Structural Integration Design For Efficient Photoluminescence In Butterfly Scales And Its Biomimetic Materials

With the advancement of the optical theory and technology, advanced optical materials and devices have been developed. The photon age is coming after the electron age. The design and fabrication of optical materials is the fundamental of developing optical devices and systems. Optical materials with structural features in light wavelength scale can efficiently manipulate optical signals. The typical optical structure is photonic crystals, which is of great potential in intricate and precise optical devices. The optimization of structural parameters of optical structures is an important step in the development of optical materials. In the meanwhile, there are diverse optical structures in biology, which are endowed with optimal parameters for a certain optical effect by a long term of natural selection. There are a multitude of delicate optical microstructures in butterfly scales like the multilayer, two dimensional photonic crystals and three dimensional crystals. Investigation of optical structures and the underlying optical mechanisms in butterfly scales for the transfer of the natural structure to artificial optical materials is an efficient way of design and fabricate advanced optical materials. In this paper, green scales of butterfly Papilio nirus and blue scales of Papilio nirus are selected for research to reveal a complex optical microstructure and its underlying optical mechanism.Structural characterization of scales of butterfly Papilio nirus and butterfly Papilio nirus via optical microscope and field emission scanning electron microscope revealed a complex optical microstructure comprised of a network layer, a two dimensional photonic crystal layer and a layer of multilayer structure. Reflection spectra of butterfly wing scales via the micro-spectrophotometer and the UV-Vis spectrophotometer with an integrating sphere got the reflection spectra from the perpendicular direction and in the full angle respectively. According to the results of the structural characterization, the structural model for computation was extracted and applied in simulation with finite differential time domain method (FDTD) and plane wave expansion (PWE) method for the reflection spectra of the network layer and the multilayer structure and the bandgap digram of the two dimensional photonic crystal layer respectively. Combining the results of the experimental optical test and the computational optical simulation, we have analyzed and discussed the optical mechanism underlying the complex optical structure in the butterfly scales and proved that the scales enhance the light harvesting for the excitation of the pigment and the photonic crystal layer filtrates and guides downwards the needed lights form the emission of the pigment which in turn was efficiently reflected upwards again by the bottom multilayer and transport out of the scale. The photoluminescence of the scale pigment is fully enhanced from the excitation to the emission.Furthermore, to explore the biomimetic optical materials with the structural integration design in the butterfly wing scales, photoluminescent materials have been fabricated through biotemplation method using butterfly scales as templates. Green scales of butterfly Papilio nirus and butterfly Papilio epiphobas being bio-templates, photoluminecent materials Y₂O₃:Eu³⁺, TiO₂ and ZnO were fabricated with proper precursors through precursor infiltration and calcination process. Precursor solution adheres to the scale templates homogenously and the calcination process removes the biotemplates and enables the crystalline growth of the optical materials. Structural characterization of the biomimetic Y₂O₃:Eu³⁺ revealed faithful microstructure compared to the original butterfly scales. The biomimetic Y₂O₃:Eu³⁺ was identified as pure Y2O3 phase through XRD testing. Emission spectrum proved that Eu3+ has been effectively doped into the Y2O3 matrix. Biomimetic anatase TiO₂ templated from green scales of butterfly Papilio nirus has been fabricated through precursor infiltration and calcination. SEM characterization of the biomimetic TiO₂ revealed an apparent two dimensional photonic crystal structure. Biomimetic ZnO was also fabricated successfully through biotemplation after several kinds of precursor were tried and the crystalline phase was identified as zencite. According to the photoluminescence testing, emission peak in the range of visible wavelength was enhanced compared to the featureless ZnO thin film.In this research, we probe into the natural delicate optical microstructures and investigate the green scales of butterfly Papilio nirus and blue scales of Papilio nirus. The optical mechanism was explained that the scales employ a complex optical microstructure comprised of anti-reflection network, two dimensional photonic waveguide and multilayered Bragg reflector to enhance the photoluminescence of the pigment. Inspired by the smart and intricate integration design of the butterfly scales, we have fabricated artificial photoluminescent materials through biotemplation method, which is of substantial referential significance for the development advanced optical materials.