Efficient generation of an isolated single-cycle attosecond pulse

A new method for efficiently generating an isolated single-cycle attosecond pulse is proposed. It is shown that the ultraviolet (UV) attosecond pulse can be utilized as a robust tool to control the dynamics of electron wave packets (EWPs). By adding a UV attosecond pulse to an infrared (IR) few-cycle pulse at a proper time, only one return of the EWP to the parent ion is selected to effectively contribute to the harmonics, then an isolated two-cycle 130-as pulse with a bandwidth of 45 eV is obtained. After complementing the chirp, an isolated single-cycle attosecond pulse with a duration less than 100 as seems achievable. In addition, the contribution of the quantum trajectories can be selected by adjusting the delay between the IR and UV fields. Using this method, the harmonic and attosecond pulse yields are efficiently enhanced in contrast to the scheme [G. Sansone {\it et al.}, Science {\bf314}, 443 (2006)] using a few-cycle IR pulse in combination with the polarization gating technique.Comment: 5 pages, 4 figure

Tunneling Ionization Rates from Arbitrary Potential Wells

We present a practical numerical technique for calculating tunneling ionization rates from arbitrary 1-D potential wells in the presence of a linear external potential by determining the widths of the resonances in the spectral density, rho(E), adiabatically connected to the field-free bound states. While this technique applies to more general external potentials, we focus on the ionization of electrons from atoms and molecules by DC electric fields, as this has an important and immediate impact on the understanding of the multiphoton ionization of molecules in strong laser fields.Comment: 13 pages, 7 figures, LaTe

Identification of the cellular mechanisms responsible for the generation of particular HLA-DR epitopes

HLA class II/peptide complexes (pHLA-II), organized into microdomains on the surface of antigen presenting cells (APCs) or on APC-secreted exosomes, engage CD4+ T cells for immune recognition. The association of the HLA-II alleles DRB1*0401/0404 with rheumatoid arthritis may be due to their propensity to present self-peptides for immune recognition. pHLA-II presentation on APCs is largely determined by HLA-DM, an intracellular chaperone, and its negative regulator, HLA-DO. Previously described DRB1*04-restricted epitopes (D11-0401, D13-0401, and D13-0404) were found dependent, sensitive, and resistant, respectively, to HLA-DM activity. The aims of this study were to determine whether (a) HLA-DO affects epitope expression; (b) cell surface microdomains concentrate these epitopes; and (c) exosomes express these epitopes. Key findings include: HLA-DO appears not essential, but its role in optimal epitope expression may be cell-context dependent; lipid raft disruption abrogated only the DM-dependent D11-0401epitope; exosomal expression of these epitopes was cell specific and independent of their cell surface expression. Altogether, this study has enhanced our knowledge of DM-dependent, -sensitive, and -resistant epitopes on rheumatoid arthritis-associated pHLA-DRB1*04 molecules

High-harmonic generation: taking control of polarization

The ability to control the polarization of short-wavelength radiation generated by high-harmonic generation is useful not only for applications but also for testing conservation laws in physics

Fabricating nanostructures on fused silica using femtosecond infrared pulses combined with sub-nanojoule ultraviolet pulses

Circular craters with diameters of 500 nm are fabricated on the surface of fused silica by femtosecond ultraviolet–infrared (UV–IR) pulse trains with 0.8 nJ UV pulse energy. UV damage thresholds at different IR energies and UV–IR delays are measured. Diameters and depths of the ablated craters can be modified by adding the IR pulse and varying the UV–IR delays. These results demonstrate the feasibility of nanomachining using short wavelength lasers with pulse energy far below normal damage thresholds

Damage formation on fused silica illuminated with ultraviolet-infrared femtosecond pulse pairs

Citation: Yu, X., Chang, Z., Corkum, P. B., & Lei, S. (2015). Damage formation on fused silica illuminated with ultraviolet-infrared femtosecond pulse pairs. Proceedings of SPIE. doi:10.1117/12.2182633We investigate damage formation on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm (UV) pulse followed by a loosely focused 800 nm (IR) pulse. We show that the damage size is determined by the UV pulse, and only a small fraction of the normal UV damage threshold energy is needed to cause damage when combined with the properly delayed IR pulse. Our results, analyzed with a rate equation model, suggest that the UV pulse generates seed electrons through multiphoton absorption and the IR pulse utilizes these electrons to cause damage by avalanche ionization. By tuning such parameters like pulse energy, time delay, IR pulse duration and polarization, we further demonstrate that damage profile can be controlled. Copyright © 2015 SPIE

Momentum space tomographic imaging of photoelectrons

We apply tomography, a general method for reconstructing 3-D distributions from multiple projections, to reconstruct the momentum distribution of electrons produced via strong field photoionization. The projections are obtained by rotating the electron distribution via the polarization of the ionizing laser beam and recording a momentum spectrum at each angle with a 2-D velocity map imaging spectrometer. For linearly polarized light the tomographic reconstruction agrees with the distribution obtained using an Abel inversion. Electron tomography, which can be applied to any polarization, will simplify the technology of electron imaging. The method can be directly generalized to other charged particles.Comment: Accepted by J. Phys.