Scientists have been grappling with the challenge of creating small, high-quality lasers that emit light in yellow and green wavelengths for many years. While red and blue lasers have long been industry standards, technology for green lasers has faced significant technical hurdles. This lack of stable and compact green lasers in the visible light spectrum has been termed the "green gap." Closing this gap could enable substantial advancements in various technological fields, including underwater communication, medical treatments, advanced color displays, and applications in quantum technology.

Unlike the mature technologies of red and blue lasers, green laser pointers have been available on the market for 25 years but only produce light in a narrow spectrum of green. Moreover, these lasers are not integrated onto chips, which would allow their use in more complex devices and systems. In an effort to overcome this technical challenge, scientists at the National Institute of Standards and Technology (NIST) recently made significant progress. Their work on modifying a small optical component known as a ring microresonator resulted in closing the green gap and creating a miniature source of green laser light that can be integrated onto a chip.

Microresonators, which are key to this technological breakthrough, are used to convert infrared laser light into other colors. This process involves pumping infrared light into the resonator, where it circulates thousands of times, becoming intense enough to cause a strong interaction with silicon nitride. This interaction, known as optical parametric oscillation (OPO), generates two new wavelengths of light, known as idler and signal.

During previous research, scientists were able to produce several individual colors of visible laser light, but the full spectrum of yellow and green colors needed to fill the green gap remained out of reach. To overcome this problem, the NIST team took two key steps. First, they slightly thickened the microresonator, allowing the creation of light that penetrates deeper into the green spectrum, all the way to a wavelength of 532 nanometers. The second step involved removing a portion of the silicon dioxide layer underneath the resonator, reducing the sensitivity of the output colors to the dimensions of the resonator and the wavelength of the infrared light. These modifications gave scientists greater control over generating different shades of green, yellow, orange, and red light.

The result of this work was the creation of over 150 different wavelengths within the green gap, with the possibility of fine-tuning each of them. Such precision opens the door to new applications in various industries. For example, advanced color displays in projection systems could significantly benefit from these new lasers, enabling a wider color range with greater precision. In medicine, green laser light can be used in treatments such as diabetic retinopathy, a condition that leads to abnormal growth of blood vessels in the eye.

One of the most promising applications of these new lasers is in quantum technology. Compact lasers that cover the green gap could enable significant improvements in data storage and processing capabilities in qubits, the fundamental units of quantum information. Currently, quantum computing technology and communications rely on larger, less efficient lasers, limiting their practicality outside the lab. By developing these new, smaller lasers, scientists have made a significant step toward portable and practical quantum computing systems.

Despite these achievements, the NIST team continues to work on improving the energy efficiency of their lasers. The current output power of these lasers represents only a small fraction of the input power, limiting their practical application. By increasing the efficiency of coupling between the input laser and the waveguide that directs light into the microresonator, as well as improving methods for extracting generated light, scientists believe they will significantly enhance the overall efficiency of these devices.

This innovation marks a continued evolution in digital technology and its impact on various applications, opening new possibilities for future advances in laser science and quantum technology. Researchers, including Jordan Stone and Xiyuan Lu from JQI, along with Zhimin Shi from Meta’s Reality Labs Research in Redmond, Washington, published their findings on August 21, 2024, in the journal Light: Science and Applications.

Source: National Institute of Standards and Technology (NIST)