Fourth-harmonic generation in a single lithium niobate-crystal with cascaded second-harmonic generation

Area of Science:

• Nonlinear Optics
• Laser Physics
• Materials Science

Background:

• Harmonic generation is crucial for extending the spectral range of lasers.
• Lithium niobate (LiNbO3) is a well-established nonlinear optical material.
• Free electron lasers (FELs) offer unique tunability for fundamental radiation.

Purpose of the Study:

• To demonstrate tunable second- and fourth-harmonic generation in a single LiNbO3 crystal.
• To investigate harmonic generation using a tunable infrared free electron laser.

Main Methods:

• Utilized a 1-cm-long lithium niobate crystal.
• Employed the Mark III infrared free electron laser (FEL) for tunable fundamental radiation.
• Tuned the fundamental laser wavelength from 2 to 2.5 µm.

Main Results:

• Successfully generated tunable second-harmonic (1-1.25 µm) and fourth-harmonic (0.5-0.625 µm) radiation.
• Achieved a fundamental-to-second-harmonic energy conversion efficiency of 66%.
• Measured a fundamental-to-fourth-harmonic energy conversion efficiency of 3.3 × 10⁻⁶, with a maximum fourth-harmonic energy of 3.3 nJ.

Conclusions:

• A single lithium niobate crystal can efficiently generate tunable second and fourth harmonic radiation.
• The results highlight the potential of FELs coupled with nonlinear crystals for versatile light generation.