Researchers at Harvard University have made significant advancements in the control and manipulation of “points of darkness” within light using metasurfaces. This breakthrough has opened up new avenues in the fields of remote sensing, precision measurement, and covert detection. By creating precise dark spots capable of capturing atoms or serving as reference points for imaging, the team has also developed resilient “polarization singularities” that maintain stable dark spots in polarized optical fields. The accompanying image depicts a metasurface, responsible for generating these point singularities, as captured through a scanning electron microscope. Credit: Harvard University.
Two recent studies detail novel techniques utilizing metasurfaces to create and control dark regions known as “optical singularities.”
Optical devices and materials have long been employed by scientists and engineers for harnessing light in research and practical applications such as sensing and microscopy. The group led by Federico Capasso at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has dedicated extensive effort to developing more advanced and powerful optical methods and tools. Their latest breakthrough involves the ability to exert control over points of darkness using metasurfaces, rather than manipulating light itself.
“Dark regions in electromagnetic fields, referred to as optical singularities, have historically presented challenges due to their complex structures and the difficulties associated with shaping and sculpting them. However, these singularities hold immense potential for groundbreaking applications in remote sensing and precision measurement,” explains Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, who also serves as the senior corresponding author for the two papers documenting their findings.
The metasurface concept was first introduced by Capasso’s lab in 2011, representing sub-wavelength-spaced arrays of nanostructures. In 2016, they expanded upon metasurface technology to develop high-performance metalenses—flat optical lenses composed of nanopillars fabricated through semiconductor lithography techniques. This innovation revolutionized the strategy for focusing light using remarkably lightweight devices.
The recent studies from the Capasso group, published in Nature Communications and Science Advances, demonstrate how metasurface technology enables control not only over light but also darkness.
“Both studies introduce new categories of optical singularities—specifically designed dark regions—employing powerful yet intuitive algorithms to inform metasurface fabrication,” states Soon Wei Daniel Lim, co-first author of the Nature Communications paper alongside Joon-Suh Park.
In one study, Lim and his collaborators designed and constructed an optical device incorporating metasurfaces composed of titanium dioxide nanopillars capable of manipulating light to generate an array of optical singularities.
To precisely determine the locations of these dark spots, Lim employed a computer algorithm to reverse engineer the metasurface’s design.
“I instructed the computer: Here’s what I want to achieve in terms of dark spots; now, tell me the shape and diameter the nanopillars should take on this metasurface to make it happen,” he elaborates.
As light passes through the metasurface and lens, it generates an array of predetermined dark spots.
“These dark spots are exciting because they can function as optical traps for capturing atoms,” adds Lim. “This could potentially simplify the optical architecture used in atomic physics laboratories, replacing conventional bulky instruments that occupy a lab table measuring 30 feet with compact and lightweight optical devices.”
Dark spots prove useful beyond atom trapping and can serve as highly accurate reference points for imaging purposes.
“Points of darkness are much smaller than points of light,” notes Lim. “In an imaging system, this makes them effective measurement points for accurately distinguishing between two distinct positions within a sample.”
In their Science Advances paper, the Capasso group presents a new class of optical singularities: remarkably stable dark spots within a polarized optical field, known as polarization singularities.
“We’ve designed points of darkness capable of withstanding a wide range of disturbances—they possess topological protection,” states Christina Spaegele, the paper’s first author. “This robustness paves the way for optical devices with enhanced reliability and durability in various applications.”
Previous research achieved certain polarization singularities; however, maintaining a perfect dark spot under those circumstances was exceedingly delicate, making them vulnerable to destruction by stray light or environmental factors.
“By directing light through a specially-designed metasurface and focusing lens, we can create an unwavering polarization singularity entirely encircled by points of light, effectively producing a dark spot within a sphere of brightness,” describes Spaegele.
The technique is remarkably robust to the point that even introducing a defect into the metasurface does not obliterate the dark spot; rather, it merely shifts its position.
“This level of control could prove invaluable for imaging samples in ‘hostile’ environments where vibrations, pressure, temperature, and stray light would typically hinder the imaging process,” Spaegele suggests.
The team asserts that these advancements in optical singularities hold significant implications for remote sensing and covert detection.
“Points of darkness could serve to mask bright sources while imaging a scene, allowing the observation of faint objects that would otherwise be overshadowed,” explains Capasso. “Objects or detectors placed in these dark positions would not reveal their presence by scattering light, enabling them to remain ‘hidden’ without affecting the surrounding light.”
References:
“Point singularity array with metasurfaces” by Soon Wei Daniel Lim, Joon-Suh Park, Dmitry Kazakov, Christina M. Spägele, Ahmed H. Dorrah, Maryna L. Meretska, and Federico Capasso, 5 June 2023, Nature Communications.
DOI: 10.1038/s41467-023-39072-6
“Topologically protected optical polarization singularities in four-dimensional space” by Christina M. Spaegele, Michele Tamagnone, Soon Wei Daniel Lim, Marcus Ossiander, Maryna L. Meretska, and Federico Capasso, 16 June 2023, Science Advances.
DOI: 10.1126/sciadv.adh0369
Harvard’s Office of Technology Development has safeguarded the intellectual property arising from these studies and is actively exploring opportunities for commercialization.
Additional authors who contributed to these papers include Dmitry Kazakov, Ahmed H. Dorrah, Maryna L. Meretska, Michele Tamagnone, and Marcus Ossiander.
This research received support from the Air Force Office of Scientific Research and the European Research Council.
Table of Contents
Frequently Asked Questions (FAQs) about metasurfaces
What are metasurfaces and how are they used in the research?
Metasurfaces are sub-wavelength-spaced arrays of nanostructures developed by the researchers at Harvard. They are utilized in this research to control and manipulate light, creating dark areas known as “points of darkness” or optical singularities.
What are the potential applications of these “points of darkness”?
The “points of darkness” created using metasurfaces have various applications. They can be used for remote sensing, allowing the detection of faint objects by masking out bright sources. They are also useful in precision measurement, serving as highly accurate reference points for imaging. Additionally, these dark spots have potential uses in covert detection and simplifying optical architectures in atomic physics labs.
How do the researchers ensure the stability and robustness of the dark spots?
The researchers have achieved stability and robustness in the dark spots through the design of “polarization singularities.” These singularities are topologically protected, making them resistant to disturbances such as stray light or environmental factors. Even with the introduction of defects, the dark spots remain intact, merely shifting their positions.
What are the advantages of using metasurfaces and optical singularities?
Metasurfaces and optical singularities offer several advantages. They provide precise control over light and darkness, enabling the creation of specific dark spots for various applications. The compact and lightweight nature of these optical devices makes them highly desirable, especially for imaging systems. Furthermore, the ability to hide objects or detectors without scattering light allows for covert operations and undetectable positioning.
More about metasurfaces
- Harvard University: Harvard University website
- Nature Communications Paper: Point singularity array with metasurfaces
- Science Advances Paper: Topologically protected optical polarization singularities in four-dimensional space
- Harvard Office of Technology Development: Harvard OTD website
3 comments
metasurfaces are like magic! they can trap atoms and create dark spots. imagine compact, lightweight optical devices replacing bulky equipment in labs. harvard’s breakthroughs always push boundaries.
harvard is always at the forefront of innovative research. these metasurfaces and optical singularities have exciting potential in various fields. imaging in “hostile” environments without interference? mindblowing!
wow harvard scientists r so smart they can control darkness? thats crazy! i wondr how they do it with metasurfes, sounds high-tech. cool research.