Fine control of the waves of light

Team from USP wins international research

Brazil is beginning to play an important role in research into diffractive optics. This segment of Physics uses techniques that modify a beam of light to create a new source of light. With this, it is possible to develop technologies for the manufacture of new optoelectronic microcircuits, as light detectors for digital photographic cameras, as well as printing the holograms credit cards for example, or creating artistic and advertising images.

International recognition in this area went to the combined work of two groups of researchers, one from the EESC, the São Carlos School of Electrical Engineering, and the other from the Polytechnic School, of the University of São Paulo (USP). In June, they won first place in the category Artistic Division, of the 2000 version of the Diffractive Beauty Contest, held in Quebec, Canada, and organized by the Optical Society of America (OSA). The work, entitled Diffractive Optical Element with Complex Amplitude Modulation, was coordinated by Professor Luiz Gonçalves Neto, who teaches at both schools.

The two groups are studying Diffractive Optical Elements (DOEs), which are optical surfaces with micro-relief structures that is capable of modifying a beam of light by means of delaying its propagation in space. The control of the light, in this case, is carried out by the micro-relief structures engraved on the surface of the DOE, which act as obstacles to the beam of light. This is the main difference between them and common lenses, which have smooth surfaces produced by abrasion or polishing.

The device shown in Canada provided the best images ever generated up to that moment by this technology, and the team demonstrated that it had the intellectual and technological capacity to develop the DOEs.  At the event, the group also showed new proposals for calculations to make these devices.

Between lines and patents
The innovative technology developed and recognized in the competition of the OSA was shown in the projection of two images, a butterfly and an eagle head, with the use of a laser light beam. They attracted the attention of the specialized public present at the exhibition, and even caused a line to see the images. After this success in Canada, Gonçalves began to draw up a patent application for the new DOEs, and to publish the results in the magazines Optics Photonics News and Applied Optics, published by the Optical Society of America.

The development of this whole process enjoys funding from FAPESP, for the project ‘Implementation of Diffractive Optical Microelements’, which is being carried out in the ambit of the Young Researcher Program, with financial support of R$ 32,000 and US$ 33,000. This program offers finance in support of infrastructure and scholarships for researchers who have recently concluded their doctorates, and show the competence to carry out research projects. Another of the Program’s functions is to open up new areas for research, and to help to place the new researcher in the traditional academic institutions. The program thereby makes it possible to create new nuclei of study, like the two groups that are studying diffractive optics at São Carlos and at the Poly, in São Paulo.

For three years, Gonçalves has headed the Micro-Optics Group, at the EESC.  There, under his supervision, a group of researchers began, two years ago, post-graduate projects in this area. They include Patrícia Cardona and Giuseppe Cirino. Also taking part, as coordinators, are Professors Ronaldo Domingues Manzzano and Patrick Verdonck, from the Integrated Systems Laboratory, of the Poly-USP.

Aluminum on the plates
It was working with techniques and tools that make it possible to control these waves that Gonçalves came to draw up something new.  “Diffractive Optic Elements have been known for at least 25 years. The novelty is that we discovered a new way of applying aluminum to the plates of the DOEs. This innovation brought a great difference over the technologies that existed up to this point. We managed a way of controlling not only the modulation of the phase of a light front, but also, simultaneously, the modulation of its amplitude (the modulation of the intensity of the light)”, Gonçalves explained.

Light front, or wave front, is a concept in optics that describes the surfaces formed by joining points in space with the same phase (the delay of light with regard to a point of reference in space). And diffraction is the interaction (deflection) of wave fronts with obstacles, which are the micro-relief structures on the surface of the DOEs, whose dimension is close to that of a wavelength. The potential application of these concepts that is taking shape in the research of the two groups from USP is the possibility of improving the quality of optical telescopes.

The widening of study and of the application of diffractive optics in industrial technological development has brought about increasing demand for researchers and for projects from the advanced research centers. A large multinational company, for example, approached the EESC-USP’s Optics Group Laboratory, to study the possible applications of the new methods for the diffraction of waves of light. A good idea in this area is capable of saving a few thousands of dollars. “Diffractive optics covers new technologies that eliminate stages in the building of optical elements”, explains the professor.

