Unidirectional Light Propagation in Natural Crystals

Abstract

In recent work, we showed that one-way light propagation, with respect to the incidence angle, can occur in simple, natural crystalline materials, such as crystal quartz, at far-infrared frequencies. Our results support the idea that natural crystals can serve as efficient and functional oriented asymmetric absorbers, features that have led to the use of materials such as the quartz we have studied in developing on-chip devices.

Asymmetric Reflection Induced in Reciprocal Hyperbolic Materials

“…Let us start by introducing the example hyperbolic material, which here is chosen to be crystal quartz. 9,29 This is a naturally occurring hyperbolic material with uniaxial anisotropy and low damping. In this crystal, the condition for hyperbolic dispersion is met in several regions across the infrared spectra due to infraredactive phonon resonances, making one of the principal components of the crystal's permittivity tensor, ( ), of opposite sign to the other two principal components.…”

“…Let us start by introducing the example hyperbolic material, which here is chosen to be crystal quartz. , This is a naturally occurring hyperbolic material with uniaxial anisotropy and low damping. In this crystal, the condition for hyperbolic dispersion is met in several regions across the infrared spectra due to infrared-active phonon resonances, making one of the principal components of the crystal’s permittivity tensor, , of opposite sign to the other two principal components. , These regions can be generally found between the transverse optical (TO) phonon frequencies and the longitudinal optical (LO) phonon frequencies. , Here, we will focus on the frequency range between 400 and 600 cm –1 (corresponding to free-space wavelengths between 16 and 25 μm) where there exist two regions where the component of parallel to the anisotropy, ε ∥ , is of an opposing sign to the two components perpendicular to it, ε ⊥ , as shown in Figure a.…”

“…This "asymmetric" light propagation is the basis for a variety of optical technological applications; an example of this is signalprocessing devices. 9,10 Nonreciprocal materials, such as magneto-optical materials and Weyl semimetals, have been investigated as a way to achieve asymmetric propagation of light because of their ability to break time-reversal symmetry. 11−15 For instance, the reflection of light has been successfully modulated using bulk magneto-optical materials, 16−19 as surface magnon-polaritons are highly nonreciprocal.…”

“…Controlling and modulating light propagation has long been a subject of intense interest, as it has enabled several key technological advances through the years. − More recently, considerable attention has been paid to the capability to select light propagating in a specific direction. − In particular, there has been tremendous interest in the type of propagation through some materials where light traveling forward shows a different behavior compared to when it is traveling backward. This “asymmetric” light propagation is the basis for a variety of optical technological applications; an example of this is signal-processing devices. , Nonreciprocal materials, such as magneto-optical materials and Weyl semimetals, have been investigated as a way to achieve asymmetric propagation of light because of their ability to break time-reversal symmetry. − For instance, the reflection of light has been successfully modulated using bulk magneto-optical materials, − as surface magnon-polaritons are highly nonreciprocal. In antiferromagnets, for instance, the hyperbolic behavior associated with magnon-polaritons has been shown to be an excellent mechanism for engineering reflectivity, and very recently, nonreciprocal reflection has been shown to be dramatically enhanced in structured magnetic gratings .…”