  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.
