Study On Broadband Polarization Converters Based On Anisotropic Metamaterials

Circularly polarized waves have wide applications in sensing structures with chirality and stereoscopic 3D imaging.Conventionally,a birefringent crystal plate with a certain thickness is employed to realize linear-to-circular polarization conversion.However,it is inconvenient to integrate birefringent crystals with other optical components in optical circuits due to their bulky volume,which originates from their small birefringence index.Metamaterials have provided many exciting methods to realize efficient polarization conversion for exhibiting a variety of exotic optical phenomena and material properties that are unattainable with conventional materials.Anisotropic metamaterials that have modal birefringence can also be employed to realize linear-to-circular polarization conversion.However,their operation bandwidth is limited due to the dispersion of the phase delay between two orthogonal directions of light.To address this issue,we propose to weaken the dispersion of the phase delay through integrating two birefringent metamaterials together whose phase delay gradients have opposite signs.It has been demonstrated that the proposed design method can be utilized to realize constant ?/2 phase delay in a broad frequency range,enabling broadband polarization conversion.The main research work of this paper includes:(1)We have realized efficient conversion between circularly-and linearly-polarized light waves in a broad frequency range by utilizing anisotropic metamaterials(AMMs)consisting of multilayered metal-dielectric structures.An AMM waveguide array with a rectangular cross-section can be used to manipulate polarization states due to its giant modal birefringence index.We demonstrate how it is possible to achieve weak dispersion in the phase delay by using double-stacked AMM waveguide arrays.The weak dispersion in the phase delay originates from the different signs of phase delay and phase delay gradients from the two different AMM waveguide arrays.The condition of the dispersion-free phase delay for the transmitted wave has been theoretically derived from the transmission matrix as the propagation characteristic of the AMM waveguide is involved.We further reveal that the designed double-stacked AMM waveguide array can function as an efficient quarterwave plate that enables the conversion between the linearly polarized light and the circularly polarized light within a broad frequency band.In addition,the bandwidth over which the degree of linear polarization is nearly unity and over which the angle of linear polarization is kept at approximately 45o is basically consistent with the phase bandwidth.This offers a promising approach for developing a practical polarization converter in the terahertz domain.(2)We have realized efficient linear-to-circular polarization conversion in a broad frequeny band numerically and experimentally by using metallic gratings(MGs)that can support spoof surface plasmon polaritons at low frequencies.A MG array has modal birefringence since the two orthogonal directions of light waves have different propagation constants.A single-sized MG array can only realize linear-ro-circular polarization conversion in a narrow bandwidth.We demonstrate numerically and experimentally how it is possible to achieve constant ?/2 phase delay by combining two different MG arrays to form a double-stacked MG array.The weak dispersion in the phase difference within a broad frequency band results from the different signs of the phase delays and phase delay gradients from the two different MG arrays.We further point out that the designed double-stacked MG array can function as an efficient quarter-wave plate that enables the conversion between linearly polarized waves and circularly polarized waves within a wide frequency band.Numerical results show that,during the operation frequency regime the degree of linear polarization is nearly unity and the angle of linear polarization is kept approximately 45o since the two orthogonal polarization states of light have high and comparable transmission coefficients.Experimental measurements match well with the numerical simulations,demonstrating the designed double-stacked MG arrays can realize broadband and efficient linear-to-circular polarization conversion.This provides a valuable method to address the bandwidth issue for other metamaterials-based polarization manipulation components.