Researchers at the Physics Institute's Optics Laboratory are using holography to manufacture photonic crystals with high optical quality. These artificially structured crystals have the ability to act on photons in optical devices, in the same way that semiconductors act on electrons in electronic devices. This group is the first in the country to produce structures of this type, using the same holography technique that is used to record and reconstruct three-dimensional images.
As they are constructed materials, their symmetry and geometry can be previously defined, unlike natural crystals, composed of arrangements of atoms, whose geometries are determined by the nature of the elements. In this way, there seems to be no limit to what can be done with them. The applications, says the coordinator of the Optics Laboratory Lucila Cescato, include everything from basic research to cutting-edge technology. Among the possibilities are improving the efficiency of light emitters and light guiding devices, and the construction of optical components and circuits similar to the electronic ones that exist today.
Photonic crystals can be one-, two- or three-dimensional. Those being manufactured at the Optics Laboratory are two-dimensional, explains Elso Rigon, who has just defended his master's degree working, under Lucila's guidance, on the development of processes for recording two-dimensional structures, with nanometric dimensions (that is, on a millionth scale). millimeter), using holographic exposures and lithography.
What is really interesting about photonic crystals is that, due to their similarity to natural crystals, they have prohibited energy bands. These are energies or wavelengths within the material where photons cannot exist. It is precisely this property that gives these structures the name photonic crystals.
This means that there is the possibility, for example, of creating a structured material, with prohibited wavelength bands, to inhibit the spontaneous emission of light. This would result in a device that harnesses much more energy by emitting light only in the region of interest. Something like having an incandescent lamp that only emits visible light, unlike current ones that emit most of the infrared light, heat. “The photonic crystal thus opens up the possibility of manufacturing light sources with an efficiency of almost 100%”, compares Lucila. There are no devices for this yet, but when they exist, it is not difficult to imagine the savings that will be generated in terms of energy.
As photonic crystals are structured, interspersing different materials (it will be more efficient the more different the materials are), therefore, the most effective way of construction is to alternate material and air. For the construction of light guiding devices and optical circuits, this material must be dielectric, such as resins, glass, oxides, in short, any transparent material in the range where it is desired to present prohibited bands for photons.
Rigon was able to make crystals using holography by projecting interference patterns onto a photosensitive material. This periodic light, dark pattern is recorded on the material, then developed as in a photograph. Thus, the luminous pattern becomes a relief pattern, resulting in a surface full of elevations, similar to arrangements of tiny posts, with air between them. The properties of this crystal depend on the material where the pattern is being engraved, the spacing of the structures and their geometry. “We are managing to obtain structures formed by arrangements of resin cylinders, to complementary structures, such as holes in a carbon film”, says Rigon. These “holes” have a diameter between 100 and 150 nanometers and the distance between the center of one “hole” and the center of the other is 1 micron. However, Rigon highlights, these dimensions can be reduced by up to 50%, using the same technique developed.
As these cylinders (posts) are aligned, if, for example, a row of them is removed, the light will be confined in that region, being able to follow any designed path “This quality is very useful for manufacturing optical devices and circuits, because, in the crystals With photonics, we can make extremely versatile trajectories, such as 90-degree curves, because the light cannot leave the constructed path,” says Rigon.
Confining light in well-defined regions of optical guides is ideal for manufacturing optical fibers. Photonic crystal fibers (photonic fibers) are much more efficient than current optical fibers. In current ones, light only propagates if it hits at an angle greater than the critical angle. If it is below, the light escapes through the “shell” of the fiber. In a photonic fiber, regardless of the angle of incidence, the light will only pass through the path opened for it, explains Lucila. If it finds a defect, it spreads it, but it continues to propagate.
sieves - At the Optics Laboratory, although the line of research into photonic crystals is recent, photosensitive materials and diffractive optical elements have been studied for a long time. The techniques developed in the laboratory allow the fabrication of nanometric structures with applications in both optics and micro-mechanics.
The group is currently also developing processes for manufacturing microsieves. The same holographic technique used to build photonic crystals is now being used to build tiny sieves, formed by a very thin membrane with nanometer-scale holes, on the order of 100 millionths of a millimeter. “We are going to get holes on a virus scale”, observes Lucila. The filtration tests will be carried out by professor Maria Aparecida da Silva, from the Faculty of Chemical Engineering.
These sieves are nanofabricated products using the holographic technique, which replaces conventional techniques with advantages. The researcher explains that, normally, to record structures in the dimensions achieved by the group, the only other process is to use electron beams, but equipment like this costs millions of dollars and only records in extremely small areas. “To record 'submicrometer' periodic structures, holography is more efficient and cheaper,” she says.
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