Development of robust optical structures made of darkness

Optical devices and materials allow scientists and engineers to harness light for research and real-world applications, such as sensing and microscopy. Federico Capasso’s group at the Harvard John A. Paulson School of Engineering Applied Sciences (SEAS) has devoted years to inventing more powerful and sophisticated optical methods and tools. Now, his team has developed new techniques to control points of darkness, instead of light, using metasurfaces.

“Dark regions in electromagnetic fields, or optical singularities, have traditionally posed a challenge due to their complex structures and the difficulty of shaping and sculpting them. However, these singularities bring potential for groundbreaking applications in fields such as remote sensing and precision measurement,” said Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior corresponding with -authored two new papers describing the work.

In 2011, Capasso’s lab introduced metasurfaces, or sub-wavelength-spaced arrays of nanostructures. In 2016, they used metasurfaces to develop high-performance metalenses—flat optical lenses composed of nanopillars that they fabricated using semiconductor lithography techniques—opening up a new approach to focus light using ultralight devices.

The latest studies from the Capasso group—published in Communication in Nature and Advances in Science—report how metasurface technology can harness not only light, but darkness as well.

“Both of these studies introduce new classes of optical singularities—regions of designed darkness—using powerful yet intuitive algorithms to inform the fabrication of metasurfaces,” said Soon Wei Daniel Lim, co -first author of the paper in Nature Communications with Joon-Suh Park.

In that study, Lim and collaborators designed and fabricated an optical device containing metasurfaces of titanium dioxide nanopillars that can control light to create an array of optical singularities.

To control exactly where these points of darkness appear, Lim used a computer algorithm to help him reverse engineer the design of the metasurface.

“I said to the computer: Here’s what I want to achieve in terms of dark spots, tell me what shape and diameter the nanopillars on this metasurface should be in order for this to happen,” he said.

As light travels through the metasurface and lens, it forms a prescribed array of dark spots.

“These dark areas are exciting because they can be used as optical traps to capture atoms,” Lim said. “It could potentially be used to simplify the optical architecture used in atomic physics labs, replacing today’s conventional equipment—instruments that take up 30 feet of lab table space—with compact, lightweight optical devices.”

Dark spots are not only handy for trapping atoms. They can also be useful as very accurate reference positions for imaging.

“The points of darkness are smaller than the points of light,” Lim said. “As part of an imaging system, these make effective measurement points to accurately discriminate between two different positions within a sample.”

Their Advances in Science paper, the Capasso group described a new class of optical singularities: extremely stable points of darkness in a polarized optical field, known as polarization singularities.

“We have designed points of darkness that can withstand a wide range of perturbations—they are topologically protected,” said Christina Spaegele, first author of the paper. “This stability opens the way to optical devices with high reliability and durability in a variety of applications.”

Previous research has achieved some polarization variations, but the conditions for maintaining the perfect area of ​​darkness are extremely fragile, making it easily destroyed by stray light or other environmental conditions.

“By shining light through a specially designed metasurface and focused lens, we can create an immobile polarization singularity completely surrounded by points of light—essentially creating a dark spot within a sphere of light,” Spaegele said.

The method is so powerful that even the introduction of a defect in the metasurface does not destroy the dark spot, but only changes its position.

“This level of control can be especially useful for imaging samples in ‘hostile’ environments, where vibrations, pressure, temperature, and stray light would normally disrupt the behavior. of imaging,” Spaegele said.

The team says these new developments in optical singularities have implications for remote sensing and covert detection.

“Points of darkness can be used to mask bright sources while imaging a scene, allowing us to see faint objects that would otherwise be obscured,” Capasso said. “Objects or detectors placed in these dark positions will also not give away their position by scattering light, allowing them to ‘hide’ without affecting the surrounding light.”