Leonardo Brugnera

High harmonic generation spectroscopy of hydrocarbons

We have demonstrated the ability of few-cycle midinfrared intense laser pulses to produce extended harmonic spectra (≥45 eV) 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.

High harmonic emission from a superposition of multiple unrelated frequency fields

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 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. These observations are explained by calculations performed in the strong field approximation. Most importantly we find that enhancement is found to arise as a consequence of both increased ionization in the sum-field and modification of the electron trajectories leading to an earlier return time. The enhanced yield even when using modest intensity fields of 5 x 10(13) Wcm(-2) is extended to the 80 eV range and is a promising route to provide a greater photon number for applications in XUV imaging and time-resolved experiments at a high repetition rate.

Enhancement of high harmonics generated by field steering of electrons in a two-color orthogonally polarized laser field

We demonstrate enhancement by 1 order of magnitude of the high-order harmonics generated in argon by combining a fundamental field at 1300 nm (10(14) W cm(-2)) and its orthogonally polarized second harmonic at 650 nm (2 × 10(13) W cm(-2)) and by controlling the relative phase between them. This extends earlier work by ensuring that the main effect is the combined field steering the electron trajectory with negligible contribution from multiphoton effects compared to the previous schemes with 800/400 nm fields. We access a broad energy range of harmonics (from 20 eV to 80 eV) at a low laser intensity (far below the ionization saturation limit) and observe deep modulation of the harmonic yield with a period of π in the relative phase. Strong field theoretical analysis reveals that this is principally due to the steering of the recolliding electron wave packet by the two-color field. Our modeling also shows that the atto chirp can be controlled, leading to production of shorter pulses.

Ultrafast science and development at the Artemis facility

The Artemis facility for ultrafast XUV science is constructed around a high average power carrier-envelope phasestabilised system, which is used to generate tuneable pulses across a wavelength range spanning the UV to the far infrared, few-cycle pulses at 800nm and short pulses of XUV radiation produced through high harmonic generation. The XUV pulses can be delivered to interaction stations for materials science and atomic and molecular physics and chemistry through two vacuum beamlines for broadband XUV or narrow-band tuneable XUV pulses. The novel XUV monochromator provides bandwidth selection and tunability while preserving the pulse duration to within 10 fs. Measurements of the XUV pulse duration using an XUV-pump IR-probe technique demonstrate that the XUV pulselength is below 30 fs for a 28 fs drive laser pulse. The materials science station, which contains a hemispherical electron analyser and five-axis manipulator cooled to 14K, is optimised for photoemission experiments with the XUV. The end-station for atomic and molecular physics and chemistry includes a velocity-map imaging detector and molecular beam source for gas-phase experiments. The facility is now fully operational and open to UK and European users for twenty weeks per year. Some of the key new scientific results obtained on the facility include: the extension of HHG imaging spectroscopy to the mid-infrared; a technique for enhancing the conversion efficiency of the XUV by combining two laser fields with non-harmonically related wavelengths; and observation of D3+ photodissociation in intense laser fields.

Molecular internal dynamics studied by quantum path interferences in high order harmonic generation

Abstract We investigate how short and long electron trajectory contributions to high harmonic emission and their interferences give access to information about intra-molecular dynamics. In the case of unaligned molecules, we show experimental evidence that the long trajectory contribution is more dependent upon the molecular species than the short one, providing a high sensitivity to cation nuclear dynamics from 100’s of as to a few fs after ionisation. Using theoretical approaches based on the strong field approximation and numerical integration of the time dependent Schrodinger equation, we examine how quantum path interferences encode electronic motion when the molecules are aligned. We show that the interferences are dependent upon which ionisation channels are involved and any superposition between them. In particular, quantum path interferences can encode signatures of electron dynamics if the laser field drives a coupling between the channels. Hence, molecular quantum path interferences are a promising method for attosecond spectroscopy, allowing the resolution of ultra-fast charge migration in molecules after ionisation in a self-referenced manner.

Development of High Power Infrared Optical Parametric Amplification Laser System Seeded by Self-difference Frequency Generation Pulses

We report on our recent development of a femtosecond infrared laser system based on self-phase-stabilized seed by difference-frequency generation of supercontinuum obtained with so-called hollow fiber compression technique and two-stage optical parametric amplification. After the final amplifier, we obtained pulses with duration of 500 μJ and as a first demonstration of this new laser system, we generated high-harmonics up to 47th order in a tube target filled with Xe gas.

Trajectory Selection in High Harmonic Generation Using Multicolor Fields

We examine trajectory selection and resulting yield modulation in high harmonic generation using a multicolor field composed of an 800nm fundamental and its perpendicularly polarized second harmonic.

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].

High-order Harmonics in Fragile Molecules

Exploiting an ultrafast IR source, we produced extended harmonic spectra in several molecules with low ionization potentials. These results pave the way to the extension of high harmonic spectroscopy to complex species like biomolecules.