Tunable Chirality Of Graphene Metamaterials In Mid-infrared Regime

Metamaterials have attracted more and more attention because of their high performance,miniaturization and integration.A chiral metamaterial is a material whose structure and mirror image cannot overlap with each other by rotation and translation,and can only overlap each other by turning over.When electromagnetic waves are spreaded on such a chiral metamaterial,the electric field and the magnetic field are cross-coupled,thereby the metamaterial can realize some optical phenomenon such as optical rotation effect.Although some nature substances in earth can be chiral,but the chirality is weak and difficult to control.So we need to achieve the obvious chiral properties and tune to the chirality by metamaterials.We also should know that not only chiral materials can achieve chiral properties,by some means,the achiral materials also can be chiral,such as when oblique incidence of light irradiate onto the achiral metamaterials,the achiral metamaterials can exhibit chiral properties.In this thesis,the graphene achiral metamaterial and the two-dimensional graphene chiral metamaterial are designed in mid-infrared regime,and the tunability of the chirality is also realized.The main work of this thesis is summarized as follows:(1)An achiral metamaterial is designed.The structure itself does not have chiral properties.So external three-dimensional chiral properties can be realized by obliquely incident planar electromagnetic waves onto the structure.When there is no graphene in the structure,the structure can exhibit the circular dichroism and the circular birefringenc.The circular dichroism of the transmission and reflection have a maximum when frequency is at 47 THz,and the maximum value is 0.3 and 0.57,respectively;the circular birefringence of the transmission and reflection has a maximum when the frequency is 48 THz,and the maximum values is 50° and 20°,respectively.When graphene is not added,the structure can exhibit circular dichroism and circular birefringence.When graphene is added,the circular dichroism and circular birefringence of the structure will have a blue shift and the intensity is also changed.After designing the basic parameters of the graphene achiral metamaterial,the circular dichroism and circular birefringence can be tuned by changing the Fermi level of the graphene material,the angle of the incident electromagnetic wave,the thickness of the substrate and so on.At the same time,through a lot of calculations,achiral metamaterial I designed has some difference about the absorption of left-handed circularly polarized light and right-handed circularly polarized light and the maxmium difference can be 30%.(2)A metamaterial with planar chirality is designed.Since the structure itself has twodimensional chiral properties,when the circularly polarized light is incident perpendicularly to the structure,the structure can exhibit asymmetric transmission phenomena for left-handed circularly polarized light and right-handed circularly polarized light.And the maximum of asymmetric transmission coefficient can reach 0.14.As the Fermi level of graphene increases from 0.3eV to 0.9eV,the extreme frequency of asymmetric transmission is blue-shifted by about 5THz,and the value of asymmetric transmission coefficient is almost constant.In order to realize the three-dimensional chiral property,that is,the optical rotation effect,circularly polarized light is obliquely incident on the two-dimensional chiral metamaterial.And the asymmetric transmission,circular dichroism and circular birefringence can be tuned by changing the Fermi level of the graphene material,the angle of the incident electromagnetic wave,and the thickness of the dielectric layer.