Metasurfaces harness the power of darkness for new applications

Scientists and engineers at the Harvard John A. Paulson School of Engineering Applied Sciences (SEAS), led by Federico Capasso, have made significant advancements in the field of optical devices and materials. Their recent work focuses on harnessing darkness, or optical singularities, using metasurfaces.

Traditional optical singularities, which are dark regions in electromagnetic fields, have presented challenges due to their complex structures and difficulties in shaping and manipulating them. However, these singularities have immense potential for applications in remote sensing and precision measurement.

Capasso's team introduced metasurfaces in 2011, which are arrays of nanostructures with sub-wavelength spacing. In 2016, they used metasurfaces to create high-performance metalenses, which are flat optical lenses consisting of nanopillars. These metalenses revolutionized light focusing capabilities with lightweight devices.

The team's latest studies, published in Nature Communications and Science Advances, demonstrate how metasurface technology can control darkness as well as light. They have developed optical devices containing metasurfaces that can create arrays of optical singularities.

To achieve precise control over the locations of dark spots, the researchers employed computer algorithms to design the metasurfaces. By specifying their desired dark spots, the algorithms determined the shape and diameter of the nanopillars on the metasurface.

These dark spots have various applications. They can serve as optical traps to capture atoms, potentially simplifying the optical architecture used in atomic physics labs. Additionally, they can act as highly accurate reference positions for imaging, allowing for precise measurement and discrimination between different positions within a sample.

In their Science Advances paper, the Capasso group introduced polarization singularities, which are extremely stable dark spots in a polarized optical field. These polarization singularities are topologically protected and can withstand various perturbations, offering durability and reliability in optical devices.

The team achieved exceptional control over polarization singularities by using a specially-designed metasurface and focusing lens. Even when introducing defects to the metasurface, the dark spot remains intact but shifts its position.

These advancements have significant implications for remote sensing and covert detection. Dark spots can be used to mask out bright sources while imaging a scene, allowing for the detection of faint objects that would otherwise be overshadowed. Moreover, objects or detectors placed at these dark positions do not scatter light, enabling them to remain hidden without affecting the surrounding light.

Source: Harvard John A. Paulson School of Engineering and Applied Sciences

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