We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse. We found experimentally that resonant emission lines between 19.7 eV and 22.3 eV (close to the 13th and 15th harmonic of the laser) exhibit a strong, asymmetric chirp dependence, with pronounced intensity modulations. The chirp dependence is reproduced by our numerical time-dependent Schrödinger equation simulations of a resonant HHG by the model indium ion. As demonstrated with our separate simulations of HHG within the strong field approximation, the resonance can be understood in terms of the chirp-dependent HHG photon energy coinciding with the energy of an autoionizing state to ground state transition with high oscillator strength. This supports the validity of the general theory of resonant four-step HHG in the few-cycle limit.

We present measurements of the spatial coherence of the high-order harmonics generated in laser-produced ablation plumes. Harmonics were generated using 4 fs, 775 nm pulses with peak intensity 3 × 1014 W cm−2. Double-slit fringe visibilities in the range of ≈0.6–0.75 were measured for non-resonant harmonics in carbon and resonantly enhanced harmonics in zinc and indium. These are somewhat higher than the visibility obtained for harmonics generated in argon gas under similar conditions. This is attributed to lower time-dependent ionization of the plasma ablation targets compared to argon during the high harmonics generation process.

Enhanced single harmonic generation is analyzed in indium laser ablation plasmas at excitation conditions of multicycle (30 fs) and few-cycle (3.5 fs) pulses. We demonstrate the strong influence of pulse duration, on the emission spectra from the indium plasma. For few-cycle pulses, the enhanced emissions do not coincide with the expected harmonic wavelengths, which is the case for multicycle pulses. We test the coherent properties of an enhanced emission around 20 eV using polarization and double-slit interference techniques. We also characterize the dynamics of the emissions from the indium plasma by tuning the laser pulse duration. A theoretical analysis is presented to describe the indium plasma emission upon excitation by few-cycle pulses.

Summary form only given. It is nowadays possible to produce isolated attosecond pulses in the XUV photon energy range (~90 eV) [1]. These can be employed in combination with infrared (IR) pulses in IR field dressed photo-ionisation experiments to investigate attosecond dynamics in XUV pump IR probe schemes [2]. Specific chromophores in large biomolecules have a high photo excitation cross-section in the VUV range (10-20eV) [3]. Therefore attosecond VUV pulses will enhance the excitation fraction of a molecular sample and allow direct atto-pump atto-probe experiments. We discuss how we plan to produce efficient HHG radiation in this range. The target is optimised to achieve high photon flux and a sub-fs pulse. We discuss the set up for an absolute measurement of the photon flux in combination with spectral and temporal characterisation of the pulse [4] necessary for such an optimisation. We discuss the modelling of the optimisation process according to phase matching calculations [5] and 2D/3D time dependent Schrödinger equation (TDSE) numerical simulations [6].