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The sensitivity of strong-field ionization to atomic orbital helicity has attracted much attention from physicists, due to its potential application in attosecond spectroscopy and spintronics. In order to intuitively observe the physical mechanisms of helicity-dependent ionization rates during photoionization, the concept of the Wigner phase can be used to characterize the different interactions between the rotating electrons and the Coulomb potential. Here, we find that in both one- and three-photon ionization schemes, the electrons liberated more easily by the circularly polarized laser field suffer less influence of the Coulomb potential during the propagation and then accumulate less Winger phase. This result indicates that the strength of the interaction between the rotating electrons and the Coulomb potential can explain the helicity-dependent ionization for different ionization mechanisms universally, which is also supported by our classical ensemble analysis. Our work provides an intuitive perspective towards the physics picture of ionization propensity rules. [ABSTRACT FROM AUTHOR]

We theoretically investigate the high-order harmonic generation (HHG) driven by dual-pulse lasers. The first pulse of 800 nm can create plasma in the high-density gas target. Then, the second pulse of 1800 nm is spatiotemporally reshaped by the plasma and generates soft x-ray high-order harmonics in the overdriven regime. The simulation results show that adding the first pulse can promote the phase matching of HHG in the cutoff region. As a result, a cutoff-extended HHG is achieved, which is one order of magnitude more intense than that in one pulse scheme reported previously. In dual-pulse driving lasers, the harmonic yield and the cutoff photon energy can be optimized effectively by increasing the gas pressure inside the water window region. [ABSTRACT FROM AUTHOR]

The high-resolution three-dimensional photoelectron momentum distributions via above-threshold ionization (ATI) of Xe atoms are measured in an intense near circularly polarized laser field using velocity map imaging and tomography reconstruction. Compared to the linearly polarized laser field, the employed near circularly polarized laser field imposes a more strict selection rule for the transition via resonant excitation, and therefore we can selectively enhance the resonant ATI through certain atomic Rydberg states. Our results show the self-reference ionization delay, which is determined from the difference between the measured streaking angles for nonadiabatic ATI via the 4f and 5f Rydberg states, is 45.6 as. Our method provides an accessible route to highlight the role of resonant transition between selected states, which will pave the way for fully understanding the ionization dynamics toward manipulating electron motion as well as reaction in an ultrafast time scale. [ABSTRACT FROM AUTHOR]

A method to produce circularly polarized attosecond pulses with tunable helicity from CO molecule by using an unidirectionally rotating laser field is proposed. It is found that broadband harmonic supercontinuum with circular polarization can be generated from the oriented CO molecule. This enables the production sub‐100 attosecond isolated pulse with the ellipticity as high as 0.9 at the macroscopic level. Moreover, the helicity of the generated high‐order harmonics and the attosecond pulse can be tuned by adjusting the orientation of the CO molecule. This method will be beneficial for the studies on chiral‐specific dynamics and magnetic circular dichroism on an attosecond time scale. [ABSTRACT FROM AUTHOR]

Ultrafast electron pulses generated using femtosecond lasers offer a direct means for investigating ultrafast processes, but the yield of electron emission remains a significant practical limitation. This study demonstrates a substantial enhancement in yield of electron emission from carbon nanotubes induced by two-color (ω + 2 ω) pulses with an 80 MHz repetition rate and this enhancement is non-delay-dependent. Compared to the sum of photocurrents induced by each color pulse individually, the combined two-color pulses result in a remarkable enhancement in yield of electron emission by more than 25 times. We attribute this enhanced yield to carrier-assisted optical field emission: carbon nanotubes sustain high carrier concentration state under laser radiation thereby enhancing yield of electron emission. We propose the carrier migration emission model to provide a semi-quantitative description for the carrier-assisted optical field emission. This phenomenon introduces a new method to increase the yield of ultrafast electron pulses from CNTs and paves the way for next-generation ultrafast electron emitters and on chip ultrafast optoelectronic device. [Display omitted] [ABSTRACT FROM AUTHOR]