Tiny Green Lasers: Brighter and Better Than Ever

tiny green lasers Better

NIST researchers have achieved a significant advancement in laser technology by bridging the so-called “green gap” with innovative microresonators that now produce a wide spectrum of green light.

For many years, the creation of small red and blue lasers has been successful, but the development of other colors has posed challenges.

Now, scientists have addressed this critical technological gap by fabricating orange, yellow, and green lasers that are compact enough to be integrated onto a chip.

These low-noise, miniature lasers within this specific wavelength range hold immense potential for quantum sensing, communication, and information processing.

Closing the Green Gap: Laser Technology Breakthroughs

Creating green lasers has not been straightforward. While small, high-quality red and blue lasers have been manufactured successfully for years, the conventional method of injecting electric current into semiconductors has proven less effective for producing tiny lasers that emit light at yellow and green wavelengths.

This scarcity of stable, small-scale lasers in this portion of the visible spectrum is known as the “green gap.”

Bridging this gap not only represents a technological triumph but also unlocks new possibilities in fields such as underwater communication and medical treatment.

Although green laser pointers have been available for 25 years, they typically emit light in only a narrow range of green wavelengths and are not suitable for integration into chips where they could function alongside other devices to perform useful tasks.

Advancements in Optical Technology

Researchers at the National Institute of Standards and Technology (NIST) have successfully closed the green gap by refining a small optical component: a ring-shaped microresonator, compact enough to be placed on a chip.

This advancement in optical technology promises to enhance underwater communication, as water is nearly transparent to blue-green wavelengths in most aquatic settings.

Additional applications include full-color laser projection displays and laser-based medical treatments, such as for diabetic retinopathy, which involves the proliferation of blood vessels in the eye.

Elevating Quantum Computing with Green Lasers

Miniaturized lasers in this wavelength range are also crucial for applications in quantum computing and communication, where they could be used to store data in qubits, the fundamental units of quantum information. Currently, these quantum applications rely on larger, bulkier lasers, which limits their use outside of controlled laboratory environments.

Over the past few years, a team led by Kartik Srinivasan from NIST and the Joint Quantum Institute (JQI)—a collaboration between NIST and the University of Maryland—has utilized microresonators made from silicon nitride to transform infrared laser light into other colors.

By directing infrared light into the ring-shaped resonator, the light circulates thousands of times until it reaches intensities that cause a strong interaction with the silicon nitride.

This interaction, termed optical parametric oscillation (OPO), results in the generation of two new wavelengths of light, known as the idler and the signal.

Refining Laser Production Methods

In earlier studies, the researchers were able to generate a few specific colors of visible laser light. Depending on the microresonator’s dimensions, which dictate the colors of light produced, they were able to create red, orange, and yellow wavelengths, as well as a wavelength of 560 nanometers, which sits on the borderline between yellow and green light.

However, they were not able to produce the complete spectrum of yellow and green colors needed to fully address the green gap.

“We didn’t want to be proficient at just a few wavelengths,” stated NIST scientist Yi Sun, a contributor to the new study. “We aimed to cover the entire range of wavelengths within the gap.”

To achieve this, the team made two key modifications to the microresonator. First, they slightly increased its thickness, which enabled the generation of light at wavelengths as short as 532 nanometers, thus penetrating deeper into the green gap. This extended range allowed the researchers to cover the entire gap.

Additionally, they exposed more of the microresonator to air by etching away some of the underlying silicon dioxide layer.

This process made the output colors less dependent on the microring dimensions and the infrared pump wavelength, granting the researchers greater control over generating a wider range of wavelengths, including various shades of green, yellow, orange, and red.

As a result, the researchers succeeded in creating over 150 distinct wavelengths across the green gap, with the ability to fine-tune each one.

“Previously, we could make broad adjustments—from red to orange to yellow to green—in the laser colors generated by OPO, but it was difficult to make fine adjustments within each color band,” noted Srinivasan.

The team is now focused on improving the energy efficiency of the green-gap laser colors they produce. Currently, the output power is only a small fraction of the input laser’s power. Enhanced coupling between the input laser and the waveguide that directs light into the microresonator, along with better techniques for extracting the generated light, could substantially boost efficiency.

The findings of this research, which include contributions from Jordan Stone and Xiyuan Lu of JQI, as well as Zhimin Shi from Meta’s Reality Labs Research in Redmond, Washington, were published online on August 21 in the journal Light: Science and Applications.

Reference

“Advancing on-chip Kerr optical parametric oscillation towards coherent applications covering the green gap” by Yi Sun, Jordan Stone, Xiyuan Lu, Feng Zhou, Junyeob Song, Zhimin Shi, and Kartik Srinivasan, 21 August 2024, Light: Science & Applications. DOI: 10.1038/s41377-024-01534-x.