Highly Reproducible, Bio-Based Slab Photonic Crystals Grown by Diatoms

Abstract

Slab photonic crystals (PhCs) are photonic structures used in many modern optical technologies. Fabrication of these components is costly and usually involves eco-unfriendly methods, requiring modern nanofabrication techniques and cleanroom facilities. This work describes that diatom microalgae evolved elaborate and highly reproducible slab PhCs in the girdle, a part of their silicon dioxide exoskeletons. Under natural conditions in water, the girdle of the centric diatom Coscinodiscus granii shows a well-defined optical pseudogap for modes in the near-infrared (NIR). This pseudogap shows dispersion toward the visible spectral range when light is incident at larger angles, eventually facilitating in-plane propagation for modes in the green spectral range. The optical features can be modulated with refractive index contrast. The unit cell period, a critical factor controlling the pseudogap, is highly preserved within individuals of a long-term cultivated inbred line and between at least four different C. granii cell culture strains tested in this study. Other diatoms present similar unit cell morphologies with various periods. Diatoms thereby offer a wide range of PhC structures, reproducible and equipped with well-defined properties, possibly covering the entire UV-vis-NIR spectral range. Diatoms therefore offer an alternative as cost-effective and environmentally friendly produced photonic materials.

Keywords: bioinspiration; diatoms; frustules; nanofabrication; photonic crystal slabs.

Figure 1

Frustule and girdle internal structure in the diatom Coscinodiscus granii. The frustule of this species contains four SiO₂ parts, i.e., two valves and two girdles. A) The girdles encircle the valves at the overlapping regions keeping the frustule together. Intact frustule from side and from top view, with a girdle band visibly separating on the left side. B) Overview demonstrating the hollow cylinder character of the SiO₂ girdle and the split ring spacing (on the left side). C) Surface micropores of the girdle in square lattice arrangement. Inset: Fast Fourier transform analysis of the lattice from the micrograph, demonstrating high periodicity of the pore arrangement. D) Cross‐section showing internal structure of the girdle along the Z‐axis. E) CAD reconstruction of the girdle crystal structure over four unit cells. Letters indicate the lattice parameter used in the optical model. F) Unit cell characterization of the C. granii girdle. Measurements were performed on SEM micrographs, representing measurements of five individual girdles of the diatom C. granii (strain K‐1834). Periods (a ₁ and a ₂), as well as pore diameter (d), were determined on ten individual SEM images of the same diatom strain. Parameters used in the optical models are indicated.

Figure 2

Micro‐ and nanoporosity defining the void filling volume in the C. granii girdle. A) SEM micrograph of a girdle sliced along the Z‐axis. The micrograph indicates nanoporous characteristics of the girdle. B) CAD illustration of the unit cell including nanoporosity (δ_i = 0.05). C) CAD illustration demonstrating the void presented by micropores.

Figure 3

Photonic properties of the C. granii girdle. A) Sketch of the experimental setup, describing the direction of the focused white light (diameter ≈2 µm) on the girdle in the Z‐direction. B) Experimental and FDTD simulations for the reflectance at normal incidence. C) Sketch illustrating how light interferes with the micropores over periods a ₁ and a ₂ in the Z‐direction. D) Pseudogap and in‐plane diffraction of guided modes, shown by reflectance as a function of angle of light incidence and wavelength, observed with an oil‐immersed objective (100×) with large numerical aperture (NA = 1.45). The dashed white line represents the numerical approximation, using the lattice parameters from Figure 1F. E) RGB photograph showing waveguiding of green light as suggested by Fuhrmann et al. in ref. [16]. F) FDTD simulation and Fourier‐micro‐spectroscopy measurements of the girdle in air.

Figure 4

Comparison of PhC properties in different C. granii cell culture strains. A) Comparison of dispersion of the pseudogap in two strains of C. granii (K‐1831 and K‐1834) immersed in water. Significant differences, as determined with T‐Test, are indicated with asterisk (*) at p ≤ 0.05 level (N = 5). B) Comparison of the girdle lattice period (a ₁,₂) in four strains of C. granii. C) Comparison of the surface micropore diameter in four strains of C. granii. Different capital letters in panels (B) and (C) indicate significant difference at p ≤ 0.05 level, tested with one factorial ANOVA and Holm Sidak posthoc tests. D) Effective refractive index approximation while varying the period of the unit cell (period a ₁), or the surface micropore diameter (pore d), respectively.

Figure 5

Internal girdle morphologies of different centric diatom species. All centric species tested in this study showed similar internal girdle crystallography, but varied in the period (a ₁) of the unit cell. A) C. wailesii, 332 ± 25 nm. B) C. radiatus, 235 ± 11 nm. C) T. pseudonana, 281 ± 8 nm.