Manipulating the polarization of light is fundamental to many emerging areas of physics. It allows the creation of holograms, helps in producing ultra-compact, flat lenses and paves the way to digital and programmable metasurfaces. In this pursuit, chiral structures are crucial, but before now no efficient and easily fabricated devices have been produced for optical frequencies.

Chiral structures lack mirror symmetry and are found across all length scales in biology and chemistry. Chirality is typically detected through a differential response to left- and right-handed circularly polarized light (LCP and RCP). This is measured as either circular dichroism (a difference in transmission intensities) or circular birefringence (a rotation of the plane of linearly polarized light). Generally termed chiroptical responses, these tend to be very small in naturally occurring chiral materials. Recent work on manufacturing artificial chiral structures, such as arrays of 3D helices or 2D gammadions, has led to some improvement. However, 3D structures are difficult to manufacture for optical frequencies, and 2D materials only work at oblique incidence due to fundamental symmetry considerations, and are considered extrinsic chiral objects.

However, researchers at Harvard University and the National University of Singapore have reconsidered this problem, and found a neat solution by inducing high-order multipole resonances in a 2D chiral structure that replicate the symmetry of a 3D chiral structure.

Interacting electric and magnetic moments

A chiroptical response originates from the interaction of electric and magnetic moments. In 2D structures, electric currents are constrained to be tangential and lie in the plane of the structure, so magnetic moments (which are orthogonal to the electric ones) are always out of the plane. Therefore typically the way to achieve overlap between the electric and magnetic moments is to illuminate these 2D structures at an oblique angle, thereby realizing an extrinsic chiral response.

In contrast, 3D chiral structures, such as helices, support electric currents out of the plane of the array. These give rise to in-plane magnetic moments that can interact strongly with the electric moments, producing strong intrinsic chiroptical activity under illumination at normal incidence.

The key to the structure proposed and studied in this recent work is that it is made of a dielectric with a high refractive index; titanium oxide. In high-index dielectric structures of a thickness comparable to the wavelength, out-of-plane electric displacement currents exist. These displacement currents mean that electric and magnetic moments can overlap, even at normal incidence in a planar structure.

Chiroptical activity from high-order multipoles

The researchers created a 2D array of planar gammadions using electron beam lithography and atomic layer deposition of TiO2. The notion that a 2D planar structure with a plane of mirror symmetry can exhibit a large chiroptical response at normal incidence is counter-intuitive. However, the out-of-plane displacement currents and associated magnetic moments provide the necessary broken symmetry. The circular dichroism measured experimentally was around 80%, with a circular birefringence of 100,000 degrees per unit length at 540 nm wavelength. These results far exceed the chiroptical responses of any natural materials and go beyond other state-of-the-art artificial chiral structures.

When 87% of RCP light is transmitted in the zeroth diffraction order, almost all LCP light is transmitted in the first diffraction order at ±460. A waveguide layer is included in the structure to increase the transmission in the zeroth diffracted order, but it does not contribute to the chiroptical response.

The large chiroptical response is attributed to higher-order multipoles excited in the gammadions, thanks to careful tuning of the geometry. This is in contrast to most systems, whose predominant electromagnetic response is a dipolar one. A dipole dominated system intuitively radiates at normal incidence, and thus cannot support the diffracted modes under LCP illumination. A quadrupole or higher dominated system primarily radiates off-axis, so it can accommodate this behaviour. In a chiral structure the light couples to these multipoles differently depending on the helicity, thereby giving rise to a difference in the RCP and LCP transmitted intensities.

For more information, read the full article at Light: Science & Applications.