Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces

Area of Science:

• Photonics and Nanophotonics
• Nonlinear Optics
• Materials Science

Background:

• Lithium niobate (LN) is a key material for nonlinear optics, particularly for second-harmonic generation (SHG).
• Previous research explored SHG enhancement in micro-ring resonators, hybrid waveguides, and nanostructures with anapole modes or plasmonic resonances.
• Improving SHG efficiency in nanostructured nonlinear materials remains an active area of research.

Purpose of the Study:

• To numerically demonstrate highly efficient frequency doubling in lithium niobate metasurfaces.
• To investigate the role of high-Q resonances, specifically symmetry-protected bound states in the continuum (BICs), in enhancing SHG.
• To explore the dependence of SHG performance on metasurface design parameters, such as asymmetry.

Main Methods:

• Numerical simulations of lithium niobate metasurfaces.
• Analysis of high-Q resonances associated with bound states in the continuum.
• Investigation of the relationship between the metasurface's asymmetric parameter and its radiative Q-factor and field enhancement.
• Calculation of second-harmonic conversion efficiency at varying pump intensities.

Main Results:

• High-Q resonances in LN metasurfaces significantly boost frequency doubling.
• The radiative Q-factor and field enhancement are tunable via the asymmetric parameter (α).
• A peak SH conversion efficiency of 0.49% was achieved at a pump intensity of 3.3 kW/cm² for a metasurface with a Q-factor of ~8x10⁴.

Conclusions:

• Symmetry-protected bound states in the continuum enable highly efficient frequency doubling in lithium niobate metasurfaces.
• These engineered metasurfaces show promise for developing practical blue-ultraviolet light sources.
• The findings provide a pathway for metadevices based on nanoengineered conventional nonlinear crystals.