This post originally appeared on CLEO BLOG by Frank Kuo and is reproduced with permission from its author.
It is an understatement when we describe nature as the most talented painter. In fact, she is not only the greatest artist, but also the most renowned scientist, in essentially all aspects. Her scientific achievements are found everywhere. For example, today, much of our knowledge in the field of bio-photonics is just a re-discovery of what she has done (Another interesting topic which relates the evolution to optical science can be found here).
Many of the astonishing color patterns we found in the insect kingdoms are manifestations of nanometre-scale architectures. These architectures are collaborative works of cells. Those cells cooperate together to create optical effects we widely apply in modern photonic science. For instance, butterflies have cells structures that look like multi-layer reflective coatings on their wings. Depending on the thickness of each layer, different colors present vividly. Same tricks have been adopted and perfected by many shiny beetles. As shown in figure 1, enchanting colors on the surface of the insects are precisely the magic of multilayer structures. The layers are mostly composed of thin parallel sheets of chitin (secreted by the epidermis and often interspersed with other organic components). These layers differ in refractive index. And again, depending on the spacing between these layers and their indices of refractions, different colors can be reflected. Furthermore, some insects have arrays of very fine elements, known as nipple arrays, which look like micro lenses with subtle variation of index of refraction, to reduce reflectivity in their compound eyes and enhance collecting the light from the environment. Nature did create optical science way before mankind stole fire from Prometheus!
Figure 1. (a) A presentation of simple cuticular multilayer reflector. (b) The cross section of a cuticular reflector. (c) A colorful buprestid. (d)-(f) Different structures of cuticular multilayer reflectors commonly seen in insects. Courtesy of A. E. Seago, P. Brady, J-P. Vignerson, and T. D. Schultz in J. R. Soc. Interface 6(supp2) S165–S184 (2008).
Recently, researchers have discovered another beautiful example. A marble berry (Pollia condensata) originates from Africa produces colors like jewels (Figure 2). Looking at the photos of it, I actually feel it looks more like a star sapphire. Maybe we could nickname it “star-sapphire-berry”? Or “jewel berry”? Each berry is about 5 mm in diameter, and according to the authors, there are several wonders of these berries: (1) its charming metallic color is rare in nature. (2) If you look carefully, it has a brilliant pixelated appearance with green, purple, and red speckles (The authors call it “pointillism”, apparently they also think nature is an artist). (3) The color, if well preserved, can last more than 40 years, and counting. (4) It reflects about 30% of the sunlight, which makes it the shiniest organism we have found in nature so far. Colorful beetles can only take the 2nd place. So how do all these happen?
Figure 2. (a) The charming marble berry (Pollia condensata). (b) A single bunch of the fruit. Courtesy of S. Vignolini, P. J. Rudall, A. V. Rowland, A. Reed, E. Moyroud, R. B. Faden, J. J. Baumberg, B. J. Glover, and U. Steinera in PNAS 109 39 15712 (2012).
The secret of this appealing demonstration lies on the structures of the cell walls of the cells that populate the surface of the fruit. Each cell has the cell wall composed of multi-layer cellulose micro-fibrils. The layers of micro-fibrils form an assembly called “helicoidal assembly” in which each layer has an orientation slightly shifted compared to the previous layer (Figure 3). The periodicity (p) of this helicoidal assembly defines the variation of the index of refraction in the cell wall, and this feature further defines what color the cell wall can reflect. The maximum reflectivity wavelength (λ) is found to be λ = p*2*n, where n is the average index of refraction of cellulose (n = 1.53). In the case of the marble berry, most of the cells have periodicity of 145 nm, which makes its blue coloration in general. Since each cell is a pixel on the surface of the berry and the value of p varies from cell to cell (from 125 to 200 nm), and, you see the fine pixilation of different colors (ranging from red, green, to purple). In addition, due to the handedness of the hellioidal assembly, the reflected light from each cell is either right-handed (RH) circular polarized, or left-handed (LH) circular polarized (Each cell has its own handedness, and on average, the total number of cells having RH equals to that having LH).
Figure 3. Top left: The layers of the cellulose micro-fibrils form the helicoidal assembly. Top right: The simulation shows the structure of each layer and how cirlular polarized light is reflected. Bottom: How light is reflected when it hits different cells on the surface of the berry. There are 6 cells shown in the figure, each one has a different periodicity, so each one reflects different colors of light. Courtesy of the same authors in figure 2.
The surface of the berry is scrutinized under a powerful microscope and the result is shown in figure 4. In A/B/C of figure 4, only LH/RH/cross polarized light is selected for detection. In A and B (especially the insets) of figure 4, the reflection from each cell is either LH or RH circular polarized light (The cell reflects a shiny strip of light in either LH or RH detection, but not both). Overall, these microscopic images clearly depict that each cell is a building block of the alluring coloration on the surface of the berry. Amazingly, another organic bio-photonics is deciphered by the scientists and presented to us.
Figure 4. (a)-(c) The images of the berry under microscope. The microscope is set up in a way that it picks up only the desired polarization as specified in the text. (d) The experimental setup. (e) The reflective spectra of two different cells. Courtesy of the same authors in figure 2.
If you look carefully enough, wonders are hidden in nature everywhere. We as human are picking up very fast in adopting the gift given by nature into technology applications. I hope by now, you are intrigued by bio-photonics. If that is the case, mark your calendar and join us in bio-photonics section in CLEO 2013. It will be a great conference for you to immerse in this astounding field.
Already doing research in this direction? Submit your abstract before January 30th. Share with us!
The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.