Counterintuitive alignment of H2(+) in intense femtosecond laser fields

The multiphoton ionization of H2 has been studied using laser pulses of 266 nm wavelength, 250 fs duration, and 5x10(13) W/cm(2) peak intensity. Dissociation of H2(+) via one-photon absorption proceeds through two channels with markedly different proton angular distributions. The lower-energy channel (2.6 eV kinetic energy release) is produced in the bond softening mechanism, which generates parallel alignment. The higher-energy channel (3.5 eV) originates from population trapping in a light-induced bound state, where bond hardening generates orthogonal, counterintuitive alignment.     

Enhanced ionization of perpendicularly aligned H2 in intense laser fields

Using three-dimensional classical ensembles, we have investigated the enhancement of double ionization of perpendicularly aligned H₂ molecules by a 800 nm laser pulse with intensity ranging from 1 × 10¹⁴ W/cm² to 6 × 10¹⁴ W/cm². The simulated results show that double ionization probability of H₂ strongly depends on R and reaches a maximum at an intensity independent critical distance R_C ≈ 5 a.u. Furthermore, the enhancement of double ionization is more pronounced in the cases of weaker or stronger fields. These results, a well indication of the influence of molecular structures and laser–molecule interactions on double ionization of diatomic molecules, are analyzed in detail and qualitatively explained based on the field-induced barrier suppression model and back analysis.     

Time-resolved imaging and manipulation of H2 fragmentation in intense laser fields

We report on the experimental realization of time-resolved coincident Coulomb explosion imaging of H2 fragmentation in 10(14) W/cm(2) laser fields. Combining a high-resolution "reaction microscope" and a fs pump-probe setup, we map the motion of wave packets dissociating via one- or two-photon channels, respectively, and observe a new region of enhanced ionization. The long-term interferometric stability of our system allows us to extend pump-probe experiments into the region of overlapping pulses, which offers new possibilities for the manipulation of ultrafast molecular fragmentation dynamics.     

Electron density changes and high harmonics generation in H2 molecule under intense laser fields

Under interaction with a high-intensity laser field, the real-time femtosecond dynamics of the electron density in the H₂ molecule has been studied quantum mechanically. For this purpose, a time-dependent generalized nonlinear Schr?dinger equation of motion, developed earlier in our laboratory by combining density functional theory and quantum fluid dynamics in real space, is solved numerically at the equilibrium internuclear distance of the molecule. By employing various time-dependent calculated properties as probes, information and insight are obtained about the phenomena of excitation, ionization, bond-softening, dipole formation and high-harmonics generation. The present approach goes beyond the linear response formalism.     

Dissociation and ionization of H+2 by ultrashort intense laser pulses probed by coincidence 3D momentum imaging

Laser-induced dissociation and ionization of H(+)(2) were simultaneously measured using coincidence 3D momentum imaging, allowing direct separation of the two processes, even where the fragment kinetic energy is the same for both processes. The results for 45 and 135 fs 790 nm pulses with an intensity of approximately 2.5 x 10(14) W/cm(2) differ from each other much more than one would expect from previous measurements with longer pulses. Ionization was negligible for the longer pulse and was strongly aligned along the laser polarization for the shorter pulse, but showed no structure in its kinetic energy distribution. In addition, the ionization to dissociation ratio was found to be much smaller than theoretically predicted for H(+)(2).     

Controlling HD+ and H+2 dissociation with the carrier-envelope phase difference of an intense ultrashort laser pulse

Carrier-envelope phase difference effects in the dissociation of the HD+ molecular ion in the field of an intense, linearly polarized, ultrashort laser pulse are studied in the framework of the time-dependent Schr?dinger equation. We consider a reduced-dimensionality model in which the nuclei are free to vibrate along the field polarization and the electrons move in two dimensions. The laser has a central wavelength of 790 nm and a pulse length of 10 fs with intensities in the range 6x10(14) to 9x10(14) W/cm(2). We find that the angular distribution of dissociation to p+D and H+d can be controlled by varying the phase difference, generating differences between the dissociation channels of more than a factor of 2. Moreover, the asymmetry is nearly as large for H+2 dissociation.     

Interaction of intense ultrashort laser pulse with shell target

Based on numerical PIC modeling and an analytical model, we analyze the interaction of a short high-intensity laser pulse with homo- and heterogeneous (two-layer) shell targets. We show that the shell target is capable of focusing ions in a narrow region. In this case, the ion energy exceeds the ion energy for a flat infinite target of the same thickness. We propose to use the effect of focusing and cumulation of ions to implement the collision of ionic beams that counterpropagate from opposite sides of the sphere and to increase the yield of ion-ion reactions. The constructed model makes it possible to calculate the energy of the ion and the number of ions in the region of focusing.     

Electronic and optical properties of asymmetric GaAs double quantum dots in intense laser fields

In the present work, we investigated the effect of an intense non-resonant laser field on the electronic structure and the nonlinear optical properties (the light absorption, the optical rectification) of a GaAs asymmetric double quantum dot under a strong probe field excitation. The calculations were performed within the compact-density matrix formalism under the steady state conditions with the use of the effective mass approximation. The obtained results show that: (i) the electronic structure and, consequently, the optical properties are sensitive to the dressed potential; (ii) the changes in the incident light polarisation lead to blue or redshifts in the intraband optical absorption spectrum; (iii) for specific values of the structure parameters and under an intense laser illumination, the asymmetric double quantum dots can be a good candidate for NOR emission of THz radiation.     

用Coulomb-Volkov方法研究强激光场中H原子的电离

利用Coulomb-Volkov方法研究了H原子在不同波长的线性极化强激光场中电离的能量谱和动量谱,并与强场近似和直接数值求解含时Schr(o|¨)dinger方程的结果进行比较,结果发现:随着激光频率的增加,由Coulomb-Volkov方法得到的结果与数值求解含时Schr(o|¨)dinger方程的结果符合得很好.     

Near-complete absorption of intense, ultrashort laser light by sub-lambda gratings

We demonstrate near-100% light absorption and increased x-ray emission from dense plasmas created on solid surfaces with a periodic sub-lambda structure. The efficacy of the structure-induced surface plasmon resonance, responsible for enhanced absorption, is directly tested at the highest intensities to date (3 x 10{15} W cm{-2}) via systematic, correlated measurements of absorption and x-ray emission. An analytical grating model as well as 2D particle-in-cell simulations conclusively explain our observations. Our study offers a definite, quantitative way forward for optimizing and understanding the absorption process.     

Symmetric fragmentation of polyatomic molecular ions in ultrashort intense laser pulses

The intense field-ionization behavior of linear homonuclear polyatomic molecular ions has been numerically investigated for different molecular configurations. The enhanced ionization effect of molecular ions is found as a general feature in a critical bond length range, but the enhancement degree of ionization is bond-selective. The more symmetric the molecular ion stretches, the more enhanced the ionization of the molecular ion. It implies that the symmetric fragmentation of linear molecular ions is most probable at moderate laser intensity. These results are explained in terms of the field-induced over-barrier ionization mechanism.     

Formation of ground-state vibrational wave packets in intense ultrashort laser pulses

The formation of coherent vibrational wave packets in the electronic ground state of neutral molecules in intense ultrashort laser pulses and their subsequent detection by means of recently developed pump-probe experiments are discussed. The wave packet formation is due to the pronounced dependence of the strong-field ionization rate on the internuclear distance. This leads to a deformation of the initial wave function due to an internuclear-distance dependent depletion. The phenomenon is demonstrated with a time-dependent wave packet study for molecular hydrogen.