Attosecond transient absorption spectroscopy in atomic species

Abstract

When high intensity laser light is focused into a gaseous target, high energy photons can be produced through the strongly nonlinear effect of high order harmonic generation. For a 30 fs, 800 nm or 1400 nm wavelength laser pulse, the result is a train of attosecond pulses produced at odd harmonic frequencies of the driving laser field, spanning energy ranges into the 100s eV. These attosecond pulses can access timescales characteristic to the movement of electrons in atoms, and by exploiting their properties as an ultrafast probe, electron dynamics in evolving atomic systems can be observed. This thesis presents the development of a pump-probe beamline capable of performing transient absorption spectroscopy experiments with resolution better than 150 as. Accompanying a full description of the experimental setup and methods, investigations are made into the attosecond transient absorption from strong field dressed helium and krypton atoms around their first ionisation edge, and 3d ionisation edge respectively. The result of the delay dependent transient absorption measurement is modulations to the recorded absorption amplitude for the harmonic orders around the respective ionisation thresholds. We investigated intensity regimes with an 800 nm laser field approaching the strong field ionisation threshold in helium. Experimental results are presented considering first the response of strong field dressed helium using an 800 nm laser field, and second the response of strong field dressed helium and krypton using a 1400 nm laser field. The use of the longer 1400 nm wavelength allows access to higher energy probe harmonics, enabling laser dressed krypton core to continuum, and core to Rydberg state transitions to be studied. By comparing the effect to the delay dependent absorption modulation as additional parameters are varied, information can be gained about the behaviour of the electrons. The parameters studied as a function of delay include: the dressing field intensity, target backing pressure and dressing field relative linear polarisation, aligned both parallel and perpendicular with respect to the probing harmonic pulse train. Key results include a strong suppression of the modulation amplitude for the above ionisation threshold harmonic orders when the dressing field linear polarisation is changed from parallel to perpendicular in the helium target. This is reproduced at both 800 nm and 1400 nm driving wavelengths.