Just as many people seek to turn back the clock on aging, scientists are on a quest to do the same for a special subset of lasers called gallium nitride lasers. These lasers, which emit near-blue light, are known for their superior performance compared to traditional red lasers and are already prized for their precision and versatility.

“These lasers are a type of laser that uses a semiconductor material called gallium nitride to produce light ranging from green to ultraviolet,” said Wayne Kang, an associate research fellow at Xiamen University in China and one of the authors of the study, explained in an email.

Lasers create powerful, focused beams of light using a core component called the active medium. When this medium gets energized — through light, electricity, or chemicals — it emits light and produces the laser beam.

The wavelength of a laser’s light depends on the quantum states of electrons inside the atoms in the active medium, which varies between materials. Shorter wavelengths offer greater precision, such as writing more data onto a compact disc with a tightly focused laser beam. This need for precision has spurred the development of short-wavelength lasers, with gallium nitride lasers being a leading example.

“Early semiconductor lasers used different materials, like gallium arsenide, and emitted red or infrared light. However, with the development of high-resolution displays, laser communication, and materials processing, the demand for shorter wavelength lasers has increased.” said Kang. “Therefore, gallium nitride lasers need to be investigated, and high-power lasers of this type, in particular, have become a research hotspot.”

However, despite all their advantages, these lasers have a significant drawback — they rapidly degrade and their radiation power drops significantly after just a few hundred hours of operation, leading to the need for their frequent replacement, which, in turn, leads to high financial costs and harm to the environment.

By making them last longer, researchers could boost their effectiveness and reduce costs, leading to advancements in technology and improvements in applications ranging from high-precision manufacturing to advanced medical treatments.

To solve this problem, Kang and his colleagues sought to identify the root cause of degradation.

What’s causing the degradation?

The team began by investigating how a gallium nitride laser diode changes during operation. In these devices, an electric current flows between two regions known as the p- and n-regions, which exist within the laser’s structure.

The p-region has a shortage of electrons, creating “holes” where electrons are missing. Electrons from the electron-rich n-region flow into these holes, and when they recombine in the gain medium, photons are emitted, generating light of a specific wavelength. These photons bounce between mirrors at each end of the cavity, stimulating more light emission and generating the laser beam, which exits through a partially transparent mirror.

To uncover the underlying causes of degradation, the researchers used advanced computer modeling to simulate the chemical and physical processes affecting the laser under intense radiation and high temperatures.

They also employed a number of experimental techniques, such as transmission electron microscopy, which allows them to analyze a material by passing a beam of electrons through it and detecting the electrons that emerge on the other side, providing a glimpse into the active medium’s internal structure.

Their findings revealed that the primary cause of degradation is the diffusion of silicon from the mirrors into the p-layer at high temperatures. This diffusion reduces the concentration of electron holes in the p-layer, which in turn decreases the electric current flowing through the active medium. As the current drops, so does the laser’s output power.

Getting around the culprit

To counteract this, the researchers replaced the silicon oxide mirror coating with aluminum oxide, known for its superior temperature resistance. They also added a layer of aluminum nitride on top of the coating. Aluminum nitride is stable, has low conductivity, and does not chemically react with gallium nitride, which significantly reduces material diffusion from the mirror coating into the laser’s active regions.

“The most significant finding of our study is that the elemental diffusion of the mirror coating could be a key factor in the laser diode degradation, which has not been reported in the lower power devices,” said Kang. 

“On the basis of this finding, we developed a new anti-aging technology to suppress this diffusion,” he continued. “This anti-aging technology was verified by a 5500-hour aging test, achieving a nearly ten-time improvement in degradation suppression.”

The scientists believe that their innovative approach has the potential to improve the durability and performance of even more powerful lasers and those operating at shorter wavelengths. 

This could lead to significant advancements in fields ranging from light detection and ranging (LiDAR) systems, which are crucial for autonomous vehicles, to quantum communications, where powerful gallium nitride lasers are used to manipulate and control quantum states of light and matter.

“We plan to test our anti-aging technology on higher power laser diodes,” concluded Kang. “Additionally, we aim to explore the degradation mechanisms of shorter-wavelength lasers, such as ultraviolet range. The ultra-violet laser diodes are widely used in biochemical analysis and excitation light sources of high photolithography resolution.”

Reference: Enming Zhang et al, High-Power GaN-Based Blue Laser Diodes Degradation Investigation and Anti-aging Solution, Advanced Photonics Research (2024). DOI: 10.1002/adpr.202400119

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