FIG 2 (IMAGE)

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FIG 2

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Fig. 2: Fundamentals of SHG. (a) Schematic energy-level diagram illustrating SHG. Efficient nonlinear harmonic generation requires both phase matching and mode matching. (b) Illustration of phase versus mode matching based on translational symmetry. In systems with preserved translational symmetry (e.g., bulk media, waveguides, and microrings), momentum is a well-defined quantum number, and phase matching is essential. When translational symmetry is broken (e.g., unitcell of metasurfaces, photonic crystals, defect cavities, and nanoparticles), phase matching is relaxed, and mode matching governs this nonlinear process.

 


 

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