A semiconductor laser developed by University of California, Berkeley (UC Berkeley) researchers accomplishes an elusive goal in the field of optics: the ability to maintain a single mode of emitted light while maintaining the ability to scale up in size and power. The work shows that size does not have to come at the expense of coherence, enabling lasers to be more powerful and to cover longer distances for many applications, the developing team said. Researchers led by Boubacar Kanté, the Chenming Hu Associate Professor in UC Berkeley’s Department of Electrical Engineering and Computer Sciences (EECS) and faculty scientist at the Materials Sciences Division of the Lawrence Berkeley National Laboratory, showed that a semiconductor membrane perforated with evenly spaced and same-size holes functioned as a scalable laser cavity. They showed that the laser emits a consistent single wavelength, regardless of the size of the cavity. Schematic of the Berkeley Surface Emitting Laser (BerkSEL) illustrating the pump beam (blue) and the lasing beam (red). The unconventional design of the semiconductor membrane synchronizes all unit-cells, or resonators, in phase so that they are all participating in the lasing mode. Courtesy of the Boubacar Kanté group/UC Berkeley. “Increasing both size and power of a single-mode laser has been a challenge in optics since the first laser was built in 1960,” Kanté said. “Six decades later, we show that it is possible to achieve both these qualities in a laser.” In lasers, the coherent single-wavelength directional light begins to break down as the size of the laser cavity increases. The standard workaround is to use external mechanisms like a waveguide to amplify the beam. This, however, takes up a lot of space. “By eliminating the need for external amplification, we can shrink the size and increase the efficiency of computer chips and other components that rely upon lasers,” Kanté said. The work is particularly relevant to VCSEL technology; in VCSELS, light is emitted vertically out of the chip. VCSELs typically measure only a few microns wide. The current strategy used to boost their power is to cluster hundreds of individual VCSELs together. Because the lasers are independent, their phase and wavelength differ, so their power does not combine coherently. “This can be tolerated for applications like facial recognition, but it’s not acceptable when precision is critical, like in communications or for surgery,” said study co-lead author Rushin Contractor, an EECS Ph.D. student. The study found that the design of the laser, dubbed BerkSEL for Berkeley Surface Emitting Laser, enabled the single-mode light emission because of the physics of the light passing through the holes in the membrane, a 200-nm-thick layer of indium gallium arsenide phosphide, a semiconductor commonly used in fiber optics and telecommunications technology. The holes, which were etched lithographically, had to be a fixed size, shape, and distance apart. These acted as Dirac points, a topological feature of two-dimensional materials based on the linear dispersion of energy. The researchers point out that the phase of light that propagates from one point to the other is equal to the refractive index multiplied by the distance traveled. Because the refractive index is zero at the Dirac point, light emitted from different parts of the semiconductor are exactly in phase and thus optically the same. “The membrane in our study had about 3000 holes, but theoretically, it could have been 1 million or 1 billion holes, and the result would have been the same,” said study co-lead author Walid Redjem, an EECS postdoctoral researcher. The researchers used a high-energy pulsed laser to optically pump and provide energy to the BerkSEL devices. They measured the emission from each aperture using a confocal microscope optimized for near-infrared spectroscopy. The semiconductor material and the dimensions of the structure used in this study were selected to enable lasing at telecommunications wavelength. Authors noted that BerkSELs can emit different target wavelengths by adapting the design specifications, such as hole size and semiconductor material. The research was published in Nature (www.doi.org/10.1038/s41586-022-05021-4).
