Most people have seen a green laser pointer. So it may be surprising to hear that achieving high-quality, compact sources of laser light in the spectral region between the blue and the red has been notoriously difficult—until now. Finally filling the visible light “green gap” for lasers, researchers at the National Institute of Science and Technology (NIST) have developed a compact device that generates the full spectrum of orange, yellow, and green wavelengths. The technology creates opportunities for advances in medical devices, quantum information technologies, and underwater communications.

For decades, the main approach for making compact visible-wavelength lasers has been injecting an electrical current into a semiconductor material, where it gets converted into optical energy that is released as photons. The wavelength of this light depends on the specific semiconductor material. One family of materials is very good in the blue range, but it becomes challenging to reach longer, greener wavelengths; the other family of materials is very good in the red range, but it struggles to get to shorter, yellow-orange wavelengths.

Many materials-based efforts have sought to create high-quality lasers in the elusive orange-yellow-green region, but despite progress, achieving a spectrally pure laser emission that can access the full range of wavelengths between blue and green remained a daunting challenge.

“For many applications that we have in mind, you not only want a laser at certain wavelengths in the green gap, but you want that laser to be high-performance, particularly in terms of narrow linewidth,” says Kartik Srinivasan, a NIST Fellow in the Photonics and Optomechanics Group in the Physical Measurement Laboratory, in Gaithersburg, Md. Instead of trying to develop a new semiconductor laser, he and his colleagues took a different approach: They used existing semiconductor laser wavelengths, such as 780 nanometers (infrared), and focused on using chip-integrated nonlinear optical processes to generate colors across the green gap.

How to Make a Green Laser

Srinivasan’s group works on silicon nitride optical devices that use a pump laser at a central wavelength to stimulate a nonlinear optical process that generates two different output wavelengths. In their setup, the near-infrared beam travels through a waveguide and enters a ring-shaped microresonator. The light circles the ring thousands of times, increasing in intensity—and also splitting into two different wavelengths, which then exit the ring via a coupled output waveguide. Due to a precise energy-conservation relationship that holds that twice the input frequency must equal the sum of the two output frequencies, one output wavelength is shorter and one is longer than the near-infrared pump. The precise frequencies that get amplified are a carefully tuned property of the resonator geometry and material.

In previous work, the team had demonstrated this process by fabricating waveguides on a silicon surface to make an integrated photonic chip (the specific waveguide material used is silicon nitride). They generated on-chip output-beam powers of a few milliwatts each, at conversion efficiencies of 15 to 20 percent. In general, the design offers two main “knobs” for adjusting the output colors. One is the device geometry, which can be varied across many devices on a single chip. The second is the pump wavelength. “For a given geometry, pump-wavelength tuning can let us access different wavelength bands, and by pumping different devices, we can fill in the green gap,” says Srinivasan.

The researchers have now improved their design to output wavelengths that cover the entirety of the green gap. To do so, they made two modifications to previous resonator geometries. First, they made the microresonators slightly thicker, which increased the overall spectral extent that they could reach. In particular, this allowed them to generate light at shorter wavelengths than before.

Next, the researchers added a chemically selective undercut etching beneath the ring, which made the specific output wavelengths slightly less sensitive to geometry variations. “This means that we not only could generate a few wavelengths within the green gap, but we could create more than 150 wavelengths within this spectral region,” says Srinivasan. That combination should provide the ability to hit any desired wavelength.

Beautiful Oranges, Yellows, and Greens

The immediate indication that their device worked in the lab, according to Xiyuan Lu, another of the researchers, was the creation of “beautiful orange, yellow, and green colors” that shifted in hue while the researchers adjusted the pump wavelength and device geometry. “The nice thing about this wavelength region is that the generated colors are very striking and quite different than our input laser color,” he says. More-precise spectral analysis pinpointed the generated wavelengths and emission linewidths, which matched model predictions.

The researchers say that the design methodology—particularly when combined with other elements, like efficient thermal heaters that continuously tune the output wavelength and methods to hit target wavelengths extremely accurately—will be used in the many fields where green lasers have been sought: marine environments, where water is nearly transparent to blue-green wavelengths; ophthalmology, where green wavelengths interact with diabetes-causing cells in the outer retina; and quantum computing, where information-storage applications still depend on bulky and power-consuming laser sources.

“I’m really excited about the ability to use nonlinear integrated photonics to augment existing compact laser sources,” says Srinivasan, since it makes accessible a much broader range of wavelengths than currently available.

The work was published in August in the journal Light: Science & Applications.