Weiyi Hong

Multi-cycle laser-driven broadband supercontinuum with a modulated polarization gating

We propose a method to generate broadband supercontinuum with a modulated polarization gating in the multi-cycle regime. The polarization gating at 1600 nm is employed to intensify the ellipticity dependence of high harmonic signal, and the adding second harmonic (SH) field is used for modulating ionization ratio between the driving field half-cycles. It is shown that longer driving pulses (6 optical cycles, 32.4 fs) can be applied to confine the efficient recollision of electron wave packet to half an optical cycle. The produced bandwidth of the extreme ultraviolet (XUV) supercontinuum is about 280 eV, corresponding to a fourier-transform-limited pulse of 10 as. In addition, an attosecond pulse of about 100 as with tunable wavelength can be straightforwardly obtained just by filtering different range of the spectrum.

Broadband water window supercontinuum generation with a tailored mid-IR pulse in neutral media

We propose a new (to our knowledge) scheme to generate a broadband water window supercontinuum in a neutral rare-gas media. Using a phase-stabilized few-cycle mid-IR pulse tailored by a much weaker 800 nm pulse, a supercontinuum from 265 to 425 eV is observed, and the ionization probability is below 0.1%. The nonionized media enable us to adopt the right phase-matching technique to select the short path of the supercontinuum, and an isolated 80 as pulse with a central wavelength of 3.2 nm is straightforwardly obtained.

Controlling nonsequential double ionization via two-color few-cycle pulses.

Using the classical three-dimensional ensembles, we have demonstrated the controlling of dynamics in nonsequential double ionization (NSDI) of helium by two-color few-cycle pulses. By changing the relative phase of the two pulses, recollisions leading to NSDI can be restricted in a time interval of several hundreds attosecond nearly before the field extremum and as a result, the correlated electron momentum distribution exhibits a so-far unobserved narrow arc-like structure. This structure reveals a novel energy correlation between the two electrons from NSDI by two-color few-cycle pulses.

Few-cycle attosecond pulses with stabilized-carrier-envelope phase in the presence of a strong terahertz field

High-order harmonic generation in the presence of a strong terahertz field is investigated, and phase stabilization of the few-cycle laser pulses is extended to the extremely ultraviolet region. It is found that the strong terahertz field significantly breaks the symmetry between the consecutive half-cycle and greatly extends the harmonic cutoff, producing both odd and even harmonics which are covered with an extremely broad bandwidth and well locked in phase. These results can support the generation of few-cycle attosecond pulse trains with stabilized carrier-envelope phase from pulse to pulse, and also enables the generation of phase-stabilized pulse train with tunable wavelength.

Control of quantum paths of high-order harmonics and attosecond pulse generation in the presence of a static electric field

The time–frequency properties of high-order harmonic generation in the presence of a static electric field are investigated. It is found that the quantum paths contributing to the harmonics can be controlled by adding a static electric field. The highest photon energies of harmonics emitted in the adjacent half-cycles of the laser field are modulated by the static electric field, and then an attosecond pulse train with one burst per optical cycle can be extracted. For the ratio between the laser and the static field of 0.39, the harmonic spectrum is extended to Ip + 9.1Up, and the harmonics above Ip + 0.7Up are emitted almost in phase. The phase-locked harmonics covered by a broad bandwidth are produced, and then a regular attosecond pulse train with a pulse duration of 80 as is generated.

Mid-infrared modulated polarization gating for ultra-broadband supercontinuum generation

We propose a method to control the harmonic process by a mid-IR modulated polarization gating for the effective generation of an ultra-broadband supercontinuum in the neutral rare-gas media. Using a mid-IR polarization gating pulse modulated by a weaker 800-nm linearly polarized pulse, the ionization, acceleration and recombination steps in the harmonic process are simultaneously controlled, leading to the efficient generation of an ultra-broadband supercontinuum covered by the spectral range from ultraviolet to water window x ray under the low ionization rate. The right phase-matching technique is employed to macroscopically select the short quantum path of the supercontinuum, then isolated sub-100-as pulses with tunable wavelengths are directly obtained. This supercontinuum also supports the pulse duration far below one atomic unit of time.

The effect of molecular alignment on correlated electron dynamics in nonsequential double ionization.

The electron-electron correlation in nonsequential double ionization (NSDI) from aligned molecules by linearly polarized 800 nm laser pulses has been investigated with the three-dimensional classical ensemble model. The result shows that for the perpendicular alignment the two electrons involved in NSDI more likely exit the molecule into the opposite hemispheres as compared to the parallel alignment, which agrees well with the experimental result [Phys. Rev. Lett. 95, 203003 (2005)]. This alignment effect is qualitatively explained based on the suppressed potential barriers which are different for parallel molecules and perpendicular molecules. Additionally, the intensity dependence of the alignment effect is also explored.

Phase-dependent nonsequential double ionization by few-cycle laser pulses

By solving the time-dependent Schrodinger equation, we investigate the phase dependence of nonsequential double ionization of one-dimensional Ar atoms by linearly polarized few-cycle pulses. The resulting two-electron momentum distributions exhibit a strong dependence on the pulses carrier-envelope phase (CEP). The calculated Ar2+ ion momentum distributions parallel to the laser polarization direction are in good agreement with the experimental measurements of Liu et al (2004 Phys. Rev. Lett. 93 263001). The ratio of the total double ionization yields from the first and third quadrants as a function of the CEP is similar to the ratio of the total single ionization yields from the left and right sides. The influence of electron–electron Coulomb repulsion on double ionization is also investigated by comparing a short-range interaction with the infinite-range interaction. Our theoretical simulations provide a prediction for determining the CEP value of few-cycle pulses by comparing with the experimental measurements.

Control the revisit time of the electron wave packet.

Ionization of molecules by strong laser fields launches an electron wave packet. This electron wave packet, which can be driven back by the field to recollide with the parent ion, has been widely explored to probe the ultrafast nuclear dynamics. We numerically demonstrate the precise control of the temporal characteristic of the recolliding electron wave packet (REWP) by orthogonally polarized two-color fields. Through changing the relative phase of the two fields, the revisit time of REWP can be manipulated with a resolution of less than 200 attos, thus significantly improving the resolution of the well known molecular clock. This provides an efficient method for real-time observation of the ultrafast molecular dynamics with attosecond resolution.

Control of quantum paths in the multicycle regime and efficient broadband attosecond pulse generation

We theoretically investigate the broadband isolated attosecond generation in a two-color multicycle laser pulse. It is shown that quantum path selection can be realized and then a sub-100-as isolated attosecond pulse is straightforwardly obtained. By adding an UV attosecond pulse to the synthesized two-color field, the continuous harmonic yield is significantly enhanced by 2 orders and the bandwidth is further broadened, which is beneficial to the efficient generation of a sub-100-as isolated attosecond pulse.