First Evidence of Purely Extreme-Ultraviolet Four-Wave Mixing.

Area of Science:

• Nonlinear optics
• Quantum optics
• Ultrafast spectroscopy

Background:

• Extending nonlinear optical techniques to the extreme-ultraviolet (EUV) spectrum is crucial for combining chemical specificity with background-free, ultrafast detection.
• Current limitations hinder the application of advanced nonlinear optical methods in the EUV, soft, and hard x-ray regimes.

Purpose of the Study:

• To report the first observation of a four-wave-mixing (FWM) response in solid-state samples exclusively stimulated by EUV pulses.
• To establish a proof-of-principle for nonlinear optical spectroscopy in the EUV regime.

Main Methods:

• Generation of an all-EUV four-wave-mixing (FWM) signal using high-order harmonics from the FERMI free-electron laser (FEL).
• Diffraction of EUV high-order harmonics from a standing wave created by interfering fundamental wavelength FEL pulses.
• Measurement of the FWM signal intensity to extract nonlinear optical properties.

Main Results:

• First observation of an all-EUV FWM signal from solid-state samples.
• Estimation of the effective third-order nonlinear susceptibility (χ(3)) in the EUV regime as approximately 6×10⁻²⁴ m²/V².
• Demonstration of a novel nonlinear optical technique in the EUV photon-energy range.

Conclusions:

• This experiment pioneers nonlinear optics in the EUV, enabling new spectroscopic techniques.
• The observed FWM response provides a pathway for advanced, ultrafast, and chemically sensitive measurements.
• Future developments include frequency and phase-resolved FWM methods in the unprecedented EUV regime.