The quest for the elusive green laser has challenged researchers for years, yet groundbreaking developments in the field may finally resolve this long-standing dilemma. While the journey towards creating stable, miniature lasers that successfully emit green and yellow light has been fraught with difficulties, recent innovations led by scientists at the National Institute of Standards and Technology (NIST) may pave the way for a wide array of applications, ranging from medical technologies to quantum computing. This article examines the scientific breakthroughs that have closed the gap in the spectrum and explores the implications of these advancements.
For decades, scientists have made significant strides in developing lasers that yield red and blue light; however, achieving similar success in the green and yellow wavelengths has proven problematic. The conventional approach of injecting electric current into semiconductors has largely fallen short in this regard, giving rise to what is referred to by researchers as the “green gap.” Without small, efficient lasers that can generate green light, the potential applications—particularly in areas like aquatic communications, advanced display technologies, and medical treatments—have remained largely untapped.
Although green laser pointers have been available for over two decades, they generate light in a very specific and narrow portion of the green spectrum. As a result, they lack the versatility and functionality required for integration with other devices on a chip. The breakthrough moment came when NIST researchers modified a ring-shaped microresonator, a chip-sized optical component that has the potential to transform this scenario.
In their recent publication in the journal Light: Science & Applications, NIST scientists announced their success in creating a miniature green laser source by utilizing a specially designed microresonator made of silicon nitride. The ingenious design enables the conversion of infrared laser light into different visible light wavelengths. As infrared light circulates within the microresonator, it undergoes an interaction known as optical parametric oscillation (OPO), generating new wavelengths in the process.
Previously, this research team, led by Kartik Srinivasan of NIST and the Joint Quantum Institute (JQI), had successfully produced various visible laser colors, including red, orange, and limited shades of yellow and green. However, the aim was always broader—to fill the entirety of the green gap spectrum. To achieve this, the researchers made two significant modifications.
First, they slightly thickened the microresonator, which facilitated the generation of light that could ‘reach’ deeper into the green spectrum—specifically targeting wavelengths as short as 532 nanometers. This move was pivotal in broadening the spectrum of colors they could produce. The second alteration involved etching away parts of the silicon dioxide layer beneath the microresonator, effectively allowing for a greater exchange of air. This adjustment made the output colors less sensitive to variations in the dimensions of the microresonator and the input infrared laser’s wavelength, thereby providing the researchers with finer control over producing a variety of wavelengths.
With their enhanced microresonator, the researchers reported the capability to create over 150 distinct wavelengths, effectively spanning the entire green gap and facilitating precise tuning of light outputs. “Previously, we could make relatively large adjustments across color bands, but the fine-tuning was a challenge,” noted Srinivasan. This newfound ability opens the door for more sophisticated applications in several fields, particularly in how lasers can be employed in medical treatments, full-color displays, and quantum technology.
Underwater communication stands out as one of the most promising applications, given that water is almost transparent to blue-green wavelengths. Additionally, there is significant potential in the realm of computer science—especially in quantum computing. The small size and efficiency of these lasers may allow for their deployment beyond traditional laboratory settings, making quantum information technology more accessible.
Despite these remarkable advancements, NIST researchers acknowledge that challenges remain in optimizing the energy efficiency of their green laser colors. Current laser output power is only a small fraction of the input, which could hinder practical applications. Enhancements in the coupling of the input laser with the waveguide that channels light into the microresonator, alongside improved extraction methods for generated light, are critical steps the team is currently pursuing.
As they continue to push the boundaries of laser technology and explore the significant implications of their work, these researchers aim not only to bridge the green gap but also to unlock transformative advancements across multiple domains, ultimately shifting the paradigm in how we harness and utilize light.