Glass-in-glass Fabrication Method Produces Complex Optics

A manufacturing technique for creating microstructures by integrating two types of glass with different physical properties could provide a path to complex infrared (IR) optics to be used in spectroscopy, imaging, sensing, and other applications. Researchers at École Polytechnique Fédérale de Lausanne (EPFL) developed the technique. It can be used with a variety of glass types to create almost any 3D shape with features measuring 1 μm or less.

The researchers created microscale, freeform, 3D chalcogenide microstructures embedded in a fused silica matrix. Chalcogenide glass has a wide transmission window in the IR range, but it is difficult to structure in 3D due to its poor mechanical properties and low chemical and environmental stability.

“Glass that transmits IR wavelengths is essential for many applications, including spectroscopy techniques used to identify various materials and substances,” said professor Yves Bellouard, who led the research. “However, infrared glasses are difficult to manufacture, are fragile, and degrade easily in the presence of moisture.”

Using femtosecond laser-assisted chemical etching, the researchers created an arbitrarily shaped 3D cavity inside a fused silica glass substrate. Using the pulsed beam of the femtosecond laser, they modified the silica glass structure so that the exposed areas of the glass could be removed with a chemical such as hydrofluoric acid to create microcavities.

Then, using pressure-assisted casting at a miniature scale, they melted and pressurized the chalcogenide so it could flow into the network of carved silica cavities. A microstructural examination confirmed the consistency of material properties before and after the infiltration of chalcogenide.

Although the researchers used chalcogenide glass to demonstrate their technique, they said that the material that flows into the carved silica cavities can be a metal, glass, or any other material that does not react with silica glass, and that has a melting point below that of the carved silica substrate.

This technique additionally enabled the EPFL team to build arbitrary, 3D glass-in-glass elements while maintaining micrometric resolution and submicron roughness. The resulting silica-chalcogenide composites offered a high refractive-index contrast over a broad spectrum, in geometries and size scales suited for complex, 3D IR optics based on total-internal-reflection optical design principles.

Fused silica can also serve as a mechanically and chemically resistant support and protection for chalcogenide glass microstructures.

“Our fabrication method can be used to protect IR glass, opening new avenues for microscale infrared optical circuits that are fully integrated in another glass substrate,” Bellouard said. “Also, because fused silica and chalcogenide offer high refractive-index contrast, we can form these materials into IR waveguides that can transmit light much like optical fibers.”

EPFL researchers used a glass-glass fabrication approach to create various structures that combined chalcogenide IR glass with fused silica glass. These included a set of pillars with different dimensions. The white dotted rectangle in the image indicates the smallest silica cavity used in this work. Courtesy of Yves Bellouard, École Polytechnique Fédérale de Lausanne.

The researchers created several different shapes with chalcogenide glass and a silica glass substrate, including the EPFL logo. With colleagues at ETH Zurich, they showed that some of these structures could be used for guiding mid-IR light emitted from a QCL at 8 μm. According to the researchers, a limited number of optical components are available for this spectral range because of manufacturing challenges.

“Our technique could open the door to a whole new range of new optical devices because it can be used to make infrared optical circuits and arbitrarily shaped IR micro-optics that were not previously possible because of the poor manufacturability of IR glass,” researcher Enrico Casamenti said. He added that the optics could be used to create an IR camera small enough to integrate into a smartphone.

The research was published in Optics Express (www.doi.org/10.1364/OE.451026).