Chen Ji-Gen

Frequencies-Selected Enhancement of the Extended High-Order Harmonic Generation Plateau from a United Two-Atom System Irradiated by a Combined Pulse

We present a high-order harmonic generation (HHG) spectrum from a united two-atom system exposed to a combined laser pulse, by numerically solving a one-dimensional time-dependent Schrodinger equation. The combined laser pulse is composed of a low-frequency femtosecond pulse and a high-frequency attosecond one with respective appropriate amplitudes. For a suitable inter-nuclear separation, the harmonic emission efficiencies near the second cutoff of the extended plateau can be effectively enhanced by more than four orders of magnitude compared with the case of the low-frequency pulse alone. Such a combined pulse irradiating on the united two-atom system ionizes each atom, in a large rate (but not to a too large ionization yield), mainly at a particular time-interval. When the ionized electron from an atom reaches at the vicinity of the other atom and recombines with it, significant HHG enhancement is achieved for particular harmonics. This result, to our knowledge, is one of the best up to now in the endeavor for dramatically extending the width and simultaneously enhancing the height of the plateau.

Simultaneous Extension and Enhancement of the HHG Plateau by Using Combined Lasers Irradiating on a United Two-Atom System

We present the high-order harmonic generation spectrum from a united two-atom system simultaneously exposed to a fundamental Ti:sapphire and its 33rd harmonic (H33) lasers by numerically solving a one-dimensional time-dependent Schrodinger equation. At a suitable inter-nuclear separation, the harmonic plateau is widened from Ip+3.2Up in the single-atom case up to Ip+5.6Up in the united-atom case. Furthermore, the plateau is heightened in excess of six orders of magnitude by using the combined lasers compared with the case of the fundamental laser alone. Such a scheme can step by step accomplish the following processes: through a single H33 photon resonance transition an appropriate amount of electrons are first populated to an excited state, in which the electrons are easily ionized and subsequently accelerated by the fundamental laser; then they can recombine with the neighbouring atom, so that high-efficiency emissions of the harmonics beyond Ip+3.2Up are successfully realized.

Field-Free Orientation of Molecules with Small Permanent Dipole Moments by Using a Train of Half-Cycle Pulses

We present a scheme aiming at the field-free orientation of molecules with smaller permanent dipole moments under the framework of the half-cycle pulse (HCP) method. Taking the HCCCl molecule as an example, we show that a train of identical HCPs with special time intervals can considerably raise the level of the orientation. The scheme is analytically interpreted in terms of intermittent Rabi oscillations by referring to a simplified two-state model.

A Time-Distinguished Analysis of the Harmonic Structure from a Model Molecular Ion

We present high-order harmonic generation spectra resulted from a single-electron model molecular ion exposed to intense laser fields by numerically solving a one-dimensional time-dependent Schr?dinger equation. There are three plateaus in the spectra and the maximal cutoff energy is Ip+8.5Up, when the inter-nuclear distance R equals ??0/2. Here Ip is the ionization potential and Up = E02/(4?2) is the ponderomotive potential with E0 and ? being the laser electric field amplitude and the central frequency. The harmonic structures are well interpreted by a modified three-step model in which the effects of the electron reflected by the non-parent ion are stressed.