Measuring Carrier-Envelope Phase of Few-Cycle Laser Pulses Using High-Order Above-Threshold Ionization Photoelectrons

We experimentally demonstrate the measurement of carrier-envelope phase of few-cycle laser pulses. We have built a stereo above-threshold ionization setup. The photoelectron energy spectra of high-order above-threshold ionization are measured in both the left and right directions in the linearly polarized laser fields. It is shown that the left-right asymmetry of the spectra is dependent on carrier-envelope phase of few-cycle laser pulses. Two asymmetry parameters from the low-energy and high-energy regions at the above-threshold ionization plateau map a phase ellipse, in which the points indicate the absolute value of carrier-envelope phase. We have calibrated the phase ellipse by comparison with the semiclassical calculation. This setup allows us to determine the value of the absolute phase of few-cycle laser pulses.     

Angle-Resolved Electron Spectra of F~- Ions by Few-Cycle Laser Pulses

The above-threshold detachment of F~- ions induced by a linearly polarized few-cycle laser pulse is investigated theoretically using the strong-field approximation model without considering the rescattering mechanism. We first derive an analytical form of transition amplitude for describing the strong-field photodetachment of F~- ions.The integration over time in transition amplitude can be performed using the numerical integration method or the saddle-point(SP) method of Shearer et al. [Phys. Rev. A 88(2013) 033415]. The validity of the SP method is carefully examined by comparing the energy spectra and photoelectron angular distributions(PADs) with those obtained from the numerical integration method. By considering the volume effect of a focused laser beam, both the energy spectra and the low-energy PADs calculated by the numerical integration method agree very well with the experimental results.     

Controlling of Slope of Carrier-Envelope Phase of Few-Cycle Laser Pulses on Propagation Distance near the Focus

The effect of focusing geometry on slope of carrier-envelope （CE） phase ФCE versus propagation distance from the focus in few-cycle laser pulses is investigated. The slope could be adjusted by changing the distance L between the waist of the incident beam and the lens. At the focus, 偏dФCE/O（Z/ZR） = 0 when L = 0, and 偏dФCE/偏d（z/zR） = -2 when L = ∞. The longer the distance L, the steeper the curve of the CE phase at the focus.     

Duration-Dependent Asymmetry in Photoionization of H atoms in Few-Cycle Laser Pulses

The photoionization of H atoms irradiated by few-cycle laser pulses is studied numerically. The variations of the total ionization, the partial ionizations in opposite directions, and the corresponding asymmetry with the carrier-envelope phase in several pulse durations are obtained. We find that besides a stronger modulation on the partial ionizations, the change of pulse duration leads to a shift along carrier-envelope （CE） phase in the calculated signals. The phase shift arises from the nonlinear property of ionization and relates closely to the Coulomb attraction of the parent ion to the ionized electron. Our calculations show good agreement with the experimental observation under similar conditions.     

激光脉冲绝对相位对产生阿秒脉冲的影响

在用超短激光脉冲产生和测量阿秒脉冲的过程中，绝对相位成为一个非常重要的影响因素。讨论了绝对相位对阿秒脉冲的产生和激光振荡的关系的影响，发现高次谐波的产生过程非常稳定，阿秒脉冲和驱动它的激光振荡之间的稳定的相位关系并不会受绝对相位的随机性的破坏，从而为采用交叉相关技术直接测量由强场原子高次谐波过程产生的阿秒脉冲宽度提供了理论依据，正确解释了文献[13]上报道的实验结果。     

Measurement of Carrier-Envelope Phase and Field Strength of a Few-Cycle Pulse by Non-sequential Double Ionization

We propose a method to measure the carrier-envelop phase （CEP） and the intensity of a few-cycle pulse by controlling the non-sequentiai double ionization （NSDI） process. By using an additional static electric field, we can change the momentum distribution of the double-charged ions parallel to the laser polarization from an asymmetrical double-hump structure to a nearly symmetrical one. It is found that the ratio between the strength of the static electric field and that of the laser field is sensitive to the CEP but robust against the intensity fluctuation. Therefore we can determine the OEP of a few-cycle pulse precisely by measuring the static electric field. Fhrthermore, if the CEP of the few-cycle pulse is fixed at a certain value, we can also calibrate the intensity of the laser pulse by the static electric field.     

Self-Induced Transparency of Few-Cycle Terahertz Pulses

JETP Letters - A theory of optical phenomena caused by the gyrotropy of semiconducting quantum wells has been developed. It has been demonstrated that the study of the reflection of light near a...     

Microscopic Dynamics of Nonsequential Double Ionization by Elliptically Polarized Few-Cycle Laser Pulses

With the classical ensemble model, we investigate nonsequential double ionization (NSDI) of xenon atoms using 780 nm, 0.25 PW/cm2elliptically polarized few-cycle laser pulses. The momentum distribution of correlated electron along the long axis of the laser polarization plane shows an obvious V-like structure locating at the third quadrant, and the momentum along the short axis of the laser polarization plane are mainly distributed in the second and fourth quadrants. Moreover, we demonstrate that the Coulomb repulsion interaction plays a decisive role to the above results.By back analyzing the classical trajectories of NSDI, we find that there are two kinds of recollision trajectories mainly contribute to NSDI, and the different microscopic dynamics for the two kinds of trajectories are clearly explored.     

