Radio Science, Volume 27, Number 2, Pages 161-168, March-April 1992 Experimental characterization of isotropic chiral composites in circular waveguides R. D. Hollinger, V. V. Varadan, D. K. Ghodgaonkar, and V. K. Varadan Research Center forthe Engineering of Electronic and Acoustic Materials and Department of Engineering Science and Mechanics, Penn State University, University Park (Received March 11,1990; revised September 9, 1991; accepted September 17, 1991.) Thispaper reports the first experimental characterization of microwave chiral composites in circular waveguides and the associated calibration that is required for accurate measurements. The experimental setup,samplepreparation, calibration, measurement technique, and data analysis arediscussed. Results are presented in the form of rotary dispersion and axial ratio and compared withthat obtained from free space measurements. It is demonstrated that aneffect similar to the Cotton effect canbe observed at microwave frequencies also. INTRODUCTION The theory of eigenmodes andE field patterns in circular waveguides containing isotropic chiral media has been previously investigated[Eftimiu and Pearson, 1989; Svedin, 1990; Hollinger, 1990; Hollinger et al., 1991]. Similar workhas been done for parallel plate waveguides containing chiral media [Varadan et al., 1988; Enghetaand Pelet, 1989, 1990; PeletandEngheta, 1990a,b]. However, until now no experimental work has beenreported for chiral materials in waveguides. Microwavechiral composites have been characterized in freespace by Tinoco and Freeman [1957] and more recentlyby Guire et al. [ 1988], Umaxi et al. [1991], and Ro [1991]. These experiments demonstrated that microwavechiral composites consisting of metal helices embedded in a homogeneous hostmaterial exhibit a phenomenon known in stereo chemistry as optical activity.Thisphenomenon encompasses both optical rotary dispersion (ORD) and circular dichroism (CD). Optical rotary dispersion can be Copyright 1992 by the American Geophysical Union. Papernumber 91RS02458. 0048-6604/92/91RS-02458508.00 explained astherotation of thepolarization of theE field by an amount dependent on the frequency and the handedness of the medium. This occurs due to a difference in the phase velocities of left circularly polarized (LCP) andfight circularly polarized (RCP) waves in the chiral medium. Circular dichroism is thedifferential absorption of LCP and RCP waves in thechiralmedium resulting in thetransmitted E field being elliptically polarized. It is hoped that the combination of theory and experiment may lead both to new techniques for characterizing chiral materials aswell aswaveguide components such as polarizers, attenuators, and terminations. The present method of characterization using a circular waveguide technique is analternative to the currently used free space techniques [Guire, 1988; Umari et al., 1991; Ro, 1991]. In this paper the experimental setup, sample preparation, calibration of the waveguide setup, measurements on chiral samples, anddata analysis will be described. The experimental results in the form of rotarydispersion (RD) andaxialratio (AR) will be discussed in relation to the Cotton effect, which is comprised of circulardichroism and the rotary dispersion that accompanies it in the absorption region[Barron, 1982],andtheeffects of concentration and handedness of the helices. A comparison is also madewith free space measured data. 161