World's Smallest Light-trapping Silicon Cavity Born: Self-assembly At The Atomic Level Generates Quantum Light Sources

Increasing the strength of the interaction between light and matter to produce better photodetectors or quantum light sources is a key goal of quantum optics and photonics.
And the best way to do that is to use optical resonators that store light for long periods of time so that their interaction with matter becomes stronger. If the resonator is also very compact, compressing the light into a very small region of space, the interaction is further amplified. In the perfect resonator, a region the size of an atom can store light for long periods of time.
The Challenge of Resonator Miniaturization
For decades, engineers and physicists have grappled with the problem of how to make small optical resonators without sacrificing their performance, similar to how small semiconductor devices have to be constructed. The semiconductor industry's roadmap for the next 15 years predicts that the smallest possible width of a semiconductor structure will be no less than 8 nm, which is the width of tens of atoms.
The self-assembled cavities described can be integrated into larger self-assembled assemblies for routing light around an optical chip. The figure shows an optical cavity embedded in a circuit containing multiple self-assembled components.
New approach overcomes extreme conditions
Last year, DTU Electro's Associate Professor S?ren Stobbe and his colleagues published a new paper in the journal Nature in which they demonstrated 8 nm cavities, but now they have proposed and demonstrated a new method to create self-assembled cavities with air voids on the scale of a few atoms. Their paper, "Self-assembled photonic cavities with atomic-scale constraints," details the findings, published in the Dec. 6 issue of Nature.
In this experiment, two halves of a silicon structure were "suspended" from a spring, and in the first step, the silicon device was firmly attached to a layer of glass. The device was made using conventional semiconductor technology, so the distance between the two halves was only a few tens of nanometers. Once the glass has been selectively etched, the structure is released and it is now simply supported by the spring.
Since the two parts are closely connected, surface forces cause them to attract each other. The result is a self-assembled resonator with silicon mirrors surrounding bow-tie shaped gaps at the atomic scale, which was created by carefully crafting the structure of the silicon structure.
The researchers are still far from a fully self-constructing circuit. But they have managed to fuse two approaches that have so far traveled along parallel tracks to create a silicon resonator that has never been miniaturized before.
Advances in silicon-based semiconductor technology have been made possible by one particular approach, known as the "top-down method". Another approach is known as "bottom-up" technology: trying to make nanotechnology systems assemble themselves. The key to their research lies in combining these two approaches.
The study demonstrates a viable technique for linking the two nanotechnology approaches by employing a new generation of fabrication techniques that combine the atomic size offered by self-assembly with the scalability of conventionally produced semiconductors.
By fabricating photonic cavities, the researchers were able to confine photons in air gaps so tiny that they could not be accurately measured, even with a transmission electron microscope. However, the smallest they built was only 1-3 silicon atoms in size, setting a new record for a small volume of light-trapping silicon cavities.
We don't need to find these cavities later and insert them into another chip architecture," Stobbe said. That's also not possible because it's so small. In other words, we're building something on the scale of an atom that has been inserted into a macroscopic circuit. We're very excited about this new direction of research, and there's a lot of work ahead."