Quantum Computing and Communication Advance with Green Laser Breakthrough

Scientists from the National Institute of Standards and Technology (NIST) have addressed the long-standing “green gap” by modifying a tiny optical component, a ring-shaped microresonator, small enough to fit on a chip. The study was published in the journal Light: Science and Applications.

For years, scientists have successfully created tiny, high-quality lasers that emit red and blue light by injecting electricity into semiconductors. However, this method has been less effective for generating small lasers that emit green and yellow wavelengths, a challenge referred to as the "green gap." Bridging this gap opens up new possibilities in various fields, such as underwater communication, medical treatments, and advanced optical devices.

While green laser pointers have existed for 25 years, their light output is restricted to a narrow range of green wavelengths, and they are not integrated into chips that could allow them to function alongside other devices for practical applications.

Small green lasers are particularly valuable for underwater communication systems, as most aquatic environments are nearly transparent to blue-green wavelengths. Other potential applications include full-color laser projection displays and medical treatments, such as those for diabetic retinopathy, which is characterized by an increase in blood vessels in the eyes.

Compact lasers in the green and yellow wavelength range are also important for quantum computing and communication, where they can store data in qubits, the fundamental units of quantum information. However, the current reliance on larger, heavier, and more powerful lasers limits the practical use of these quantum applications outside of laboratories.

For several years, a group headed by Kartik Srinivasan of NIST and the Joint Quantum Institute (JQI), a collaboration between NIST and the University of Maryland, has been utilizing silicon nitride microresonators to change the color of infrared laser light. Light infrared is blasted thousands of times around the ring-shaped resonator until it reaches intensities high enough to interact with silicon nitride.

This interaction, known as optical parametric oscillation (OPO), produces two additional light wavelengths: the idler and the signal.

In previous studies, the researchers successfully generated a few different colors of visible laser light, including red, orange, and yellow, as well as a wavelength of 560 nm, which sits right at the boundary between yellow and green light. However, they were unable to produce the full range of green and yellow shades necessary to completely close the green gap.

We did not want to be good at hitting just a couple of wavelengths. We wanted to access the entire range of wavelengths in the gap.

Yi Sun, Study Collaborator and Scientist, National Institute of Standards and Technology

The research team made two key modifications to the microresonator to close the green gap. First, they slightly increased its thickness. By adjusting the microresonator's size, the scientists were able to more easily produce light that passed through the green gap, reaching wavelengths as low as 532 nm. This adjustment allowed them to cover the entire green gap spectrum.

The scientists also removed a portion of the silicon dioxide layer beneath the microresonator through etching, exposing it to additional air. This change reduced the sensitivity of the output colors to variations in the infrared pump wavelength and the microring diameters. As a result, the researchers gained greater control over producing slightly different green, yellow, orange, and red wavelengths, which allowed them to generate and fine-tune more than 150 distinct wavelengths across the green gap.

Previously, we could make big changes—red to orange to yellow to green—in the laser colors we could generate with OPO, but it was hard to make small adjustments within each of those color bands.

Kartik Srinivasan, National Institute of Standards and Technology

The team is now working to improve the energy efficiency of the green-gap laser colors. Currently, the output power is only a small fraction of the laser's input power. By enhancing the way light is extracted from the microresonator and improving the coupling between the input laser and the waveguide that directs light into the resonator, the researchers hope to achieve a significant increase in efficiency.

The research was co-authored by Jordan Stone and Xiyuan Lu from JQI, along with Zhimin Shi from Meta’s Reality Labs Research in Redmond, Washington.

Journal Reference:

Sun, Y., et al. (2024) Advancing on-chip Kerr optical parametric oscillation towards coherent applications covering the green gap. Light Science & Applications. doi.org/10.1038/s41377-024-01534-x.