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ETH Team Leads Way to More Precise Diffraction Gratings

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For decades, scientists have tried to improve the design and production of diffraction gratings, one of the oldest and most important technologies for controlling light. An ETH Zürich team, working with colleagues at Utrecht University and Heidelberg Instruments Nano, has found a way to achieve more efficient diffraction gratings. To make their gratings, the researchers produced wavy surfaces with nanometer precision.

Diffraction gratings use the principle of interference to deflect light of different wavelengths in precisely determined directions. When a lightwave hits a grooved surface, it is divided into many smaller waves, each emanating from an individual groove. When these waves leave the surface, they can either add together or cancel each other, depending on the direction in which they travel and on their wavelength.

For a diffraction grating to work properly, its grooves need to have a separation similar to the wavelength of the light. “Traditionally, those grooves are etched into the surface of a material using manufacturing techniques from the microelectronics industry,” researcher Nolan Lassaline said. “This means, however, that the grooves of the grating are rather square in shape. On the other hand, physics tells us that we should have grooves with a smooth and wavy pattern, like ripples on a lake.”

The ETH Zürich approach is based on scanning tunneling microscopy, which scans material surfaces with the sharp tip of a probe to achieve high resolution. The researchers found that the probe’s sharp tip could also be used to pattern a material, so as to create a wavy surface. The researchers heated the tip of a scanning probe to almost 1000 °C and pressed it into specific locations on a polymer surface, causing the polymer’s molecules to break up and evaporate at the locations where the probe was exerting pressure, and thus allowing the surface to be precisely sculpted.

Diffraction grating produced with a hot scanning probe. The red line shows the surface profile of the grating. Courtesy of ETH Zurich/Nolan Lassaline.

Diffraction grating produced with a hot scanning probe. The red line shows the surface profile of the grating. Courtesy of ETH Zürich/Nolan Lassaline.

The researchers were able to write surface profiles point-by-point into the polymer layer, almost arbitrarily, with a resolution of a few nm. When they completed the pattern, they transferred it to an optical material by depositing a silver layer onto the polymer. They then detached the silver layer from the polymer and used it as a reflective diffraction grating.

At ETH diffraction gratings are produced by patterning a polymer layer (green) with a hot scanning probe. A silver layer (grey) is then deposited, which is finally detached with a glass slide (blue). Courtesy of ETH Zurich/Nolan Lassaline.
At ETH Zürich, diffraction gratings are produced by patterning a polymer layer (green) with a hot scanning probe. A silver layer (gray) is then deposited, which is finally detached with a glass slide (blue). Courtesy of ETH Zürich/Nolan Lassaline.

Unlike traditional square-shaped grooves, the gratings developed using this approach can be shaped so that the interference of the reflected lightwaves creates precisely controllable patterns. “This [method] allows us to produce arbitrarily shaped diffraction gratings with a precision of just a few atomic distances in the silver layer,” professor David Norris said. 

A two-dimensional diffraction grating with a wavy surface, produced using the ETH technique (electron microscope image). Courtesy of ETH Zurich/Nolan Lassaline.
A two-dimensional diffraction grating with a wavy surface, produced using the ETH Zürich technique (electron microscope image). Courtesy of ETH Zürich/Nolan Lassaline.

The researchers used their method to produce an ultrathin grating that simultaneously coupled red, green, and blue light at the same angle of incidence. They demonstrated a variety of diffractive surfaces by using their method to analytically design and replicate 2D moiré patterns, quasicrystals, and holograms.

The researchers believe that their method for producing “perfect” gratings could lead to new possibilities for controlling light. “The new technology can be used, for instance, to build tiny diffraction gratings into integrated circuits with which optical signals for the internet can be sent, received, and routed more efficiently,” Norris said.

Lassaline believes that the diffraction gratings made using the team’s approach could be used to make miniaturized optical devices such as on-chip microlasers. Those miniaturized devices, he said, could range from ultrathin camera lenses to compact holograms with sharper images. The new method for producing diffraction gratings could have a broad impact in optical technologies such as smartphone cameras, biosensors, or autonomous vision for robots and self-driving cars.

The research was published in Nature (www.doi.org/10.1038/s41586-020-2390-x). 

EuroPhotonics
Autumn 2020
GLOSSARY
nanophotonics
The study of how light interacts with nanoscale objects and the technology of applying photons to the manipulation or sensing of nanoscale structures.
Research & TechnologyeducationEuropeETH Zurichlaserslight sourcesmaterialsopticsscanning tunneling microscopyCommunicationsConsumernanodiffraction gratingslight diffractionOptical Fourier surfacesnanophotonicsapplied opticsEuro News

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