Illustration of the new bowtie structure, which can be seen in the middle of the picture. The bowtie structure compresses light spatially, and the nanostructures around it store it temporally. The result is a compression of light to the smallest scale to date – the world's smallest photon in a dielectric material. (Illustration: DTU)

Until recently, it was widely believed among physicists that it was impossible to compress light below the so-called diffraction limit, except when using metal nanoparticles, which unfortunately also absorb light. It therefore seemed impossible to compress light strongly in dielectric materials such as silicon, which are key materials in information technologies and come with the important advantage that they do not absorb light. Interestingly, it was shown theoretically in 2006 that the diffraction limit also does not apply to dielectrics.

Still, no one has succeeded in showing this in the real world, simply because it requires such advanced nanotechnology that no one has been able to build the necessary dielectric nanostructures until now. A research team from DTU (Technical University of Denmark) has successfully designed and built a structure, a so-called dielectric nanocavity, which concentrates light in a volume 12 times below the diffraction limit. The result is ground-breaking in optical research.

"Although computer calculations show that you can concentrate light at an infinitely small point, this only applies in theory. The actual results are limited by how small details can be made, for example, on a microchip," says Marcus Albrechtsen, PhD-student at DTU Electro.

“We programmed our knowledge of real photonic nanotechnology and its current limitations into a computer. Then we asked the computer to find a pattern that collects the photons in an unprecedentedly small area - in an optical nanocavity – which we were also able to build in the laboratory.”

Optical nanocavities are structures specially designed to retain light so that it does not propagate as we are used to but is thrown back and forth as if you put two mirrors facing each other. The closer you place the mirrors to each other, the more intense the light between the mirrors becomes. For this experiment, the researchers have designed a so-called bowtie structure, which is particularly effective at squeezing the photons together due to its special shape.

The nanocavity is made of silicon, the dielectric material on which most advanced modern technology is based. The material for the nanocavity was developed in cleanroom laboratories at DTU, and the patterns on which the cavity is based are optimized and designed using a unique method for topology optimization developed at DTU. Initially developed to design bridges and aircraft wings, it is now also used for nanophotonic structures.

The discovery could be decisive for developing revolutionary new technologies that may reduce the amount of energy-guzzling components in data centers, computers, telephones, etc. The energy consumption for computers and data centers continues to grow, and there is a need for more sustainable chip architectures that use less energy. This can be achieved by replacing the electrical circuits with optical components. The researchers' vision is to use the same division of labor between light and electrons used for the Internet, where light is used for communication and electronics for data processing. The only difference is that both functionalities must be built into the same chip, which requires that the light be compressed to the same size as the electronic components. The breakthrough at DTU shows that it is, in fact, possible.