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

Molecular high-order harmonic spectra and its application to the generation of the isolated attosecond pulse

Molecular high-order harmonic generation (HHG) spectra from H₂⁺ and its application to the generation of the isolated attosecond pulses (IAPs) have been numerically investigated. Results show that (i) the 7th harmonic order is enhanced with the nuclei around the equilibrium internuclear, and as the internuclear distance increased, this enhanced harmonic produces a red-shift (even disappearance). Theoretical analyses show that the electronic transition between the ground and the 1st excited states is responsible for the red-shift enhanced harmonic. (ii) The harmonic spectra exhibit several maxima and minima, and a red-shift of these points is predicted as the internuclear distance increased. (iii) By properly choosing the internuclear distance, the harmonic yield is enhanced, and there is only the single quantum path contributing to the harmonic spectra. (iv) Further, by properly adding the half-cycle pulse as well as the spatial inhomogeneous effect, a 375 eV supercontinuum with a pulse enhancement of 3.9 dB and some attosecond X-ray pulses shorter than 60as can be produced.

Ultra-broadband water window supercontinuum generation with high efficiency in a three-color laser field

We propose an efficient method to generate an ultra-broadband supercontinuum by using a three-color field synthesized by a 1600 nm driving pulse and two weak (3200 nm and 400 nm) control pulses. The results show that the acceleration and ionization steps in the HHG process are simultaneously controlled in our scheme, which can not only broaden the bandwidth but also enhance the harmonic yields of the generated spectrum. Thus a high-efficiency supercontinuum with the photon energies ranging from 205 eV to 480 eV is observed. Based on our 3D propagation simulations, we show that the short quantum path of the supercontinuum is well phase-matched and can be selected under suitable propagation conditions. Thereby, the modulation due to quantum path interference is largely reduced and intense isolated sub-100 as pulses with tunable central wavelengths are directly obtained within water window spectral region.

Broadband isolated attosecond pulse with high spatiotemporal quality in pre-excited medium by multi-cycle two-color fields

We propose a scheme that combines multi-cycle two-color pump pulses and pre-excited medium to simultaneously control the ionization and acceleration steps of electron wave packet. Intense two-plateau supercontinua are observed at microscopic level, which support isolated attosecond pulse with tunable central wavelength. We further investigate the propagation effects and find that the macroscopic response is mainly associated with the initial population of the excited state. Large initial population can distort the pump field and weaken the phase-matching of the supercontinua due to the high density of free electrons. On the contrary, small initial population of 3% can generate well phase-matched supercontinua, and an efficient isolated attosecond pulse with gaussian-like spatial distribution is directly achieved. The divergence angle of the generated pulse is less than 0.2 mrad, which indicates its beam quality is good. This pulse benefits for some potential applications of ultrafast measurements with high spatiotemporal resolutions.

Coherent control of high-order harmonic generation by phase jump pulses

We theoretically investigate the high-order harmonic generation driven by laser pulses with a π-phase jump. The cutoff of high-order harmonic spectrum extends dramatically due to the phase jump which enlarges the asymmetry of the laser field. We find that the intensity and the coherence of the continuum can be controlled by the jump time. By selecting proper frequency of the continuum spectrum, a sub-100-attosecond pulse can be generated without any phase compensation.

Efficient generation of high beam-quality attosecond pulse with polarization-gating Bessel-Gauss beam from highly-ionized media

Single attosecond pulse generation with polarization gating Bessel-Gauss beam in relatively strongly-ionized media is investigated. The results show that Bessel-Gauss beam has the ability to suppress the spatial plasma dispersion effects caused by high density of free electrons, thus the laser field can maintain its spatial profile through highly-ionized medium. This indicates the use of Bessel-Gauss beam has advantages over Gaussian beam in high harmonic generation under high ionization conditions. In our scheme, significant improvement of spatiotemporal properties of harmonics is achieved and an isolated attosecond pulse with high beam quality is filtered out using polarization gating.

Dramatic cutoff extension and broadband supercontinuum generation in multi-cycle two color pulses

We propose a method to markedly increase the electron-recollision kinetic energy, using the combination of a 0.8 μm/13 fs driving pulse with a much weaker multi-cycle mid-infrared pulse at 10.4 μm. The results show that the synthesized field effectively lengthens the accelerated distance of electron wave packet and the harmonic cutoff is significantly extended to IP+26UP, which is covered with the water-window spectral region. In addition, only one single quantum path contributes to harmonics, and those higher than IP+15UP become continuous. This supercontinuum can support the generation of an isolated sub-100 as pulse with tunable central wavelength and also the pulse with the duration below one atomic unit of time (24 as). Moreover, our scheme can further extend to more longer driving pulses, which provides a dramatic approach for cutoff extension and broadband supercontinuum generation with multi-cycle pump pulses.

