimes of relativistic HHG from overdense plasmas are considered.     

Harmonic Enhancement Mechanism of a Superposition State Atom Irradiated by Short Pulses

We investigate the high-order harmonic generation （HHG） of a model atom whose initial state is prepared in a superposition of its ground state and an excited state irradiated by different duration laser pulses. Compared to the HHG generated from an atom whose initial state is in its ground state, its conversion efficiency obtains some enhancement. The enhancement originates from the higher ionization rate （rather than the ionization yield） of the atom with superposition initial state.     

利用43飞秒的强激光脉冲实现单个阿秒脉冲输出的新机理

研究了多个光周期的长脉冲(43 fs)激光与氖原子相互作用产生的高次谐波的辐射特性. 通过计算机数值模拟，发现当激光场的强度超过饱和光强时，原子将会在几个光周期内被迅速电离，使得原子的高次谐波谱在截止区呈现出平滑的多平台结构. 同时，在高频波段离子产生的高次谐波的影响将变得十分显著. 通过选取合适的子平台进行频率叠加，在消除了离子的高次谐波的影响后，可以获得单个的阿秒软X射线脉冲. 关键词： 高次谐波 阿秒脉冲     

Extending the High-Order Harmonics Plateau and Isolated Attosecond Pulse Generation in an Intense Three-Color Laser Field

We propose a method to extend the high-order harmonics plateau and generate an isolated sub-10-as pulse by adding a weak control pulse (10 fs, 1600 nm) to modify a two-color laser field (5 fs, 800 nm; 10 fs, 400 nm). The numerical results show that the plateau is extended obviously in the three-color laser field regime. Additionally, the ionization rate and classical returning kinetic-energy maps are calculated to better understand the physical origin of the high-order harmonics generation (HHG). By means of the ionization probability and the time-frequency distributions, more features of the HHG are revealed. Furthermore, our simulations show that the width of the plateau and the relative conversion efficiency of the harmonic spectra are highly dependent on the relative phase. Finally, by adjusting the relative phase and superposing a properly selected range of the HHG spectrum, an isolated attosecond pulse with a duration of 7 as is obtained.     

Control of the supercontinuum spectrum bandwidth and emission efficiency for intense-isolated sub 40-as pulse generation by using a coherent superposition of states

We present a systematic study for generation of intense and short single as pulse in pre-excited He-ionic medium with the use of a same-frequency laser field synthesis. By preparing He+ ions in a coherent superposition of the ground and excited states, a depth of spectral modulation efficiently decreases and the efficiency of harmonic plateau increases. By means of optimizing the laser parameters, the broadband XUV supercontinuum achieve by the superposition state scheme. It shows that by increasing the harmonic order of the supercontinuum, emission efficiency increases. The detailed view shows that the short-trajectory contributes to each supercontinuous harmonic and long-trajectory fully suppresses. Finally, by superposing some properly selected harmonics from optimal spectrum, an intense and almost transform-limited isolated 38-as pulse can be generated without phase compensation. A single isolated 5-as pulse can be achieved after phase compensation. These results are explained by using classical and quantum time-frequency analyses.     

激光驱动晶体发射高次谐波的特性研究

通过数值模拟激光驱动下电子在周期性势阱中的动力学行为, 研究了晶体在激光场中发射高次谐波的特性. 研究发现在一定的激光波长和光强驱动下, 晶体发射的谐波谱会呈现出双平台结构, 经分析后得知第一个平台主要来自于最低导带与价带间的电流(电子-空穴对复合), 第二个平台主要来源于较高导带与价带间的电流(电子-空穴对复合), 且两个平台的截止位置处的能量都与激光场的振幅呈线性关系. 在少周期激光驱动下, 晶体谐波第二平台的截止位置与激光的载波相位呈单调变化, 由此我们提出可以利用晶体谐波第二平台的截止位置来确定少周期激光的载波相位. 进一步研究发现, 在啁啾激光驱动下, 晶体发射谐波谱的第二平台有较大变化, 第二平台的发射效率会随啁啾参数而改变, 能够通过改变啁啾激光场来提高晶体谐波第二平台的发射效率.     

Carrier-envelope phase measurement using plasmonic-field-enhanced high-order harmonic generation of H atom in few-cycle laser pulses

We investigate the plasmonic-field-enhanced high-order harmonic generation (HHG) of H atom driven by few-cycle laser pulses, by solving the time-dependent Schrödinger equation (TDSE). Compared with the homogeneous field, HHG spectra generated by inhomogeneous field exhibit two-plateau structure. We analyze the origin of the two plateaus by using the semiclassical trajectory method. Our results from both classical and TDSE simulations show that the cutoffs of the two plateaus are dramatically affected by the carrier-envelope phase (CEP) of laser pulse in the inhomogeneous field, even for a little longer pulse. Thus, we can determine the CEP of driving laser based on the cutoff position of HHG generated in the inhomogeneous field.     

