Broadband second harmonic generation in aperiodic nonlinear photonic crystals: 1D projection from 2D Vogel sunflower spiral array

The structure of the sunflower head can be used to realize broadband applications in optics. However, this 2D structure, with a ring-shaped reciprocal space, only has a normalized Fourier coefficient peak around 0.033, which leads to a relatively low conversion efficiency and may restrict its applications. We tried to maintain its broadband features while with larger Fourier coefficients by structure dimension reduction. We obtained an aperiodic 1D structure from a 2D Vogel sunflower spiral array by a cut-and-projection method. Workable reciprocal vector bands were found with this 1D structure in the vicinity of a pre-set central wavelength λ0 = 1.4 µm, and its peak Fourier coefficients can be 5–7 times as large as the original 2D structure. With this, we investigated broadband quasi-phase matching (QPM) second harmonic generation (SHG) in samples with different reversed ratios D. To illustrate in more detail, three samples were closely examined with D = 0.4, 0.5, and 0.6. Bandwidths of these three samples for first-order QPM SHG are 90, 70, and 30 nm, respectively, with a fundamental wave in the vicinity of λ0 = 1.4 µm. The exact SHG solution of coupled-wave equations was used in the evaluation of conversion efficiencies. Calculations showed broadband high conversion efficiency.