Berkeley engineers have invented a new semiconductor laser that achieves an elusive goal in optics: maintaining a single mode of emitted light while maintaining amplification capabilities in size and power. This is an achievement that means size does not have to come at the expense of coherence and , making the laser more powerful and able to cover longer distances for many applications.
Boubacar Kanté, Chenming Hu, associate professor in the Department of Electrical Engineering and Computer Science (EECS) at the University of California, Berkeley, and faculty scientist in the Materials Sciences Division at Lawrence Berkeley National Laboratory (Berkeley Lab) The research team's results showed that a semiconductor film with uniformly spaced and same-sized holes can serve as a perfect stretchable laser cavity. They demonstrated that the laser emits a consistent single wavelength regardless of the size of the cavity.
The researchers describe their inventions, called Berkeley Surface Emission Lasers (BerkSELs), in a study published Wednesday, June 29, in the journal Nature.
"Increasing the size and power of single-mode lasers has been a challenge in the field of optics since the introduction of the first laser in 1960," Kant said. "Sixty years later, we have shown that it is possible to achieve both of these qualities in a laser. I think this is the most important paper my group has published to date."
Although the invention of the laser led to a wide range of applications, from Surgical tools to barcode scanners to precision etching, but optical researchers have been facing a long-standing limitation. As the size of the laser cavity increases, the coherent single-wavelength parallel light that is the defining characteristic of lasers begins to break up. The standard solution is to use external mechanisms, such as waveguides, to amplify the beam.
"Using another medium to amplify the laser takes up a lot of space," Kant said. "By eliminating the need for external amplification, we can shrink the size and increase the efficiency of computer chips and other components that rely on lasers."
Schematic showing a "Dirac cone" in which light is emitted simultaneously from an entire semiconductor cavity due to the Dirac point singularity
The results of this study are particularly relevant to the Vertical Cavity Surface Emitting Laser (VCSEL), in which the laser is emitted vertically out of the chip. This laser has a wide range of applications, including fiber optic communications , computer mice, laser printers and biometric identification systems.
VCSELs are typically small, only a few microns across. The current strategy used to enhance their capabilities is to cluster hundreds of individual VCSELs together. Because the lasers are separate, they differ in phase and wavelength, so their power does not combine coherently.
"This is tolerable in applications such as facial recognition, but not in situations where accuracy is critical, such as communications or surgery," said Rushin Contractor, co-lead author of the study and an EECS doctoral student.
Kanté compared the extra efficiency and power provided by BerkSEL's single-mode laser to a group of people getting a stalled bus to move. Multimode lasers are similar to people pushing in different directions, he said. Not only does this reduce efficiency, but it can also be counterproductive if people push in the opposite direction. Boxell's single-mode laser is comparable to everyone in the crowd pushing a bus in the same direction. This is far more efficient than existing lasers, which only have a subset of people pushing the bus.
Research found that the reason why the BerkSEL design is able to achieve single-mode light emission is due to the physical properties of light passing through the membrane pores. The membrane hole is a 200-nanometer-thick layer of indium gallium phosphide semiconductor, a semiconductor commonly used in optical fiber and telecommunications technology. Holes etched using photolithography must have a fixed size, shape, and spacing.
The researchers explained that the periodic holes in the membrane turned into Dirac points, which are topological features of two-dimensional materials based on linear dispersion of energy. They are named after the British physicist and Nobel Prize winner Paul Dirac, known for his early contributions to quantum mechanics and quantum electrodynamics.
Top view of a Berkeley Surface Emitting Laser (BerkSEL) scanning electron micrograph. Hexagonal lattice photonic crystals (PhC) form electromagnetic cavities
The researchers pointed out that the phase of light propagating from one point to another is equal to the refractive index multiplied by the propagation distance. Because the refractive index of the Dirac point is zero, the light emitted from different parts of the semiconductor is completely in phase and therefore optically identical.
"The membrane in our study has about 3,000 pores, but in theory it could have a million or a billion pores and the results would be the same," said Walid Redjem, co-lead author of the study and an EECS postdoc.
The researchers used high-energy pulsed lasers for optical pumping and powering the BerkSEL device. They measured the emission at each aperture using a confocal microscope optimized for near-infrared spectroscopy.
The semiconductor materials and structure dimensions used in this study were designed to enable lasers at telecommunication wavelengths. The authors note that BerkSELs can emit different target wavelengths by adjusting design specifications such as pore size and semiconductor materials.
Other study authors are Wanwoo Noh, co-lead author, who received his PhD from EECS in May 2022; Wayesh Qarony, Scott Dhuey, and Adam Schwartzberg of Berkeley Lab; and Emma Martin, a doctoral student at EECS.