Strategic reason
The first work in this field was published in 1965, by the American Alfred Lohman, who made use of these principles to make holograms with a computer.  Later on, the American government decided to invest resources in this area for strategic, and above all military, reasons, adopting more precise artificial vision for intercepting targets by means of missiles. “Today, the optical and electronic industries generally are beginning to use this technology”, says Patrícia. Laser distance detectors and even “robot vision” are being implemented with this technology.

There are hundreds of uses for diffraction technology, Gonçalves explains. They all depend on a Diffractive Optic Element. In its raw form, this device looks like a round glass biscuit. But they are much more than they seem to be. They are round silicon dioxide laminas, 1 millimeter thick, on which information is engraved in extremely small dimensions, which obey specifications expressed in microns, proportional to the wave used. “As it is necessary to introduce just one phase delay in each region of the light front that falls on it, the Diffractive Optic Elements are thinner and lighter than any material with refractive characteristics, such as the common lenses that we use in glasses and in photographic cameras”, he explains.

These characteristics make the manufacturing process for a biscuit like this something very similar to what happens in the manufacture of digital electronic circuits, which are always processed in tiny structures, like the various kinds of chips used in profusion in, for example, computers.  The techniques for the engraving of the micro-information on the relief on a DOE are basically the same as those used in assembling those circuits, that is, optic lithography, corrosion by plasma, and electron beam lithography.

One of the points of support for USP’s School of Engineering is the CTI, the Technological Center for IT Foundation, located in Campinas, which makes the masks needed to produce the optic micro-elements. The CTI has three institutes (computing, automation, and microelectronics), where the crucial stages of the process are carried out, such as making the prototypes (drawing) of the integrated circuits and micro-systems, micro-milling and photolithographic masks and electronic packaging, among others.

Team victory
Gonçalves himself recognizes: “To take part in the event in Quebec, we started from scratch.  I had the written concept, and students for their doctorates working on the project. To set up everything, test it, set it up again in Canada, and do it all again, was a fine example of teamwork. We did all the work in two weeks”. Working with these devices that control the wave of light represents an extra challenge, because the resulting image was always irregular, with a low standard in terms of definition, particularly if compared with the image concept generated by the cinema or television. In the competition won by the Brazilian team, the differentiating detail, regarding the image, was precisely the quality standard that had never before been achieved to such an extent.

More than the correct use of good equipment, the diffractive microelements depend on numerous calculations. This point also constitutes a special dimension of the progress attained.  “We didn’t actually revolutionize the calculations, on the contrary, the more we tried to make the process functional and flexible, the simpler they became.  I always tell my pupils that if they don’t know calculations and the process of manufacture in depth, to know where they can bring in a simplification, they can forget the issue”, Gonçalves tells.

With the technique for the construction of DOEs mastered, the country is qualified to carry out such processes as optic filtering, laser light modeling, and even the construction by computer of holograms of the highest quality.  According to studies made by the team, however, costs are today a considerable obstacle in this field.  The industrial investments are high.  “To set up a research laboratory of any size, equipped to handle the specification of the material used, besides manufacturing, the tests, and the specialized labor, amounts to a figure of US$ 1 million.”

Separating channels
Correcting optical telescopes may be the most humdrum of the possibilities for the use of these new diffractive technologies, explains the professor. “It is a field of work that is always expanding”, Gonçalves believes. One of the areas to benefit is the telecommunication industry.  They need to use devices like demultiplexers in optic fiber networks, to make the separation of information throughout these kinds of network safer and more efficient.  Demultiplexers are devices that separate the various channels of information that exist in a given medium.  They serve to separate the various wavelengths transmitted through a fiber, for example.

After this first international recognition, the two groups from USP are back to grappling with their studies of diffractive optics, an area that is just starting to develop.

• Luiz Gonçalves Neto, aged 38, a graduate in Electrical Engineering, from USP’s São Carlos School of Engineering.  Took a master’s degree at the São Carlos Institute of Physics, also of the USP.  Took his doctor’s degree at Laval University, in Quebec, Canada.
Project: Implementation of Diffractive Optic Microelements
Investment: R$ 32,000 and US$ 33,000