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Lasers & Sources

Vitreous Forms of Crystal Could Enhance Ultrafast Lasers

Eye on Technology - INVERT GLASS

From oemagazine May 2004
30 May 2004, SPIE Newsroom. DOI: 10.1117/2.5200405.0003

Using a unique heating and cooling process for making molten glass, researchers have created a vitreous form of forsterite (Mg2SiO4) in hopes of gaining understanding into the chemical bonds that hold invert glass materials together, potentially leading to new types of doped optical materials.

Lasers and Levitation

Researchers suspended forsterite samples in pure oxygen streams before melting the samples with a C02 laser to make vitreous forsterite.

Although conventional wisdom says that alkaline and alkaline-earth orthosilicate materials such as Mg2SiO4 cannot be vitrified, researchers at Japan Synchrotron Radiation Research Institute (JASRI; Sayo, Hyogo, Japan), Japan Atomic Energy Research Institute (Ibaraki, Japan), Tokyo University of Science (Oshamambe, Hokkaido, Japan), Argonne National Laboratory (Argonne, IL), and Containerless Research Inc. (Evanston, IL) have done just that using a containerless manufacturing method. Developed at Containerless Research Inc., the levitator suspends raw material in a process gas. A continuous-wave CO2 laser then quickly heats the material; blocking the laser light from heating the raw material leads to fast cooling and vitrification without the adverse effects of contaminants, such as those found in a melting crucible or container. The method also eliminates crystallization on container walls, enabling many new types of glass to be made.

The resulting vitreous Mg2SiO4 (v-Mg2SiO4) is a so-called "invert glass," explains Shinji Kohara, principal investigator at JASRI. Invert glass occurs when the relative amount of SiO2 is reduced to where only isolated SiO4 and Si2O7 dimers exist, causing the short-range order structure and the relaxation phenomena to change drastically.

Distorted Chemical Bonds

According to Kohara, forsterite's low SiO2 content (33 mol%) prevents formation of an extended SiO4-based network. Mg2SiO4 melt exhibits a very rapid rise of viscosity near the glass transition point (Tg), which means it is a very "fragile" liquid. "Thus any interpretation of the physical properties of the forsterite must delineate the network-modifying nature of MgO," says Kohara. "That, in turn, demands understanding of the SiO4 and MgOx subunit organization in terms of short-to-intermediate order structure. This we accomplished with a combination of novel synthesis, diffraction, spectroscopy, and computer simulation."

Neutron and x-ray structure analysis indicated that v-Mg2SiO4 has a unique short- and intermediate-range order structure. A broad, skewed Mg-O peak indicated a distribution of Mg-O distances from 1.8 to 2.5 ŗin other words, highly distorted MgO polyhedra including 4-, 5-, and 6-oxygen coordinations. These results differ markedly from the long-range order structure in crystalline Mg2SiO4 (c-Mg2SiO4), Kohara explains.

Furthermore, the team determined the key difference between v-Mg2SiO4 and c-Mg2SiO4 stems from the Mg-O correlation. The team took Raman measurements to test the connectivity with SiO4 tetrahedra, and the results supported the existence of connected SiO4 tetrahedra in v-Mg2SiO4. The team found that the intermediate-range structure of v-Mg2SiO4 arises almost entirely from connectivity of Mg-O polyhedra that provides the structural network. Also, the structure of the glass differs from its crystalline counterpart in that Si2O7-type dimers fill in the space within the network. Kohara concludes, "Our study of vitreous forsterite prepared by the containerless method may provide insights into the highly nonequilibrium materials processing that occurs in interplanetary and interstellar environments."

Closer to Earth, Richard Weber, vice president for materials research and development at Containerless Research Inc., says the vitreous glass could lead to improved Cr4+:forsterite lasers. "We were interested in synthesizing forsterite to understand the structure of the glass and see if we might be able to plug ions into the structure to make optically active materials," Weber explains, adding that an improved Mg2SiO4 could lead to ultrafast lasers with even shorter pulses and broader tunability. The Cr4+:forsterite laser emits between 1100 nm and 1400 nm and is centered around 1250 nm—the zero-dispersion wavelength of optical fiber, and a low-scattering, low-absorption area of the near infrared spectrum, making the source attractive for applications that include telecommunications and biological studies.