Abstract
Abstract We establish a simple quantitative criterium for the search of new dielectric materials with high values of refractive index in the visible range. It is demonstrated, that for light frequencies below the bandgap, the latter is determined by the dimensionless parameter η calculated as the ratio of the sum of the widths of conduction and valence bands and the bandgap. Small values of this parameter, which can be achieved in materials with almost flat bands, lead to dramatic increase of the refractive index. We illustrate this rule with a particular example of rhenium dichalcogenides, for which we perform ab initio calculations of the band structure and optical susceptibility and predict the values of the refractive index n > 5 $n{ >}5$ in a wide frequency range around 1 eV together with comparatively low losses. Our findings open new perspectives in search for the new high-index/low-loss materials for all-dielectric nanophotonics.
Highlights
All-dielectric photonics [1, 2] is arguably the most rapidly evolving field of modern nano-optics
Small values of this parameter, which can be achieved in materials with almost flat bands, lead to dramatic increase of the refractive index. We illustrate this rule with a particular example of rhenium dichalcogenides, for which we perform ab initio calculations of the band structure and optical susceptibility and predict the values of the refractive index n > 5 in a wide frequency range around 1 eV together with comparatively low losses
We start from considering a simplified model of interband polarization in bulk material and show that the condition of the weak dependence of the bandgap on the wave vector in the whole Brillouin zone (BZ) results in a high value of optical susceptibility close to the absorption edge
Summary
All-dielectric photonics [1, 2] is arguably the most rapidly evolving field of modern nano-optics. The key factor defining the functionality of resonant all-dielectric nanostructures, besides their shape, is the value of the refractive index of the material forming resonant nanoantennas. We derive a simplified estimation for the susceptibility and demonstrate that the refractive index can be substantially increased if η becomes small, as it happens in materials with flattened valence and conduction bands. This situation takes place in ReSe2, for which we perform the ab initio analysis of the optical properties and predict that for the photon energies around 1 eV, the real part of the refractive index n > 5, which is the current record high value, and its imaginary part remains small. We predict extreme biaxial optical anisotropy in this material
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