A laser that breaks repeating patterns could reshape semiconductor design

A laser does not always need a perfectly repeating pattern to produce a clean beam. Researchers at the University of Illinois Urbana-Champaign (UIUC)n have demonstrated a semiconductor laser that deliberately breaks with that convention, replacing the regular structure of a photonic-crystal surface-emitting laser (PCSEL) with a quasi-periodic one. The device produced single-mode lasing at room temperature at 1.5 micrometers, showing that non-repeating patterns can be built into a laser while preserving precise control over light. “We demonstrate photopumped lasing from a buried dielectric QPCSEL (quasi-periodic photonic-crystal surface-emitting laser) at room temperature with emission wavelength at 1.5 μm,” the researchers note in their study. The approach could give engineers more freedom to design lasers for applications such as sensing, communications, aerospace and defense. Breaking the repeating pattern PCSELs use a photonic crystal — a carefully designed pattern that controls how light moves through a semiconductor. Unlike conventional lasers, the optical structure can spread light interaction across a large area and direct some of it out through the surface. The design can produce a narrow, well-controlled beam and single-mode emission, which are useful properties when a laser must deliver light precisely rather than in many competing optical modes. Over the past two decades, PCSELs have attracted interest for advanced semiconductor laser applications, including aerospace and defense systems. However, the geometry is also a limitation. Conventional PCSELs generally depend on repeating patterns, and changing the shape or spacing of those features can make fabrication difficult. Tiny structures can also be distorted during semiconductor regrowth, making it harder to reproduce the exact geometry that was designed. The Illinois team previously developed a buried-dielectric platform to address that problem. Instead of etching holes directly into the semiconductor, researchers patterned a silicon dioxide layer and then covered it with epitaxial semiconductor. The dielectric features were therefore buried inside the device, helping preserve their shape during fabrication. Turning periodic into quasi-periodic The study authors wanted to push the idea and test whether the photonic-crystal pattern itself could be made non-periodic. Drawing inspiration from non-repeating, topologically protected patterns, they created a quasi-periodic structure. Rather than placing identical features at strictly repeating intervals, the pattern varies in a controlled way. The researchers used tiny, low-index silicon dioxide features surrounded by high-index semiconductor material to create the quasi-photonic-crystal layer. This is because the dielectric pattern is embedded rather than left exposed, the semiconductor regrowth process can preserve a more complicated geometry. The resulting device was photopumped, meaning an external light source supplied the energy needed for lasing. At room temperature, it emitted at 1.5 micrometers and demonstrated single-mode operation. This is significant because it shows a quasi-periodic photonic crystal can produce the controlled laser emission normally associated with more rigid periodic designs. “We’ve demonstrated that we can have a non-periodic pattern and more flexibility to tune it. It’s a different way of engineering the refractive index variation to get the properties we want from our lasers,” Erin Raftery, one of the study authors and PhD candidate at UIUC, said. A more flexible laser platform The current device is still a proof of concept, and its performance has not yet established that it is better than conventional PCSELs in every measure. It was photopumped rather than electrically injected, so it is not yet a practical diode laser. Still, the buried-dielectric approach could allow different photonic-crystal patterns to be fabricated on the same substrate, giving engineers more freedom to optimize lasers for different purposes. Such flexibility could be relevant to technologies ranging from silicon-photonics lidar to other compact optical systems. “Right now, you can only grow one kind of structure at a time, whereas we can mix and match on the same substrate. This could allow us to build more reliable, better-performing lasers,” Kent Choquette, one of the study authors and an engineering professor at UIUC, said. The team’s next goal is an electrically injected device, which would bring the technology closer to real-world use. “We’ve demonstrated the physics. Now we need to demonstrate a practical device,” Choquette added. The study is published in the journal Applied Physics Letters. 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