A detailed study of the interaction between fluorescence and nanostructure in naturally evolved photonic systems

The living world has been producing and using colour for hundreds of millions of years. By means of various forms of evolutionary selection pressures, nature has developed many ingenious techniques and practices with which it can control colour in highly advantageous ways. Our research team would like to learn from nature; to discover its unique techniques for manipulating the flow of light and its specialised methods for generating bright colour. We will then synthetically apply these techniques for use in modern technology . The colours displayed by animals and plants can be produced in several distinctly different ways. The majority of colour we see in the living world around us is produced using chemical pigments such as chlorophyll and melanin. These pigments absorb some colours and scatter others. Of special relevance to this project is a particularly distinctive sort of pigment that is said to be fluorescent. This sort of pigment usually absorbs ultra-violet light and then strongly reemits a different colour that can be blue, green or red. In certain cases, very small regularly spaced structures, a few millionths of a millimetre wide, can produce colour without the need for any pigment. This way of producing colour is the same as that which creates the colour in soap bubbles and it relies on light waves interacting with the regular arrangements of these tiny structures. If they are just the right size, they can completely control what colour of light is reflected. This manipulation of light, even in three dimensions if the structure has the right form, is an incredibly important feature of modern optical technology and is usefully employed in many different areas of high-tech industry. The project we propose to undertake in this investigation is exceptionally exciting because it comprises the detailed study of something which has only just been discovered but only superficially documented. Until this discovery, science didn't know that nature has evolved the simultaneous use of fluorescent pigment and nanostructure to produce a system that controls the emission of the light it produces. In previous BBSRC-funded work (published in November 2006), our research group discovered that some butterflies produce coloured light by fluorescence and then control the way in which this light is emitted from their wings by using a specialised form of very small but regular structure. The really exceptional part of this discovery is that this butterfly's system is very similar to a form of light emitting system that is found in technology, one of the new generation of high-efficiency light emitting diodes (LEDs). In other words, nature and technology have converged on very similar designs that appear to optimise the way that light can be produced and emitted. With this proposed study, we want to understand this and other fluorescent natural systems in far greater detail; to see if their photonic designs can be used to make synthetic optical devices much more efficient. The results of the project will be communicated to the international scientific community and to photonics companies. The project will provide fundamental new information about the biology of specialised natural brightly coloured animals such as butterflies, beetles, scorpions and birds. It will also provide new ideas for technology about efficient ways to control the colour of surfaces and to manipulate the flow of light in communication applications such as LEDs and light-guiding networks.

The main aim of this project is to characterise in detail the nature of controlled emission of fluorescence in natural systems resulting from interaction between fluorescent pigment and photonic nanostructure. Our recent original research has shown that the emission of fluorescence in some lepidopteran systems is directionally-controlled. This control is exerted by the presence in the butterfly wing scales of both a photonic crystal slab containing high quantum yield fluorescent material and a distributed Bragg reflector. After a preliminary investigation we were able to describe superficially the mechanism which underpins this phenomenon. The research we proposed here comprises a very detailed study to describe fully the form and function of this lepidopteran fluorescence. It will also incorporate a broader investigation to consider other animate systems known already to be fluorescent, but whose possible interaction with photonic nanostructure is completely uncharacterised. The specific form of the periodicity associated with the photonic crystal slab in the fluorescent Lepidoptera is of significant interest to photonics technology. Instead of exhibiting perfect 2D periodicity in refractive index, these Lepidoptera use a distinct quasi-periodicity to manipulate light. In other words, whereas technological photonics operates a fundamentally fault-intolerant approach to nanostructure (using perfectly periodic systems), these lepidopteran systems appear to use a fault-tolerant approach. This minimises their 'developmental' energy use. They appear to employ the minimum amount of order and periodicity with which they can still sufficiently control the flow of light for desired effect. With this approach in mind, in the second half of the project we will aim to apply the design protocols we learn from nature's fault-tolerant nanostructure to the design of nanostructure responsible for light manipulation in synthetic light emission systems we will fabricate ourselves.