Berkeley engineers have created a kind of semiconductor laser accomplishes an elusive goal in the field of optics: the ability to emit a single mode of light while maintaining scalability and power. It’s an achievement that means size doesn’t have to come with mounting, allowing for more powerful lasers and longer distance coverage for many applications.
Diagram of the Berkeley Surface Emitting Laser (BerkSEL) illustrating the pump beam (in blue) and the lens beam (in red). The unique design of the semiconductor film synchronizes all unit cells (or resonators) in phase so that they all engage in lens mode. Illustrated by the Kanté . team
A research team led by Boubacar Kanté, Chenming Hu, Associate Professor in UC Berkeley’s Department of Electrical Engineering and Computer Science (EECS) and scientist in the Department of Materials Science at the Lawrence Berkeley National Laboratory (Department of Science and Technology). Berkeley experiment), showed that a perforated semiconductor film with equally spaced and same-sized holes acts as a perfect, expandable laser cavity. They demonstrated that the laser emitted a consistent wavelength, regardless of the size of the cavity.
The researchers describe their invention, dubbed Berkeley Surface Emitting Lasers (BerkSELs), in a published study in the journal Nature.
Top view of the scanning electron microscopy image of the Berkeley Surface Luminescent Laser (BerkSEL). The hexagonal lattice photonic crystal (PhC) forms an electromagnetic cavity. Image credit: team Kanté
“Increasing both the size and power of single-mode lasers has been a challenge in optics since the first laser was built in 1960,” says Kanté. “Six decades later, we demonstrate that It is possible to achieve both of these qualities in one laser. I consider this the most important paper my team has published to date”.
Despite the numerous applications opened up by the invention of the laser – from surgical tools to barcode scanners to precision engraving – there is a nagging limitation that optical researchers must face. face. The coherent directional light, of a single wavelength, is a defining characteristic of the laser that begins to break down as the size of the laser cavity increases. The standard workaround is to use external mechanisms, such as waveguides, to amplify the beam.
“Using another medium to amplify laser light takes up a lot of space,” says Kanté. “By eliminating the need for external amplification, we can shrink the size and increase efficiency of computer chips and other laser-based components.”
The results of the study specifically concern vertical cavity surface emission lasers, or VCSELs, in which laser light is emitted vertically on the chip. Such lasers are used in a wide range of applications, including fiber optic communications, computer mice, laser printers and biometric identification systems.
VCSELs are usually very small, measuring a few micrometers wide. The current strategy used to increase their power is to gather hundreds of individual VCSELs together. Because the lasers are independent, their phases and wavelengths are different, so their power is not closely matched.
“This may be acceptable for applications like facial recognition, but it is unacceptable when accuracy is important, like in communications or surgery,” said study co-author Rushin Contractor, a Dr. EECS said. student.
Kanté compares the efficiency and extra power enabled by BerkSEL’s solo mode to a crowd that has a stalled bus to navigate. Multi-mode lasers are like people pushing in different directions, he said. Not only is it inefficient, but it can also backfire if people push in opposite directions. Single mode in BerkSELs is comparable to having each person in a crowd push a bus in the same direction. This is much more efficient than what is done in existing lasers, where using the same analogy, only part of the crowd contributes to pushing the bus.
The study found that the BerkSEL design allows for single-mode light emission due to the physics of light passing through holes in the film, a 200-nanometer thick layer of indium gallium arsenide phosphide, a commonly used semiconductor. in fiber optics and telecommunications technology. The holes, engraved using the lithography technique, should be of a fixed size, shape, and spaced far apart.
Diagram showing “Dirac cone.” Light is emitted synchronously from the entire semiconductor cavity due to the Dirac singularity. Image credit: team Kanté
The researchers explain that the cyclic holes in the membrane become the Dirac point, a topological feature of two-dimensional materials based on the linear dispersion of energy. They are named after British physicist and Nobel laureate Paul Dirac, known for his early contributions to quantum mechanics and quantum electrodynamics.
The researchers showed that the phase of light traveling from one point to another is equal to the refractive index times the distance traveled. Because the refractive index at the Dirac point is zero, the light emitted by the different parts of the semiconductor is exactly in phase and therefore optically the same.
“The membrane in our study had about 3000 holes, but theoretically it could be 1 million or 1 billon holes, and the results would be the same,” said study co-author Walid Redjem, a researcher. EECS postdoctoral researcher, said.
The researchers used a high-energy pulsed laser to pump the optics and power the BerkSEL devices. They measured the emission from each aperture using a confocal microscope optimized for near-infrared spectroscopy.
The semiconductor materials and structure sizes used in this study were selected to be able to generate lasers at telecommunications wavelengths. The authors note that BerkSEL can emit different target wavelengths by adjusting design specifications, such as hole size and semiconductor material.
Source: UC Berkeley