Temporally Modulated Phase Retrieval Method for Weak Temporal Phase Measurement of Laser Pulses

We propose a simple iterative algorithm based on a temporally movable phase modulation process to retrieve the weak temporal phase of laser pulses.This unambiguous method can be used to achieve a high accuracy and to simultaneously measure the weak temporal phase and temporal profile of pulses,which are almost transform-limited.A detailed analysis shows that this iterative method has valuable potential applications in the characterization of pulses with weak temporal phase.     

Spider Structure of Photoelectron Momentum Distributions of Ionized Electrons from Hydrogen Atoms for Extraction of Carrier Envelope Phase of Few-Cycle Pulses

The spider structure in the photoelectron momentum distributions(PMDs) of ionized electrons from the hydrogen atom is simulated by solving the time-dependent Schrodinger equation(TDSE).We find that the spider structure exhibits sensitive dependence on carrier envelope phase(CEP) of the few-cycle pulses.To elucidate the striking CEP dependence of the spider structure,we select three physical parameters IL,IR,and IR/IL to quantitatively characterize the variations of the spider structure induced b...     

Electromagnetically Induced Gratings Created by Few-Cycle Light Pulses (Brief Review)

JETP Letters - Electromagnetically induced gratings (EMIGs) of atomic populations are formed in the resonant media at the interference of two or more overlapping monochromatic beams. They are of...     

Transverse Inhomogeneous Carrier-Envelope Phase Distribution of Idler Generated through Difference-Frequency-Generation

The transverse inhomogeneous carrier-envelope phase （CEP） distribution of idler generated through difference-frequency-generation （DFG） in quadratic nonlinear crystals is theoretically studied. In practical CEP stabilized DFG setups, the pump and the signal are usually Gaussian beams with non-uniform intensity distribution. Since the idler CEP is dependent on gain, this non-uniform intensity distribution leads to inhomogeneous gain across the aperture of the idler beam, resulting in a varying transverse idler CEP. Simulation results show that in practical settings, in the high-gain regime, transverse inhomogeneous CEP can be much smaller compared with π/2. However, when gain on the propagation axis reaches saturation, CEP difference can well exceed π/2.     

Pedestal Measurement of Laser Pulses

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Laser Wakefield Acceleration Using Mid-Infrared Laser Pulses

We study a laser wakefield acceleration driven by mid-infrared(mid-IR) laser pulses through two-dimensional particle-in-cell simulations.Since a mid-IR laser pulse can deliver a larger ponderomotive force as compared with the usual 0.8 μm wavelength laser pulse,it is found that electron self-injection into the wake wave occurs at an earlier time,the plasma density threshold for injection becomes lower,and the electron beam charge is substantially enhanced.Meanwhile,our study also shows that quasimonoenergetic electron beams with a narrow energy-spread can be generated by using mid-IR laser pulses.Such a mid-IR laser pulse can provide a feasible method for obtaining a high quality and high charge electron beam.Therefore,the current efforts on constructing mid-IR terawatt laser systems can greatly benefit the laser wakefield acceleration research.     

复杂激光脉冲波形的整形

介绍了用时空变换的方法对激光脉冲整形的原理；提出了由于硬边光通量调制器引入的动态的光脉冲强度空间分布的不一致性及其克服的方法，理论上用二维快速傅里叶变换（ＦＦＴ）进行了验证；最后计算了系统光强透过率和光束焦斑大小，调制狭缝宽度的关系，为了显示本系统的复杂脉冲整形能力，在实验上获得了惯性约束核聚变（ＩＣＦ）感兴趣的复杂激光脉冲波形。     

空间走离和相速度补偿的高效飞秒脉冲倍频

采用两块大小均为10 mm×10 mm×0.2 mm的BBO晶体级联的实验方案对中心波长为800 nm、脉宽70 fs、重复频率10 Hz的激光进行了倍频,针对BBO晶体的空间走离和相速度失配,分别调节两块倍频晶体的光轴夹角和基频光的角度匹配失谐量.实验表明,两块倍频晶体光轴反向平行可以补偿空间走离效应,一定的角度失谐可以补偿基频光和倍频光的相速度失配.在无聚焦情况下,使用两块BBO晶体级联相互配合优化将8.1 mJ的入射基频光倍频后得到带宽6.7 nm(傅里叶转换极限脉冲宽度为35 fs)、2.8 mJ以上的倍频光.其倍频效率高达35.7%,是同等条件下单块晶体倍频转换效牢的1.7倍.     

Envelope Area and Electric Pulse Area Interference in Excitation of Quantum Systems by Few-Cycle Attosecond Light Pulses

JETP Letters - In this letter, excitation of discrete levels in a quantum system is considered, when the duration of the driving pulse is changed from multicycle to ultrashort half-cycle regime. It...     

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.