Soft X-Ray Second Harmonic Generation as an Interfacial Probe

Area of Science:

• Nonlinear optics
• Soft x-ray physics
• Materials science

Background:

• Nonlinear optical processes at soft x-ray wavelengths are underexplored due to limitations in light source intensity and coherence.
• Developing advanced light sources is crucial for advancing nonlinear optics in the soft x-ray regime.

Purpose of the Study:

• To observe and analyze soft x-ray second harmonic generation (SHG) in graphite thin films.
• To investigate the potential of soft x-ray SHG as a surface and interface probing technique.

Main Methods:

• Utilizing high-intensity, coherent soft x-ray pulses from the FERMI free electron laser.
• Experimentally generating soft x-ray SHG near the carbon K edge (∼284 eV) in graphite.
• Performing first-principles theoretical analysis to understand the observed phenomena.

Main Results:

• Successfully observed soft x-ray second harmonic generation in graphite thin films.
• Demonstrated resonant enhancement of SHG above the carbon K edge.
• Showed that the technique is interfacially sensitive, with SHG intensity originating from the top atomic layer in a centrosymmetric sample.

Conclusions:

• Soft x-ray SHG is a viable nonlinear optical process with potential applications in materials science.
• The technique offers elemental specificity and sensitivity to interfaces, including buried ones.
• This work establishes a new tool for probing interfaces with high precision.