Maria Babu, Deaglan Farren, Thomas Long, and Myles Power.
We cannot look outside of our windows without noticing the beauty and vibrancy of the natural world, from the crisp red petals of a tulip to the interesting purple and greens of a pigeon’s plumage. The range of colours we observe correspond to light of different wavelengths, with shorter wavelengths corresponding to violet light and longer ones corresponding to red light with the whole visible spectrum of light in-between. Usually objects have their colour because they absorb some wavelengths of light but reflect others. However, some creatures in nature have found ways of manipulating light in such a way as to make them stand out against the more ordinary backdrop of the natural world, contributing to their biological processes and behaviours. These biological structures are the products of millions of years of evolution and, in some cases, are so sophisticated that modern technology can be explored and even advanced through studying them.
This process of observing and adapting structures found in nature to our own technology is called biomimetics, or biomimicry. Biomimetics in photonics involves combining the aforementioned ways in which naturally occurring structures can interact with light and the application of this knowledge to research and development.
Some animals such as the blue morpho naturally exhibit a unique wing structure that manipulates incident light, rather than simply having pigment in their cells as we usually find with colourful animals. This results in their signature and uncanny blue colour. The butterfly’s wings accomplish this by causing specific interference, diffraction and iridescence of the incident light to reflect blue light of a highly specific wavelength. Usually, these iridescent colours vary depending on the angle at which light is incident, such as in a soap bubble. When observed from different angles, a soap bubble displays multiple colours. However, thanks to the wing’s specific structure, it manages to reflect only the dazzling blue colour. In order for this to work, these structures must be on the nanometre scale, comparable to the wavelength of the light it is manipulating.
The wings themselves are composed of stacked layers of almost crystal-like structures, with a protective scaled layer on top that give them their glossy appearance. The spacing between these layers is constant, at about 300 nanometres. This specific spacing, along with how the different shelves diffract light is the key to the butterfly’s iridescent blue at all observable angles. By manipulating the light as it passes through these structures, the wing diffracts the blue light into a larger than normal angular range. Reflecting light in this structural manner has several benefits. For example, it allows them to appear much brighter in their vibrant, natural habitats, enabling them to frighten off even the fiercest of predators.Several other less dainty creatures such as the Lamprochypus augustus green weevil also exhibit a similar iridescent colouration. Unlike the butterfly, weevils and beetles owe their colouration to chitinous photonic crystal scales that coat their protective shells. This particular weevil has already been the subject of interest to scientists studying photonic crystals. These weevils, unlike butterflies, take on a brilliant iridescent emerald green rather than blue, perhaps as a means of camouflage, rather than drawing attention to themselves. This variety shows the versatility of this tiered photonic crystal structure in reflecting a wide array of brilliant iridescent colours.
We can also see similar structures and photonic effects in plants, the most obvious of which are flowers. Flowers are not just colourful, vibrant, beautiful additions to our gardens; they also serve an important reproductive function in angiosperms, or flowering plants. The anther of the flower is the structure that produces pollen, which is then transferred to the stigma, the female gonad, of a different flower. The vehicles of pollen transport are insects such as bees and butterflies. In order to attract these animal pollinators, flowers have developed various visual cues such as displaying striking colours, iridescence and glossiness.
As previously mentioned, colour mainly comes from pigment which absorbs certain colours of light whilst reflecting others. It is this reflected colour that we see exhibited by the petals. For example, in the buttercup flower, the yellow colour comes from carotenoid pigments that absorb blue-green light and reflect light that correspond to a yellow colour .
Microscopically, the shape of the cells of the flower can affect how intense the colour is. If the cells are flat, this means that only light that travels straight down into the cell can interact with the pigments. Most of the light that arrives at an angle gets bounced off the cell and is lost. However, if the shape of the cell is conical, then this can act as a lens, concentrating the light into the cell. Any light that gets reflected will travel into a neighbouring cell, increasing the contact with the pigment .
Buttercups also have a very distinct glossiness to their petals. If you hold a buttercup under your chin, the brilliant yellow hue is reflected onto your skin. This special quality is unique to this flower and is a result of the combination of the pigments and the different layers within the petals interacting. This effect utilises the same mechanism that allows you to see the sheen of a soap bubble or an oil slick. The top layer of the flower is called the epidermal layer, and in the buttercup, the epidermis, only one cell thick, is covered by a thin wax coating allowing for an ultra-smooth surface. Below that, there is a starch layer that is separated from the top cells by pockets of air. Interference of light between the smooth top surface of cells and the air layer creates the mirror-like reflection of the buttercup .
Another visual feature that plants use to attract insects and other pollinators is iridescence. In the Queen of the night tulip, the shimmering quality is due to cellulose, the main structural component of plant cells, arranging itself into ridges that act as a diffraction grating . A diffraction grating is a surface with slits occurring at regular intervals. These slits can disperse white light into its spectrum. The spectrum is comprised of different wavelengths of light, each corresponding to a different colour. Iridescence is caused by the light waves interfering with each other, either constructively or destructively. Constructive interference is when the crests and troughs of the waves match and amplify, reinforcing the colour. On the other hand, destructive interference reduces the vibrancy of the colour as the troughs and crests cancel each other. As we change our viewing angle, there is a change in the degree of constructive and destructive interference, leading to a change in the colours observed, i.e. iridescence .
