Engineering of Dual Wavelength, Polarization Selective Metalenses in Silicon Carbide

Quantum technology often feels like a distant future, but its building blocks are already taking shape in the solid materials around us. At the heart of this emerging field are tiny imperfections inside crystals, known as defects. Imagine a perfect diamond or a block of silicon carbide as a vast, orderly city of atoms. Occasionally, an atom is missing, or a different kind of atom has moved into the wrong spot. These missing or misplaced spots are the defects. While they might sound like flaws, in the world of quantum physics, they are actually powerful tools. These defects can trap electrons in specific states that act like tiny magnets, holding information for long periods. They can also be made to glow with light, and the color of that light depends on the specific type of defect. Scientists are eager to use these glowing defects to build quantum computers and sensors, but there is a significant hurdle: the material that holds them is often so dense with atoms that the light they emit gets trapped inside, bouncing around until it fades away. To make these quantum devices useful, researchers must find a way to pull that trapped light out and guide it exactly where it needs to go.

A team of researchers has now developed a solution that integrates directly into the material itself, creating a compact optical interface that can catch and steer light from two different types of quantum defects at the same time. Working with silicon carbide, a hard, durable crystal used in everything from power electronics to quantum research, the team designed and built a specialized surface called a metalens. Unlike traditional glass lenses that are thick and curved, a metalens is a flat, microscopic pattern etched directly onto the surface of the material. This new device is unique because it is monolithic, meaning it is carved from the same block of silicon carbide as the defects it is trying to catch, rather than being a separate piece glued on top. The researchers engineered this surface to work with two very specific colors of light: one at 860 nanometers and another at 1240 nanometers. These colors correspond to the glow of two different quantum defects, the silicon vacancy and the nitrogen vacancy, which are promising candidates for quantum communication.

The challenge the team faced was not just collecting the light, but doing so while also sorting it based on how the light waves are oriented, a property known as polarization. Light waves can vibrate in different directions, and for quantum information to be read correctly, these directions must be controlled. The researchers designed their metalens so that it would treat light vibrating horizontally differently from light vibrating vertically. When light hits the surface, the tiny pillars that make up the metalens pattern bend the waves in specific ways. For the 860-nanometer light, the lens directs horizontally vibrating waves to one spot and vertically vibrating waves to a slightly different spot. It does the same for the 1240-nanometer light, but with a different pattern of pillars. This allows the device to act as a traffic controller, separating the different types of information carried by the light without needing bulky external equipment.

To test their creation, the team first simulated how the light would behave using powerful computer models. These simulations showed that the design could indeed separate the different polarizations and focus the light effectively. They then moved to the laboratory to build the device. Using a process similar to printing a circuit board but with far greater precision, they used a beam of electrons to draw the pattern of tiny pillars onto the silicon carbide block. They then etched away the material to leave behind a forest of microscopic pillars, each standing about 1.6 micrometers tall. The final device looked like a flat square divided into two sections, one optimized for the shorter wavelength and the other for the longer one. When they examined the finished product under a microscope, the pillars were uniform and smooth, confirming that the manufacturing process had succeeded in creating the complex structures required.

The true test came when they introduced the quantum defects into the silicon carbide. They used a beam of nitrogen ions to implant atoms into the crystal, creating the vacancies needed for the defects to form. Once the defects were in place, they shone a laser on the back of the crystal to make the defects glow. The light traveled through the crystal and hit the metalens on the front. The device successfully collected the light and focused it into distinct spots, proving that it could pull the photons out of the dense crystal and direct them into free space. The researchers measured how much light made it through and found that the metalens captured about half of the available light. While this is lower than what a perfect lens might achieve, it is a significant improvement over having no lens at all, where most of the light would be trapped inside the crystal due to its high density.

Beyond simply collecting light, the team demonstrated that the device could be used to read the quantum state of the defects. They placed the sample in a magnetic field and measured how the light intensity changed as they tuned the magnetic field. This technique, known as optically detected magnetic resonance, allows scientists to see the spin state of the electrons inside the defects. The metalens successfully collected the signal from both the silicon vacancy and the nitrogen vacancy defects, showing clear peaks in the data that matched what is expected from these quantum systems. The fact that the device could distinguish between the two different colors of light and handle the polarization of each meant that it could potentially read multiple quantum bits simultaneously. This capability is a crucial step toward building scalable quantum networks, where many quantum devices need to communicate with each other through a single, compact interface.

The work establishes that it is possible to carve complex, multi-functional optical tools directly into the very material that houses the quantum sources. By operating at two distinct wavelengths and managing polarization, the metalens offers a way to simplify the optical systems needed for quantum technologies. Instead of needing separate lenses and filters for every different type of defect, a single, flat surface can handle the job. The researchers showed that this approach works not just in theory or simulation, but in a real, fabricated device that can collect light and read quantum states. This integration of the optical interface with the quantum source suggests a path forward for creating smaller, more efficient quantum devices that could eventually be used in everything from ultra-secure communication networks to highly sensitive magnetic sensors.