Second-harmonic flat-top beam shaping via a three-dimensional nonlinear photonic crystal

Area of Science:

• Nonlinear optics
• Photonics
• Laser physics

Background:

• Gaussian beams are fundamental in optics but have limitations for applications requiring uniform intensity.
• Flat-top beams offer advantages like uniform illumination but are challenging to generate, especially in two dimensions.
• Nonlinear photonic crystals provide a versatile platform for manipulating light properties through engineered material responses.

Purpose of the Study:

• To experimentally demonstrate two-dimensional (2D) flat-top beam shaping.
• To achieve simultaneous nonlinear frequency conversion with 2D flat-top beam generation.
• To extend previous one-dimensional (1D) nonlinear beam shaping techniques to a full 2D wavefront.

Main Methods:

• Utilizing a three-dimensional (3D) nonlinear photonic crystal fabricated in lithium niobate.
• Employing a near-infrared femtosecond laser to induce a spatially controlled modulation of the second-order nonlinear optical coefficient.
• Independently adjusting the flat-topped wavefront truncation in mutually perpendicular coordinates.

Main Results:

• Successful generation of 2D flat-topped beams with an optimal flatness of 97.1%.
• Achieved a nonlinear conversion efficiency of 10^-2 at a peak power of 37 kW and an interaction length of 630 µm.
• Demonstrated simultaneous full-wavefront flat-top distribution and nonlinear frequency conversion in two dimensions.

Conclusions:

• The study successfully extends nonlinear beam shaping to 2D, overcoming previous limitations.
• The developed method enables precise control over wavefront flatness and nonlinear optical processes.
• This work opens new possibilities for advanced optical systems requiring tailored beam profiles and frequency conversion.