Interview

Abstract

Xiaofei Xu Xiaofei Xu from the School of Communication and Information Engineering, Shanghai University, talks to Electronics Letters about the paper ‘Double waveguide method to retrieve the electromagnetic parameters of biaxial anisotropic materials’, page 1224. My research field focuses on electromagnetics and microwave engineering. In this area, an interesting problem is how to accurately determine the unknown constitutive parameters of complex materials. Nearly 50 years ago, pioneers such as A. M. Nicolson, G. F. Ross, and W. B. Weir proposed a method to characterise simple isotropic materials by measuring their reflection/transmission coefficients. This method was subsequently named after their initials (NRW) and has been adapted for a variety of applications focused on anisotropic, dispersive and lossy materials. In consideration of these complex properties, e.g. biaxial anisotropy, new technical approaches are needed based on the original NRW method. This is what I find interesting and currently work towards. As is known, the material under test (MUT) in the original NRW method is assumed isotropic. For a biaxial MUT, the permittivity or permeability are anisotropic along different principal axes. To characterise the biaxial anisotropic properties, a spontaneous approach is rotating the MUT sample. Another solution is designing transverse electric TE10-TE20 waveguide fixtures, but these are complicated to fabricate. Alternatively, this work aims to address the problem by using simple measuring fixtures without changing the MUT orientation, in which a double waveguide (DW) method is proposed and demonstrated. The letter reports a DW method to characterise a biaxial anisotropic medium in which only two ordinary waveguides are needed. The measuring fixtures are simple to fabricate. The orientation of the MUT does not need to change, which is especially suitable for characterising a thin sample and simplifies the measurement. This work in this Letter is developed in two sections. First, the frame work of the DW method is analytically introduced. Then a proof-of-concept biaxial anisotropic medium is numerically presented in validation of the method. The inverse problem for biaxial materials is addressed by utilising simple DWs. In the mathematics, the biaxial parameters are coupled in a dispersion equation. These unknown parameters can be solved by introducing an additional equation. To make this possible, the DWs work as the physical counterparts of the mathematical equations. The correlation relationship between the DWs is used to obtain accurate closed form equations and predict the biaxial anisotropic properties of a MUT. Future work seeks to demonstrate the method with experiment, and to make this experimental demonstration more general and convincing, the MUT can be dispersive and with loss. Some problems regarding technique will need to be considered as part of this future experiment; e.g. the calibration, the evaluation of measuring error, and elimination of ambiguities etc. I believe that the DW method will become a promising and useful tool for microwave engineers in their practical tasks to measure biaxial anisotropic media. In addition to this work, I am also interested in studying new microwave components and antennas with enhanced performances using these complex materials. In recent decades, there has been interest in extending the NRW method, with its associated challenges, to characterise composite structures. These composites are homogenised as effective metamaterials. Some exotic behaviours, e.g. negative refractive index, are predicted from particular composites. A variety of applications such as directive antennas, miniaturised antennas, and microwave filters have been reported made of metamaterial element structures. To design the functional elements, the NRW method provides an important tool. In the next ten years, I think the method will be developed and become even more powerful. The complexity of both natural and artificial complex materials will be studied to make the method more robust and suitable for various applications.