Optical properties of nanoporous silica frustules of a diatom determined using a 10 µm microfiber probe

A microfiber spectroscopic method was developed to reveal the photonic crystalline characteristics of a cylindrical frustule of the diatom Melosira variance. The spectroscopy apparatus used was composed of two quartz fibers with fine ends, approximately 10 µm in diameter, set on micromanipulators to control the position of the fine ends of each fiber, which were focused at the edge of a frustule. The method had fine space resolution and sensitivity toward the propagation direction of incident light, which is suitable for analyzing photonic crystallinity. A typical absorption was observed at wavelengths of 400–500 nm, which corresponded to a quasi-stop band calculated by a finite-difference time-domain (FDTD) method for a three-dimensional (3D) slab model constructed based on an SEM image of the frustule. These results suggest that the photonic crystalline characteristics of the frustule can assist the absorption of incident light near 420 nm, which is crucial for the efficient photosynthesis of the diatom.