Silicon nanodisks can polarise light at any angle

When polarised light passes through air into an insulating dielectric crystal, it is partly reflected at the interface. But at one particular angle, known as Brewster’s angle, incoming light travels through the dielectric with no reflection at all.

Studies have shown that this trick arises when electric dipoles, induced by light in the dielectric, align with the reflection direction to inhibit optical reflection. However, this only happens naturally for a particular polarisation of light.

Ramón Paniagua-Domínguez from the A*STAR Data Storage Institute and co-workers investigated ways to expand the Brewster effect beyond the restrictions of specific light polarisations and angles. To do so, they turned to metasurfaces — devices that use micro- and nanoscale surface patterns to manipulate electromagnetic radiation in ways not seen in nature.

The team realised that if magnetic dipoles could be excited at the same time as the electric dipoles, the subsequent interference patterns would produce other directions and polarisations where Brewster’s effect would occur.

Because natural materials have very weak magnetic dipoles at optical frequencies, Paniagua-Domínguez developed a theoretical model to engineer this kind of response from a metamaterial.

“Silicon has one of the largest refractive indices at optical frequencies, together with very low losses,” he notes. “And when you shape it into nanospheres, with sizes smaller than the excitation wavelength, a high-quality magnetic-dipole response presents itself.”

The researchers fabricated an array of protruding disks, a few hundred nanometers in scale, onto a silicon surface.

Reflection tests with several different wavelengths and angles revealed results strikingly different from conventional runs — light polarised both parallel and perpendicular to the interface produced zero-reflection points, while Brewster angles could be controlled by selecting appropriate light wavelengths.

Paniagua-Domínguez then developed a dipole-based model to understand the observed phenomenon.

“The generalised Brewster’s effect may add numerous new functionalities to those already achieved with metasurfaces,” says Paniagua-Domínguez. “In particular, one could foresee devices that will perform particular operations, such as light-beam shaping, which will be controllable by the angle of incidence and polarisation state of the incoming light.”

The A*STAR-affiliated researchers contributing to this research are from the Data Storage Institute.