椭圆偏振强激光场中准直的乙炔分子的高次谐波发射

We perform an experimental study on high-order harmonic generation (HHG) of aligned acetylene molecules induced by a 35-fs 800-nm strong laser field, by using a home-built HHG spectrometer. It is observed that the molecular HHG probability declines with increasing the laser ellipticity, which is in consistence with the deduction from the well-known tunneling-plus-rescattering scenario. By introducing a weak femtosecond laser pulse to nonadiabatically align the molecules, we investigated the molecular orbital effect on the HHG in both linearly and elliptically polarized driving laser fields. The results show that the harmonic intensity is maximum for the molecular axis aligned perpendicularly to the laser electric field. It indicates that both the highest occupied molecular orbitals (HOMO) and HOMO-1 contribute to the strong-field HHG of acetylene molecules. Our study should pave the way for understanding the interaction of molecules with ultrafast strong laser fields.     

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.     

Tunable spectral shift of high-order harmonic generation in atoms using a sinusoidally phase-modulated pulse

High-order harmonic generation (HHG) from an atom illuminated by a sinusoidally phase-modulated pulse is investigated by solving the time-dependent Schrödinger equation. The spectral shift that occurs in atomic HHG can be achieved easily using our laser pulse. It is shown that the photon energy of the generated harmonics is controllable within the range of 1 eV. The shift of the frequency peak position is rooted in the asymmetry of the rising and falling parts of the laser pulse. We also show that by varying the phase parameters in the frequency domain of the laser one can adjust and control the shift in atomic harmonic spectra.     

High-order harmonic generation of pulses with multiple timescales: selection rules,carrier envelope phase and cutoff energy

ABSTRACT

High harmonic generation (HHG) is sensitive to the carrier envelope phase (CEP) of its driving laser field if it is a sufficiently short pulse (several-cycle pulse). Here we show that strong CEP effects can also be found in HHG from long duration multi-cycle pulses (up to 200?fs at 800?nm central wavelength). We find that HHG from multi-cycle pulses may be CEP dependent when the driving pulse exhibits two distinct timescales (multi-timescale pulse): (i) a short timescale associated with the average frequency, and (ii) a long timescale associated with the pulse’s temporal periodicity. The interplay of these timescales results in significant changes to both the cutoff frequency, and the appearance of symmetry allowed harmonics in the spectrum as function of CEP, similar to HHG from several-cycle pulses. We relate this effect to the multi-timescale intensity variations in the driving pulse, and construct an analytical condition to access the phenomenon. Lastly, we numerically demonstrate reconstruction of the CEP through HHG from long duration multi-timescale pulses. Our work may be useful in several areas of strong-field physics and attosecond science, for example, allowing spectroscopy of multi-timescale processes (e.g. HHG from vibrationally active media), and paving the way towards CEP characterisation using long pulses.     

高次谐波发射中的超拉曼发射

本文研究了单电子分子离子与强激光场相互作用下的高次谐波发射谱。从光谱中观察到除了存在入射激光频率奇数倍的谐波谱线外,还存在其他频率的超拉曼谱线。在理论上阐明了超拉曼谱线不同于高次谐波谱线的物理产生机制。研究发现当体系初态为基态和较高激发态的叠加态时能够较清晰的观察到超拉曼谱线,从而说明激发态电子的布居对超拉曼谱线的产生起着关键作用。因此在实验上,制备一个叠加态使得电子从激发态电离,是观察到超拉曼谱线的重要条件。     

Isolated ultrashort attosecond pulse generation from a coherent superposition state in a few-cycle chirped laser pulse

We theoretically demonstrate an efficient method for producing an isolated ultrashort attosecond pulse by high-order harmonic generation in an intense few-cycle chirped laser pulse. Our simulation calculations show that the harmonic spectrum reveals an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared in a coherent superposition of the ground and first excited states. It is also shown that the method can enhance the short quantum path and eliminate the long one, and then an isolated 23-attosecond pulse is produced. By properly adjusting the chirp parameter, a clean single attosecond pulse as short as 11 attoseconds is directly obtained.     

双色激光脉冲辐照叠加态生成强的38 as孤立脉冲

采用多周期800 nm激光组合1 600 nm红外激光脉冲辐照氦离子He⁺叠加态生成强、短孤立阿秒脉冲. 结果表明,相对于800 nm单色激光辐照基态情形,截止频率由70次谐波大幅度展宽到280次谐波,且获得频宽为108 eV并由单一短量子路径贡献的连续辐射谱,叠加该连续谱210次到280次谐波获得了脉宽为38 as的强、短孤立脉冲,其强度比单色场情形提高了11个数量级. 研究进一步表明,两束激光脉冲的时间延迟从0.05π到－0.05π之间获得的孤立阿秒脉冲是最优化的,且脉冲持续时间 关键词： 组合激光脉冲 相干叠加态 阿秒脉冲