Four-wave-mixing experiments with seeded free electron lasers.

Area of Science:

• Ultrafast spectroscopy
• Free electron laser (FEL) applications
• Materials science

Background:

• Free electron lasers (FELs) enable advanced optical techniques with X-ray radiation.
• Four-wave-mixing (FWM) processes can be stimulated by coherent extreme ultraviolet (XUV) radiation.

Purpose of the Study:

• To demonstrate and characterize the X-ray transient grating (X-TG) technique on silicon nitride (Si3N4).
• To investigate the ultrafast electron dynamics and relaxation mechanisms in Si3N4 using X-TG.

Main Methods:

• Utilized a seeded FEL to generate coherent XUV radiation.
• Performed X-TG measurements on a silicon nitride sample.
• Analyzed signal decay on sub-picosecond and slower timescales (up to 100 ps).

Main Results:

• Observed two distinct decay mechanisms: sub-picosecond and slower dynamics (> ns), indicative of thermal diffusion.
• Estimated a significantly larger effective third-order susceptibility in Si3N4 compared to SiO2.
• Determined a linear dependence of the time-coincidence peak on input beam intensity, confirming weak field regime.
• Found a dependence of ultrafast relaxation timescale on XUV intensity, suggesting free-electron/hole population grating dynamics.

Conclusions:

• X-TG is a powerful technique for probing FEL-induced electron dynamics with high sensitivity.
• The observed dynamics in Si3N4 involve ultrafast electron-hole relaxation followed by lattice excitation.
• The method provides insights into material responses to intense XUV radiation, surpassing conventional techniques.