Giant intrinsic chirality from planar dielectric nanostructures

February 23, 2018

Harvard researchers have developed a metasurface, comprised of a single planar layer of nanostructures, which exhibits strong optical chirality in transmission. This means it can let circularly polarized light of one polarization pass through almost unhindered, while light of the opposite helicity is completely diffracted away. Such capabilities are incredibly useful for a host of applications, such as circular dichroism spectroscopy in the analysis of drug samples, and polarization filters in telecommunications.

This work challenges some long-held notions about chiral metamaterials and metasurfaces. 'Previously people thought that to achieve a strong, intrinsic chiro-optical response, the structures had to be complicated three-dimensional shapes, such as corkscrews or helices, in order to break the symmetry ', says Prof. Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard University. 'These 3D metamaterials were extremely difficult to fabricate on a large-scale. With this work, we showed that even a planar layer of dielectric nanostructures whose thickness is on the order of the incident wavelength can exhibit strong intrinsic chirality. This offers a practical way for such devices to be implemented in various applications as they can now be made in a single lithographic step. '

The authors were able to achieve this using gammadion-shaped nanostructures made of a relatively high-index dielectric material: titanium oxide. "This allows us to create planar structures with a strong in-plane magnetic moment, without resorting to a 3D geometry. By further optimizing the in-plane parameters of the gammadions, we can achieve the necessary coupling between the electric and magnetic moments to observe strong intrinsic chiro-optical activity," says Alexander Zhu, first author of the study. The authors experimentally achieved up to 80% circular dichroism in transmission at green wavelengths, with more than 90% of light with the correct helicity being transmitted at normal incidence. This result is on par with the state-of-the-art 3D metamaterials and greatly exceeds planar counterparts under similar conditions.

Further analysis points to some rich physics underlying this phenomenon of giant intrinsic chirality in planar structures. The authors found that the optical response of the gammadion structures is dominated by higher-order multipoles, such as the toroidal quadrupole and magnetic octupole. In naturally occurring media, such high orders are vanishingly small, such that only dipole responses are typically observed. However, their existence is critical since dipole modes radiate primarily along normal incidence, whereas the primary radiation direction for higher-order modes is off-normal. This provides some insight into the design and optimization of these nanostructures. The authors are now seeking to further improve these results and develop a fast, efficient sensor for spectroscopic detection of chiral compounds.
This research has been published in the journal Light: Science and Applications, an open access journal from Nature Publishing Group. This paper was coauthored by Wei Ting Chen, Aun Zaidi, Yao-Wei Huang, Mohammadreza Khorasaninejad, Vyshakh Sanjeev, and Cheng-Wei Qiu. It was supported by the Air Force Office of Scientific Research. This work was performed in part at the Center for Nanoscale Systems (CNS) at Harvard, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation.

Changchun Institute of Optics, Fine Mechanics and Physics

Related Nanostructures Articles from Brightsurf:

Unlocking PNA's superpowers for self-assembling nanostructures
Researchers at Carnegie Mellon University have developed a method for self-assembling nanostructures with gamma-modified peptide nucleic acid, a synthetic mimic of DNA.

Machine learning enhances light-matter interactions in dielectric nanostructures
The discovery has promising possibilities for the development of a wide range of photonic devices and applications including those involved in optical sensing, optoacoustic vibrations, and narrowband filtering.

Electron correlations in carbon nanostructures
Graphene nanoribbons are only a few carbon atoms wide and have different electrical properties depending on their shape and width.

Paving a way to achieve unexplored semiconductor nanostructures
A research team of Ehime University paved a way to achieve unexplored III-V semiconductor nanostructures.

Nanostructures help to reduce the adhesion of bacteria
Scientists has shown how bacteria adhere to rough surfaces at the microscopic level.

Diamonds are forever: New foundation for nanostructures
Researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) have fabricated a novel glass and synthetic diamond foundation that can be used to create miniscule micro -- and nanostructures.

How do atoms vibrate in graphene nanostructures?
Researchers from the University of Vienna, the Advanced Institute of Science and Technology in Japan, the company JEOL and La Sapienza University in Rome have developed a method capable to measure all phonons existing in a nanostructured material.

Heterophase nanostructures contributing to efficient catalysis
In the research on phase engineering of noble metal nanomaterials, amorphous/crystalline heterophase nanostructures have exhibited some intriguing properties.

Dresden physicists use nanostructures to free photons for highly efficient white OLEDs
Thanks to intensive research in the past three decades, organic light-emitting diodes (OLEDs) have been steadily conquering the electronics market -- from OLED mobile phone displays to roll-out television screens, the list of applications is long.

Self-healing DNA nanostructures
DNA assembled into nanostructures such as tubes and origami-inspired shapes could someday find applications ranging from DNA computers to nanomedicine.

Read More: Nanostructures News and Nanostructures Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to