A graduate student set out to make a repeating pattern non-repeating. The result may change how semiconductor lasers are built.
Researchers at the University of Illinois have demonstrated a quasi-periodic photonic-crystal surface-emitting laser, a device type that breaks from the standard repeating crystal patterns that have defined the field for two decades. Their findings appear in Applied Physics Letters, as reported by Phys.org.
Photonic-crystal surface-emitting lasers, known as PCSELs, have shown promise for defense and aerospace applications since the early 2000s. But a persistent problem has limited their development. Because the photonic crystal pattern must be optimized for specific geometric configurations, researchers have had little flexibility when trying to fabricate devices with different pattern shapes or sizes. Each design locked them into a narrow set of options.
Graduate student Erin Raftery approached the problem directly. Her goal was to take a periodic structure and make it nonperiodic. Drawing on work in topologically protected patterns, she integrated a new patterning method with her group's existing buried dielectric platform, which the lab first demonstrated in 2025.
Most semiconductor lasers are built by etching tiny holes vertically through the device. Raftery took a different approach. She etched a silicon dioxide layer, which was then covered with epitaxial semiconductor material, embedding it inside the device. The resulting partially periodic structure lased successfully at room temperature, validating the concept.
The key advantage is flexibility. "We've demonstrated that we can have a nonperiodic pattern and more flexibility to tune it," Raftery said. "It's a different way of engineering the refractive index variation to get the properties we want from our lasers."
Kent Choquette, the electrical and computer engineering professor who leads the lab, pointed to another practical benefit. Current fabrication methods can only produce one type of structure at a time on a given substrate. The new platform removes that constraint. "Right now, you can only grow one kind of structure at a time, whereas we can mix and match on the same substrate," Choquette said. "This could allow us to build more reliable, better-performing lasers."
The buried dielectric approach embeds the patterned layer inside the device rather than leaving it exposed at the surface. That structure gives the quasi-periodic laser its uniformity and makes the pattern geometry less dependent on precise external conditions during fabrication.
The team now plans to push the design toward a practical application. The next target is an electrically injected diode, which would be a more commercially viable device than the optically pumped version demonstrated so far. "We've demonstrated the physics," Choquette said. "Now we need to demonstrate a practical device."
That step carries commercial implications. Semiconductor lasers are used across telecommunications, sensing, manufacturing, and defense. A platform that allows flexible, geometry-independent fabrication of high-performance lasers could reduce production complexity and open new design possibilities for engineers working across those fields.
