The samples designed and the results obtained are opening up a means for developing the right technique to obtain materials of this type in the future, although the outcome of the tests, which were carried out using low-cost, traditional techniques, was not what had been expected. This has been recorded in the PhD thesis by Paola Morales entitled "Efectos de filtrado por recubrimiento de cristal fotónico" (Effects of filtering using photonic crystal coating) read at the NUP/UPNA.
Photonic crystals are materials with structures that are repeated within the space and which have specific characteristics allowing radiation to be filtered in a different way, among other properties. Materials of this type exist naturally as in the wing structure of Morpho menelaus butterflies, on the skin of chameleons or in precious stones, such as opals. "Butterflies have a structure that interacts with light. For example, the blue butterfly has no blue pigments but adopts this colour whenever its structure interacts with the light," pointed out Dr Morales.
The research carried out by the physicist Paola Morales was based on the use of spheres to create a photonic crystal coating with a filtering effect. The first part of her thesis focussed on the analysis of the behaviour of the spheres when faced with variations such as the distance between them, the order, the shape and the material of the spheres. "I used marbles and decorative gemstones with a diameter of a few centimetres to take the place of spheres and spheroids, respectively, with radiation of a "large size": microwaves. These tests were compared with models carried out on computer and the coincidence was found to be very good. This, added to the fact that despite the size the behaviour of the structure when dealing with radiation is the same, in other words, it can be applied to nanometres and to metres, or what boils down to the same thing, to light as well as to sound waves, and this has enabled us to design filters for radiation of a "low size": visible infrared," pointed out the researcher.
Low-cost manufacturing processes
The second part of the thesis consisted of trying to manufacture the materials and create a monolayer of spheres that would filter visible and infrared radiation. To cut costs, the researcher used spheres of three different types of materials, including titanium dioxide, an economic material that has high dielectric permittivity. "It is the material that for example is a component of chewing gum and even toothpaste and it is cost-effective, but the problem it has is that when the spheres are small, they get stuck to each other. What we are proposing, unlike the ones that have been done so far using dielectric spheres without any gaps between them, was to create a coating using spheres with gaps. However, we did not manage to disperse them".
The materials obtained were however useful in verifying the filtering effect, but the researcher acknowledges that they did not obtain the desired decompressed monolayers, either. "Our plan was to design a colourless monolayer with total visibility, and a filtering effect. In most cases current coatings tend to be made up of various layers. In sunglasses, for example, a fairly thick pigmented film is applied to achieve filtering. If we were to apply a monolayer, we would need a much smaller quantity of material but the effect would be the same".
Although the results of the manufacturing cannot be regarded as highly successful, the study of the tests and the detailed characterisation of the samples obtained are opening up the door to future development of a suitable technique to obtain materials of this type.