Bio-Templating for 3D Second-Harmonic Photonic Crystals

Three-dimensional nonlinear (second-harmonic) photonic crystals can simultaneously generate different nonlinear processes such as second-harmonic generation, optical parametric amplification, or other sum- and difference-frequency processes along different photonic crystalline directions. ¹ However, creating these crystals presents a considerable challenge due to the chemical inertness of metal oxides.² Laser-poling and laser-erasing have been utilized in materials like lithium niobate and Ba_0.77Ca_0.23TiO₃.³ We have demonstrated the first bottom-up fabricated 3D second-harmonic woodpile photonic crystal using a combination of sol-gel chemistry and soft-nanoimprint lithography.⁴ Our objective is to study the emission of second-harmonic generated light near a complete photonic band gap and explore inhibited spontaneous emission.⁵<br/>Soft-nanoimprint lithography is limited in its ability to stack sufficient layers to achieve a complete photonic band gap. Therefore, we applied a bio-templating approach. Previous research has utilized bio-templating of biological beetle of butterfly scales with sol-gel derived titanium dioxide, zinc oxide, and silicon dioxide.⁶ Researchers have previously replicated scales from <i>Eupholus schoenherri</i>, <i>Pachyrynchus moniliferus</i>, <i>Eudiagogus pulcher,</i> and <i>Lamprocyphus augustus</i> with diamond-based symmetries among others.⁶<br/>This study shows the first demonstration of bio-templating into a nonlinear optical material. We selected a barium titanate sol-gel with a refractive index of around 2.0 and a tetragonal phase.⁴ We utilized green and red scales with rod-connected diamond symmetry from <i>Eupholus schoenherri</i> (band gap at 500 nm), and <i>Pachyrynchus niitasoi</i> (band gap at 690 nm).⁷ The beetle scales were opened by plasma etching and then infiltrated with barium titanate sol-gel. The samples were subsequently heated to 400 °C to burn away the chitin of beetle scales. The resulting rod-connected diamond structure was heated to 700 °C for ten minutes to form tetragonal barium titanate.<br/>We successfully replicated the photonic network into a nonlinear material and demonstrated, for the first time, a linear photonic band gap from a three-dimensional photonic crystal made out of a nonlinear optical material. By optimizing the filling factor through adjusting the ratio of barium titanate sol-gel to methanol, we found that a ratio of 1:0.5 (BTO:MeOH) achieved the largest reflectivity of 50 % for samples heated up to 400 °C. Due to shrinkage and the different refractive index, the band gap of the replicated network is blue shifted by 120 nm compared to the original biological scale.<br/>1. Slusher, R. E. & Eggleton, B. J. Nonlinear Photonic Crystals. in (eds. Slusher, R. E. & Eggleton, B. J.) 1–12 (Springer Berlin Heidelberg, Berlin, Heidelberg, 2003). doi:10.1007/978-3-662-05144-3_1.<br/>2. Vogler-Neuling, V. V. <i>et al.</i> Photonic Assemblies of Randomly Oriented Nanocrystals for Engineered Nonlinear and Electro-Optic Effects. <i>ACS Photonics</i> vol. 9 2193–2203 Preprint at https://doi.org/10.1021/acsphotonics.2c00081 (2022).<br/>3. Zhang, Y., Sheng, Y., Zhu, S., Xiao, M. & Krolikowski, W. Nonlinear photonic crystals: from 2D to 3D. <i>Optica</i> <b>8</b>, 372–381 (2021).<br/>4. Vogler-Neuling, V. V. <i>et al.</i> Large-Scale Bottom-Up Fabricated 3D Nonlinear Photonic Crystals. <i>Adv Photonics Res</i> (2024) doi:10.1002/adpr.202400058.<br/>5. Yablonovitch, E. Inhibited Spontaneous Emission in Solid-State Physics and Electronics. <i>Phys Rev Lett</i> <b>58</b>, 2059–2062 (1987).<br/>6. Jorgensen, M. R. & Bartl, M. H. Biotemplating routes to three-dimensional photonic crystals. <i>J Mater Chem</i> <b>21</b>, 10583–10591 (2011).<br/>7. Parisotto, A., Steiner, U., Cabras, A. A., Van Dam, M. H. & Wilts, B. D. Pachyrhynchus Weevils Use 3D Photonic Crystals with Varying Degrees of Order to Create Diverse and Brilliant Displays. <i>Small</i> <b>18</b>, (2022).