Ultrabroadband reflective polarization convertor for terahertz waves

Abstract

We design and experimentally demonstrate an ultrathin, ultrabroadband, and highly efficient reflective linear polarization convertor or half-wave retarder operating at terahertz frequencies. The metamaterial-inspired convertor is composed of metallic disks and split-ring resonators placed over a ground plane. The structure exhibits three neighboring resonances, by which the linear polarization of incident waves can be converted to its orthogonal counterpart upon reflection. For an optimal design, a measured polarization conversion ratio for normal incidence is greater than 80% in the range of 0.65-1.45 THz, equivalent to 76% relative bandwidth. The mechanism for polarization conversion is explained via decomposed electric field components that couple with different resonance modes of the structure. The proposed metamaterial design for enhancing efficiency of polarization conversion has potential applications in the area of terahertz spectroscopy, imaging, and communications. V C 2014 AIP Publishing LLC. [http://dx.Terahertz science and technology have seen rapid development, underpinned by many promising applications in imaging, sensing, and communications. 1 Towards these applications, high-performance terahertz components become essential for manipulating terahertz waves. One important group of components is related to polarization manipulation, including polarizers, wave retarders, and polarization rotators. In particular, conventional wave retarders can be achieved by using waveplates made of natural birefringent materials with a retardation effect. 2,3 Those wave plates require a relatively long propagation distance to obtain sufficient phase accumulation, despite the limited operation bandwidth and availability. Thus, more convenient and flexible approaches are desirable to fully manipulate the polarization state of electromagnetic waves.Over the past decade, metamaterials as artificial composite materials have attracted great attention due to their exotic electromagnetic properties unavailable to natural materials. 4 Such unique properties open up significant opportunities, including an alternative approach to manipulating the polarization of electromagnetic waves. 5-9 Several high-efficiency wave retarders have been demonstrated through different metamaterial microstructure designs, and these polarization wave retarders were demonstrated for conversion between different polarization states, such as linear to linear, 10-14 linear to circular, 15,16 and circular to circular polarization. 17 Compared with the traditional wave plates, these metamaterial-based wave retarders have advantages including subwavelength thickness, high conversion efficiency, angular tolerance, and scalability. In most of the existing wave retarders, the polarization states are manipulated in the transmission mode with a limited number of designs operating in the reflection mode. 11,12,15,18 For most retarders in the reflection mode, the undesirable high co-polarization reflection severely limits the polarization conversion efficienc...

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