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
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 Eupholus schoenherri, Pachyrynchus moniliferus, Eudiagogus pulcher, and Lamprocyphus augustus with diamond-based symmetries among others.⁶
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 Eupholus schoenherri (band gap at 500 nm), and Pachyrynchus niitasoi (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.
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.
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.
2. Vogler-Neuling, V. V. et al. Photonic Assemblies of Randomly Oriented Nanocrystals for Engineered Nonlinear and Electro-Optic Effects. ACS Photonics vol. 9 2193–2203 Preprint at https://doi.org/10.1021/acsphotonics.2c00081 (2022).
3. Zhang, Y., Sheng, Y., Zhu, S., Xiao, M. & Krolikowski, W. Nonlinear photonic crystals: from 2D to 3D. Optica 8, 372–381 (2021).
4. Vogler-Neuling, V. V. et al. Large-Scale Bottom-Up Fabricated 3D Nonlinear Photonic Crystals. Adv Photonics Res (2024) doi:10.1002/adpr.202400058.
5. Yablonovitch, E. Inhibited Spontaneous Emission in Solid-State Physics and Electronics. Phys Rev Lett 58, 2059–2062 (1987).
6. Jorgensen, M. R. & Bartl, M. H. Biotemplating routes to three-dimensional photonic crystals. J Mater Chem 21, 10583–10591 (2011).
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. Small 18, (2022).