Wavelength dependence of electron localization in the laser-driven dissociation of H2(+)

We theoretically investigate the laser wavelength dependence of asymmetric dissociation of H2(+). It is found that the electron localization in molecular dissociation is significantly manipulated by varying the wavelength of the driving field. Through creating a strong nuclear vibration in the laser-molecular interaction, our simulations demonstrate that the few-cycle mid-infrared pulse can effectively localize the electron at one of the dissociating nuclei with weak ionization. Moreover, we show that the observed phase-shift of the dissociation asymmetry is attributed to the different population transfers by the remaining fields after the internuclear distances reach the one-photon coupling point.

Broadband large-ellipticity harmonic generation with polar molecules

We investigate the polarization properties of high harmonic generation from polar molecules with a linearly polarized field. It is found that elliptically polarized harmonics are observed in a wide spectral range from the plateau to the cutoff. Further analyses show that the nonsymmetric structure of the highest occupied molecular orbital is the origin of ellipticity of the harmonics. The results provide a method for generation of large-ellipticity XUV pulses, which will benefit the application of HHG as a tool of detection in materials and biology science.

Laser-polarization-dependent photoelectron angular distributions from polar molecules

Photoelectron angular distributions (PADs) of oriented polar molecules in response to different polarized lasers are systematically investigated. It is found that the PADs of polar CO molecules show three distinct styles excited by linearly, elliptically and circularly polarized lasers respectively. In the case of elliptical polarization, a deep suppression is observed along the major axis and the distribution concentrates approximately along the minor axis. Additionally, it is also found that the concentrated distributions rotate clockwise as the ellipticity increases. Our investigation presents a method to manipulate the motion and angular distribution of photoelectrons by varying the polarization of the exciting pulses, and also implies the possibility to control the processes in laser-molecule interactions in future work.

Mid-infrared laser-driven broadband water-window supercontinuum generation from pre-excited medium

We theoretically investigate the broadband water-window supercontinuum generation from pre-excited medium with a mid-infrared pulse. We find that the wavelength scaling of the harmonic yield from near-visible (0.8 μm) to mid-infrared (1.8 μm) in single-atom level is λ(-2.7). Using an intense phase-stabilized few-cycle 1.6 μm laser pulse, a broadband water window supercontinuum with bandwidth of approximately 140 eV is obtained. We also investigate the macroscopic effects and find that large initial population of the excited state leads to the high-density of free electrons, which shift the carrier-envelop phase of the driving pulse and further diminish the water-window supercontinuum generation. The highly-ionized medium also results in poor temporal and spatial properties of the attosecond pulse. Instead, small initial population of the excited state can produce well phase-matched xuv supercontinuum in water-window region and an 100-as pulse with central wavelength of 2.8 nm and pulse energy of 0.15 nJ can be filtered out.

Phase dependence of electron localization in HeH²⁺ dissociation with an intense few-cycle laser pulse

The electron localization in the dissociation of the asymmetric charged molecular ion HeH²⁺ exposed to an intense few-cycle laser pulse is studied by solving numerically the 3D time-dependent Schrödinger equation. By varying the carrier-envelope phase (CEP) and the intensity of the pulse, the upward shift of the localization probability and the suppression of the dissociation channel He²⁺+H are observed. Our analysis shows that the phenomenon is attributed to the asymmetric structure of the molecule as well as the recollision-assistant field-induced ionization of the electron wave packets localized on H⁺ in the trailing of the pulse.

Spatial coherence control of xuv supercontinuum generation by two-color laser field

We investigate the spatial characteristics of xuv supercontinuum generation in the two-color laser field consisting of a fundamental and a weak second harmonic field. By optimizing the synthesized two-color field, the spatial profile of the xuv supercontinuum varies from annular-like to Gaussian-like and then the spatial quality is improved effectively, which is beneficial for its potential applications. Moreover, our calculation shows that the spatial quality of the supercontinuum is stable when the intensity of the controlling field varies in the acceptable fluctuation.

Broadband supercontinuum generation method combining mid-infrared chirped-pulse modulation and generalized polarization gating

We present a method to control the harmonic process by a mid-infrared modulated generalized polarization gating for the generation of the broadband supercontinuum. Using a mid-IR generalized polarization gating modulated by a weaker mid-IR linearly polarized chirped field, the ionization, acceleration and recombination steps in the HHG 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. Using this method we expect that isolated sub-100 attosecond pulses with tunable wavelength could be obtained straightforwardly.

Infrared two-color multicycle laser field synthesis for generating an intense attosecond pulse

We propose and demonstrate the generation of a continuum high-order harmonic spectrum by mixing multicycle two-color (TC) laser fields with the aim of obtaining an intense isolated attosecond pulse. By optimizing the wavelength of a supplementary infrared pulse in a TC field, a continuum harmonic spectrum was created around the cutoff region without carrier-envelope phase stabilization. The obtained harmonic spectra clearly show the possibility of generating isolated attosecond pulses from a multicycle TC laser field, which is generated by an 800 nm, 30 fs pulse mixed with a 1300 nm, 40 fs pulse. Our proposed method enables us not only to relax the requirements for the pump pulse duration but also to reduce ionization of the harmonic medium. This concept opens the door to create an intense isolated attosecond pulse using a conventional femtosecond laser system.

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.