The relationship of selective surrounding response and the nanophotonic structures of Morpho butterfly scales

doi:10.1016/j.mee.2011.12.017

Microelectronic Engineering

Volume 95, July 2012, Pages 42-48

https://doi.org/10.1016/j.mee.2011.12.017 Get rights and content

Abstract

This paper investigates the sensitivity of micro/nano structures in Morpho butterfly scales to surrounding media. The theory of multilayer-thin-film interference is introduced to explain the mechanism of sensitivity. Two-dimensional optical models with three different geometrical designs and similar size were constructed to authenticate the theory. The rigorous coupled-wave analysis (RCWA) technique was employed for theoretical calculation of vector diffraction. By comparing the modeling results from RCWA with the experimental results, we identified the key to sensitivity, multilayer-thin-film interference occurred over a structure composed of alternating films with high and low refractive indexes. Further, we identified the main characteristics of the scales that were relevant to sensitivity. These characteristics should be applied to designing and fabricating artificial nanostructures for highly sensitive and selective bio-inspired chemical sensors.

Graphical abstract

Introduction

Many insects (particularly butterflies), birds, fish, and marine animals have structural colors that make their surface colors change with the viewing angle (iridescence) or appear as “metallic” [1], [2], [3], [4], [5], [6]. The visibility of up to half a mile of some Morpho butterflies has long been a key topic. The nanostructures on its wing hold the secret of the famous structural color, and are sensitive to surrounding media [7]. The striking blue is attributed to the nanostructure formed by discrete multi-layers of cuticle and air [8], [9], [10], [11], [12], [13], and is seriously influenced by the environment surrounding the nanostructures. The color and brightness change as the surrounding media alter [14]. This can be implemented into the engineering designs of artificial butterfly scales for highly sensitive and selective detection of closely related chemicals. It was reported by Potyrailo et al. that the iridescent scales of the Morpho sulkowskyi butterfly reacted to different individual chemical vapors and responded differently [7]. They claimed that there is a notable difference between the reflectance spectra from the scales in different vapors, and the optical response dramatically outperformed that of presently known nano-engineered photonic sensors. Traditional photonic chemical vapor sensors need to add specific chemical compounds into their nanostructures, which is typically consist of colloidal photonic crystal films, porous silicon or other nanostructures. In order to selectively sense more than one vapor, arrays of such sensors are needed. However, the Morpho butterfly scale provides high selectivity for several vapors purely based its single photonic nanostructure, and can discriminate closely related chemical vapors through changes in the reflectance spectra [15]. Also, these nano-engineered photonic chemical sensors have many advantages over the traditional electrical signal chemical sensors, such as high sensitivity and selectivity, very fast response speed and safe in inflammable, explosive, and dangerous chemicals inspection.

In this work, the optical sensitivity of two kinds of Morpho scales to different media was investigated. The theory of multilayer-thin-film interference is introduced to explain the sensitivity to surrounding media. The complex nanostructure was modeled, and the multilayer RCWA was utilized to analyze the models in different background media. The simulation is reciprocally authenticated with the results from the multilayer-thin-film interference theory and the experiments. What is more, by comparing the modeling results, the key characteristics of the scales relevant to sensitivity were identified. These characteristics should be applied to designing and fabricating artificial nanostructures for highly sensitive and selective bio-inspired chemical sensors.

Section snippets

Experiments

The Morpho butterfly families are generally conspicuous due to their brilliant iridescent color. Morpho didius and Morpho rhetenor are the most famous butterflies. The material of the scales is cuticle, which is a composite of rods of chitin set in a matrix of proteins [16]. The cuticle is transparent with little pigment, and the color is produced by their structures [17]. M. didius has two types of scales: ground scales and glass scales. The glass scales lie above the ground scales and exhibit

Models and simulations

To figure out what was behind the differences in the sensitivity of the nanostructures, three 2D optical models simplified from Morpho scales were designed, as shown in Fig. 4. The models have x-invariant structures, in which there are infinite periods along the x-axis [19], [20], [21]. Here only two periods are set, as an example. The lamellae of model A are rectangular and parallel to the base. The rectangular lamellae of model B, arranged interlaced, are also parallel to the base. In model

Analysis and discussion

As mentioned above, Morpho scales and the optical models A, B, and C are sensitive to their surrounding mediums. We used the theory of multilayer-thin-film interference to acquire the explanation. Consider a plane wave of light that is incident on a thin layer of thickness d. The reflected light beams from the two interfaces may interfere with each other. Constructive interference takes place if the optical path difference is an integral multiple of half-wavelength. Otherwise, destructive

Conclusions

The nanostructures of Morpho butterfly wing scales demonstrate highly selective chemical vapor response, which has distinct advantages over traditional chemical sensors. The sensitivity of Morpho nanostructure to surrounding media was investigated in this paper. It is found from the reflectance spectra that the peak of diffraction efficiency and the wavelength of the peak efficiency are in relation to the refractive index of the surrounding media and the structural color of the

Acknowledgments

This project was funded by National Key Basic Research Special Fund of China (Grant No. 2009CB724204) and National Natural Science Foundation of China (Grant No. 51175210 and 90923019).

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Citation Excerpt :
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Excellent Color Sensitivity of Butterfly Wing Scales to Liquid Mediums
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The relationship of selective surrounding response and the nanophotonic structures of Morpho butterfly scales

Abstract

Graphical abstract

Introduction

Section snippets

Experiments

Models and simulations

Analysis and discussion

Conclusions

Acknowledgments

Nature

Phil. Mag.

J. Opt. A

Phil. Mag.

Nat. Photonics

Proc. R. Soc. Lond. A

J. Phys. Chem.

Rev. Sci. Instrum.

Nature

Nature