by 2) within a contour d is uniformly exposed by electron beam lithography. Therefore, the air characteristics that define the device are uniformly tuned. e.g. 40° tilt SEM images of the bowtie region for δ = { − 2, − 4, − 6} nm. We measure the average width of the fabricated bow ties to be (8 ± 5) nm, (10 ± 5) nm, and (16 ± 5) nm for shapes e, f, and g, respectively, noting the variation in width along the z-direction caused by the scallops and ∼1° negative sidewall angle represented by the uncertainty as discussed in the main text. Special offer: Nature Communications (2022). DOI: 10.1038/s41467-022-33874-w” width=”800″ height=”383″/> Fabrication of silicon topology-optimized dielectric bow tie cavity (DBC). a rendering of the DBC design generated by a tolerance-constrained topology optimization. The normalized field ∣E∣ is projected onto the faces defining the three symmetry planes of the design. b Magnification of the solid silicon bowtie showing strong field confinement due to the 8 nm bowtie bridge dimension. c 40° scanning electron microscopy (SEM) image of a fabricated cavity. d Global geometry-tuning, d. Each air pixel (black) (1 nm2) within a contour d is uniformly exposed by electron beam lithography. Therefore, the air characteristics that define the device are uniformly tuned. e.g. 40° tilt SEM images of the bowtie region for δ = { − 2, − 4, − 6} nm. We measure the average width of the fabricated bow ties to be (8 ± 5) nm, (10 ± 5) nm, and (16 ± 5) nm for shapes e, f, and g, respectively, noting the variation in width along the z-direction caused by the scallops and ∼1° negative sidewall angle represented by the uncertainty as discussed in the main text. Credit: Nature communications (2022). DOI: 10.1038/s41467-022-33874-w
Until recently, it was widely believed among physicists that it was impossible to compress light below the so-called diffraction limit (see below), except by using metallic nanoparticles, which unfortunately also absorb light. Therefore, it seemed impossible to strongly compress light into dielectric materials such as silicon, which are key materials in information technologies and have the significant advantage of not absorbing light.
Interestingly, it was theoretically shown in 2006 that the diffraction limit also does not apply to dielectrics. However, no one has been able to demonstrate this in the real world, simply because no one has been able to fabricate the necessary dielectric nanostructures until now.
A research team from DTU has successfully designed and fabricated a structure, a so-called dielectric nanocavity, which concentrates light into a volume 12 times below the diffraction limit. The result is groundbreaking in optical research and has just been published in Nature communications.
“Although computer calculations show that you can concentrate light into an infinitesimally small spot, this is only true in theory. Actual results are limited by how small details can be made, for example, on a microchip,” says Marcus Albrechtsen, Ph.D. .-student at DTU Electro and first author of the new article.
“We programmed our knowledge of actual photonic nanotechnology and its current limitations into a computer. We then asked the computer to find a pattern that collects the photons in an unprecedentedly small area—an optical nanocavity—which we were also able to create in the lab.”
Optical nanocavities are structures specially designed to hold light so that it doesn’t spread out like we’re used to but bounces back and forth like if you put two mirrors facing each other. The closer you place the mirrors together, the brighter the light between the mirrors becomes. For this experiment, the researchers designed a so-called bow-tie structure, which is particularly effective at squeezing photons together due to its special shape.
The nanocavity is made of silicon, the dielectric material on which the most advanced modern technology is based. The material for the nanocavity was developed in cleanroom laboratories at DTU and the patterns underlying the cavity are optimized and designed using a unique topology optimization method developed at DTU. Originally developed for the design of aircraft bridges and wings, it is now also used for nanophotonic structures.
“It took a lot of joint effort to achieve this breakthrough. It was only possible because we were able to combine world-leading research from various research groups at DTU,” says Associate Professor Søren Stobbe, who led the research work.”
Major breakthrough for energy efficient technology
The discovery could be decisive for the development of revolutionary new technologies that may reduce the amount of power-consuming components in data centers, computers, phones, etc.
Power consumption for computers and data centers continues to increase, and there is a need for more sustainable chip architectures that use less power. 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 functions must be built into the same chip, which requires compressing the light to the same size as the electronics. The breakthrough at DTU shows that it is, in fact, possible.
“There is no doubt that this is an important step in developing more energy-efficient technology for, say, nanolasers for optical links in data centers and future computers—but there is still a long way to go,” says Marcus Albrechtsen.
The researchers will now work further and refine the methods and materials to find the optimal solution.
“Now that we have the theory and the method, we will be able to produce increasingly intense photons as the surrounding technology develops. I am convinced that this is only the first in a long line of important developments in physics and photonic nanotechnology centered around from these principles,” says Søren Stobbe.
The diffraction limit
Diffraction limit theory describes that light cannot be focused into a volume smaller than half a wavelength in an optical system – for example, this applies to resolution in microscopes.
However, nanostructures can be composed of elements much smaller than the wavelength, which means that the diffraction limit is no longer a fundamental limit. Bowtie structures, in particular, can compress light into very small volumes that are limited by the sizes of the bowtie and thus the quality of the nanofabrication.
When light is compressed, it becomes more intense, enhancing the interactions between light and materials such as atoms, molecules, and two-dimensional materials.
Dielectric materials
Dielectric materials are electrical insulators. Glass, rubber, and plastic are examples of dielectric materials, in contrast to metals, which are electrically conductive.
An example of a dielectric material is silicon, which is often used in electronics as well as photonics.
Finding high Q resonant modes in a dielectric nanocavity
Marcus Albrechtsen et al, Nanometric photon confinement in topologically optimized dielectric cavities, Nature communications (2022). DOI: 10.1038/s41467-022-33874-w
Provided by the Technical University of Denmark
Reference: Researchers compress light 12 times below diffraction limit in a dielectric material (2022, October 26) Retrieved October 26, 2022 from https://phys.org/news/2022-10-compress-diffraction-limit- dielectric-material.html
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