New materials

Crystalline Light

Researchers develop prototype of a compact laser for wide commercial use

eduardo cesarCrystal developed in São Carlos emits an invisible beam, but when coupled to another crystal produces a green lighteduardo cesar

Brazil may shortly take a technological leap forward in the production of lasers. The novelty lies in the crystal used to generate the beam of light, which was developed by researchers from the São Carlos Physics Institute (IFSC), of the University of São Paulo (USP). This piece is the heart of laser equipment, and it may lead the country to join the select group of nations that make components used in compact apparatuses, which arouse a strong commercial interest. This kind of equipment is used in ophthalmologic surgeries, dental treatments, and other medical procedures, like the removal of facial expression marks and tattoos. “Our researches are opening up room for Brazil to produce compact lasers”, explains physicist Luiz Antônio de Oliveira Nunes, the coordinator of the IFSC’s Laser and Applications Laboratory (LLA).

To consider the importance of the research carried out at São Carlos, it has to be understood that a laser (a word made up by the initials of Light Amplification by Stimulated Emission of Radiation) is made up of an active medium (in this case a crystal, but it could be a gas or a liquid) adapted to an optic cavity (a space between two mirrors where the light is confined and passes several times through the crystal), where the generation of light occurs. To do so, it is necessary for the active medium to be given an external source of energy, like a flash bulb or another laser (a smaller and cheaper diode laser).

The larger the capacity for absorbing and converting the light energy from the external source of laser light, the more efficient the active medium is. In the case of the laser developed by the researchers from USP, the crystal is a monocrystalline fiber. This means that it is not an aggregate of small crystals, as in the case, for example, of ceramics. It is merely a grain that has grown in three dimensions. To get the crystal, the researchers used chemical compounds with oxides of yttrium, neodymium and vanadium, minerals that react amongst themselves and make up the formula YVO4:Nd3+.

“The monocrystalline fiber with a cylindrical format, measuring about 0.5 millimeters in diameter and 1 millimeter in length, can replace the so-called bulk crystals – larger pieces obtained using sophisticated and costly techniques for growing crystals – normally used for producing compact laser apparatuses”, explains chemist Andrea Simone Stucchi de Camargo, from the team of researchers. The two materials, the fiber and bulk crystals, have identical optical, physical and mechanical properties, differing only in size. “Our work consisted of studying the optical characteristics and behavior of the active ions in this fiber, to optimize the processes for absorbing and emitting light”, says Andrea. The laser with the fiber emits a beam of light in the infrared region, with a wavelength of 1,064 nanometers, which corresponds to a frequency that is invisible. Adapted to another kind of crystal, this laser doubles its frequency, to 532 nm, and displays a green luminosity.

Fundamental cooperation
The fiber in its raw format was developed by the Crystals Growth Group, from the same IFSC, under the supervision of physicist José Pedro Andreeta. “We prepared the material, but we had no way of measuring all its properties. The work in cooperation with the Laser and Applications Laboratory was fundamental for the success of the work”, explains Andreeta. The information exchanged between the researchers made it possible for the fiber to be perfected and to be turned into something just as efficient as a commercial bulk crystal. “We took four years to achieve a good result.”

Besides its smaller size, one of the advantages of the monocrystalline fiber over bulk crystals is that its production is much quicker and far cheaper. The fibers are ready in some minutes or hours, while bulk crystals take days or weeks to reach the ideal size.

To get them, the researchers resorted to a process called the Laser Heated Pedestal Growth Technique (LHPG). “This technique had already been known for a few years, but nobody had succeeded in developing a fiber like ours with the ideal characteristics of bulk crystal”, Andrea says. “This crystal is very difficult to get, because during its growth, a formation of structural defects frequently occurs.” The LHPG process takes some 40 to 50 minutes to grow a fiber of roughly 3 cm. “The equipment for growing the fiber was developed by us with funds from FAPESP and is showing better results than those to be found in many countries”, claims physicist José Andreeta.

The unprecedented nature of the research has earned it the publication of articles in international scientific publications like theOptics Letters magazine, in January this year. In February, the work was also the subject of an article published in the American magazinePhotonics Spectra , which is specialized in the commercial laser area. But the greatest proof of the success of the work, according to the researchers, was the presentation of the results at the Conference on Lasers and Electro Optics (Cleo2003) held in the city of Baltimore, in the United States, in June last year. “My presentation aroused great interest, so much so that I was invited by American researcher Steve Payne, one of the most renowned in this area, to repeat the presentation at the Lawrence Livermore National Laboratories, a research center for national security themes, located in the state of California”, says Andrea.

Despite the success and commercial viability of the equipment – although it still needs further developments to reach the market -, the group has not yet requested a patent for the device nor for the monocrystalline fiber, but is carrying on conversations with businessmen interested in manufacturing and selling the novelty.The next challenge for the researchers from the Lasers and Applications Laboratory is to study a transparent ceramic made up of lanthanum-doped lead zirconium titanate, known as PLZT, to be used as an active medium for laser generation.

This ceramic has been known since the 1960’s for its electrical properties, but until a short time ago there was no concern for studying its optical properties. The interest for this new material is that, once the manufacturing process is mastered, it will be cheaper and quicker to produce than crystals. Furthermore, there will be no limitation of size – neither the bulk crystals nor the monocrystalline fibers can be grown in very large proportions, unlike ceramics, which, in principle, can be made in any dimension.

The projects
1. Growth and Assessment of the Physical Properties of Monocrystalline Fibers Prepared in Controlled Atmospheres (nº 98/16319-4); Modality Regular Line of Research Grants; Coordinator José Pedro Andreeta – IFSC/USP; Investment R$ 47,967.60 and US$ 59,919.00
2. Optical Monocrystalline Fiber Spectroscopy (nº 99/06830-6); Modality
Regular Line of Research Grants; Coordinator Luiz Antonio de Oliveira Nunes – IFSC/USP; Investment R$ 36,327.56 and US$ 67,250.00