Abstract
Noncollinear wave-mixing spectroscopies with attosecond extreme ultraviolet (XUV) pulses provide unprecedented insight into electronic dynamics. In infrared and visible regimes, heterodyne detection techniques utilize a reference field to amplify wave-mixing signals while simultaneously allowing for phase-sensitive measurements. Here, we implement a self-heterodyned detection scheme in noncollinear wave-mixing measurements with a short attosecond XUV pulse train and two few-cycle near infrared (NIR) pulses. The initial spatiotemporally overlapped XUV and NIR pulses generate a coherence of both odd (1snp) and even (1sns and 1snd) parity states within gaseous helium. A variably delayed noncollinear NIR pulse generates angularly-dependent four-wave mixing signals that report on the evolution of this coherence. The diffuse angular structure of the XUV harmonics underlying these emission signals is used as a reference field for heterodyne detection, leading to cycle oscillations in the transient wave-mixing spectra. With this detection scheme, wave-mixing signals emitting from at least eight distinct light-induced, or dressed, states can be observed, in contrast to only one light induced state identified in a similar homodyne wave-mixing measurement. In conjunction with the self-heterodyned detection scheme, the noncollinear geometry permits the conclusive identification and angular separation of distinct wave-mixing pathways, reducing the complexity of transient spectra. These results demonstrate that the application of heterodyne detection schemes can provide signal amplification and phase-sensitivity, while maintaining the versatility and selectivity of noncollinear attosecond XUV wave-mixing spectroscopies. These techniques will be important tools in the study of ultrafast dynamics within complex chemical systems in the XUV regime.
Highlights
Spectroscopy in the extreme ultraviolet (XUV) and x-ray regimes provides fundamental insights into the nature of matter through its element specificity and charge-state sensitivity
A short XUV pulse train generated by high harmonic generation (HHG) and the time-coincident, collinear near infrared (NIR) driving pulse induce a coherent superposition of the ground state and both one-photon, dipole-allowed 1snp and one-photon, dipole-forbidden 1sns and 1snd states, as demonstrated in previously reported four-wave mixing experiments [22,28,30]
Diffuse XUV harmonics serve as a reference field for divergence angle-dependent four-wave mixing emission signals generated in helium gas
Summary
Spectroscopy in the extreme ultraviolet (XUV) and x-ray regimes provides fundamental insights into the nature of matter through its element specificity and charge-state sensitivity. Attosecond pulses in the XUV and x-ray regimes have been produced with high harmonic generation (HHG)-based table-top set-ups [3,4] and free electron lasers [5,6,7]. Nonlinear wave-mixing techniques are regularly employed at infrared and visible wavelengths to disentangle complex spectra in multiple dimensions, yielding time-resolved information about coupling in complicated ensembles [11,12,13,14]. These techniques can be broadly classified by their detection scheme.
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