Metamaterials are artificial electromagnetic media that could exhibit several exotic phenomena that cannot be normally found in nature[
Chinese Optics Letters, Volume. 16, Issue 8, 081202(2018)
Experimental realization of a switchable filter based on a dynamically transformable array
We introduce a geometrically reconfigurable metasurface whose artificial “atoms” will reorient within unit cells in response to a thermal stimulus in the microwave spectrum. It can alternate between two contrasting behaviors under different temperatures and serve as a switchable filter that allows the incident energy to be selectively transmitted or reflected with an excess of 10 dB isolation at certain frequencies for both polarizations. The experimental results are consistent with the theoretical simulations, verifying the availability of an innovative method for manipulating electromagnetic waves with the merits of higher controllability for dynamic behavior and greater flexibility in the design process.
Metamaterials are artificial electromagnetic media that could exhibit several exotic phenomena that cannot be normally found in nature[
In this present work, we provide a feasible method to realize a switchable metasurface with superior performance and convenience in practical applications. We design and fabricate a geometrically reconfigurable array using shape memory alloys (SMAs), which suffer from a high temperature sensitivity that can cause changes to the geometry of the unit cells with the proper variation in temperature. Utilizing the reversible conversion between the two kinds of arrays through thermoregulating, an all-metal metasurface could realize the working mode of transmission or reflection at the frequencies of interest, as we expect, with a good isolation. We use equivalent circuit models to predict the electromagnetic behavior of the surface, and the switching function is demonstrated by full-wave analysis. The measured results coincide with the simulations, making the dynamically transformable array a practicable method for greatly improving the flexibility and simplicity in the design process of a reconfigurable metasurface in different electromagnetic spectra with alternative resonant structures.
When the configuration of the unit cell is a square loop aperture element, as shown in Fig.
Sign up for Chinese Optics Letters TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
Figure 1.Equivalent circuit model for the TE polarization of (a) a square loop aperture element, (b) a square aperture element, (c) a shorted square loop aperture element; and for TM polarization of (d) a shorted square loop aperture element.
Qualitative analysis from equivalent circuit models implies that we could modify the transmission characteristics of the surface by moving the patch and then switching the morphology of the unit cell between two different geometrical shapes corresponding to the transmissive mode and the reflective mode, respectively, at a certain frequency. One possible method is connecting the patch and the bar with a cantilever that could bend out of the aperture, as shown in Fig.
Figure 2.Operational principle of the model: morphology transforming between two different elements.
For getting a vivid illustration of the behavior of the equivalent circuit models, full-wave analysis of the metasurface is performed by using Computer Simulation Technology (CST) software. The transmission spectrum of the filter is determined by solving Maxwell’s equations with the finite integration technique; the numbers of cells per wavelength and cells per max model box edge are both 15 for the whole model structure. A three-dimensional (3D) view of the unit cell structure is shown in Fig.
Figure 3.(a) 3D schematic of the unit cell, where
In order to change the shape of the original unit cell properly, one simple and effective method is that we machine the sample using SMA that have been characterized by the changes in its transformation temperatures and its two-way shape memory effect. It can recover a shape not only on heating but also during cooling[
The experimental alloy is prepared by induction melting of high-purity initial elements with a nominal composition of copper (69.61wt.%), zinc (25.7wt.%), aluminum (3.81wt.%), nickel (0.8wt.%), lanthanum (0.04wt.%), and cerium (0.04wt.%) under an argon atmosphere at 1250°C and homogenized at 850°C for 24 h. The percentage value is the mass fraction of every component element. After furnace cooling the ingot is hot-rolled and cold-rolled into a plate with a thickness of 0.5 mm, then a sample of
By using the method mentioned in Ref. [
The process of thermal cycling under an applied stress to induce the two-way shape memory effect is commonly referred to as two-way training. First, heat the surface higher than 70°C to the high temperature phase with electric heaters, then bend the patch of each unit cell out of the aperture slowly to the deformation position where the value of
By repeating the above cycle for 6 times, the surface possesses the ability of switching its morphology, alternating between two different shapes, without an external force, on heating and cooling: when the sample is heated over 58°C, the appearance of the unit cell begins switching to the transmissive morphology by depressing the cantilever downward until the temperature is higher than 70°C and then the value of
Figure 4.Morphologies of the sample with external stimulation of (a) high temperature and (b) low temperature.
