J. P. Marangos

High harmonic generation spectroscopy of hydrocarbons

We have demonstrated the ability of few-cycle midinfrared intense laser pulses to produce extended harmonic spectra (≥45u2002eV) suitable for high harmonic spectroscopy in aligned hydrocarbons with ionization potentials in the range 9.07–11.52 eV. Modulations in the spectra measured with different alignment angles show signatures of the molecular structure. These results pave the way for the extension of high harmonic spectroscopy to complex biomolecules.

Spatio-temporal characterization of optical waveforms

Near single-cycle lasers have opened new frontiers in nonlinear optics and are essential tools for attosecond science [1]. In the last few years development in pulse characterization has been aimed both at improvements in fidelity and ease of use and also the ability to characterize ever shorter pulses down to near-single-or sub-cycle pulse durations [2, 3]. It is usually assumed that the electric field of a laser pulse can be factorized into a spatial and a temporal part E(x, y, t) = E(x, y) E(t). Most currently known and established pulse characterization techniques average over one or more spatial dimension of the beam or measure a sub-sample of the beam selected by an aperture, which can lead to errorneous measurements even in case of benign sources like hollow fiber compressors due to the inherent radially varying spectrum in the collimated beam. For most experiments space-time couplings are detrimental but for some e.g. the attosecond gating scheme by ultrafast wavefront rotation, the introduction of deliberate space-time couplings where E(x, y, t) ≠ E(x, y) E(t) is desirable [4].

Controlling ionisation and fragmentation processes in CO2 via laser driven inelastic electron recollisions

For the first time, the angular dependence of nonsequential double ionisation and dissociation induced by laser driven inelastic electron rescattering was investigated experimentally in aligned CO2. A strong dependence on the recollision angle was found.

Toward the Extension of High Order Harmonic Spectroscopy to Complex Molecules: Investigation of Aligned Hydrocarbons

High-order harmonic generation is as a powerful tool for the study of molecular properties. Up to now this investigation tool has been confined to simple molecules, with a relatively high ionization potential, since ionization saturation hindered its exploitation to fragile molecules. In this work we show that such limitation can be overcome by using mid-IR ultrashort driving pulses; as prototypical molecules we considered hydrocarbons. Clear signatures of the highest occupied molecular orbital were found in the harmonic spectra generated in unsaturated aligned hydrocarbons like acetylene, ethylene, allene and 1,3-butadiene. Our findings demonstrate that high-order harmonic generation spectroscopy can be extended to complex molecular species.

Yield enhancement in multicolour high-order harmonic generation: Superposition of multiple un-related frequencies

We report observations and analysis of high harmonic generation driven by a superposition of fields at 1290 nm and 780 nm. These fields are not commensurate in frequency, as in previous experiments such as Ref. [1], and the superposition leads to an increase in the yield of the mid-plateau harmonics of more than two orders of magnitude compared to using the 1290 nm field alone. Significant extension of the cut-off photon energy is seen even by adding only a small amount of the 780 nm field. Fig. 1 plots the observed yield enhancements against harmonic energy and scanned over the temporal delay between the peaks of the two pulse envelopes. The figure shows two different intensity ratios, as stated in the caption. In both cases yield enhancement of up to two orders of magnitude is seen below 40 eV, together with smaller enhancements up to 80 eV, the limit of the MCP detector. This is a promising route to provide a greater photon number in attosecond pulse production [2], for applications in XUV imaging and time-resolved experiments [3].