Nonlinear Spectroscopy with X-Ray Two-Photon Absorption in Metallic Copper.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Conventional one-photon absorption (OPA) spectroscopy is a standard technique for probing electronic transitions in materials.
• Two-photon absorption (TPA) offers complementary information but is often challenging to measure, especially in the X-ray regime.
• Free-electron lasers (XFELs) provide high-intensity, tunable X-ray pulses, enabling new spectroscopic investigations.

Purpose of the Study:

• To measure the X-ray two-photon absorption (TPA) spectrum of metallic copper.
• To compare the TPA spectrum with the conventional one-photon absorption (OPA) spectrum.
• To investigate the influence of XFEL pulse properties on the measured spectrum and confirm the intrinsic TPA signal.

Main Methods:

• Utilizing a free-electron laser (XFEL) to generate intense, tunable X-ray pulses.
• Performing X-ray two-photon absorption (TPA) spectroscopy on metallic copper.
• Analyzing the dependence of the absorption spectrum on the XFEL pulse energy.

Main Results:

• The measured X-ray TPA spectrum of copper exhibits distinct features compared to its OPA spectrum.
• A prominent peak is observed below the Fermi level in the TPA spectrum.
• This peak is assigned to electronic transitions originating from the 3d states.
• Pulse-energy dependence studies confirm the measurement of the intrinsic TPA spectrum, unaffected by saturation effects.

Conclusions:

• X-ray TPA spectroscopy using XFELs is a viable technique for probing electronic structures.
• The observed spectral features provide insights into the electronic band structure of metallic copper, particularly concerning 3d state transitions.
• The methodology ensures the accurate measurement of intrinsic TPA spectra, paving the way for future studies on various materials.