For characterizing the behavior of the sample, an experiment is carried out in a microwave anechoic chamber; two electric heaters are used for heating the sample to assure that the temperature is high enough. When heating the sample to the high temperature phase for TE polarization, we find that the passband is centered at 15 GHz with a small insertion loss of 0.4 dB, as shown in Fig.
Figure 5.Measurement results of the sample at different temperatures under (a) TE polarization and (b) TM polarization.
Figure 6.Measurement results of the sample at different temperatures under (a) TE polarization and (b) TM polarization for different cycles.
To better understand the physical picture for the variation of the transmission characteristics, we can gain some insight into the changes of the electric field strength and the surface current distribution on the surface at the resonance frequency, as shown in Figs.
Figure 7.Distributions of (a), (c) the surface currents and (b), (d) the electric fields for TE polarization under normal incidence when (a), (b)
Figure 8.Distributions of (a), (c) the surface currents and (b), (d) the electric fields for TM polarization under normal incidence when (a), (b)
For TE polarization, the maximal surface current is induced around the two flanks of the aperture and flows along the edges of the inner patch and outer bars in the opposite directions at 14.3 GHz, as shown in Fig.
The change rules of the electric field strength and surface current distribution on the surface for TM polarization at 6.8 GHz are like that for TE polarization when the value of
In this article, we propose a geometrically reconfigurable metasurface as a flexible platform for realizing a switchable spatial filter with the features of good reversibility and high switch isolation in the microwave part of the spectrum. This is accomplished by fabricating a transformable array whose unit cells could convert their structure from one geometrical shape to the other spontaneously and repeatedly in response to a thermal stimulation. Both simulations and measurements have been carried out to validate its performance, demonstrating that a switchable attenuation of more than 10 dB will occur at disparate frequencies for different polarizations during the cycle of heating and cooling. A further physical mechanism is shown by contrasting the distributions of the electric field and surface current on the surface at the same frequency for the two working modes. With the advantages of more flexibility for the design procedure and higher controllability for the dynamic behavior, the design methodology in this paper can be applied to other diverse functions such as high-sensitivity sensing, tunable filtering, frequency-selective detection, and multispectral imaging.
[2] N. Engheta, R. W. Ziolkowski. Metamaterials: Physics and Engineering Explorations(2006).
[3] W. Cai, V. Shalaev. Optical Metamaterials: Fundamentals and Applications(2009).
[17] K. P. Chen, S. C. Ye, C. Y. Yang, Z. H. Yang, W. Lee, M. G. Sun. Opt. Express, 24, 16815(2016).
[22] B. A. Munk. Frequency Selective Surfaces: Theory and Design(2000).
[24] G. F. Xu, N. C. Si, Y. X. Li. Chin. J. Nonferrous Met., 14, 825(2004).
[25] Y. Bellouard. Proceedings of the 7th European Symposium on Martensitic Transformations, 582(2008).
[26] M. Koyama, T. Sawaguchi, K. Ogawa, T. Kikuchi, M. Murakami. Mater. Sci. Eng. A, 497, 353(2008).
[27] G. F. Xu. Res. Stud. Foundry Equip., 23, 20(2001).
Get Citation
Copy Citation Text
Xin Chen, Jinsong Gao, Bonan Kang, "Experimental realization of a switchable filter based on a dynamically transformable array," Chin. Opt. Lett. 16, 081202 (2018)
Category: Instrumentation, measurement, and metrology
Received: May. 7, 2018
Accepted: Jun. 13, 2018
Published Online: Aug. 2, 2018
The Author Email: Bonan Kang (kbonan@163.com)