Living photonic crystals

G. I. Márk1, K. Kertész1, Zs. Bálint2, Z. Vértesy1, Z. E. Horváth1, J. Balázs1, D. Méhn3, I. Kiricsi3, V. Lousse4, J. -P. Vigneron4, L. P. Biró1

1 Nanotechnology Departement , Research Institute for Technical Physics and Material Science , 1121 Budapest, Konkoly Thege Miklós út 29-33, Hungary
2 Hungarian Natural History Museum, 1088 Budapest, Baross utca 13, Hungary
3 Department of Applied And Environmental Chemistry , University of Szeged, 6720 Szeged, Rerrich Béla tér 1, Hungary
4 Laboratoire de Physique du Solide , Facultés Universitaires Notre-Dame de la Paix , 61 rue de Bruxelles, B-5000 Namur, Belgium

Click on the image to see an Indeo compressed AVI animation (1.3 M)


Photonic crystals are periodically structured dielectric media, generally possessing ranges of frequency in which light cannot propagate through the structure (photonic bandgap). This periodicity, whose lengthscale is proportional to the wavelength of light in the band gap, is the electromagnetic analogue of a crystalline atomic lattice.

Butterfly scale structure
Butterfy wing structure in different magnifications

Efforts for producing artificial photonic crystals are motivated by possible applications in optical filtering and optical computing, or in the design of very compact lasers, and by phenomena such as the spectral manipulation of light by shock waves. However, photonic-crystal-like structures have been developed in several species of butterflies and beetles during their evolution.


Lycaenid butterfly populations in mountainous regions at altitudes of 2000-2500 m show an interesting phenomenon described by Balint and Johnson in 1997, called discoloration. Males of these populations do not show bright structural coloration on their wings, (like low altitude ones) but they have the warm brown color typical of females.

Geographical origin of blue and brown butterflies
Geographical origin of blue and brown butterflies

Electron microscopic investigation of two male butterflies (Polyommatus daphnis - low altitude, and Polyommatus marcidus - over 2500 m Elbrus Mountains) revealed that the blue color can be attributed unambiguously to the fine, spongelike medium, called by the entomologists pepper-pot structure present between the ridges and the cross ribs in the scales of the colored butterfly. Only traces of this structure can be found on the scales of the discolored butterfly.

Blue butterfly
Brown butterfly
A blue butterfy
A brown butterfly
Blue butterfly scale structure
Brown butterfly scale structure
Blue butterfy scale structure
Brown butterfly scale structure
Blue wing SEM image
Brown wing SEM image
SEM image of blue wing. Inset: FFT power spectrum
SEM image of brown wing. Inset: FFT power spectrum

Thermal measurements

The explanation given to the phenomenon of discoloration is based on the thermal-regulation mechanism of butterflies. Males emerge several days earlier before the emergence of females for setting up perching area. Prior to their early morning activities, they have to spend considerable time to heat their bodies using the energy of solar radiation. The discolored males are able to use in a more efficient way the energy of the sunlight, therefore they have better survival rates. To verify this hypothesis, we measured the temperature difference between the identical illuminated blue and brown wings. The ratio of butterfly wing temperatures compared to white paper was calculated according to the equation

Temperature ratios

where TBR(Pi), TBL(Pi) and TWH(Pi) are the temperatures of brown, blue wings and white paper, and Pi is the incident light power.

Temperature measurements
R(Pi), and temperature difference between the brown wing and white paper. As inset, the experiment setup.

Optical measurements

The transmission spectra shows that practically all the UV-VIS light incident on the upper wing is either  absorbed by the wing or reflected by the scales both for the blue and the brown butterfly. Comparative specular reflectance measurements between the wings were carried out at 27,5o. One may remark a significant difference around 490 nm with an extended shoulder towards the red region. The differences in the reflectance then indicate that, due to the smaller reflectivity of the brown wing, this sample is absorbing a larger amount of energy.

Transmission spectra of the blue (continuous line) and the brown (broken line) butterfly wings.
Specular reflectance difference of the blue and brown male butterfly wings, measured at 27.5°. The spectral regions in which the colored glass filters have a transmission higher than 50% are indicated.

In support of the interpretation of the reflectance data, a computer model was build and reflection coefficients computed. The theoretical structure consists of a stack of four ordered layers with 0,2 mm thickness. Each of the chitin walls are 0,066 mm thick, with refracting index of 1,5. This structure provides the theoretical result shown as a solid line. The dotted line refers to a variant model. In this case the parallelepipedic cavities have been replaced by hollow ellipsoids. The convergence of the results provided by these models, confirms that the coloration spectrum most likely originates from the pepper-pot structure.

Calculated reflectance
Model structure built on the basis of high-resolution SEM images used in the computer simulation of the photonic crystal. Reflectance values (solid line) of the pepper-pot structure as calculated from the computer model. The dotted line refers to a second, independent model, where parallelepipedic cavities are replaced by ellipsoids.


The pepper-pot structure acts as a natural photonic band gap (PBG) causing the increased reflectance in the spectral range from blue to near UV. Beyond their applicability in optical devices, PBG materials may find useful applications in thermal management too. The pepper-pot structure is far from being as perfect as artificially produced PBG materials, it is likely that the manufacturing conditions are less strict for these materials to be used in thermal applications.


The work in Hungary was supported by OTKA grant T 042972.


Biró, L., P.; Bálint, Zs.; Kertész, K.; Vértesy, Z.; Márk, G., I.; Horváth, Z., E.; Balázs, J.; Méhn, D.; Kiricsi, I.; Lousse, V.; Vigneron, J.-P.: Role of photonic-crystal-type structures in the thermal regulation of a lycaenid butterfly sister species pair; Phys. Rev. E 67, 021907-1(2003).

Last updated: June 28, 2004 by Géza I. Márk ,
This page was accessed  times since June 28, 2004.