Local polariton states in impure ionic crystals

TL;DR

This paper investigates how a single impurity in an ionic crystal near polariton resonance can create unique local states within the polariton gap, dominated by long-wavelength polaritons, differing from traditional vibrational states.

Contribution

It introduces a new class of local polariton states within the gap of the polariton spectrum caused by impurities, highlighting their distinct properties from vibrational states.

Findings

Local states appear within the polariton gap due to impurities.

These states are dominated by long-wavelength polaritons.

Properties differ from vibrational local states due to the density of states behavior.

Abstract

We consider the dynamics of an ionic crystal with a single impurity in the vicinity of the polariton resonance. We show that if the polariton spectrum of the host crystal allows for a gap between polariton branches, the defect gives rise to a novel kind of local states with frequencies within the gap. Despite the atomic size of the impurity we find that new local states are predominated by long-wavelength polaritons. The properties of these states are shown to be different from the properties of the well-known vibrational local states. The difference is due to the singular behavior of the density of states of polaritons near the low-frequency boundary of the polariton gap. Assuming cubic simmetry of the defect site we consider a complete set of the local states arising near the bottom of the polariton gap.