Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

Area of Science:

• Nonlinear optics
• Solid-state physics
• Materials science

Background:

• Optically active phonon modes in ferroelectrics (KTP, KTA) are crucial for Raman lasing and terahertz wave generation.
• Previous studies showed picosecond pulse interactions lead to stimulated Raman scattering and polariton-phonon scattering.

Purpose of the Study:

• Investigate infrared optical parametric amplification (OPA) in KTP and KTA using ultrashort pulses.
• Analyze the impact of phonon oscillation periods on OPA dynamics.
• Understand the coupling between nonlinear optical polarizations and parametrically amplified waves.

Main Methods:

• Utilized ~800-nm ultrashort pulses with durations comparable to optical phonon oscillation periods.
• Employed KTP and KTA crystals for infrared OPA experiments.
• Analyzed OPA output spectra, pump intensity dependence, and temporal gain evolution.

Main Results:

• Demonstrated coherent impulsive Raman excitation of phonons.
• Observed significant spectral modulations and overall redshift in parametrically amplified waves (especially the signal).
• Identified pump intensity dependence and spectral modulations indicating coupling between phonon and parametric polarizations.

Conclusions:

• The effective nonlinear susceptibility (χ((2))) cannot be treated as instantaneous under these conditions.
• Coupling between impulsively excited phonons and parametric waves significantly influences OPA output.
• Findings are vital for optimizing terahertz wave generation and Raman lasing applications.