Detecting electronic coherences by time-domain high-harmonic spectroscopy.

Area of Science:

• Atomic, Molecular, and Optical Physics
• Quantum Optics
• Spectroscopy

Background:

• Ultrafast spectroscopy typically requires X-ray laser sources to monitor electronic states separated by a few eV.
• Existing methods face limitations in accessibility and cost associated with X-ray sources.

Purpose of the Study:

• To propose and theoretically model an alternative method for ultrafast spectroscopy using only optical lasers.
• To enable the monitoring of electronic and vibrational states with high temporal resolution.

Main Methods:

• Time-domain high-order harmonic spectroscopy (HHS) using strong optical laser pulses to prepare coherent superposition of electronic states.
• Probing coherent dynamics using higher-order harmonics generated by a delayed optical probe pulse.
• Development of a semiperturbative model based on the Liouville space superoperator approach to analyze nonlinear response orders in multi-pulse HHS.

Main Results:

• Demonstration that high-order harmonic generation (HHG) can be utilized to create and probe coherent electronic dynamics.
• Monitoring of coherence between bound electronic states observed in harmonic spectra from both first- and second-order nonlinear responses.
• Validation of the proposed method's capability to provide insights into electronic state dynamics using only optical lasers.

Conclusions:

• The proposed time-domain HHS method offers a viable, X-ray-free alternative for ultrafast spectroscopy.
• This approach allows for the study of electronic coherences with broad bandwidth using accessible optical laser technology.
• The developed theoretical model provides a framework for understanding and optimizing multi-pulse HHG for spectroscopic applications.