Tiny new lasers fill a long-standing gap in the rainbow of visible light colors, opening the way to new applications

Making green lasers isn’t easy. Scientists have been making small, high-quality lasers that generate red and blue light for years. But the method they usually use—injecting electrical current into semiconductors—doesn’t work as well for building tiny lasers that emit light in the yellow and green wavelengths.

Researchers call the shortage of stable miniature lasers in this region of the visible light spectrum a “green hole.” Filling this hole opens new possibilities in underwater communications, medical treatments and more.

Green laser pointers have been around for 25 years, but they produce light only in a narrow green spectrum and aren’t integrated into chips where they could work with other devices to perform useful tasks.

Scientists at the National Institute of Standards and Technology (NIST) have now managed to fill this ecological gap by modifying a tiny optical component: a ring-shaped microresonator, small enough to fit on a chip. The research is published in the journal Light: science and applications.

A miniature source of green laser light could improve underwater communication, because water is nearly transparent to blue-green wavelengths in most aquatic environments. Other potential applications include color laser projection screens and laser treatment of diseases including diabetic retinopathy, a proliferation of blood vessels in the eye.

Compact lasers in this wavelength range are also important for quantum computing and communications applications, as they could potentially store data in qubits, the fundamental unit of quantum information. Currently, these quantum applications rely on lasers of larger size, weight, and power, which limits their ability to be deployed outside the laboratory.

For several years, a team led by Kartik Srinivasan of NIST and the Joint Quantum Institute (JQI), a research partnership between NIST and the University of Maryland, has been using microresonators made of silicon nitride to convert infrared laser light into other colors. When infrared light is pumped into the ring-shaped resonator, the light rotates thousands of times until it reaches intensities high enough to interact strongly with the silicon nitride. This interaction, known as optical parametric oscillation (OPO), produces two new wavelengths of light, called idler and signal.

In previous studies, the researchers generated a few individual colors of visible laser light. Depending on the dimensions of the microresonator, which determine the colors of light generated, the scientists produced red, orange, and yellow wavelengths, as well as a 560-nanometer wavelength, right on the border between yellow and green light. However, the team was unable to generate all of the yellow and green colors needed to fill the green gap.

“We didn’t want to just look at a few wavelengths,” says Yi Sun, a NIST scientist and collaborator on the new study. “We wanted to access the entire range of wavelengths in the interval.”

To fill this gap, the team modified the microresonator in two ways. First, the scientists made it slightly thicker. By changing its dimensions, the researchers were able to more easily generate light that penetrated deeper into the green gap, at wavelengths as short as 532 nanometers (billionths of a meter). With this extended range, the researchers were able to cover the entire gap.

Additionally, the team exposed the microresonator to more air by removing some of the silicon dioxide layer underneath. This had the effect of making the output colors less sensitive to the dimensions of the microring and the wavelength of the infrared pump. The lower sensitivity gave the researchers more control to generate slightly different green, yellow, orange, and red wavelengths from their device.

The researchers found that they could create more than 150 distinct wavelengths in the green band and fine-tune them. “Previously, we could make big changes (from red to orange, from yellow to green) in the laser colors we could generate with the OPO, but it was difficult to make small adjustments within each of these color bands,” Srinivasan noted.

Scientists are currently working to improve the energy efficiency with which they produce green-space laser colors. Currently, the output power is only a few percent of that of the input laser. Better coupling between the input laser and the waveguide that channels the light into the microresonator, as well as better methods for extracting the generated light, could significantly improve efficiency.

Authors include Jordan Stone and Xiyuan Lu of JQI, as well as Zhimin Shi of Meta’s Reality Labs Research in Redmond, Washington.