Tunably controlling waveguide behaviors are always desirable for various kinds of applications. In this work, we theoretically propose the possibility to realize a tunable high- pass waveguide by magnetically controlling magnetorheological fluids filled inside. Through computer simulations and numerical calculations, we find that the low-pass or high-pass behavior of such waveguides can be manually switched. Furthermore, the cutoff frequency and transmission band of the waveguides can be smoothly controlled by an applied magnetic field. It is revealed that the underlying mechanism lies in the field-induced anisotropic structure of magnetorheological fluids. By combining soft materials, this work shows a way to obtain magnetocontrollable properties of waveguides, which may help to achieve tunable properties for other metamaterial-based devices like invisible cloaks and photonic crystals. ENG metamaterials ENG metamateirals, namely, materials with negative permittivity, may show high-pass plasma-like behavior for an incoming electromagnetic field whose electric field is parallel to the direction which anisotropic property is shown (TE polarized). In gigahertz spectrum, ENG metamaterials are usually realized by metallic wire array. The effective permittivity in the z direction satisfies:, with to be the cutoff frequency under consideration. Proposed waveguide structure Conclusions X. W. Li 1, C. Z. Fan 1,2, and J. P. Huang 1 1 Department of Physics, State Key Laboratory of Surface Physics, and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Fudan University, Shanghai , China 2 School of Physical Science and Engineering, and Key Laboratory of Materials Physics of Ministry of Education of China, Zhengzhou University, Zhengzhou , China Bibliography [1] Y. Gao, J. P. Huang, Y. M. Liu, L. Gao, K. W. Yu, and X. Zhang, Phys. Rev. Lett. 104, (2010). [2] L. Zhou, W. Wen, and P. Sheng, Phys. Rev. Lett. 81, 1509 (1998). [3] N. Engheta and R. W. Ziolkowski, Metamaterials: Physics and Engineering Explorations (Wiley-Interscience, Hoboken, 2006). The second period: if H field continues to intensify, the microparticle chains would be elongated and extend throughout the waveguide cross section. This may open a conducting path between the waveguide upper and lower boundary, thus generating the metallic wire configuration, like the ENG metamaterials. The equivalent metallic wires are considered to be arranged as square lattice, with equal spacing and radii. The equivalent metallic wire model can be simplified into two dimensions when only TE polarized microwave is present for the sake of translational symmetry. FEM simulations are conducted to find out the high-pass behavior. Furthermore, if H field intensifies, the chains or columns would aggregate in to thicker columns, thus increasing the wire radius of the metallic columns. Through FEM simulation and numerical calculation, we observed that the cutoff frequency as well as the transmission band vicinity would suffer red shift when H field intensifies. The high-pass behavior of our proposed waveguide is hence tunable. Introduction Magnetocontrollable high-pass behavior of waveguides with magnetorheological fluids Theoretical analysis Properties of MR fluid waveguide (H field=0) Assuming the sphere composite inclusions are homogeneously distributed in the MR fluid with H=0, we carry out effective medium approximation (EMA) to derive the effective permittivity of the MR fluid. Maxwell-Garnett theory is applied to estimate the dielectric behavior of an individual composite particle (Fe 3 O Cu). Bruggman’s formula is used to estimate the effective permittivity of the whole MR fluid. It turns out when H=0, the MR fluid exhibits isotropic property and its effective permittivity remains to be positive. The finite element method (FEM) simulation result of the condition when H=0 shows that the waveguide is propagation supportive. Properties of MR fluid waveguide (H field ≠ 0) When a magnetic field, H, is exerted upon the MR fluid, the dielectric property of the entire waveguide medium would be absolutely different due to the presence of chains or columns formed by the microparticles. When H increases, the dielectric behavior and hence the waveguide behavior can be distinguished into two periods. The first period corresponds to the application of comparatively weak H which initiates microparticles to form single strand microparticle chains as the embryo for the metallic wires extending through the whole waveguide cross section. The second period of our system is introduced by larger H. In this situation, the existing chains start to form columns; each column acts as a conducting wire keeping good contact with the upper and lower boundaries of the waveguide. Here the wires are regarded to be composed of the effective medium of composite microparticles, which has metallic properties (equivalent metallic wires). In this period, the fluid behavior resembles that of ENG metamaterials. The first period: H-induced single strand chains can be considered as prolate spheroids randomly distributed in the host fluid. The spheroids have uniform size and orientation, which can be characterized by the longitudinal demagnetization factor g z. g z is a function of particle number per chain, n, and can reflect the intensity of magnetic field. Appling Maxwell Garnett EMA for spheroid inclusions, we may find that the effective permittivity of the MR fluid would remain negative ever since the particle number per chain (n) exceeds a certain value (critical threshold). Before that, the effective permittivity is positive, and the waveguide supports propagation. Our proposed waveguide structure consists of three zones, filled with oil (1 cm long), MR fluid (4cm), and oil (1cm). Parameters: length=6cm, height=0.9cm, width= 2.5cm. The suspensions in the MR fluid are composite core shell sphere particles with Fe 3 O 4 core (10 μm in radius) and Cu shell (10 μm in thickness ). The volume fraction of the particles among the MR fluid is 5%. H=0H≠0 The first periodThe second period InclusionsCore-shell spheresAcicular spheroidsEquivalent metallic wires H field intensityRelatively weakRelatively strong Effective medium property IsotropicAnisotropic If H intensifies Chain growth and elongation Chain aggregation and larger wire radius Waveguide behavior Propagation supportive Supports propagation when n n c High-pass (Cutoff frequency and mid-band frequency decreases as H intensifies)