Isolated attosecond pulses using a detuned second-harmonic field

Abstract

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 ...

Gating attosecond pulse train generation using multicolor laser fields

“…It only requires that the three outputs from an OPA be combined with the appropriate phase. This is similar to calculations [25] in which a slightly detuned second harmonic was added to the fundamental field, leading to an isolated attosecond pulse. Our present method gives better contrast because of the reduced periodicity of the electric field and does not rely on depletion to reduce post-pulses.…”

“…Combined with PG in a technique called double optical gating, an XUV supercontinuum capable of supporting 130-as pulses was generated using 9-fs driving pulses [23]. Other approaches of combining different frequencies have been studied theoretically [24][25][26]. A recent experiment demonstrated increased spectral density when two incommensurate laser frequencies were combined [27], creating the conditions necessary for temporal confinement of attosecond pulse generation.…”

Control continuum bandwidth and attosecond pulse generation by infrared laser parametric source

“…In Figure 1d, continuum bandwidth dependence on relative strength ratio k is studied. In the figure, the linear dependence of the continuum bandwidth on the relative strength ratio is shown from k = 0 to 0.1 which is consistent with heterodyne mixing theory proposed by Merdji et al [21], generated continuum bandwidth from our quasiclassical description through equation (13) is verified. Increasing relative strength ratio k further, the continuum bandwidth deviated from linear dependence on k. They began to decrease around k = 0.4, 0.3 and 0.3 respectively.…”

“…Polarization gating technique by utilizing the pulse's polarization angle to modulate the HHG process has been realized which relax the laboratory requirement for generating attosecond pulse [13][14][15][16][17][18][19]. Temporal gating is another technique for generating continuum with broader spectral width by superposing two few-cycle pulses [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Recently, Feng et al [19] generate an isolated attosecond pulse by using 20 to 28 fs driving lasers pulse employing polarization gating technology.…”

Quantum wavepacket exploration of isolated high-order harmonic attosecond pulse generation: from two-color scheme to three-color scheme

“…Great efforts have been made in the generation of shorter isolated attosecond pulses [6][7][8][9][10][11], among which high-order harmonic generation (HHG) based technology is the most promising [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. The HHG process can be understood through the well known three-step model (TSM) [29].…”