Towards Single-Mode Continuous-Wave InP Based Photonic-Crystal Surface-Emitting Lasers at 2 µm

Light is a powerful tool for seeing the world, but not all light is created equal. For decades, scientists have engineered lasers that fire beams of light in specific colors, or wavelengths, to perform tasks ranging from reading barcodes to guiding autonomous vehicles. Most common lasers operate in the near-infrared range, a band of light just beyond what the human eye can see. However, there is a different region of the spectrum, stretching from two to two and a half micrometers, that offers unique advantages. This range, known as the extended shortwave infrared, passes through the atmosphere with remarkable clarity, making it ideal for long-distance sensing and communication. Crucially, this specific color of light is also safe for human eyes because the eye's internal fluids absorb it before it can reach the sensitive retina. To harness these benefits, researchers need lasers that are not only powerful but also emit a single, pure color of light without spreading out into a messy beam.

A team of researchers at The Ohio State University, working alongside colleagues from the Air Force Research Laboratory and national laboratories, has taken a significant step toward creating these specialized lasers. Their work focuses on a new type of laser design called a photonic-crystal surface-emitting laser. Unlike traditional lasers that shoot light out the side or from the top in a way that often creates a wide, diverging beam, these devices use a microscopic grid of holes etched into the semiconductor material. This grid acts like a mirror that forces the light to travel in a single direction and emit straight up from the surface, creating a tight, high-quality beam. While similar devices have been built for shorter wavelengths, the researchers set their sights on the difficult two-micrometer range. They chose to build these lasers using a material called indium phosphide, a choice that required them to prove it could perform as well as, or better than, the more common gallium antimonide material used in previous attempts.

The journey began with a careful comparison of the two material options. The researchers used advanced computer models to simulate how light would behave inside lasers made from either indium phosphide or gallium antimonide. They found that while gallium antimonide produced slightly stronger light at lower power levels, indium phosphide offered a distinct advantage when the laser was pushed harder. As the electrical current increased, the indium phosphide design maintained its focus on the target two-micrometer wavelength, whereas the other material began to shift its color. Furthermore, the researchers conducted separate simulations to calculate how heat would move through the layers of the laser. They discovered that while the indium phosphide structure conducted heat through its internal layers more efficiently than its competitor—due to the indium phosphide cladding having a thermal conductivity of 68 W/(m·K) compared to 32 W/(m·K) for gallium antimonide—the overall temperature rise in the device is primarily determined by the packaging and the interface where the laser is mounted, rather than the semiconductor layers themselves. Consequently, the gallium antimonide device showed a slightly higher junction temperature, with a difference of up to 7.9°C at high power loads, but the thermal management challenge for both platforms is largely dictated by the mounting interface rather than the material choice alone.

With the material choice confirmed, the team turned to the physical design of the laser. They created a microscopic pattern of air holes, arranged in a square grid, etched into a thin layer of the semiconductor. The size and spacing of these holes were calculated to resonate with light at exactly two micrometers. A key challenge in building these devices is the process of regrowing the semiconductor material over the etched holes. If the material does not fill the holes perfectly, the laser's performance suffers. The researchers modeled two scenarios: one where the holes were filled with perfectly vertical walls, and another where the walls were slightly slanted, resembling an inverted teardrop shape, which is what actually happens during the manufacturing process. Their simulations showed that even with these realistic, slightly slanted walls, the laser would still function correctly, with the light naturally selecting the strongest, most stable mode to emit. This gave them confidence that the design was robust enough for actual fabrication.

To prove their design worked, the team built and tested standard edge-emitting lasers using the same indium phosphide material and active region they planned to use for the final surface-emitting device. These test lasers were not the final product but served as a crucial proof of concept. When powered on, these devices successfully emitted light at a wavelength of 2061 nanometers, confirming that the material could indeed generate the desired color. The lasers operated continuously without interruption, a demanding test for any semiconductor device. At room temperature, they produced a steady output of 42.5 milliwatts of power. As the temperature rose, the lasers remained stable, though their efficiency dropped slightly, a behavior that is typical for this type of technology. The team also measured how the laser's color shifted as the temperature changed, finding a predictable relationship that allowed them to calculate the device's thermal resistance, a measure of how well it could handle heat.

The results of this work provide a clear roadmap for the future of two-micrometer lasers. By demonstrating that indium phosphide can support high-power, continuous-wave operation at this wavelength, the researchers have validated a material platform that is easier to manufacture and offers favorable thermal properties compared to the alternatives. They have shown that the microscopic engineering required to create these surface-emitting lasers is feasible, even with the imperfections inherent in the manufacturing process. While the team has not yet built the final single-mode surface-emitting laser at two micrometers, the successful testing of the underlying components and the thorough validation of the design through simulation and preliminary experiments suggest that such a device is within reach. This progress brings the scientific community closer to having compact, powerful, and eye-safe light sources capable of operating in the challenging but highly useful extended shortwave infrared spectrum.