Researchers have developed a new manufacturing process that allows infrared (IR) glass to be combined with another glass and shaped into complex miniature shapes. The technique can be used to create complex infrared optics that could make IR imaging and sensing more widely accessible.

“Glass that transmits IR wavelengths is essential for many applications, including spectroscopy techniques used to identify various materials and substances,” said research team leader Yves Bellouard of Ecole Polytechnique. Federal of Lausanne (EPFL) in Switzerland. “However, infrared glasses are difficult to manufacture, fragile and degrade easily in the presence of moisture.”

In the magazine Optica Publishing Group Express Optics, the researchers describe their new technique, which can be used to embed fragile infrared goggles into a durable silica matrix. The process can be used to create virtually any interconnected 3D shape with features measuring a micron or less. It works with a wide variety of lenses, providing a new way to fine-tune the properties of 3D optics with subtle lens combinations.

“Our technique could open the door to a whole new range of new optical devices as it can be used to fabricate arbitrarily shaped infrared optical circuits and IR micro-optics that were not possible before due to the low manufacturability of the IR glass,” said Enrico Casamenti, first author of the paper. “These optics could be used, for example, for spectroscopy and sensing applications or to create an infrared camera small enough to fit into a smartphone.”

Merge Materials

The new fabrication process grew out of previous work in which Bellouard’s research team collaborated with Andreas Mortensen’s team, also at EPFL, to develop a method of forming highly conductive metals inside of an insulating 3D silica substrate.

“Our team started looking for innovative ways to achieve broadband light confinement in arbitrarily shaped 3D optical circuits,” Bellouard said. “It was then that we decided to explore the possibility of modifying a process that we had first demonstrated using metal so that it could be used to produce structures that combine two types of glass.”

For the new approach, the researchers start by creating an arbitrarily shaped 3D cavity inside a fused silica glass substrate using femtosecond laser-assisted chemical etching. This uses the pulsed beam of a femtosecond laser – which can be focused to a spot about a micrometer wide – to change the structure of the glass in such a way that exposed areas can be removed with a chemical such as hydrofluoric acid.

Once done, the tiny cavity must be filled with another material to create a composite structure. The researchers achieved this by using a miniaturized version of pressure-assisted casting, in which a second material is melted and pressurized so that it can flow and solidify in the network of sculpted silica cavities. The second material can be a metal, glass or any material having a melting point lower than that of the etched silica substrate and which does not react with the silica glass.

Creation of complex optics

“Our fabrication method can be used to protect infrared glass, opening new avenues for micro-scale infrared optical circuits that are fully embedded in another glass substrate,” Bellouard said. “Furthermore, because fused silica and chalcogenide provide high refractive index contrast, we can turn these materials into infrared waveguides capable of transmitting light like optical fibers.”

The researchers demonstrated the new method by creating various complex shapes, including an EPFL logo, using chalcogenide IR glass and a silica glass substrate. They also showed, with the help of colleagues at ETH Zurich, that some of the structures they created could be used effectively to guide the mid-infrared light emitted by an 8-micron quantum cascade laser. Few optical components are available for this spectral range due to manufacturing challenges.

They continue to explore the capabilities of the new process in terms of combining different glasses and plan to test the composite parts in spectroscopy and other applications.

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