Vargas William E, Avendano Esteban, Hernández-Jiménez Marcela, Azofeifa Daniel E, Libby Eduardo, Solís Ángel, Barboza-Aguilar Cynthia
Centro de Investigación en Ciencia e Ingeniería de Materiales, Escuela de Física, Universidad de Costa Rica, San José 2060-11501, Costa Rica.
Academia Nacional de Ciencias de Costa Rica, San José 1367-2050, Costa Rica.
Biomimetics (Basel). 2018 Oct 11;3(4):30. doi: 10.3390/biomimetics3040030.
A unified description involving structural morphology and composition, dispersion of optical constants, modeled and measured reflection spectra and photonic crystal characterization is devised. Light reflection spectra by the cuticles of scarab beetles ( and ), measured in the wavelength range 300-1000 nm, show spectrally structured broad bands. Scanning electron microscopy analysis shows that the pitches of the twisted structures responsible for the left-handed circularly polarized reflected light change monotonically with depth through the cuticles, making it possible to obtain the explicit depth-dependence for each cuticle arrangement considered. This variation is a key aspect, and it will be introduced in the context of Berreman's formalism, which allows us to evaluate reflection spectra whose main features coincide in those displayed in measurements. Through the dispersion relation obtained from the Helmholtz's equation satisfied by the circular components of the propagating fields, the presence of a photonic band gap is established for each case considered. These band gaps depend on depth through the cuticle, and their spectral positions change with depth. This explains the presence of broad bands in the reflection spectra, and their spectral features correlate with details in the variation of the pitch with depth. The twisted structures consist of chitin nanofibrils whose optical anisotropy is not large enough so as to be approached from modeling the measured reflection spectra. The presence of a high birefringence substance embedded in the chitin matrix is required. In this sense, the presence of uric acid crystallites through the cuticle is strongly suggested by frustrated attenuated total reflection and Raman spectroscopy analysis. The complete optical modeling is performed incorporating the wavelength-dependent optical constants of chitin and uric acid.
设计了一种统一的描述，包括结构形态和组成、光学常数的色散、建模和测量的反射光谱以及光子晶体表征。在300-1000nm波长范围内测量的金龟子角质层的光反射光谱显示出光谱结构的宽带。扫描电子显微镜分析表明，负责左旋圆偏振反射光的扭曲结构的间距随穿过角质层的深度单调变化，这使得能够获得所考虑的每种角质层排列的明确深度依赖性。这种变化是一个关键方面，将在Berreman形式理论的背景下引入，这使我们能够评估反射光谱，其主要特征与测量中显示的特征一致。通过从传播场的圆偏振分量满足的亥姆霍兹方程获得的色散关系，为所考虑的每种情况建立了光子带隙的存在。这些带隙取决于穿过角质层的深度，并且它们的光谱位置随深度而变化。这解释了反射光谱中宽带的存在，并且它们的光谱特征与间距随深度变化的细节相关。扭曲结构由几丁质纳米纤维组成，其光学各向异性不够大，无法通过对测量的反射光谱进行建模来接近。需要在几丁质基质中嵌入高双折射物质。从这个意义上说，受挫衰减全反射和拉曼光谱分析强烈表明角质层中存在尿酸微晶。结合几丁质和尿酸的波长依赖性光学常数进行了完整的光学建模。
