Miniature Lasers Fill The "Green Gap"

Researchers in the United States have demonstrated a chip-based platform that is capable of creating and tuning laser light at all wavelengths across the so-called green gap (Light Sci. Appl., doi: 10.1038/s41377-024-01534-x). The design offers a high-performing alternative to green laser diodes, which lack the power and stability of their red and blue counterparts, and provides a more compact and versatile solution than laser sources that produce green light through frequency doubling.

Infrared to visible

The approach developed by the team, led by Kartik Srinivasan at the National Institute of Standards and Technology (NIST), exploits a ring-shaped microresonator made from silicon nitride to convert infrared light into visible wavelengths. As the infrared light circulates around the resonator, it reaches the high intensities needed to drive nonlinear interactions through third-order optical parametric oscillation―in which two pump photons combine to produce light at both a longer infrared wavelength, called the idler, and the targeted wavelength in the visible range.

In previous studies, the researchers used this scheme to generate a series of distinct colors, but chromatic dispersion within the silicon nitride prevented access to wavelengths shorter than 560 nm at the yellow–green boundary. The specific output color was also extremely sensitive to the exact dimensions of the microresonator, presenting a fabrication challenge for creating reproducible chip-level devices.

Crossing the boundary

In this new work, the researchers modified their design to reduce the impacts of dispersion. They found that a small increase in the height of the resonator makes it possible to widen the separation between the idler and the desired signal, allowing them to produce green wavelengths down to 532 nm. They also etched away some of the dielectric substrate to expose part of the bottom edge of the resonator to the air, which made the output wavelength less sensitive to the specific geometry of the microring.

With these modifications, the researchers showed that more than 150 wavelengths throughout the green gap can be created by using just four devices with slightly different dimensions. They also demonstrated continuous tuning of the output wavelength through small changes to the frequency of the pump laser, as well as the narrow linewidths that are needed for coherent applications. While the system is not yet optimized to achieve high conversion rates, the team believes that existing light-coupling strategies could be deployed to achieve more efficient green emission.