3RD CONGRESS OF PORTUGUESE COMMITTEE OF URSI, LISBON, 3RD AND 4TH OF NOVEMBER 2009 1 Developing Chiral Media Based on the Inclusion of Metallic Cranks Gregorio J. Molina-Cuberos 1 , Angel J. Garc´ ıa-Collado 2 , Jos´ e Margineda 1 , Mar´ ıa J. N´ u˜ nez 1 , Ernesto Mart´ ın 1 . Abstract—A review of the manufacturing techniques for devel- oping chiral media based on the inclusion of cranks is presented. Chirality is usually obtained by a randomdistribution of helices in a host medium. Here the chiral elements are obtained by bending thin metal-wires in three segments. We will show that a random distribution of cranks is able of rotate the polarization angle of the transmitted linearly polarized wave at microwave frequencies. This effect can be enhanced by the precise location of cranks forming a 2D periodic lattice. Several structures are analyzed looking for isotropy, reciprocity and homogeneity. Index Terms—Chiral media, Materials science and technology, Microwave measurements. I. I NTRODUCTION E LECTROMAGNETIC activity is the property of some materials to rotate a linearly polarized incident wave. These media are composed by elements with chiral symmetry, and the phenomena occurs at frequencies depending on the size of the elements. In such material the electric and magnetic fields are coupled, which can be macroscopically described through the following constitutive relations [1]:  D =   E - jκ  H  B = μ  H + jκ  E where  =  0  r is the permittivity, μ = μ 0 μ r is the permeability and κ is the chirality parameter. For a linearly polarized incident wave, the transmitted wave passing through such medium is rotated and elliptically polarized. This is a consequence of the differences on phase velocities and on absorption for the left -and right- circularly polarized components. These propagation constants can be determined by: k ± = ω √ μ (1 ± κ r ) where + and - subscripts are related with the right- and left circularly polarized waves, respectively, and κ r is the relative chirality κ r =( r μ r ) -1/2 . For homogeneous and isotropic media, the chirality param- eter is a complex number and its real part is related to the rotation angle by: θ =2dω √  0 μ 0 κ′ This work was supported by the Direcci´ on General de Investigaci´ on of the Spanish Ministerio de Educaci´ on y Ciencia, under the projects/grants TEC2006-13268-C03-01. TEC2006-13268-C03-02 and TEC2006-13268- C03-03. 1 Grupo de Electromagnetismo, Facultad de Qu´ ımica, Universidad de Murcia, Campus Espinardo, E-30100, Murcia, Spain. 2 Dpto. de Ciencias Polit´ ecnicas, Universidad Cat´ olica San Antonio, Guadalupe, E-30107, Murcia, Spain. therefore the experimental determination of the rotation angle provides information about the real part of the chirality. Materials with electromagnetic activity at microwave fre- quencies are usually made by spreading helical inclusions into a host medium. The helices must be randomly oriented with no special direction in order to assure isotropy, otherwise the result is a macroscopically bianisotropic material. We have developed a new kind of materials based on the inclusion of three segment wire hooks, “cranks”. In a previous paper, we show that a random distribution of cranks produces girotropy, and the rotation angle depends on the frequency following a Condon model, with only one frequency [2]. However, a helix is a much compact structure, and for the same total wire length helix are smaller than cranks, which makes the number density of cranks to be lower than the one using helices and increases the inhomogeneity. Therefore, the material can present both local density varia- tions and accidental alignments of the cranks, which causes fluctuations of the transmitted wave when the sample is rotated or when a different sample area is illuminated by the incident beam. To experimentally determine the rotation angle, we had to measure several samples and a mean value of the rotation angle was carried out. In order to enhance the isotropic degree, we have searched the chiral behavior by placing the cranks forming 2D periodic structures. We have designed several lattices composed by cranks with different sizes and orientations. In this paper we make a short review of some of our achievements in the development of chiral media based on metallic cranks. II. DESIGN OF CHIRAL MEDIA The most widely used fabrication technique consists on distributing, as randomly as possible, helices in a dielectric host medium. Following this procedure, chiral media using cranks instead of helices was manufactured, by first time [2]. Chiral elements were produced from a 0.4 mm diameter and 12.6 mm length copper wire. Wires are bent in three segments by two 90 degrees angles, all with the same handedness. The elements were dispersed in an epoxy resin with a low curing temperature. Special attention was paid to obtaining a random and homogeneous distribution without privileged directions. The samples were 30 cm diameter and 1.5 cm wide disks, with an average inclusion number density around 2 cm -3 . The density results much lower than the inclusion number density obtained when helices were used [3] and therefore a higher inhomogeneity degree. The homogeneity problem can be solved by locating the cranks forming 2D structures. For example, by introducing