Fu-Ming Guo

Single rotational state preparation via coherent control of laser pulse

We present a scheme to prepare a single rotational state with high probability from an arbitrary superposition state. The NO molecule is used as a computational example. By using a series of linearly and circularly polarized laser pulses and adjusting their parameters, a ground rotational state with probability of 99.982% can be obtained. Starting from the ground rotational state prepared, we can prepare any desired single rotational state with high probability and some rotational superposition states. On this basis, the molecular orientation with initial thermal distribution can be considerably enhanced and well controlled.

Influence of Higher-Order Kerr Effect on Femtosecond Laser Filamentation in Air at Different Pressures

Utilizing the pressure dependent high-order kerr indices, the propagation of intense ultrashort laser pulse in atmosphere at different pressures can be investigated. The results of spectroscopic investigation provide an effective way to generate supercontinuum as well as higher conversion efficiency high-order harmonic generation.

Probing dynamic interference in high-order harmonic generation from long-range molecular ion: Bohmian trajectory investigation

Using Bohmian trajectory (BT) method, we investigate the dynamic interference in high-order harmonic generation from diatomic molecular ions. It is demonstrated that the main characteristics of the molecular harmonic spectrum can be well reproduced by only two BTs which are located at the two ions. This happens because these two localized trajectories can receive and store the whole collision information coming from all of the other re-collision trajectories. Therefore, the amplitudes and frequencies of these two trajectories represent the intensity and frequency distribution of the harmonic generation. Moreover, the interference between these two trajectories shows a dip in the harmonic spectrum, which reveals the molecular structure information.

Identifying the Multielectron Effect on Chemical Bond Rearrangement of CH3Cl Molecules in Strong Laser Fields

Strong field double ionization that triggers the chemical bond rearrangement of CH3Cl is investigated by impulsive control of the alignment of molecules. The alignment and laser intensity dependent H2+ and H3+ yields in linearly polarized femtosecond laser have been measured, and the obtained data show that the maximum signal of H2+ appears at the laser polarization parallel to the C-Cl axis of molecules and H3+ species are more likely to eject at the laser polarization parallel to the C-Cl axis at low laser intensity while the H3+ signal peaks at laser polarization perpendicular to the C-Cl axis at high laser intensity. The measurements indicate that electrons from HOMO - 1 and HOMO - 2 orbitals have been ionized for the generation of bond rearrangement at different laser intensity. Our results demonstrate the importance of multielectron effects and also provide an effective control method in the process of chemical bond rearrangement of the molecules in strong laser fields.

Light emission induced by an XUV laser pulse interacting resonantly with atomic hydrogen

The resonant interaction between XUV ultra-short laser pulses and atomic hydrogen is systematically studied by numerically solving the time-dependent Schrodinger equation in this paper. Triple-peak structures are found to appear in the harmonics emitted provided that the incident laser is resonant with the 1 s-2p transition of the hydrogen atom. Moreover, the energy difference between neighboring peaks is the same and turns out to be proportional to the peak field strength E 0. Based on the theory of strong field approximation, and taking the interactions of the 1 s-2p bound energy levels into consideration, theoretical interpretations of the phenomena mentioned are successfully presented. This work provides a possible approach for generating XUV radiation with a tunable frequency via the interaction between atoms and XUV laser pulses.

Accurate prediction of interference minima in linear molecular harmonic spectra by a modified two-center model*

We demonstrate that the interference minima in the linear molecular harmonic spectra can be accurately predicted by a modified two-center model. Based on systematically investigating the interference minima in the linear molecular harmonic spectra by the strong-field approximation (SFA), it is found that the locations of the harmonic minima are related not only to the nuclear distance between the two main atoms contributing to the harmonic generation, but also to the symmetry of the molecular orbital. Therefore, we modify the initial phase difference between the double wave sources in the two-center model, and predict the harmonic minimum positions consistent with those simulated by SFA.

Electron excitation from ground state to first excited state: Bohmian mechanics method*

The excitation process of electrons from the ground state to the first excited state via the resonant laser pulse is investigated by the Bohmian mechanics method. It is found that the Bohmian particles far away from the nucleus are easier to be excited and are excited firstly, while the Bohmian particles in the ground state is subject to a strong quantum force at a certain moment, being excited to the first excited state instantaneously. A detailed analysis for one of the trajectories is made, and finally we present the space and energy distribution of 2000 Bohmian particles at several typical instants and analyze their dynamical process at these moments.

Isolated attosecond pulse generation via the interference of ionized multi-recollision wave-packets

We propose and theoretically demonstrate a method for generating an intense isolated attosecond pulse by a few-cycle strong laser pulse. The numerical simulations show that a broadband supercontinuum spectrum can be obtained by the interference of the ionized multi-recollision wave-packets with different energies, which are produced from the laser field at different ionization instants. By controlling the peak intensity of the few-cycle laser pulse, the atom can be completely ionized in the rising edge of the few-cycle laser pulse. Therefore, the probability of ionized wave packet is large enough to ensure the continuous harmonics with high efficiency, and an intense isolated 77 as pulse can be achieved successfully. Moreover, it is shown that our scheme can modulate the duration of the isolated attosecond pulse by adjusting the initial population of the atom.

Chirp-free isolated attosecond pulse generation from an atom irradiated by a fundamental terahertz pulse synchronizing an infrared laser pulse.

We theoretically study high-order harmonic generation (HHG) and attosecond pulses from an atom irradiated synchronically by a terahertz (THz) pulse and an infrared laser pulse. For the HHG spectrum from the THz pulse alone and the combined pulse, an apparent peak-valley structure appears the platform region. Specially, for the periodic structure generated by an atom under the action of the combined pulse is originated from the interference between the electrons ionized at different instants in the laser field, which undergo many recollision and return to the core at the same time. Therefore, continuum harmonics with few chirps from the interference enhancement region can be achieved, which result in a chirp-free isolated attosecond pulse.

Tunneling Ionization of Aligned CH3X (X=I, Br, Cl) Molecules in Intense Femtosecond Laser Fields

We perform an experimental investigation on tunneling ionization of CH3X (X=I, Br, Cl) molecules in intense femtosecond laser fields (1013 to 1015 W/cm2). The angular distribution of the parent and fragment ions from the well aligned molecular targets has been measured. Compared with theoretical calculation, the results show some new insights into the ionization mechanism of these molecules in intense laser fields.