Biomimetics
The application of systems and methods found in nature to the study and design of enginering technology. Biomimetics is also known as bionics, biognosis, biomimicry and bionical creativity engineering http://en.wikipedia.org/wiki/Bionics .
The Morpho Rhetenor butterfly wing strongly reflects blue light over a large range of angles. The reflection is due to nanostructure in the wing itself, it is not due to pigmentation. The fact that the same colour is reflected over a large range of angles shows that the underlying structure is not a simple multilayer dielectric. In fact, the underlying optical nanostructure has much in common with photonic crystals and it is this fact that allowed us to design and Patent an entirely new class of photonic crystal devices.
In the Nanosystems Integration (NSI) Group, we have used Biomimetic techniques in the design of new nano-optical systems since 2001. The spin-out company Mesophotonics Ltd. was established to exploit photonic crystal technology to supply innovative solutions to current-day optical engineering problems. By studying natural systems, in particular the Morpho Rhetenor butterfly wing structure, we were able to create a number of Patents describing a totally new type of photonic crystal structure – the fundamental design supplied by the butterfly courtesy of a few tens of millions of years of evolution.
I really cannot overstate the importance of Biomimetic research. Nature has been conducting evolutionary experiments for tens of millions of years, so if we’re lucky enough to find a design close to what we require in the natural world, then it’s very likely to have been highly optimised (in the design), and we’re unlikely to do much better. However, we can “beat” Nature, as we are not restricted to biological materials in the fabrication of our copied design! Nature is highly constricted in the optical/mechanical devices she can fabricate due to the biological processes that create the basic materials. Nature’s optical devices, for example, are highly restricted in the range of refractive indices available, but even so, some pretty remarkable optical sensors, reflectors, and even emitters have been manufactured by Nature. As Engineers, we are able to use Nature’s fundamental designs and to create similar devices using highly optimised materials. By these means we can “outperform” Nature, even with her 500 million year “knowledge base” advantage.
A triangular lattice photonic crystal formed by etching (air) holes in a thin silicon nitride waveguiding membrane. Although this device has many useful optical properties, the conventional triangular lattice means that it is not possible to create a full photonic band gap in this structure.
Photonic crystals are interesting optical devices that utilise a sub-wavelength periodic (or quasi-periodic) refractive index contrast to create materials with optical properties that mimic semiconductors. Usually the photonic crystal is fabricated by etching sub-micron sized holes in a dielctric material as shown. For highly innovative photonic crystal devices it is necessary to fabricate crystal geometries that have a very high degree of symmetry in the plane. This is difficult to achieve using “conventional” crystalline patterns, as we are limited to 6-fold symmetry in the plane, as shown in the above triangular lattice. How do we fabricate structures that have more than 6-fold symmetry in the plane? The answer is we have to use quasi-crystal structures rather than conventional crystals. The well-known Penrose tiling is a quasi-crystal structure, but unfortunately it has only 5-fold rotational symmetry in the plane, and we want something with 6 or more symmetries. Researchers in the NSI Group at the University of Southampton(2) fabricated a unique 12-fold symmetric quasicrystal way back in 2000 which demonstrated unusual optical properties. However, this was only a factor of 2 increase in the number of symmetries that conventional crystals could already offer. The question now became, is it possible to fabricate photonic quasicrystals with symmetries much greater than 12? Is it possible to fabricate photonic quasicrystals with infinite rotational symmetry in the plane??
A planar waveguiding photonic quasicrystal based on Conway’s Pinwheel tiling. The mathematician John Conway discovered that a 1, 2, v5 triangle could be decomposed into 5 congruent triangles. These 5 triangles could therefore be decomposed into 5 more traingles, and so on ad infinitum. By taking the vertices of all these triangles and etching a hole at these points, you end up with the structure shown.
I was already aware of an extraordinary quasicrystal structure known as the Pinwheel tiling shown, which is stated as having infinite rotational symmetry in the plane. If an optical structure has infinite rotational symmetry in the plane, then when we diffract a laser of the appropriate wavelength off the structure, we should see a circular diffraction pattern with a bright (zero order) spot in the center. Although the Pinwheel structure does produce a circular pattern, within the circular ring there are 8 bright spots indicating an underlying 8-fold symmetry.
An inifinitely symmetric photonic quasicrystal based upon the pattern seen in the seed head of the Sunflower. Diffracting a laser off this structure produces a circular diffraction pattern with no apparent underlying lower-order symmetry. Once again, Biomimetics provides us with a solution originally found tens of millions of years ago!
Can we produce a purely circular diffraction pattern from a quasicrystal structure with no apparent lower-order underlying symmetry? It appears that we can, and Nature shows us how. Figure 4 below is a quasicrystal pattern of holes etched into a silicon nitride planar waveguide. The pattern might look familiar to you. The Sunflower pattern shown, does indeed seem to possess infinite rotational symmetry. This extraordinary property has many applications in optical nanodevices, and may even be the first artificially fabricated structure to demonstrate the efficient localisation of light.
This has been a very brief look at the power of the Biomimetic approach applied to problems in optical engineering. We are not limited to optical engineering! Nature also provides solutions to problems in mechanical engineering, aviation, and submarine technology. There is much to be gained by a detailed study of the structure of flora and fauna in much the same way that chemists study the chemicals produced by Nature in order to come up with revolutionary new pharmaceutical products.