Research On The Realization Of Arbitrary Phase Transformation By Impedance-matched Dielectric Metamaterials And The Application To Optical Devices

Metamaterials,a kind of artificially subwavelength structures with extraordinary electromagnetic properties,have triggered enormous interest.Transformation optics theory provides a general method and design tool for metamaterials to flexibly manipulate from the phase and path of electromagnetic wave.Utilizing the metamaterials,a host of striking electromagnetic devices have been proposed,such as beam shifter,electromagnetic concentrator and polarization rotator.The beam splitter and deflector based on finite embedded coordinate transformation have been proposed.However,their realization rely on irregular shape,which is difficult to integrate.In this work,we propose an indirect transformation method(ITM)to design a slab based on transformation optics.The slab can not only control the phase of the electromagnetic wave,but also convert a wavefront into any desired one and realize the polarization split.By extending the two-dimensional(2D)phase transformation into three-dimensional(3D)one,a transformation cylinder is designed to generate a hollow beam with controllable transverse electric distributions.The innovative work in the paper are as follows:1.A deflector and a polarization splitter with controllable phase are designed.Based on the principle of equal optical path length,the indirect transformation method is proposed to obtain a flat device.According to the space transformation,the transformation relationship between the original and transformed spaces are established.Then the materials of an impedance-matched nonmagnetic slab are obtained.In the transformation,we adopt a nonlinear coordinate transformation to ensure impedance match,and preserve the dispersion relation to render the device nonmagnetic.Based on the demonstrated phase transformer,we choose distinct deflection angles for two orthogonal polarizations and combine the two sets of material parameters into an anisotropic slab.As such,a wavefront-controllable polarization splitter is achieved.The splitter can effectively control the phase and the deflected direction of the output beam,which are independent of the device shape.The results are further validated by numerical simulations.The proposed method can be used to develop all-dielectric,lossless and broadband devices that can be easily constructed at optical frequencies.2.Based on 3D phase transformation,an effective scheme to produce a hollow beam is investigated.The proposed scheme can controllably convert the wave front of an arbitrary incident beam into another one by an impedance-matched slab cylinder.Applying the ITM,we establish a 3D phase transformation to obtain the relative material parameters about a plane wavefront converted into a deflected one.Utilizing the material parameters to manipulate the phase,a diverging and converging hollow beam are generated.The transverse optical field distribution of this beams presents a single annulus.The device is theoretically reflectionless because the 3D transformation achieves the impedance match,so the conversion efficiency is high.Then,based on the above created beam,two distinct deflection angles are set and the two sets of material parameters are combined into a cylinder,whereby the transverse optical field distribution of a hollow beam with two annulus is generated.The results are validated by 3D finite-difference time-domain(FDTD)simulations.