New micro laser for application and research

Schematic representation of a microlaser with a nanometer-precisely structured surface grid. The compact semiconductor laser produces a directed beam of light with a beam profile precisely tuned to the specific application.

By the thickness of the bars in the grid, their spacing, and the depth of the grooves, the wavelength of the laser beams can be adjusted. Below the grid, you can see the few Bragg mirrors above the active layer (light gray), where the radiation is generated. Beneath that are many layers of Bragg mirror layers. (Electron microscope image)

Schematic structure of the new micro laser with the lower layer of Bragg mirror pairs (DBR), the active layer in red, the few layers of Bragg mirror pairs above it (DBR), and the diffraction grating (MHCG).

An innovative microlaser concept based on so-called surface emitters (VCSEL) has been developed by the Department of "Optoelectronics and Quantum Devices" at TU Berlin under the leadership of Prof. Dr. Stephan Reitzenstein. The new microlasers are vertically structured laser diodes in which the upper, light-reflecting layers have been largely replaced by an optical grating embedded in the semiconductor material. This approach has made it possible, on the one hand, to nearly halve the production time of the laser diodes. On the other hand, inserting an optical grating provides a method that can produce many diodes with different laser wavelengths in a single manufacturing step. The results were achieved in collaboration with researchers from the University of Łódź (Poland) and are now published in the journal "Optica".

They have been in use for more than two decades: laser diodes that emit their light perpendicular to the semiconductor chip in which they are embedded. These "VCSEL" (Vertical Cavity Surface Emitting Laser) can, unlike laser diodes where the light exits from a side facet, very effectively couple the laser beam into an optical fiber. The reason for this is their round geometry with a small form factor, as well as good beam quality and focusability. VCSELs are therefore used, for example, in large data centers, where they enable efficient data transfer between individual servers. They are also used in smartphones for facial recognition.

Time savings for industrial production

"A VCSEL essentially consists of an active layer where the photons are generated, as well as layers of so-called Bragg mirrors above and below. These reflect the light repeatedly back into the active layer to stimulate the generation of more photons. This is the typical laser effect, which causes all the light beams to oscillate perfectly in phase at the end," explains Stephan Reitzenstein. "The top mirror layers reflect the laser light slightly less. This allows part of the laser beam to exit and be utilized."

The researchers have now replaced a large part of the top mirror layers with a fine grating etched into the semiconductor material using a lithographic process. Diffraction of the light waves at the grating and their subsequent superposition can achieve effects similar to those of the elaborate mirror layers. "The major difference is that we no longer need to deposit so many semiconductor layers sequentially. That is a lengthy and expensive process that takes about twelve hours. By using the grating, we save roughly half of that," explains Niels Heermeier, the lead author of the study. This time and cost savings are an important factor for industrial manufacturing.

Manufacturing different laser diodes in a single production step

"The even more significant advantage of the grating is that multiple laser diodes with different emission wavelengths can be produced simultaneously on a single semiconductor wafer in one manufacturing step," Heermeier explains. To do this, several geometric parameters of the grating are varied depending on the diode during the etching process: the width of the grating bars, their spacing, and the depth of the etched grooves between them. "This flexibility is not possible when depositing mirror layers, as they must grow uniformly across the entire wafer, setting the properties for all diodes identically."

To precisely manufacture the new microlasers with their individual properties, an extremely high accuracy is required: the dimensions must deviate less than five nanometers from the target value. Compared to the nearly 400,000 kilometers distance from Earth to the Moon, this corresponds to a maximum permissible deviation of two meters. This performance was only made possible by an electron beam lithography system available at the department, acquired with funds from the German Research Foundation (DFG) and TU Berlin, which the researchers must adapt through a complex process for their specific task.

Applications for data centers, autonomous driving, and optical computing

In addition to their use in data centers for efficient signal coupling into fiber optic cables, the microlasers could also be used for the so-called LIDAR (Light Detection and Ranging) method, which measures distances using laser beams and plays a significant role in autonomous driving. Arrangements with laser diodes of different wavelengths—such as those easily produced with the new method—offer much better resolution.

The new VCSEL diodes could also become an important component of hardware for neuromorphic computing with optical processors, developed by Stephan Reitzenstein's group together with colleagues at the University of Berkeley and MIT in Boston. These computers are modeled after the human brain and operate not with electrical circuits but with optical components and light as the information carrier. Usually, this light comes from an arrangement of multiple laser diodes on the optical chip. "It is crucial that the wavelength of the light from the laser diodes is exactly the same. Even the smallest deviations during manufacturing can lead to different wavelengths," explains Reitzenstein. With the new method, it would be possible to measure the wavelengths after manufacturing of the optical chip and then precisely match them by adding grating structures afterward.

On the way to ultracompact laser diodes

The new microlasers are also interesting for basic research, as researchers from the collaborating group in Poland have, by chance, realized the exotic quantum state of a "Bose-Einstein condensate" with photons in conventional VCSEL diodes for the first time. This phenomenon can now be studied more precisely with the new, easily adjustable laser diodes.

"The next step will be to replace the lower mirror layers of the new VCSEL diodes with a second grating," says Reitzenstein. "This would make the component even more compact and faster to produce." "However, this task is significantly more complex because the active material of the diode must then be removed from the bottom." From Reitzenstein's perspective, the new "Center for Integrated Photonics Research" (CIPHOR), as a central part of the new experimental physics building on TU Berlin's East Campus, will play a special role. This facility is scheduled to open in 2028 and will offer new manufacturing possibilities with state-of-the-art cleanrooms.