Now we turn our attention to the biomimetic photonics, such as those we’ve talked about in animals in plants, being put to practical use in technology. One area that has benefited greatly from this field is that of optical sensing which deals with the detection of light. Most animals, including us humans, have sensors that can detect light. The kind of light that our eyes can see is known specifically as visible light, because there are other types of light that exist. One of these other types is known as infrared (IR) radiation, which some animals such as some beetles, snakes, and vampire bats can detect. So, of course scientists have taken great interest in studying how these creatures are able to detect IR radiation, both out of curiosity and to help with developing technology that can help us ‘see’ in the infrared. Let’s briefly look at how a team at CAESAR mimicked the IR-sensing capabilities of a certain kind of beetle with their technological design. Now, anyone who has been outside on a sunny day will be aware of how effective light from the sun is at heating things up – that is, the light can raise the temperature of objects. So if IR ‘light’ is shone on, for example, a liquid, it’s temperature will increase. Now imagine if we wanted to detect the temperature change of the liquid, one might introduce a device which can do so in one way or another, which then means we have a way to detect the IR radiation itself. It is through this kind of setup that the Melanophila acuminata beetles (a species of fire beetle) can see IR radiation.These IR-sensing organs are called sensilla, and are filled with a liquid that expands when heated by the IR radiation, with the resulting pressure rise being detected by the beetle. The technology that emulated this utilised a similar setup, with only the devices used to detect the pressure changes differing from the beetle’s, naturally. While we may be inspired by the interesting quirks of other creatures in the animal kingdom, looking at ourselves and how our own eyes work is also an area that can bring about the development of impressive technologies. One recent project has been that of VODKA or Vibrating Optical Device for the Kontrol of Autonomous robots . This technology attempted to replicate a phenomenon termed ‘hyperacuity’. Visual acuity refers to the ‘sharpness’ of our eyesight and is measured during those vision tests at the optician’s that we all know and love, where one must try and read out all the letters through the mirror. Hyperacuity occurs when we are able to see details with our vision to a much better degree than is predicted by looking at just the biology of our eyes’ receptors. One example, known as vernier acuity, is our ability to notice the misalignment of borders with a precision up to ten times better than what is expected with our normal visual acuity.
So what gives us this ability? There are two main factors that have been identified, one is the brain itself processing the information and enhancing our sight with its mysterious algorithms. Another is what has been dubbed ‘tremor’, which refers to irregular movements of the eye. It was these movements that inspired the VODKA optical sensor, which used a pair of very small sensors behind a lens and had them move repetitively while signal processing worked with the information to track a moving target. The results were impressive – with only two pixels, the device was able to locate the angular position of an edge with a precision 900 times greater than if the sensors hadn’t been moving. However, while applying these biological structures to technology is undoubtedly promising in some areas, nothing is perfect and there are some notable disadvantages to go with the advantages of these applications.
There are many advantages and disadvantages associated with biomimetics. One of the key advantages is how easy and efficient it is to apply design principles from nature to new technologies. Some photonic structures observed in nature might never be achieved through ordinary research. It is much more efficient to use inspiration from naturally occurring structures which already display desirable properties. Sources of inspiration from naturally occurring structures are also extremely abundant. This means that similar properties may be achieved in different animal and plant species providing alternative routes to the desired effects. It follows from this that biomimetics accelerates the development of new technologies. Natural design principles not only have the potential to be directly copied into new technologies but may also inspire new research. Biological materials, structures and processes also have the advantage of being intensively trialled and gradually improved and developed over millions of years and have stood the test of time. This knowledge can be applied to new technologies to avoid unnecessary testing and trialling processes.
Another advantage of biomimetics is sustainability. Sustainability is now one of the key considerations in the development of new technologies. Biological organisms have managed to establish efficient structures and processes using sustainable materials and with virtually no adverse effects on the environment. Most of these materials are organic compounds and are generally nontoxic minerals. Therefore, biomimetics is currently, and will continue to play a vital role in climate change mitigation.
However, there are a number of disadvantages. The main obstacle to biomimetics is the complexity of structures and processes found in nature. The elaborate architecture of naturally occurring structures can make replication using current technologies almost impossible. Simplification may allow for structures to be reproduced but this often is accompanied by some loss in efficiency or functionality. And even if it is possible to develop biomimetic devices, factors such as expense and time constraints come into play. Furthermore, biomimetics is a highly interdisciplinary field. It requires expertise from biologists, physicists, chemists, material scientists and engineers. Effective collaboration between different disciplines may be impeded by conflicting technical languages and approaches to solving problems.
In conclusion, we can see that there’s much to learn about photonics from the natural world. Creatures and plants that we would normally consider mundane such as weevils, butterflies and buttercups boast incredible, nanometre accurate, yet completely organic structures capable of manipulating light. Some of these naturally occurring structures are so impressive that they still overshadow even modern technology. These specialized abilities and incredibly complex structures have been developed over the course of millions of years of evolution. It seems natural then that we humans, a relatively young species, would have much to learn from these evolutionary veterans in the field of photonics.
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