Hongchuan Du

Attosecond ionization control for broadband supercontinuum generation using a weak 400-nm few-cycle controlling pulse

We theoretically demonstrate a method for generating the broadband supercontinuum. It is found that a weak 400-nm few-cycle pulse can be used to replace the ultraviolet attosecond pulse for controlling the ionization dynamics of the electron wave packets when a long-wavelength driving pulse is adopted. By adding a 400-nm few-cycle laser pulse to a 2000-nm driving pulse at proper time, only a quantum path can be selected to effectively contribute to the harmonics, leading to the efficient generation of a broadband supercontinuum. Moreover, our scheme is stable against nearly all the small parameter shift of the driving pulse and the controlling pulse. The macroscopic investigation reveals that the macroscopic supercontinuum with the bandwidth of about 165eV can be obtained. Then isolated sub-110-as pulses can be directly generated. Moreover, the generated attosecond pulse has a divergence angle of about 0.1mrad in the far field, which indicates its beam quality is good. Besides, it is also found that a near-field spatial filter can be used to select the different quantum paths (short or long) in the far field.

Intense supercontinuum generation exceeding 300eV using a two-color field in combination with a 400-nm few-cycle control pulse

We propose a method to control the harmonic process by using a two-color field in combination with a 400-nm few-cycle control pulse for the generation of an ultra-broadband supercontinuum with high efficiency. The ionization and acceleration steps in the harmonic process can be simultaneously controlled by using a three-color field synthesized by a 2000-nm driving pulse and two weak 800-nm and 400-nm control pulses. Then an intense supercontinuum covered by the spectral range from 140 eV to 445 eV is produced. The 3D macroscopic propagation is also employed to select the short quantum path of the supercontinuum, then intense isolated sub-100-as pulses with tunable central wavelengths are directly obtained within water window region. In addition, the generation of isolated attosecond pulses in the far field is also investigated. An isolated 52-as pulse can be generated by using a filter centered on axis to select the harmonics in the far field.

Isolated attosecond pulse generation from pre-excited medium with a chirped and chirped-free two-color field

We theoretically investigate the isolated attosecond pulse generation from pre-excited medium with a chirped and chirped-free two-color field. It is found that the large initial population of the excited state can lead to the high density of the free electrons in the medium and the large distortion of the driving laser field after propagation, though it benefits large enhancement of harmonic intensity in single atom response. These effects can weaken the phase-match of the macroscopic supercontinuum. On the contrary, the small initial population of 4% can generate well phase-match intense supercontinuum. We also investigate an isolated attosecond pulse generation by using a filter centered on axis to select the harmonics in the far field. Our results reveal that the radius of the spatial filter should be chosen to be small enough to reduce the duration of the isolated attosecond pulse due to the curvature effect of spatiotemporal profiles of the generated attosecond pulses in the far field.

Electron acceleration in vacuum with two overlapping linearly polarized laser pulses

A novel electron acceleration approach with two overlapping linearly polarized laser pulses in vacuum is proposed. By our simulation, the energy and space spreads can reduce greatly comparing with the acceleration with only one laser pulse having doubled peak laser intensity for realistic laser parameters and the average energy gain from our scheme can be doubled for certain pulse lengths, at the same time. Using numerical simulation, analytical criteria for optimal regimes of our acceleration scheme is found.

Enhancement of the second plateau in solid high-order harmonic spectra by the two-color fields

We theoretically investigate high-order harmonic generation (HHG) from solids in two-color fields. It is found that under the premise of maintaining the same amplitude, the intensity of the second plateau can be enhanced by two to three orders in a proper two-color field compared with the result in the monochromatic field with the same frequency as the driving pulse of the two-color field. This can be attributed to the fact that most excited electrons can be driven to the top of the first conduction band due to the larger vector potential of the two-color fields, which leads to the higher electron population of upper conduction bands. Moreover, we also find that isolated attosecond pulses can be generated from solids by choosing a proper two-color field that allows the electrons to reach the top of the first conduction band only once. This work provides a promising method for extending the range of solid HHG spectra in experiments.

Reexamining the high-order harmonic generation of HD molecule in non-Born-Oppenheimer approximation

The high-order harmonic generation of the HD molecule is studied in non-Born-Oppenheimer approximation. It is found that there are only the odd harmonics in the harmonic spectrum of the HD molecule though the generation of even harmonics is possible in principle. Theoretical analysis [T. Kreibich et al., Phys. Rev. Lett. 87, 103901 (2001)] reveals that the nuclear dipole moment can contribute to the generation of the even harmonics, but the acceleration of the nucleus is about three orders of magnitude less than that of the electron. Hence, the even harmonics cannot be observed in the harmonic spectrum of the HD molecule.

Wavelength scaling of high-order harmonic yield from a Rydberg atom in a few-cycle pulse

We theoretically investigate the wavelength scaling of high-order harmonic yield from a Rydberg atom driven by a few-cycle pulse. It is found that the harmonic yield decreases with a slow scaling λ−2.8 as the increase of the laser wavelength for a Rydberg atom compared with λ−4.6 for a ground atom though the traveling time of the electron in the continuum state is longer for a Rydberg atom. Further analysis reveals that the initial large spatial width of the Rydberg atom is responsible for the slow wavelength scaling of the high-order harmonic yield.

Theoretical study of the odd–even-order harmonic generation for asymmetric ions in non-Born–Oppenheimer approximation

We calculated the harmonic spectra generated from the asymmetric molecules of HD~+ and HeH~(2+). It is found that HD~+ 0produces only odd harmonics, while HeH~(2+) produces both odd and even harmonics. Further analysis reveals that for both HD~+ and HeH~(2+), the nuclear dipole acceleration can generate even harmonics, but it is three orders of magnitude lower than that of the electron. Hence, the electronic dipole acceleration dominates the harmonic generation. For HD~+, the electronic dipole acceleration only contributes to the generation of odd harmonics, but for HeH~(2+) it contributes to the generation of both odd and even harmonics. Besides, one concept of the broken degree of system-symmetry is proposed to explain the different odd-even property between the harmonic spectra of HD~+ and HeH~(2+).

Theoretical scheme for simultaneously observing forward-backward photoelectron holography.

Photoelectron angular momentum distribution of He+ driven by a few-cycle laser is investigated numerically. We simultaneously observe two dominant interference patterns with one shot of lasers by solving the 3D time-dependent Schrodinger equation. Analysis of a semiclassical model identifies these two interference patterns as two types of photoelectron holography. The interference pattern with Pz>0 is a type of forward rescattering holography, which comes from the interference between direct (reference) and rescattered (signal) forward electrons ionized in the same quarter-cycle. The interference pattern with Pz<0 is a type of backward rescattering holography, which comes from the interference between a direct electron ionized in the third quarter-cycle and rescattered backward electron ionized in the first quarter-cycle. Moreover, we propose a method to distinguish this backward rescattering holography and intracycle interference patterns of direct electrons.

Role of excited states in the elliptically polarized harmonic generation

We theoretically investigate the contribution of the excited state to the ellipticity of the harmonics from H2+ at different orientation angles irradiated by a linearly polarized laser pulse. It is found that the first excited state has a significant influence to the ellipticity of the harmonics, and the contribution of higher excited states to the ellipticity can be neglected. Moreover, the conclusion is not dependent on the laser intensity.