Zhu Qi-Ren

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.

A Theoretical Strategy to Generate an Isolated 80-Attosecond Pulse

Using a linearly polarized, phase-stabilized 2.66-femtosecond driving pulse of 400 nm central wavelength orthogonally combined with another linearly polarized long pulse of 800 nm central wavelength irradiating jointly on the helium atom, we demonstrate theoretically the generation of a clean isolated 80-attosecond pulse in the spectral region of 93–155 electron volts in a two-dimensional model.

Generation of a Super Strong Attosecond Pulse from an Atomic Superposition State Irradiated by a Shape-Optimized Short Pulse

Using a linearly polarized, phase-stabilized 3-fs driving pulse of 800 nm central wavelength shape-optimized on its ascending edge by its an amplitude-reduced pulse irradiating on a superposition state of the helium atom, we demonstrate theoretically the generation of a super strong isolated 176-attosecond pulse in the spectral region of 93–124 eV. The unusually high intensity of this attosecond pulse is marked by the Rabi-like oscillations emerging in the time-dependent populations of the ground state and the continuum during the occurrence of the electron recombination, which is for the first time observed in this work.

Cyclic State Orientation of Polar Molecules Produced by a Train of Half-Cycle Pulse Clusters of a Long Repetition Period

Using a variational method, we derive the optimal population distribution of angular momentum eigenstates for any given population range in a rotational wavepacket within the field-free cyclic state orientation framework. Correspondingly, we devise a train of half-cycle pulse clusters to purposively make the structure of the computed wavepacket approach the optimal population distribution, so that we can now utilize much more powerful means to realize an ideal orientation goal.

Field-Free Molecular Orientation Generated from Cyclic Rotational States by Using Two Trains of Half-Cycle Pulses

We show that two trains of half-cycle pulses (HCPs) with different amplitudes irradiating alternately on polar molecules can achieve a remarkable enhancement of field-free orientation compared with the case of an equal amplitude HCPs train for the same pulse separation. This kind of orientation enhancements is mainly due to an optimal adjustment of the population distribution on every field-free angular momentum eigenstate, in which the populations on the undesired states of high angular momenta are effectively suppressed, and the populations on the desired states of low angular momenta are correspondingly promoted.

Harmonic Enhancement Mechanism of a Superposition State Atom Irradiated by Short Pulses

We investigate the high-order harmonic generation (HHG) of a model atom whose initial state is prepared in a superposition of its ground state and an excited state irradiated by different duration laser pulses. Compared to the HHG generated from an atom whose initial state is in its ground state, its conversion efficiency obtains some enhancement. The enhancement originates from the higher ionization rate (rather than the ionization yield) of the atom with superposition initial state.

Marked Advance in Widths and Heights of Harmonic Plateau

We propose a scheme to simultaneously widen and heighten the high-order harmonic plateau on a large scale. More specifically, by adopting a united two-atom system with a suitable inter-nuclear separation instead of a single-atom, the harmonic plateau is widened from Ip + 3.2Up to Ip + 8.5UP; further, by adopting the combined pulse, the extended plateau (harmonics near Ip + 5.6Up) is selectively heightened in excess of 4 orders of magnitude compared with the case of the low-frequency pulse alone. By means of the wavelet transform for the induced dipole of these harmonics, a single x-ray pulse as short as 210 asec is achieved.

Ultrahigh Harmonic Generation from an Atom with Superposition of Ground State and Highly Excited States

We investigate the high-order harmonic generation from an atom prepared in a superposition of ground state and highly excited state. When the atom is irradiated by an ultrashort pulse, the cutoff position of the plateau in the harmonic spectrum is largely extended compared with the case that the atom is initially in the ground state. The physics of the extension of the high-order harmonic plateau can be interpreted by the spatial structure of the atomic initial wave packet. We can optimize the generation of high-order harmonics by substituting the excited state for a particular coherent superposition of some highly excited states to form a spatially localized excited wave packet.

Dynamic Mechanism of Sustainable Molecular Orientation Generated From Cyclic Rotational States

Recently, two papers presented by Ortigoso et al. [Phys. Rev. Lett. 93 (2004) 073001 and Phys. Rev. A 72 (2005) 053401] develop a novel strategy in which the best sustainable molecular alignment/orientation has been achieved. We intend to analyse the dynamic mechanisms, including those Ortigoso et al. have not clarified.

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.