On-chip lasers that emit green light are notoriously difficult to make. But researchers at the National Institute of Standards and Technology (NIST) and the NIST/University of Maryland Joint Quantum Institute may now have found a way to do just this, using a modified optical component known as a ring-shaped microresonator. Green lasers are important for applications including quantum sensing and computing, medicine and underwater communications.

In the new work, a research team led by Kartik Srinivasan modified a silicon nitride microresonator such that it was able to convert infrared laser light into yellow and green light. The researchers had already succeeded in using this structure to convert infrared laser light into red, orange and yellow wavelengths, as well as a wavelength of 560 nm, which lies at the edge between yellow and green light. Previously, however, they were not able to produce the full range of yellow and green colours to fill the much sought-after “green gap”.

More than 150 distinct green-gap wavelengths

To overcome this problem, the researchers made two modifications to their resonator. The first was to thicken it by 100 nm so that it could more easily generate green light with wavelengths down to 532 nm. Being able to produce such a short wavelength means that the entire green wavelength range is now covered, they say. In parallel, they modified the cladding surrounding the microresonator by etching away part of the silicon dioxide layer that it was fabricated on. This alteration made the output colours less sensitive to the dimension of the microring.

These changes meant that the team could produce more than 150 distinct green-gap wavelengths and could fine tune these too. “Previously, we could make big changes – red to orange to yellow to green – in the laser colours we could generate with OPO [optical parametric oscillation], but it was hard to make small adjustments within each of these colour bands,” says Srinivasan.

Like the previous microresonator, the new device works thanks to a process known as nonlinear wave mixing. Here, infrared light that is pumped into the ring-shaped structure is confined and guided within it. “This infrared light circulates around the ring hundreds of times due to its low loss, resulting in a build-up of intensity,” explains Srinivasan. “This high intensity enables the conversion of pump light to other wavelengths.”

Third-order optical parametric oscillation

“The purpose of the microring is to enable relatively modest, input continuous-wave laser light to build up in intensity to the point that nonlinear optical effects, which are often thought of as weak, become very significant,” says team member Xiyuan Lu.

The specific nonlinear optical process the researchers use is third-order optical parametric oscillation. “This works by taking light at a pump frequency n_p and creating one beam of light that’s higher in frequency (called the signal, at a frequency n_s) and one beam that’s lower in frequency (called the idler, at a frequency n_i),” explains first author Yi Sun. “There is a basic energy conservation requirement that 2n_p= n_s+ n_i.”

Simply put, this means that for every two pump photons that are used to excite the system, one signal photon and one idler photon are created, he tells Physics World.

Towards higher power and a broader range of colours

The NIST/University of Maryland team has been working on optical parametric oscillation as a way to convert near-infrared laser light to visible laser light for several years now. One of their main objectives was to fill the green gap in laser technology and fabricate frequency-converted lasers for quantum, biology and display applications.

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“Some of the major applications we are ultimately targeting are high-end lasers, continuous-wave single-mode lasers covering the green gap or even a wider range of frequencies,” reveals team member Jordan Stone. “Applications include lasers for quantum optics, biology and spectroscopy, and perhaps laser/hologram display technologies.”

For now, the researchers are focusing on achieving higher power and a broader range of colours (perhaps even down to blue wavelengths). They also hope to make devices that can be better controlled and tuned. “We are also interested in laser injection locking with frequency-converted lasers, or using other techniques to further enhance the coherence of our lasers,” says Stone.

The work is detailed in Light: Science & Applications.