Shedding Light on Optical Properties of Stable Glass

Although glass has been around for at least 5,000 years, many of its properties are not well understood. But researchers at the University of Pennsylvania are learning more about the properties of stable glasses. Stable glasses are closely packed forms of glasses produced by depositing molecules from a vapor phase onto a cold substrate.

“There have been a lot of questions about whether this is analogous of the same amorphous state of naturally aged glasses such as amber, which are formed by just cooling a liquid and aging it for many, many years,” said Zahra Fakhraai, associate professor of chemistry in Penn’s School of Arts & Sciences.

Dr. Fakhraai and PhD student Tianyi Liu studied a new, spherical molecule designed and synthesized by chemistry professor Patrick Walsh. Because this special molecule cannot align itself with any substrate as it is deposited, the researchers expected the glasses to be amorphous and isotropic, with constituent particles arranged in no overarching pattern or order.

Their results surprised them: The researchers found that these stable glasses are birefringent, so that the index of refraction of light is different in directions parallel and normal to the substrate. The light shined in one direction on birefringent glass will break differently than light shined from a different direction, which changes the orientation of the material and can produce optical effects. Birefriengence patterns are not common in material made of round molecules. 

The researchers were able to confirm that the material had no orientation by conducting photoluminescence experiments with physics professor James Kikkawa and PhD student Annemarie Exarhos, and by calculating the index of refraction of the crystal and determining the degree of birefringence in the amorphous state with help from chemistry professor Joseph Subotnik and PhD student Ethan Alguire.

While the glass measured at zero order, it displayed birefringence analogous to having up to 30 percent of the molecules perfectly ordered. The researchers found this was due to the layer-by-layer nature of the deposition, which allowed the molecules to pack more tightly in the direction normal to the surface during the deposition. They found they could control this by changing the substrate temperature, which controls the degree of densification: The denser the glass, the higher the value of birefringence.

The research sheds light on the process of accessing lower state phases and also offers a way to engineer optical properties without having to induce an order or structure in the material.

“I think understanding how shape, orientation and packing could affect the mechanics of these coatings is one of the places where interesting applications could emerge,” Dr. Fakhraai said.

Now that the researchers have successfully showed that there can be amorphous phases that are high density, Dr. Fakhraai hopes to apply this understanding to find out what would happen in highly aged glass.

“This tells us that we can actually make glasses that have packings that would be relevant to very well-aged glass,” she said. “This opens up the possibility of better fundamentally understanding the process by which we can make stable glasses.”