Sideband and Angular Distribution Oscillations in XUV / IR Pump-Probe Experiments
Focussing ultra short infrared (IR, omega) laser pulses on rare gas atoms can lead to the generation of Higher Harmonics of the fundamental laser field, shifting the spectral range far into the extreme ultraviolet (XUV) region. If the IR field consists of several cycles, the emitted Harmonics will be confined in short attosecond pulses, which are continuously generated every half cycle of the IR pulses, resulting in attosecond pulse trains (ATP). By reasons of symmetry only odd Harmonics can be observed in this process (Fig. 1). A way to characterize the APT is given by a type of experiments, often referred to as Sideband Oscillations.
Fig 1: Typical measured High Harmonic spectrum.
In the single-photoionization process of noble gas atoms by spectrally and temporally filtered XUV-APTs, the measured electrons exhibit kinetic energies, which are separated by two times omega, thus reflecting the corresponding High Harmonic photon energies. By simultaneously irradiating the atoms with a moderately intense IR field, sidebands peaks will appear in the electron energy spectrum (Fig. 2). For each sideband there are two indistinguishable contributions, where one comes from the absorption of an IR-photon together with a primary XUV harmonic transition (q), and the other corresponds to the emission of an IR-photon from the consecutive primary XUV harmonic transition (q + 2). Provided that the individual pulses within the attosecond pulse train are short, i.e. the intra-harmonic dispersion is small enough, the observed sideband intensity as well as the high-harmonic ionization lines show a periodic modulation with a period equal to the half-cycle period of the fundamental laser pulse, when the XUV IR delay is varied (Fig. 3).
Fig 2: Generation of sideband-photoelectrons with a moderately intense IR field.
Sideband Oscillations for the APT characterization
However, as the width and especially the relative temporal position of a sideband maximum is determined by the phase difference between two neighbouring fundamental harmonic peaks, only the oscillation of the High-Harmonic ionization signal in the delay-dependent electron-energy spectrum contains the exact spectral phase information of the XUV pulses, allowing a precise characterization of the attosecond pulse trains. This technique is usually referred to as RABBIT (reconstruction of attosecond beating by interference of two- photon transitions).
Fig 3: Delay-dependent energy distribution of the photoelectrons together with a projection onto the energy axis, in order to see the ratio between sidebands and harmonic photoionization lines.