The Carrier-Envelope Phase of Focused Few-Cycle Laser Pulses

The Gouy phase shift modulates the carrier-envelope phase of a focused few-cycle laser pulse. We investigate the variations of the carrier-envelope （CE） phase of a pulsed Gaussian beam on and off the axis by the first-order approximation and numerical calculation. The CE phase undergoes a π shift from minus infinity to infinity, and it changes fastest with distance at the focus. The variation of CE phase on propagation distance is nearly independent of pulse duration At when At is greater than lO fs and the first order approximation fits this result very well. The maximum difference of CE phase between a pulse with duration of one cycle and the result of first-order approximation is 0.09.     

Above-Threshold Ionization of Xenon by Chirped Intense Laser Pulses

We experimentally investigate above-threshold ionization of xenon subject to chirped intense laser pulses. While for few-cycle laser pulses free of chirp, the Freeman resonance narrow peaks in the photoelectron spectra become strongly suppressed, they will re-emerge for laser pulses with a certain chirp. Moreover, the position of the resonant peaks exhibits a strong dependence on the direction of the chirp. The experimental features can be understood by a two-step ionization process; that is, a multi-photon excitation plus subsequent ionization process.     

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.     

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.     

Identification of Above-Threshold Ionization by Imaging Photoelectrons from Ammonia Molecules in an Intense Femtosecond Laser Field

The above-threshold ionization process of ammonia molecules induced by a femtosecond laser field at 800 nm is studied in the intensity range from 1.6 × 10¹³ to 5.7 × 10¹³ W/cm². Channel switching under different laser intensities is observed and identified in the photoelectron kinetic energy spectra of ammonia. Based on the photoelectron kinetic energy distributions and the photoelectron angular distributions, the characteristic peaks observed are exclusively ass...     

Role of XUV Photons in Atomic High-Order Above-Threshold Ionization Processes in IR+XUV Two-Color Laser Fields

We investigate the above-threshold ionization of an atom in a combined infrared(IR) and extreme ultraviolet(XUV) two-color laser field and focus on the role of XUV field in the high-order above-threshold ionization(HATI)process. It is demonstrated that, in stark contrast to previous studies, the XUV laser may play a significant role in atomic HATI process, and in particular, the XUV laser can accelerate the ionized electron in a quantized way during the collision between the electron and its parent ion. This process cannot be explained by the classical three-step model. Our results indicate that the previously well-established concept that HATI is an elastic recollision process is broken down.     

Above-Threshold Photoemission of Compound Nanoantennas Irradiated by Femtosecond Laser Pulses

A theoretical study of electron emission processes that occur at interaction of femtosecond laser pulses with bowtie compound nanoantennas is performed. A numerical analysis is conducted of the dependences of the spectra of ejected electrons on the features of the light pulses for different geometric parameters of the antennas. The calculation results show that asymmetric nanoantennas have distinct advantages over symmetric configurations of the elements of compound antennas.     

High-Order Above-Threshold Ionization of H2+ in Intense Laser Field

The nonperturbative quantum electrodynamies method proposed by Fu et al. [Phys. Rev. A 75 （2007） 063419] is employed to study the high-order above-threshold ionization （ATI） of a diatomic molecule. Based on this frequency-domain theory, the high-order ATI process can be regarded as ATI followed by laser-assisted collision, where the total transition amplitude is the coherent summation of the contributions from each ATI channel. The angular-resolved ATI spectrum, which agrees with the results by Becket et al. based on the time-domain method, is obtained by this frequency domain theory. Furthermore, it is demonstrated that the interference characteristics representing the molecular structure in the ATI spectrum originates from the recollision of the electron with two-centre ion in each ATI channel.     

Phase-Matched High-Order Harmonics from N2 Using kHz Femtosecond Laser Pulses

We report the phase-matched high-harmonic generation from N2 driven by kHz femtosecond laser pulses. By varying the position of the laser focus in the gas tube, we observe the radiation of different harmonics. Our results provide a potential source for studying ultrafast molecular dynamics.     

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。     

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...     

Laser Phase Relations of High-Order Harmonic Generation

Laser phase relations of high-order harmonic generation （HHG） are calculated with the saddle point theory. The results show a two-peak structure including a symmetric and an asymmetric temporal distributions of the instantaneous frequency of HHG radiation. These distributions are fitted with Gaussian and polynomial functions, respectively. Two calculation formulae are presented to analyse attosecond x-rays with specified time duration and energy bandwidth in experiments.     

High-Order Temporal Corrected Fields of Ultra-Short Laser Pulses and Laser-Driven Acceleration

Up to third-order temporal correction in terms of a small dimensionless temporal parameter ε=1/（ωoto） （ω0=ck0 the central oscillatory frequency, to the pulse duration of half period）, the field expressions of ultra-short focused laser pulses are explicitly presented. To evaluate the correction efficacy, electric amplitudes of zeroth-order and higher-order corrected fields are compared for different pulse durations. Furthermore, electron interaction with ultra-short laser pulses is simulated using both the zeroth-order and higher-order corrected field equations. Our simulation results show that the third-order correction terms should be considered for investigating the interaction if the laser pulse duration decreases to a few optical periods.     

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.     

阈上电离过程中光电子角分布的精细结构

应用非微扰散射理论并考虑自发辐射 ,研究了阈上电离过程中光电子的角分布。由于自发辐射的影响 ,电子的角分布具有精细结构 ,成功地解释了Nandor等人的实验观测。     

Coulomb Potential Recapture Effect in Above-Barrier Ionization in Laser Pulses

The Coulomb potential recapture effect in above-barrier ionization with ultrashort long-wavelength laser pulses is investigated theoretically by solving the one-dimensional time-dependent Schrodinger equation. We find that electrons can be recaptured with considerable possibility by the Coulomb potential near the end of the pulse though atoms are ionized almost completely within the first few half optical cycles. Therefore there is a high probability of the atom surviving after the pulse. We also check this process in the three-dimensional case and find that this kind of stabilization can still exist in three-dimensional atoms.