1 Abstract— Chiral metamaterials are widely used to implement devices that transform the polarization of an incident electromagnetic wave, such as polarization rotators and linear to circular converters, but rarely used as circular polarization converters. This letter presents a chiral metamaterial that changes the handedness of a circularly polarized incident wave. This structure, which presents subwavelength thickness, provides polarization conversion with independence on both the front and back directions and the handedness of the incident wave. High polarization conversion efficiency is shown by both simulations and experiments. Index Terms— Circular polarization, Circular polarization converter, Chiral Metamaterial I. INTRODUCTION OLARIZATION is a characteristic key of electromagnetic (EM) waves in many fields such as wireless communications, liquid crystal display [1], optical data storage [2] or quantum information [3]. Since these applications are polarization sensitive, it is very important to control and manipulate the polarization state of the propagating EM wave. Polarization rotators and linear to circular polarizers are two traditional devices, comprised by multilayer structures, which transform the polarization state of an incident wave at microwave frequencies. The first one rotates the polarization plane of an incident linearly polarized wave a certain angle without changing its polarization state [4], [5]. The second one converts a linearly polarized wave into a circularly polarized one [6], [7]. Another way to manipulate the polarization is by using chiral media. It is well known that these kind of media present different refractive indices for each of the two circularly polarized eigenwaves. Different real parts of the refractive indices produce different phase delays for the right- and left- handed circularly polarized eigenwaves and, consequently, a rotation of the polarization plane (optical activity). Meanwhile, different imaginary parts result in different absorption for each eigenwave, causing a transformation on Manuscript received December 21, 2016. Revision submitted May 10, 2017. Accepted June 12, 2017. This work has been partially supported by the Spanish Government MINECO through the FEDER co-funded Research Project TEC2014-55463- C3-3-P and the Research Project TEC2012-33321. All the authors are with the Departamento de Ingeniería de Comunicaciones, University of Cantabria, Santander, Spain (e-mail: fernanos@unican.es, gomezal@unican.es, basterrj@unican.es, vegasa@unican.es). the polarization conditions (circular dichroism). Multilayer structures [4]-[7] and bulky chiral media [8] require large thickness, similar or even greater than the operation wavelength. This limitation can be a drawback at certain frequency regimes. Fortunately, with the introduction of metamaterials, there have appeared new structures, as planar chiral metamaterials (CMMs), which provide high optical activity with subwavelength thicknesses. It can be found in the literature several planar CMM based structures printed on PCB that act as polarization rotators [9]-[12] or linear to circular converters [13]-[16]. Besides the aforementioned applications, another less common device that acts over the polarization is the circular polarization converter. This device changes circularly polarized incident waves into the opposite handedness ones. There are few references in the literature that present circular polarization converters implemented on PCB at microwave and optical frequencies [17], [18]. Reference [17] presents a bi-layered planar CMM comprised of asymmetrical split rings that shows circular polarization conversion that depends on both the handedness and the direction of the incident wave. In this paper, we feature a new CMM based circular polarization converter at microwave regime (X-band). The CMM structure is constituted by short metallic strips orthogonally placed to continuous metallic ones. The proposed circular polarization converter provides high polarization conversion ratio with independence of the incoming circular polarization type. Due to the reciprocity of its geometrical pattern, the response of this metamaterial remains unchanged for the opposite direction of propagation. The content of the paper is organized as follows. First, Section II details the conditions that the linear and circular transmission coefficients of a structure must fulfil so that it can behave as a circular polarization converter. Following the aforementioned requirements, a novel asymmetric chiral meta- material structure is introduced. Next, Section III presents simulation and experimental results that show the efficiency of the proposed converter by means of the circular transmission coefficients, the axial ratio and the polarization conversion ratio. Finally, conclusions are drawn in Section IV. II. CIRCULAR POLARIZATION CONVERTER A. Circular polarization converter characteristics This section introduces the conditions that a circular polarization converter must fulfil to transform left- or right- handed circular polarization into its orthogonal one. Reciprocal Circular Polarization Handedness Conversion using Chiral Metamaterials O. Fernández, Á. Gómez, J. Basterrechea, Member, IEEE, A. Vegas, Member, IEEE P