Control of the high-order harmonics cutoff through the combination of a chirped laser and static electric field

Abstract

Abstract：The high harmonic generation from atoms in the combination of chirped laser pulse and static field is theoretically investigated. For the first time, we explore a further physical mechanism of the significant extension of high harmonic generation cutoff based on three-step model. It is shown that the cutoff is substantially extended due to the asymmetry of the combined field. If appropriate parameters are chosen, the cutoff of high harmonic generation can reach 42 p p I U + . Furthermore, an ultrabroad super-continuum spectrum can be generated. When the phases are properly compensated for, an isolated 9 attosecond pulse can be obtained. PACS numbers: 42.65.KyIt is well known that high harmonics are generated when atoms or molecules are irradiated by intense laser field. In recent years, high harmonic generation (HHG) has become a very interesting topic of laser-atom interaction [1-3] because of a very important application of attosecond (as) pulses generation. Usually, there are two techniques to generate attosecond pulses [4]. One is to use few-cycle laser pulse, then the cutoff is continuous and by spectrally selecting it a single attosecond pulse can be obtained [5]. The other is to use multi-cycle intense infrared pulse, then select many harmonics in the plateau and an attosecond pulse train can be generated [6]. Many control ways have been studied in order to gain an isolated attosecond pulse [7][8][9]. In the presence of an intense static electric field, one can get a pulse of 220 attosecond [10]. Using a chirped few-cycle laser field, a pulse of 108 attosecond can be generated [11]. From the works mentioned above, one may want to know what would happen if the combination of chirped laser field and a static electric field is adopted in the HHG process. Based on this idea, we now investigate the HHG spectrum of atoms in this combined field.We use one-dimensional time-dependent Schrödinger equation (TDSE) to describe the interaction between an atom and the combination of chirped laser field and the static electric field in the single active electron and dipole approximations (The atom units are used in all equations in this paper, unless otherwise mentioned.): Generally, the physical origin of the HHG from linearly polarized laser field can be qualitatively understood in the framework of semi-classical model which is called three-step-method (STM) [15]: first, the electron tunnels through the barrier formed by the Coulomb potential and the laser field; next, it oscillates almost freely in the laser field; finally, it may return back and recombine with the parent ion. During the recombination, a photon is emitted.The returning kinetic energies of the electron in different laser fields are shown in Fig. 2. For 2 comparison, the ionization potential is added. It is apparent that the trajectories of the electron in these field cases are quite different. Usually, there are two dominant quantum paths with different emission times contributing to each harmonic in each half optical cycle (O.C.) [10]: the p...

Quantum path control on the harmonic emission in the presence of a terahertz field

“…The work [26] also concisely presented the harmonic spectrum in single Ne atom by chirp-free driving pulse of 5.2 fs/ 800 nm and 8 fs/800 nm on the basis of the numerical integration of the Lewenstein integral. By numerical solution of time-dependent Schrödinger equation, static-electric-field assisted harmonic generation in a model He atom with an orthogonally polarized two-color laser field [25] and with a few-cycle pulse [28] as well as in a single H atom with chirped laser pulse [29] have been studied and reported. To understand the effect of the THz field on harmonic generation at a microscopic level, perhaps a combination of having frequencies of 30THz-60THz and intensities around 10 13 w/ cm 2 [23].…”

Single attosecond pulse from terahertz-assisted high-order harmonic generation

“…The effect of THz fields on high-order harmonic generation has been investigated at the single atom level using the classical model [20], semi-classical model [33], using zero range potential calculations [19], SFA together with saddle point approximation [24] and by solving the time dependent Schrödinger equation for a model atom [20][21][22][23]. In this section we summarize these results briefly and present the single atom response calculated with our parameters by numerically integrating the Lewenstein integral (Equation 1).…”

“…The effects of using THz and static electric fields in HHG at single atom level have been investigated in several papers [19][20][21][22][23][24]. In this work, we further investigate the idea at the macroscopic level, by considering all the important effects arising during the propagation of electromagnetic fields, simulating a scenario with realistic focusing geometry.…”

Generation of Broadband Supercontinuum and Isolated Attosecond Pulse in a Chirped Two-Colour Laser Field

“…With this controlling technique, some authors have successfully synthesized broadband isolated attosecond pulses, such as an isolated 108 as pulse via coherent control of an intense few-cycle chirped laser pulse [32], a single 58 as pulse using a chirped fewcycle laser and static electric field [33], an isolated 26 as pulse using an intense few-cycle chirped laser and its high-order harmonic pulses [34], an isolated 38 as pulse using an intense few-cycle linearly chirped laser and a subharmonic laser pulse [35], an isolated 57 as pulse with a chirped two-colour laser field [36], and an isolated 31 as pulse using two-colour laser pulses with the combined chirp effects [37].…”