Probing Electron Dynamics With Ultrafast Extreme Ultraviolet Transient Absorption and Four-Wave Mixing Spectroscopies

In recent years, femtosecond and attosecond laser pulses have enabled scientists to probe ultrafast electron dynamics through various pump-probe techniques. One technique which has emerged in the past decade is extreme ultraviolet transient absorption spectroscopy (XTAS). In XTAS, extreme ultraviolet (XUV) pulses are produced through high-harmonic generation (HHG) of a femtosecond IR pulse. A portion of the same IR pulse is used to probe the sample with a variable time delay, modifying the absorption of the XUV spectrum through the sample. So far, XTAS experiments have been driven entirely by fixed-frequency IR pulses. Because of this, the odd harmonics which appear in the XUV spectrum cannot be shifted in energy, making the areas in between the harmonics inaccessible to experiments. Also, the fixed IR frequency means that one does not have control over IR-driven couplings between excited states. These couplings, which have profound effects on the XUV absorption spectrum, have never been studied as a function of the IR frequency in XTAS. In this work, I introduce new innovations to the XTAS technique. First, I send the IR probe pulse through an optical parametric amplifier (OPA) which allows me to tune the IR pulse energy without changing the XUV spectrum. This pulse is used to resonantly couple two excited states of Helium, leading to Autler-Townes splitting. By changing the detuning of the probe pulse, I was able to observe a smooth transition from resonant Autler-Townes splitting to transient light-induced states. I also observed spatially isolated four-wave mixing emissions which are not visible using the traditional XTAS pulse sequence. Second, I use high-order frequency mixing of two IR pulses to generate XUV pulses with tunable spectra. These tunable XUV pulses were applied to XTAS experiments, allowing me to probe XUV transitions in between the odd harmonics of a single IR pulse. Lastly, I used the tunable IR pulse to probe couplings between autoionizing states in Argon. These couplings are found to generate transparency in the XUV spectrum which can be shifted by changing the IR frequency. They also produce avoided crossings in the frequency-dependent spectrum which are signatures of polaritons