9 Symmetry Approach to Chiral Optomagnonics in Antiferromagnetic Insulators
209
materials [27]. This revived interest in optical chirality [28–31], which has found a
number of applications in optics and plasmonics [32–37].
The results of Tang and Cohen [27] can be understood as follows. In order to
observe effects related to the chirality of light, we have to put the electromagnetic
field in contact with a chiral environment. This principle suggests a way for finding similar effects in other systems. For example, spin-wave dynamics in collinear
antiferromagnets can be represented in a form that closely resembles the SilbersteinBateman formulation of the Maxwell’s equations. Since collinear antiferromagnets
have two magnetic sublattices, the concept of electromagnetic duality and nongeometric symmetries can be generalized to transformations between the antiferromagnetic sublattices [38]. This allows to establish a conservation law for a spin-wave
analogue of the optical chirality. Injection of a spin current into the antiferromagnet
in this case has an effect similar to a chiral environment for light-matter interactions
inside a metamaterial [38].
It is also remarkable that both the Maxwell’s equations [39] and the dynamics of
antiferromagnetic spin waves [40] allow a formulation in the form of the Dirac equation for an ultra-relativistic particle. Such particles are characterized by conserving
helicity—a projection of spin on the linear momentum [41], which also satisfies the
definition of true chirality. Breaking the symmetry between right and left, in this case,
corresponds to a Weyl material [42], wherein quasi-particles with different helicities
are spatially separated. Symmetry considerations suggest that as far as single particle
dynamics is concerned, there should be some analogy between optical metamaterials,
Weyl semimetals, and chiral antiferromagnets. There has been several proposals in
these directions. For example, one can emulate the chiral magnetic effect in metallic
antiferromagnets [43].
These arguments have a direct impact on optospintronics. Since optical chirality
and spin currents share the same symmetry properties, it is possible to use polarized
light to excite magnon spin-photocurrents in antiferromagnetic insulators [44]. Circular polarized light in this case creates a direct flow of magnon angular momentum,
whose direction is controlled by helicity of light. This effect resembles the circular
photogalvanic effect in metals [45], which recently attracted attention in topological
electron materials [46]. It has been demonstrated that for a separated Weyl node, the
photocurrent excitation rate is determined by the product of the topological charge
of the node and the helicity of light [46].
In this Chapter, we review chiral excitations in optics and antiferromagnetic insulators together with their applications in optomagnonics. Our discussion is organized
as follows. In Sect. 9.2, we give a brief review of optical chirality and nongeometric
symmetries, which is generalized to antiferromagnetic spin-waves in Sect. 9.3, where
we discuss potential applications such as spin-current induced magnon dichroism.
Section 9.4 is reserved for photo-excitation of magnon spin currents with polarized
light. Summary and conclusions are in Sect. 9.5.
209
materials [27]. This revived interest in optical chirality [28–31], which has found a
number of applications in optics and plasmonics [32–37].
The results of Tang and Cohen [27] can be understood as follows. In order to
observe effects related to the chirality of light, we have to put the electromagnetic
field in contact with a chiral environment. This principle suggests a way for finding similar effects in other systems. For example, spin-wave dynamics in collinear
antiferromagnets can be represented in a form that closely resembles the SilbersteinBateman formulation of the Maxwell’s equations. Since collinear antiferromagnets
have two magnetic sublattices, the concept of electromagnetic duality and nongeometric symmetries can be generalized to transformations between the antiferromagnetic sublattices [38]. This allows to establish a conservation law for a spin-wave
analogue of the optical chirality. Injection of a spin current into the antiferromagnet
in this case has an effect similar to a chiral environment for light-matter interactions
inside a metamaterial [38].
It is also remarkable that both the Maxwell’s equations [39] and the dynamics of
antiferromagnetic spin waves [40] allow a formulation in the form of the Dirac equation for an ultra-relativistic particle. Such particles are characterized by conserving
helicity—a projection of spin on the linear momentum [41], which also satisfies the
definition of true chirality. Breaking the symmetry between right and left, in this case,
corresponds to a Weyl material [42], wherein quasi-particles with different helicities
are spatially separated. Symmetry considerations suggest that as far as single particle
dynamics is concerned, there should be some analogy between optical metamaterials,
Weyl semimetals, and chiral antiferromagnets. There has been several proposals in
these directions. For example, one can emulate the chiral magnetic effect in metallic
antiferromagnets [43].
These arguments have a direct impact on optospintronics. Since optical chirality
and spin currents share the same symmetry properties, it is possible to use polarized
light to excite magnon spin-photocurrents in antiferromagnetic insulators [44]. Circular polarized light in this case creates a direct flow of magnon angular momentum,
whose direction is controlled by helicity of light. This effect resembles the circular
photogalvanic effect in metals [45], which recently attracted attention in topological
electron materials [46]. It has been demonstrated that for a separated Weyl node, the
photocurrent excitation rate is determined by the product of the topological charge
of the node and the helicity of light [46].
In this Chapter, we review chiral excitations in optics and antiferromagnetic insulators together with their applications in optomagnonics. Our discussion is organized
as follows. In Sect. 9.2, we give a brief review of optical chirality and nongeometric
symmetries, which is generalized to antiferromagnetic spin-waves in Sect. 9.3, where
we discuss potential applications such as spin-current induced magnon dichroism.
Section 9.4 is reserved for photo-excitation of magnon spin currents with polarized
light. Summary and conclusions are in Sect. 9.5.