Direct generation of continuous-wave and passively Q-switched visible vortex beams from a doughnut-shaped diode-pumped Pr: YLF laser.

We demonstrate the direct generation of visible vortex beams (LG01 mode) from a doughnut-shaped diode-pumped Pr:YLF laser. In continuous-wave mode, the maximum vortex output power was 36 mW at 523 nm, 354 mW at 607 nm, 838 mW at 639 nm, 722 mW at 721 nm, respectively. Moreover, based on this operation, the orange and red passively Q-switched vortex lasers were also achieved by inserting a Co:MgAl2O4 crystal into the laser cavity as a saturable absorber. The shortest pulse width of Q-switched vortex laser was 58 ns for 607 nm, and 34 ns for 639 nm, respectively. Our work provides a reliable and efficient method for the direct generation of visible vortex lasers for potential applications.

Passively mode-locked red Pr:LiYF4 laser based on a two-dimensional palladium diselenide saturable absorber.

We report a passively mode-locked Pr:LiYF4 (Pr:YLF) visible laser using a palladium diselenide (PdSe2) as a saturable absorber (SA) for the first time, to the best of our knowledge. The nonlinear optical properties of two-dimensional (2D) PdSe2 nanosheets in the visible band were studied by the open-aperture Z-scan technique. The results indicate the significant saturable absorption properties of PdSe2 nanosheets in the visible region. Furthermore, the continuous wave mode-locked (CWML) visible laser based on PdSe2 SA was successfully realized. Ultrashort pulses as short as 35 ps were obtained at 639.5 nm with a repetition rate of 80.3 MHz and a maximum output power of 116 mW. The corresponding pulse energy was 1.44 nJ and peak power was 41.3 W. These results indicate that 2D PdSe2 SA is a promising high stability passively mode-locked device for ultrafast solid-state visible lasers.

Imaging of electron transition and bond breaking in the photodissociation of H2+ via ultrafast X-ray photoelectron diffraction.

We theoretically investigate the photodissociation dynamics of H2+ using the methodology of ultrafast X-ray photoelectron diffraction (UXPD). We use a femtosecond infrared pulse to prompt a coherent excitation from the molecular vibrational state (v = 9) of the electronic ground state (1sσg) and then adopt another time-delayed attosecond X-ray pulse to probe the dynamical properties. We have calculated photoionization momentum distributions by solving the non-Born-Oppenheimer time-dependent Schrödinger equation (TDSE). We unambiguously identify the phenomena associated with the g - u symmetry breakdown in the time-resolved photoelectron diffraction spectra. Using the two-center interference model, we can determine the variation in nuclear spacing with high accuracy. In addition, we use a strong field approximation (SFA) model to interpret the UXPD profile, and the SFA simulations can reproduce the TDSE results in a quantitative way.

Tellurium as the saturable absorber for the passively Q-switched laser at 1.34 µm.

The novel two-dimensional (2D) elementary tellurium is currently of great interest in optoelectronic and photonic applications. In this contribution, 2D tellurium nanosheets were successfully created by using the liquid-phase exfoliation method. With the as-prepared tellurium nanosheets as the saturable absorber (SA), we realized a passively $ Q $Q-switched ${\rm Nd} \text:{{\rm YVO}_4}$Nd:YVO4 laser operating at 1342 nm with a pulse width of 947 ns and single pulse energy of 2.25 µJ. Our work indicated the tellurium SA could be an efficient $ Q $Q-switcher for a near-infrared solid-state laser.

Enhanced double ionization rate from O2 molecules driven by counter-rotating circularly polarized two-color laser fields.

We report that the nonsequential double ionization (NSDI) probability of an O 2 target can be enhanced greatly in a counter-rotating circularly polarized two-color driving field. The field is composed of a fundamental frequency and its third harmonic, and the combined electric field traces out a four-leaf-clover pattern. The electron ionized by such a field has more chances to collide with the valence electrons in the O 2 molecule, which significantly enhances the NSDI probability. This effect is more evident in low-intensity fields. We also find that the enhancement appears in a broad range of the field ratio of two colors and that both the NSDI yield and the underlying electronic behavior varies notably with the field ratio.

Half Kapitza-Dirac effect of H2+ molecule in intense standing-wave laser fields.

The half Kapitza-Dirac effect of H2+ molecule in an intense standing-wave laser field is studied with a focus on the influence of the molecular orbital symmetry and the molecular alignment on the photo-electron angular distributions (PADs). In standing-wave laser fields, the PADs split along the scattering angle due to the momentum change of electron with photons when it escapes from the laser fields. The structures and the symmetry of PADs are severely affected by the molecular orbital symmetry and the molecular alignment. For H2+ molecule in ground state (σg), the PADs are severely changed by the molecular alignment only when the photoelectron kinetic energy is sufficiently high. For H2+ molecule in the first excited state (σµ), the molecular alignment distinctively changes the PADs, irrelevant to the kinetic energy of photoelectrons. When the molecules are aligned either parallel with or perpendicular to the laser polarization, the PADs are symmetric about an axis. In other cases, the PADs do not show any symmetry. These results indicate that the molecular alignment can be used to control the splitting in the half Kapitza-Dirac effect.