An enormous refractive power of disordered perovskite is discovered

A research team led by Eugenio Del Re and Fabrizio Di Mei from the Physics Department of Sapienza in collaboration with Aharon J. Agranat from the Hebrew University of Jerusalem has found in the material an enormous refractive index able to transmit visible light. The research has been published in Nature Photonics and paves the way to new innovative applications

Finding materials with a high refractive index is a crucial goal in the development of many tools. In the making of optical lenses, for example, the higher the refractive index of a lens, the lower the thickness required to correct a given eyesight. For visible light, one of the highest known values is that of diamond, equal to approximately 2.4, while glass has 1.5 and water 1.3. To date, materials with an index greater than 5 are not known.

A research team led by Eugenio Del Re and Fabrizio Di Mei from the Physics Department of Sapienza, in collaboration with Aharon J. Agranat from the Hebrew University of Jerusalem, showed the first experimental evidence of a material with a giant refractive index, estimated above 26, for the whole spectrum of white light in the visible.

The disordered ferroelectric perovskite, a synthesis oxide with a crystalline structure, is so particular that it can accommodate a wide spectrum of elements and therefore show a great variety of physical properties.

When the material is kept at a certain temperature, 15 degrees Celsius, it manifests an anomalously high index of refraction. "At this critical temperature - explains Eugenio Del Re - a three-dimensional ordered mosaic of spontaneous polarization, called a super-crystal, is formed. This mosaic gives rise to the enormous increase in the dielectric susceptibility necessary to obtain a giant refractive index and realizes the conditions to allow light to enter and exit it."

The index of refraction is a dimensionless macroscopic quantity that allows us to describe how light propagates and which path it takes through a material. This index determines how a light beam bends through the interface that separates one material from another (refraction) and how much light is reflected at that interface (reflection). It also determines how light spreads in space (diffraction), and how it separates into its constituent colors (dispersion).

"The greater the value of the refractive index, the more diffraction, dispersion and angle of refraction are reduced - continues Del Re - for this reason, the experimental evidence shown suggests how materials with perovskitic structure can be used in innovative devices."

The high index of refraction allows a beam of white light to propagate in the crystal without diffraction and without dispersion, as if the space occupied by the substance is absent. The result is a beam that propagates perpendicular to the face of the crystal in which it enters regardless of the angle of incidence, a property that has never been observed before and which could allow, for example, the development of self-aligning solar panels.

These results open the way to applications in the field of devices for for the harvesting of solar energy, for optical devices without chromatic dispersion and diffraction, and and for a futuristic white-light photonics.

References:

Giant broadband refraction in the visible in a ferroelectric perovskite - Fabrizio Di Mei, Ludovica Falsi, Mariano Flammini, Davide Pierangeli, Paolo Di Porto, Aharon J. Agranat and Eugenio DelRe - Nature Photonics https://dx.doi.org/10.1038/s41566-018-0276-3