Research On Polarization Converter Based On Metamaterial Structure

Terahertz wave have attracted wide attention in recent decades,and it is highly desired for researchers to control the polarization state of terahertz wave,while polarization is one of the basic characteristics of electromagnetic wave,which can be applied in many areas.The traditional methods to control polarization state of light make use of polarizer and wave plates,which are made of optical activity crystals with birefringence effect,always suffer from the disadvantages of narrow response frequency band and difficulties in optical system integration.In recent years,metamaterials have opened a new way to manipulate polarization state with the advantage of ultrathin devices and broadband performances.Generally,there are two kinds of metamaterial structures can be used for polarization conversion,anisotropic metamaterials and chiral metamaterials.It is always difficult for a single layer of these kinds of metamaterial structures to keep a high conversion efficiency in a wide bandwidth.While the operating frequency band can be broadened by stacking multi-layer metamaterial structures,which have unique resonance in each layer at neighboring frequency bands,the coupling of these resonances realize broadband response.But the broadband response is achieved by the price of bulkier devices,as multi-layer structures prejudice the device integration and complicate the fabrication process.What’s more,there is no reasonable explanation for the physical mechanism of the couple in these multi-layer structures up till now.Given this picture,three kinds of broadband polarization converters were proposed in this paper as follow: 1.Broadband linear polarization converter based on the coupling of bilayer metamaterials in the terahertz region,which can rotate the polarization direction of the incident wave with 90° at the range of 0.71 to 1.44 THz,with polarization conversion rate(PCR)higher than 90%.2.Broadband terahertz half-wave plate based on anisotropic polarization conversion metamaterials,which can rotate the polarization direction of linearly polarized incident wave for an angle of 2b in the range of 0.64 to 1.67 THz,with PCR higher than 85%.3.Broadband bilayer metamaterials polarization converter,which can rotate the polarization direction of the incident wave with 90° at the range of 0.55 to 1.37 THz,with PCR higher than 99.8%.And the Fabry-Pérot-like cavity model was established to explain the coupling mechanism of the polarization conversion in the bilayer metamaterial structures.The clear expressions including ideal and approximate model were deduced to provide an excellent explanation of how the Fabry-Pérot cavity formed in the sub-wavelength bilayer metamaterials structure.