Optical resonators increase the strength of light-matter interaction by storing light over a long period of time. The smaller the resonator, the tighter the confinement of light will be, resulting in an even stronger interaction that can be used to produce, for example, better photodetectors or quantum light sources. Strengthening the interaction of light with matter is a central goal of quantum optics and photonics. The development of an optical resonator that can store light for a long time in a region the size of a single atom would be a big step toward reaching this goal. An illustration of the core of the photonic cavity that was fabricated as two halves that assembled themselves into one unit. The cavity confines light inside the gap, which is only a few atoms wide, as indicated in the field of view of the magnifying glass. Courtesy of Thor A.S. Weis. To create a resonator that provides strong light-matter interaction at a very small scale, researchers at the Technical University of Denmark (DTU Electro) built self-assembled, bowtie optical resonators at the atomic scale and embedded the self-assembled cavities in a larger architecture consisting of self-assembled waveguides, springs, and photonic couplers. The self-assembled cavity can be integrated into larger self-assembled components for routing light around an optical chip. “We use the new self-assembly concept for photonic resonators, which may be used in electronics, nanorobotics, sensors, quantum technologies, and much more,” researcher Guillermo Arregui said. The fabrication of functional materials and devices at the micro- and nanoscale typically follows either a top-down approach based on planar technology or a bottom-up approach where structures are self-assembled using various effects, such as the van der Waals force. The top-down approach, used in silicon-based semiconductor technologies, essentially takes blocks of silicon and makes nanostructures from them. Planar semiconductor technology offers scalability, but it cannot achieve the atomic dimensions enabled by self-assembly. The bottom-up approach aims to replicate the hierarchical self-assembly found in biological systems, but synthetic self-assembly methods that bridge the nanoscopic to macroscopic dimensions remain unscalable and inferior to biological self-assembly. Combining the scalability of top-down planar technology with the resolution of bottom-up approaches would open many potential opportunities for nanotechnological research. But so far, there has been no connection between the two approaches, and no clear pathway for their direct integration. The new approach demonstrated by the DTU Electro team allows devices to be built by using the surface forces that act on objects separated by a few tens of nanometers. The researchers harnessed these forces, including Casimir-van der Waals interactions, to fabricate nanostructures with dimensions of a few nanometers and subnanometer dimensions. The self-assembled cavity can be integrated into larger, self-assembled components for routing light around an optical chip. The figure shows the optical cavity embedded in a circuit containing multiple self-assembled elements. Courtesy of Thor A.S. Weis. The researchers attached a silicon device to a layer of glass and suspended two halves of silicon structures on springs. The devices were made using conventional semiconductor technology, so the two halves were a few tens of nanometers apart. After selective etching of the glass, the structure was released. At this point, the structure was only suspended by the springs, and because the two halves were fabricated so close to each other, they were attracted to each other due to surface forces. The Casimir force was harnessed for attracting the two halves, and the van der Waals force was used to make them stick together. The carefully engineered design of the silicon structures resulted in a self-assembled resonator with bowtie-shape gaps at the atomic scale, surrounded by silicon mirrors. The researchers used the surface forces to deterministically self-assemble and self-align the suspended silicon nanostructures with void features well below the length scales possible with conventional lithography and etching, using only conventional lithography and etching techniques. The photonic cavities built by the researchers confined photons to air gaps so small that the exact size of the gaps could not be determined, even with a transmission electron microscope. The smallest cavities that were built were 1 to 3 silicon atoms. “We don’t have to go in and find these cavities afterward and insert them into another chip architecture,” professor Søren Stobbe said. “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.” Researcher Ali Nawaz Babar said that even if the extreme dimensions were taken care of by the self-assembly process, the requirements for the nanofabrication were still extreme. “For example, structural imperfections are typically on the scale of several nanometers,” he said. “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.” The leading authors at work in the lab. Left to right: Ph.D. student Ali Nawaz Babar, postdoc Guillermo Arregui, and associate professor Søren Stobbe. Courtesy of Ole Ekelund. In all, the team fabricated 2688 devices across two microchips, each containing a platform that would either collapse onto a nearby silicon wall or not, depending on the surface area details, spring constant, and distance between platform and wall. The researchers made a map of the parameters that would lead to deterministic self-assembly. In the experiments, only 11 devices failed due to fabrication errors or other defects. To illustrate the potential of their approach, the researchers fabricated nanostructures that are impossible to make with any other known method: waveguide-coupled, high-Q silicon photonic cavities that confined telecom photons to 2-nm air gaps with an aspect ratio of 100, corresponding to mode volumes more than 100 times below the diffraction limit. “We are far from a circuit that builds itself completely,” Stobbe said. “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. We are very excited about this new line of research, and plenty of work is ahead.” The research was published in Nature (www.doi.org/10.1038/s41586-023-06736-8).
