Song-Feng Zhao

Controlling of strong tunable THz emission with optimal incommensurate multi-color laser field

Based on the photocurrent model, we study terahertz (THz) emission from argon plasmas induced by incommensurate-frequency two- and three-color laser fields. In order to enhance the THz radiation at an arbitrary frequency efficiently, a genetic algorithm is applied to search for the optimum laser parameters. For the longer two-color field, our optimizations show that the THz tunability is mainly determined by two laser frequencies, which approximately meets the law Ω = 2ω1−ω2. However, for the shorter laser pulse, the tunability of the THz wave with lower frequency also depends on the relative phase. To control the tunable THz emission, we systematically investigated how to generate the stronger THz wave with the shorter spectrum width using the optimal synthesized waveform. We found that the THz intensity can be enhanced by about an order with three-color field compared with the two-color cases. We also show that the tunable single ultrashort THz pulses can be obtained by using an optimized 50-fs two-colo...

Analysis of interference effects in the direct photodetachment from H− in a two-color laser pulse

We extend the adiabatic saddle-point (SP) method of Shearer et al (2011 Phys. Rev. A 84 033409) to study electron detachment from H− exposed to a two-color few-cycle linearly polarized laser pulse that consists of a fundamental frequency and its half harmonic. We show that the photoelectron angular distributions (PADs) are dominantly contributed from several central SPs that correspond to the strongest part of two-color pulses. By separately calculating the PADs formed in the different detachment steps, the rich oscillatory patterns in the PADs can be mainly attributed to the interference effects of coherent quantum wave packets in the classical propagation step of detachment processes, whereas the interference effects in the quantum tunneling step are weaker because the corresponding shape of the PADs is almost independent of the relative time delay of two pulses. The interference effects in the classical propagation step are also confirmed by the angle-dependent averaged cosine value of phase differences for each two dominant SPs. We find that the PADs of H− ions can be efficiently controlled by properly adjusting the relative time delay and intensity ratio of a controlling field to the driving field.

The optimal control and dynamical analysis of high-order harmonic generation from atom

High-order harmonic generation (HHG) is a frequency up-conversion process in which an ultrafast femtosecond laser pulse at high intensity interacts with atom or molecule. HHG covers the wider wavelength spectral regions of extreme ultraviolet (XUV), soft X-ray and even hard X-ray. With the high photon flux, the HHG promises to be a attractive broadband tabletop light source, and a number of interesting applications of HHG have been demonstrated in many research fields. In this paper, we review some common methods and results on the optimal control of HHG to improve its low photon flux. HHG can now be controlled for different purposes, i.e., extending the cutoff energy, increasing the efficiency of the high-harmonic conversion process, and selecting single or range of harmonics, by combining the technologies of femtosecond temporal and spatial laser pulse shaping, waveform synthesizing and evolutionary algorithm, such as genetic algorithm and optimal control theory. Selective generation of a single harmonic can now be achieved both in hollow fiber, long gas cell and gas jet. We show that by using the newly developed waveform synthesis with two-color laser fields, the harmonic yield can be enhanced by more than one order without increasing the total laser energy, or the harmonic cutoff can be extended about two times without losing harmonic yield, even after considering the propagation effects. These progresses will exert a far-reaching impact on strong field physics. To optimize the HHG processes, a detailed understanding of the corresponding dynamics is essential. We also introduce a new time-frequency (TF) analysis technique, the synchrosqueezing transform (SST), which is used to reveal the quantum dynamics of HHG. Compared with the classical type of TF methods, such as the Gabor transform, the Morlet wavelet transform, the SST can be applied to explore the dynamical origin of near- and below-threshold harmonic emission.

Accurate Structure Parameters for Tunneling Ionization Rates of Gas-Phase Linear Molecules*

In the molecular Ammosov–Delone–Krainov (MO-ADK) model of Tong et al. [Phys. Rev. A 66 (2002) 033402], the ionization rate depends on the structure parameters of the molecular orbital from which the electron is removed. We determine systematically and tabulate accurate structure parameters of the highest occupied molecular orbital (HOMO) for 123 gas-phase linear molecules by solving time-independent Schrodinger equation with B-spline functions and molecular potentials which are constructed numerically using the modified Leeuwen–Baerends (LBα) model.

Synthesis of Two-Color Laser Pulses for the Harmonic Cutoff Extension*

Increasing simultaneously both the cutoff energy and efficiency is a big challenge to all applications of high-order harmonic generation (HHG). For this purpose, the shaping of the waveform of driving pulse is an alternative approach. Here, we show that the harmonic cutoff can be extended by about two times without reducing harmonic yield after considering macroscopic propagation effects, by adopting a practical way to synthesize two-color fields with fixed energy. Our results, combined with the experimental techniques, show the great potential of HHG as a tabletop light source.