Increasing the intensity of the interaction between light and matter to create, for example, better photodetectors or quantum light sources is a key objective in quantum optics and photonics.
Utilizing optical resonators, which hold light for extended periods of time and increase its interaction with matter, is the most effective technique to do accomplish this. The interaction is further amplified if the resonator is likewise extremely compact, squeezing light into a very small area of space. A single atom-sized area would be the storage of light for an extended period of time in the perfect resonator.
For decades, engineers and physicists have grappled with the question of how small optical resonators can be manufactured without sacrificing their performance, which is analogous to asking how small a semiconductor device can be constructed. According to the semiconductor industry’s strategy for the next 15 years, a semiconductor structure's minimum width will be 8 nm, or many tens of atoms wide.
Associate Professor Søren Stobbe and his colleagues at DTU Electro produced 8 nm cavities last year; now, they propose and show a unique way to create a self-assembling cavity with an air void at the scale of a few atoms.
The discoveries are detailed in their publication “Self-assembled photonic cavities with atomic-scale confinement,” which was published in Nature.
In a nutshell, the experiment involves suspending two halves of silicon devices from springs, but not until the silicon device is securely fastened to a glass layer. The two halves are separated by a few tens of nanometers since the devices are constructed using traditional semiconductor techniques. The structure is freed once the glass is selectively etched, and it is now just supported by the springs.
Because the two parts are made so closely together, surface forces lead them to attract. The outcome is a self-assembling resonator with silicon mirrors around bowtie-shaped gaps at the atomic scale, created by meticulously crafting the architecture of the silicon structures.
We are far from a circuit that builds itself completely. But we have succeeded in converging two approaches that have been traveling along parallel tracks so far. And it allowed us to build a silicon resonator with unprecedented miniaturization.
Søren Stobbe, Associate Professor, Department of Electrical and Photonics Engineering, DTU Electro
Two Separate Approaches
A particular methodology, known as the top-down approach, is responsible for the advancements in silicon-based semiconductor technology; you start with a silicon block and work up from there to create nanostructures.
The other method, known as the bottom-up technique, involves attempting to have a nanotechnological system put itself together. It attempts to imitate biological systems that are created by chemical or biological processes, such as those seen in plants or animals.
The fundamental ideas behind these two methods are what characterize nanotechnology. The issue lies in the fact that these two methods were not previously combined: self-assembled structures have long operated at atomic sizes, but they provide no architecture for the interconnects to the outside world, whereas semiconductors are scalable but cannot reach the atomic scale.
The interesting thing would be if we could produce an electronic circuit that built itself—just like what happens with humans as they grow but with inorganic semiconductor materials. That would be true hierarchical self-assembly. We use the new self-assembly concept for photonic resonators, which may be used in electronics, nanorobotics, sensors, quantum technologies, and much more. Then, we would really be able to harvest the full potential of nanotechnology. The research community is many breakthroughs away from realizing that vision, but I hope we have taken the first steps.
Guillermo Arregui, Marie Curie Postdoctoral Researcher, Department of Photonics Engineering, Technical University of Denmark
The team at DTU Electro set out to construct nanostructures that go beyond the capabilities of standard lithography and etching, assuming that a combination of the two methods is feasible. They used just conventional lithography and etching.
They planned to use two surface forces: the van der Waals force to hold the two parts together and the Casimir force to draw them together. The same fundamental phenomenon underlies each of these forces: quantum fluctuations.
By creating photonic cavities, the researchers were able to confine photons to air gaps so tiny that they could not be precisely measured, not even with a transmission electron microscope. However, the smallest ones they constructed were 1-3 silicon atoms in size.
Even if the self-assembly takes care of reaching these extreme dimensions, the requirements for the nanofabrication are no less extreme. For example, structural imperfections are typically on the scale of several nanometers. Still, if there are defects at this scale, the two halves will only meet and touch at the three largest defects. We are really pushing the limits here, even though we make our devices in one of the very best university cleanrooms in the world.
Ali Nawaz Babar, Study First Author and PhD Student, NanoPhoton Center of Excellence, DTU Electro
Babar added, “The advantage of self-assembly is that you can make tiny things. You can build unique materials with amazing properties. But today, you can’t use it for anything you plug into a power outlet. You can’t connect it to the rest of the world. So, you need all the usual semiconductor technology for making the wires or waveguides to connect whatever you have self-assembled to the external world.”
Robust and Accurate Self-Assembly
The research demonstrates a viable technique to connect the two nanotechnology approaches by adopting a new generation of fabrication technology that blends the atomic dimensions offered by self-assembly with the scalability of conventionally produced semiconductors.
Stobbe stated, “We don’t have to go in and find these cavities afterwards and insert them into another chip architecture. That would also be impossible because of the tiny size. In other words, we are building something on the scale of an atom already inserted in a macroscopic circuit. We are very excited about this new line of research, and plenty of work is ahead.”
Babar, A. N., et. al. (2023) Self-assembled photonic cavities with atomic-scale confinement. Nature. doi:10.1038/s41586-023-06736-8