We report a mechanism and a realizable approach for the coherent control of the generation of an isolated and ultrashort attosecond (as) laser pulse from atoms by optimizing the two-color laser fields with a proper time delay. Optimizing the laser pulse shape allows the control of the electron quantum paths and enables high-harmonic generation from the long-and short-trajectory electrons to be enhanced and split near the cutoff region. In addition, it delays the long-trajectory electron emission time and allows the production of extremely short attosecond pulses in a relatively narrow time duration. As a case study, we show that an isolated 30 as pulse with a bandwidth of 127 eV can be generated directly from the contribution of long-trajectory electrons alone. The study of attosecond (as) physics in intense ultrashort laser fields is now a forefront subject of much current interest and significance in science and technology [1,2]. Attosecond pulses can be produced by means of high-harmonic generation (HHG) via atoms in intense laser fields [2,3], and the time profile of the attosecond pulses can be controlled by tuning the carrier-envelope phase [4,5]. Recent progress of attosecond physics includes control of electron wave packets [6], probing of nuclear [7] and electronic dynamics [8], attosecond timeresolved spectroscopy [9], tomographic imaging of molecular orbitals [10], etc. One of the most novel features on the attosecond time scale is the real-time observation of the motion of electrons in atoms and molecules [11,12]. The generation of ever-shorter attosecond pulses has continued to attract much interest and has become one of the most active research directions in attosecond metrology today.It has been demonstrated experimentally that, by superposing the supercontinuum harmonic spectrum, one can obtain a train of attosecond pulses (ATP) or an isolated pulse of 107 as [13] and 80 as [14]. There are more and more experiments that use two-color lasers to analyze the contribution of the long and short quantum paths of the HHG spectrum [15,16]. Recently, several methods have been proposed for the generation of extremely short attosecond pulse (sub-50 as) and that use two-color laser fields or few-cycle chirped laser pulses. These methods are based on the semiclassical three-step model [17] or the solution of the time-dependent Schrödinger equation (TDSE) of one-dimensional (1D) model systems with a soft potential. However, such short attosecond pulses have not yet been realized experimentally by these methods. In this paper, we present a mechanism and a realizable approach for efficiently generating ultrashort attosecond pulses by controlling the short and long quantum paths by means of optimizing the pulse shape and time delay of two-color laser fields. Our proposed procedure is different from other control * r97222060@ntu.edu.tw † lipc@nwnu.edu.cn ‡ sichu@ku.edu methods where only one trajectory is enhanced and the other is suppressed. Our method can enhance the radiation for the two quantum paths and split...

Abstract:We present an efficient and realizable scheme for the generation of an ultrashort single attosecond (as) pulse. The feasibility of such a scheme is demonstrated by solving accurately the time-dependent Schrödinger equation using the time-dependent generalized pseudospectral (TDGPS) method. This scheme involves the use of the optimization of the three-color laser fields. The optimized laser pulse is synthesized by three one-color laser pulses with proper relative phases. It can provide a longer acceleration time for the tunneling and oscillating electrons, and allows the electrons to gain more kinetic energy. We show that the plateau of high-order harmonic generation is extended dramatically and a broadband supercontinuum spectra is produced. As a result, an isolated 23 as pulse with a bandwidth of 163 eV can be obtained directly by superposing the supercontinuum harmonics near the cutoff region. We will show that such a metrology can be realized experimentally.