Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field

A new method to coherently control the electron dynamics is proposed using a few-cycle laser pulse in combination with a controlling field. It is shown that this method not only broadens the attosecond pulse bandwidth, but also reduces the chirp, then an isolated 80-as pulse is straightforwardly obtained and even shorter pulse is achievable by increasing the intensity of the controlling field. Such ultrashort pulses allow one to investigate ultrafast electronic processes which have never be achieved before. In addition, the few-cycle synthesized pulse is expected to manipulate a wide range of laser-atom interactions.PACS numbers: 32.80. Qk, 32.80.Wr, 42.65.Re, 42.65.Ky In the past decade, there has been a great interest to extend the pulse duration to the attosecond (as) domain. This is sprung by the great potential of attosecond pulses to trace the electronic dynamics in atoms and molecules [1, 2, 3].The straightforward attosecond metrology prefers an isolated attosecond pulse rather than a train of attosecond pulses [3, 4]. Hence much effort is paid to produce the isolated pulse [3, 4, 5]. It has been shown that an isolated attosecond pulse can be generated with a few-cycle laser pulse by selecting the continuous harmonics in the cutoff [2, 3, 5]. The intractable problem is that the bandwidth of the continuous harmonics in the cutoff is less than 20 eV, and thus the minimum duration of the isolated attosecond pulse is ∼ 250 as [2]. This is greater than the characteristic timescale of the electronic process in atoms (152 as, i.e., the period for electrons in the Bohr orbit of ground-state hydrogens), and then the application of the 250-as pulse is significantly limited. Therefore, it is urgently desired to produce an isolated attosecond pulse with broader bandwidth and shorter duration [6, 7, 8, 9]. It is theoretically demonstrated that broadband continuous harmonics can be produced using a few-cycle laser pulse with a modulated polarization [7]. This scheme has been implemented recently [8, 9], and broadband continuous harmonics are observed. However, there is a chirp in these harmonics, then an isolated 280-as pulse is produced straightforwardly. With the technology of chirp compensation [9], the pulse duration is reduced to 130 as. In this letter, we propose a new facile method for isolated sub-100-attosecond pulse generation via controlling electron dynamics using a few-cycle laser pulse in combination with a controlling field.High harmonic generation (HHG) is well understood in terms of the semiclassical "three-step" model [10]. In detail, the electron is first ionized from the atom, then it oscillates almost freely in the laser field and gains kinetic energy E k , finally, it recombines with the parent ion by releasing an energetic photon of I p + E k where I p is the ionization

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“…Then, alike previous works [13,14], it only supports to an isolated attosecond pulse greater than 200 as.…”

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confidence: 71%

Isolated sub-100‐aspulse generation via controlling electron dynamics

et al. 2007

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“…Another approach involves control of the wave packet recollision using a two-color field. Proposals based on a mixing scheme of a strong fundamental field and a weak [6] or strong [7] second-harmonic field have been investigated computationally. Experimental indications of an isolated attosecond pulse have been observed in the latter scheme explored experimentally.…”

