It’s not easy being green. For many years, scientists have successfully created compact red and blue lasers, but developing lasers in other colors, especially green, has been challenging. A new research team has tackled this issue by developing tiny lasers that emit orange, yellow, and green light, which can be incorporated onto a chip. This advancement opens up numerous possibilities in quantum sensing, communications, and information processing.
Making green lasers has been a difficult task.
For a long time, researchers have produced small, high-quality lasers that emit red and blue light. Nonetheless, the standard approach of injecting electric current into semiconductors hasn’t been as effective for creating tiny lasers that generate yellow and green light. This scarcity of stable, miniature lasers in the green spectrum is referred to as the “green gap.” Bridging this gap paves the way for new opportunities in underwater communication, medical therapies, and more.
Though green laser pointers have been around for 25 years, they emit a limited range of green light and lack integration on chips, where they could work synergistically with other devices for practical applications.
Researchers at the National Institute of Standards and Technology (NIST) have now addressed the green gap by modifying a small optical component: a ring-shaped microresonator that is tiny enough to be placed on a chip.
A small source of green laser light can enhance underwater communication, as blue-green wavelengths can transmit effectively through water in many marine settings. Additionally, such lasers could be beneficial in creating full-color laser projection displays and in treating various medical conditions like diabetic retinopathy, which involves the abnormal growth of blood vessels in the eye.
Furthermore, compact lasers operating in this wavelength region are crucial for quantum computing and communication, potentially enabling data storage in qubits, the essential units of quantum information. Currently, existing quantum technologies rely on larger, heavier, and power-hungry lasers, restricting their deployment outside laboratory environments.
For several years, a team led by Kartik Srinivasan from NIST and the Joint Quantum Institute (JQI) has been utilizing silicon nitride microresonators to change infrared laser light into other colors. When infrared light is introduced into the ring-shaped resonator, the light circulates thousands of times until it reaches high intensities, allowing it to interact more vigorously with the silicon nitride. This process, known as optical parametric oscillation (OPO), results in the generation of two additional wavelengths of light, termed the idler and the signal.
In earlier research, the team had managed to produce a select few colors of visible laser light. Depending on the size of the microresonator, which dictates the colors created, they achieved red, orange, and yellow wavelengths, along with a wavelength of 560 nanometers, right near the boundary between yellow and green light. However, the complete spectrum of yellow and green necessary to fully bridge the green gap had not been achieved.
“Our goal was not just to generate a couple of wavelengths effectively,” said NIST scientist Yi Sun, a collaborator on the current project. “We aimed to cover the entire range of wavelengths in the gap.”
To successfully fill the gap, the researchers made two modifications to the microresonator. They first slightly thickened it. This alteration allowed for the production of light that reached deeper into the green gap, achieving wavelengths as short as 532 nanometers (billionths of a meter). With this expanded range, they successfully covered the whole gap.
Additionally, the team increased the microresonator’s exposure to air by etching away some of the silicon dioxide beneath it. This change made the output colors less impacted by the dimensions of the microring and the infrared pump wavelength, giving the team greater control over the generation of various green, yellow, orange, and red wavelengths from their device.
As a result, the researchers were thrilled to discover they could create more than 150 distinct wavelengths in the green gap and adjust them finely. “In our previous work, we could switch between big color shifts — from red to orange to yellow to green — but fine-tuning within those specific color bands was quite challenging,” noted Srinivasan.
Currently, the scientists are focused on enhancing the energy efficiency of generating laser colors in the green gap. At present, the output power is only a small fraction of the input laser power. Enhancements in coupling the input laser with the waveguide channeling the light into the microresonator, along with improved extraction methods for the generated light, could lead to significant efficiency gains.
The team, including Jordan Stone and Xiyuan Lu from JQI and Zhimin Shi from Meta’s Reality Labs Research in Redmond, Washington, shared their findings online on August 21 in Light: Science and Applications.
