By adding tiny, periodic bumps to a microresonator, researchers were able to convert near-infrared (NIR) laser light into specific desired wavelengths of visible light with high accuracy and efficiency. Developed by the National Institute of Standards and Technology (NIST) and its colleagues at the Joint Quantum Institute (JQI), a research partnership between the University of Maryland and NIST, the technique has potential applications in precision timekeeping and quantum information science, which require highly specific wavelengths of visible laser light that cannot always be achieved with diode lasers. Ideally, the wavelengths should be generated in a compact device, such as a photonic chip, so that quantum sensors and optical atomic clocks can be deployed outside the laboratory, without being tethered to bulky optical equipment. In previous experiments, NIST researcher Kartik Srinivasan and his colleagues used smooth microresonators, which are ring-shaped devices with a diameter about one-quarter the thickness of a human hair, to transform a single wavelength of NIR light into two other wavelengths. The resonator, which is small enough to fit on a microchip, can be designed so that one of the two output wavelengths falls within the spectrum of visible light. The transformation occurs when the NIR laser light, confined to circle the ring-shaped resonator thousands of times, reaches intensities high enough to strongly interact with the resonator material. NIST’s earlier experiments used ring microresonators that would split NIR laser light into both long and short wavelengths. Courtesy of S. Kelley/NIST. In theory, by choosing a particular radius, width, and height of the resonator, which determine the properties of the light that can resonate in the ring, researchers can select any among a rainbow of colors possible with the technique. In practice, however, the method, known as optical parametric oscillation (OPO), is not always so precise. Deviations as small as a few nanometers from the specified dimensions of the micro-ring produce visible-light colors that differ significantly from the desired output wavelength. As a result, researchers previously had to fabricate as many as 100 of the silicon nitride micro-rings to be confident that at least some would have the right dimensions to generate the target wavelength. That being said, it still does not guarantee success. Now, Srinivasan and his collaborators, led by Jordan Stone of JQI, have demonstrated that by introducing imperfections — tiny, periodic corrugations, or bumps — along the surface of a microresonator, they can select a specific output wavelength of visible light to an accuracy of 99.7%. With improvements, Stone said, the technique should produce visible-light wavelengths accurate to better than 99.9% of their target values, a requirement for powering optical atomic clocks and other high-precision devices. “In our previous experiments, we reached the general range of a wavelength of interest, but for many applications that isn’t good enough. You really have to nail the wavelength to a high degree of accuracy,” Stone said. “We now achieve this accuracy by incorporating a periodic arrangement of corrugations on a micro-ring resonator.” The principle governing the optical transformation of a single-wavelength input into two outputs of different wavelengths is the law of conservation of energy. The energy carried by two of the input photons from the NIR laser must equal the energy carried by the output photons: one with a shorter wavelength and one with longer wavelength. In this case, the shorter wavelength is visible light. In addition, each of the input and output wavelengths must correspond to one of the resonant wavelengths permitted by the dimensions of the micro-ring, just as the length of a tuning fork determines the note at which it resonates. In their new study, the researchers designed a micro-ring whose dimensions, without corrugations, would not have allowed the photons to resonate in the ring and produce new wavelengths because the process would not have conserved energy. However, when the team sculpted the ring with tiny, periodic corrugations, altering its dimensions, it allowed OPO to proceed, transforming the NIR laser light into a specific wavelength of visible light plus one other, significantly longer, wavelength. These OPO-generated colors, unlike those previously created by smooth micro-rings, can be precisely controlled by the spacing and width of the bumps. The corrugations act like tiny mirrors that collectively reflect back and forth visible light racing around the ring, but only for one particular wavelength. The reflections result in two identical waves traveling around the ring in opposite directions. Inside the ring, the counterpropagating waves interfere with each other to create a pattern known as a standing wave, a waveform whose peaks remain fixed at a particular point in space as the wave vibrates, like a plucked guitar string. The current design for the experiment includes corrugations cut into the ring that act as mirrors. The corrugations reflect wavelengths of visible light that are exactly twice the periodicity of the corrugations so that it circulates in both directions around the ring and produces the desired color. Courtesy of S. Kelley/NIST. This translates into a shift toward a longer or shorter wavelength, depending on whether the standing wave interacts more with the peaks or troughs of the corrugations. In both cases, the magnitude of the shift is determined by the height of the bump. Because the bumps act only as a mirror for a specific wavelength of light, the approach guarantees that when OPO occurs, the generated signal wave has the exact desired wavelength. By slightly altering the wavelength of the infrared laser that drives the OPO process, any imperfections in the corrugations can be compensated for, Stone said. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01326-6).