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confidence: 99%

Isolated attosecond pulses using a detuned second-harmonic field

et al. 2007

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Calculations are presented for the generation of an isolated attosecond pulse in a multicycle two-color strong-field regime. We show that the recollision of the electron wave packet can be confined to half an optical cycle using pulses of up to 40 fs in duration. The scheme is proven to be efficient using two intense beams, one producing a strong field at and the other a strong field detuned from 2. The slight detuning ␦ of the second harmonic is used to break the symmetry of the electric field over many optical cycles and provides a coherent control for the formation of an isolated attosecond pulse.High-order harmonics generation (HHG) of intense laser pulses in gases is attracting much attention due to both fundamental and applied interests. One of the most attractive properties of the HHG source is the generation of attosecond pulses. This field has been growing spectacularly (for reviews see [1,2]). Trains of attosecond pulses are naturally produced in the HHG process, whereas the generation of a single or isolated attosecond pulse is more challenging. Isolated attosecond pulses have been, however, achieved using absolute phase-stabilized few-optical-cycle state-of-the art laser systems. The methods rely either on the laser intensity [3] or ellipticity (polarization state) [4,5] dependencies of the harmonic emission. Another approach involves control of the wave packet recollision using a two-color field. Proposals based on a mixing scheme of a strong fundamental field and a weak [6] or strong [7] second-harmonic field have been investigated computationally. Experimental indications of an isolated attosecond pulse have been observed in the latter scheme explored experimentally. These schemes are very promising but require very short pulses ͑Ͻ12 fs͒ except if harmonic heterodyning with a weak lower frequency field is used [8], which is, however, more difficult to produce experimentally than the second harmonic.In this Letter, we propose to generate single attosecond pulses in the long optical cycle regime by mixing the fundamental field to its detuned second harmonic. To illustrate the principle, we perform onedimension quantum mechanical calculations using an argon atom with a smoothed Coulomb potential. Our approach is based on the numerical solution of the time-dependent Schrödinger equation. The calculated attosecond time structure is shown in Fig. 1a when only the fundamental ͑ 1 ͒ is present. The laser intensity is constant. We see that an attosecond pulse is emitted every 1 / 2 optical cycle. This reflects the symmetry of the HHG process. In Fig. 1b we perform the same calculations but add an intense spectrally detuned second-harmonic field. The detuning is chosen so that 2 = 1 ͑2+1/N͒, with N = 5. Due to the new periodicity of the electric field we show that an attosecond pulse is now emitted every 5 / 2 optical cycle. This property of HHG can be used to isolate a single attosecond pulse. Indeed, if the laser envelope duration is of the same order as (or a few times) the delay between two consecutive ...

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“…Several methods have been proposed for the generation of the supercontinuum harmonics spectra and the generation of single attosecond pulses, such as using a few-cycle laser pulse [8], double optical gating [14], the chirped laser pulse [15], the multicycle driving pulses [16], and so on. In addition, several alternative methods have been suggested by using the two-or multicolor laser scheme for generating an intense single attosecond pulse [17][18][19]. In particular, coherent control of the electron quantum paths is an efficient method for the production of the broadband supercontinuum harmonics spectra and isolated attosecond pulse [20][21][22][23].…”

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confidence: 99%

Effects of macroscopic propagation on spectra of broadband supercontinuum harmonics and isolated-attosecond-pulse generation: Coherent control of the electron quantum trajectories in two-color laser fields

Chu

2012

Phys. Rev. A

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Recently it was shown that broadband supercontinuum harmonics can be produced from the long-trajectory electrons in the single-atom response by the coherent control of the electron trajectories through optimized two-color laser fields. Such supercontinuum harmonics can be superposed to generate an isolated sub-30-attosecond (as) pulse [Liu et al., Phys. Rev. A 84, 033414 (2011)]. In this paper, we investigate the effect of macroscopic propagation on the supercontinuum harmonic spectra and the subsequent attosecond-pulse generation of atomic hydrogen. The time-dependent Schrödinger equation is solved accurately and efficiently by means of the time-dependent generalized pseudospectral method. The effects of macroscopic propagation are investigated in near and far field by solving Maxwell's equation. The results show that the contribution of short-trajectory electron emission is increased when the macroscopic propagation is considered. However, the characteristics of the dominant long-trajectory electron emission (in the single-atom response case) are not changed, and an isolated 53 as pulse can be generated in the near field. Moreover, in the far field, the contribution of long-trajectory electron emission is still dominant for both on-axis and off-axis cases. As a result, an isolated 42 as pulse can be generated directly. Similar results are obtained when the atomic target position is changed. Therefore, the proposed method for the single ultrashort attosecond-pulse generation can be realized by means of the coherent control of the electron quantum paths in appropriately optimized two-color laser fields.

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Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field

Cited by 126 publications

References 18 publications

Isolated sub-100‐aspulse generation via controlling electron dynamics

Isolated sub-100‐aspulse generation via controlling electron dynamics

Isolated attosecond pulses using a detuned second-harmonic field

Effects of macroscopic propagation on spectra of broadband supercontinuum harmonics and isolated-attosecond-pulse generation: Coherent control of the electron quantum trajectories in two-color laser fields

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