We report the design, fabrication, and measurement of a microwave triple-band absorber. The compact single unit cell consists of three nested electric closed-ring resonators and a metallic ground plane separated by a dielectric layer. Simulation and experimental results show that the absorber has three distinctive absorption peaks at frequencies 4.06GHz, 6.73GHz, and 9.22GHz with the absorption rates of 0.99, 0.93, and 0.95, respectively. The absorber is valid to a wide range of incident angles for both transverse electric (TE) and transverse magnetic (TM) polarizations. The triple-band absorber is a promising candidate as absorbing elements in scientific and technical applications because of its multiband absorption, polarization insensitivity, and wide-angle response.
© 2011 OSA
The near perfect absorption component is one of the fundamental building blocks for a microbolometer, a photodetector, hyper-spectral imaging, or as a coating material to minimize reflections from objects [1–6]. Metamaterial (MM)-based absorbers may have promising potential in these areas because of their high performance to absorb almost all incident radiations [7,8]. The MM absorbers are made by patterning periodic electric resonators in a single layer to perform electric resonance, spaced by a dielectric substrate with another MM layer or metal plane. The coupling between the two layers leads to the magnetic resonance. By properly tailoring the top electric resonator, the spaced dielectric substrate and the bottom layer, an MM absorber can be impedance matching to free space, minimizing reflection and transmission simultaneously, and all the incident power is consumed inside . Since the first MM absorber with near unity absorptivity was proposed, the design, fabrication, and characterization of MM absorbers have attracted considerable interest. There have been several demonstrating MM absorbers performed at microwave, terahertz, and infrared frequencies [1–14]. Although with high absorptivity, most of the existing MM absorbers are single-band, polarization-sensitive, and narrow accepted angles, which limits their potential applications to spectroscopic detection and phase imaging. Multiband, polarization-insensitive and wide-angle absorbers are urgently desired. Recently, the dual-band MM absorber has been reported by Tao et al. and Wen et al., which shows two distinct absorption peaks using a hybrid unit structure [11,12]. Owing to asymmetry geometry, they are dependent on the polarization of an incident wave. Here we report the triple-band MM absorber using three nested closed-ring resonators (CRRs) to form a compact single particle. The absorber exhibits dipolar response at three frequencies with near perfect absorption for both TE and TM polarizations. Moreover, the thickness of the proposed design is merely 2.4% of the shortest working wavelength. The presented design has several important advantages, such as having polarization insensitivity, having a wide angle with more than 0.9 absorption rate as the incident angle is up to 50°, being super thin, and having a compact design. These features make it a good candidate for potential applications.
2. Triple-band absorber design
The absorber’s efficiency is characterized as, where is the absorptivity and and are the reflectivity and transmissivity as functions of frequency ω, respectively. Clearly, the higher performance of an absorber is equivalent to minimize the and simultaneously. It’s convenient and efficient to use metal film as ground plane to minimize. Impedance matching to free space is a crucial step to ensure the incident power couple to the absorber with lower reflection. To design a multiband perfect MM absorber, the parameters of unit cell must be carefully optimized to make sure the impedance to free space is approximately one at every absorption frequency.
Figure 1(a) shows schematically the unit cell of the triple-band absorber as well as the propagation configurations of the incident electromagnetic (EM) wave. The MM absorber consists of three layers. The top layer consists of an array of three nested copper CRRs, which is primarily responsible for the electric response to the incident field. The bottom layer is a copper plane, which is used to zero the transmission and is responsible for the magnetic response, exciting antiparallel surface current to that of the top layer.
We performed computer optimization and simulation of the MM absorber using the commercial program CST Microwave StudioTM 2009. Periodic boundary conditions are set in the x and y directions, and an open boundary is defined in the z direction for the electromagnetic wave incidence and transmission. A loss-metal model is utilized for copper, the electric conductivity of which is s/m. The dielectric layer is simulated with electric permittivity and loss tangent. The boundary conditions are set as perfect electric conductor (PEC) and perfect magnetic conductor (PMC) on two pair faces to approximate the normal incident TEM wave propagating the planar CRRs array. The transmission and reflection are obtained from two waveguide ports placed in front and back of the simulated unit cell, , and due to the metal ground plane; then the absorptivity is calculated as . The optimized MM absorber had the dimensions, in millimeters, of L 1 = 9.6, L 2 = 7.3, L 3 = 5.5, a = 10, w = 0.5, t 1 = 0.018, and t 2 = 0.78.
Figure 2 shows the simulated absorptivity (black) as well as those of outer, middle, and inner rings alone. There are three absorption peaks at frequencies 4.02GHz, 6.75GHz, and 9.24GHz with absorptivity 0.99, 0.96, and 0.98, respectively. The full width at half-maximum (FWHM) of each absorption peak is 0.16GHz, 0.23GHz, and 0.33GHz, respectively, which showing a tendency that the higher the frequency is, the wider the FWHM will be. We also simulated the absorptivity of each single CRR with the same parameters of the triple-band absorber, as shown in Fig. 2. The red, blue, and green curves represent the absorptivity of the solely outer, middle, and inner CRR, respectively. Every CRR is responsible for one resonance peak; the bigger the CRR is the lower the resonance frequency is, which is approximately in inverse ratio to the side length of the ring . They match well with their composite’s response with little blueshift for the two peaks of higher frequencies, and the absorptivity almost keeps the same. The shifts of the absorption peaks result from the coupling with neighboring rings and we will discuss it in the latter section. Every ring responses for one resonant frequency and is almost independent with each other. The response of the composite CRRs is the sum of that of each single ring’s. This characteristic of the design can be used to design single, dual, triple and even more resonance peaks by virtue of combining one, two, three, or more CRRs together (not shown). Especially, the resonant frequency can be designed to the desiring frequencies by varying the ring’s perimeter. Since the unit cell is four-fold symmetrical, it has almost the same responses to transverse electric (TE) and transverse magnetic polarizations (TM) for normally incident EM waves, and the absorption effect is also robust for non-normal incident angles.
The triple-band absorber shown in Fig. 1(c) was fabricated using stand print circuit board (PCB) technology with outer dimensions of 200mm × 200mm. We experimentally verified the absorption performance of the MM absorber in a microwave anechoic chamber. An Agilent N5230C vector analyzer and two broadband double-ridged horn antennas are used to emit and receive the EM wave. Owing to the metal ground plane, the transmission is zero, only the S11 is measured, and the absorptivity. As shown in Fig. 3 , the experimental result (blue curve) has three absorption peaks at frequencies 4.06GHz, 6.73GHz and 9.22GHz with absorptivity 0.99, 0.93, and 0.95, respectively, which agrees well with the simulation results as replotted in Fig. 3 (red curve) for comparison. We also characterized the MM absorber’s wide-angle performance as shown in Fig. 4 . With the increasing of incident angle, all these three absorptions remain above 0.9 at 50°, which result from the weaker magnetic coupling between the top and bottom layers as the incident angle increases [13,14]. As these experiment results reveal, the designed MM absorber operates quite well over a wide range of angles of incidence for all three absorption peaks.
To get an insight into the origin of the triple-band absorption, we focus on the EM response of the triple CRRs and the metal ground plane at normal to plane incidence with the electric field polarized along the vertical sides and the magnetic field along the horizontal sides of the rings. (See Fig. 1. Owing to the symmetry design, it’s the same with another polarization). We monitor the electric fields, surface current densities on the top and bottom metal layers at resonance frequencies as shown in Fig. 5 . The ring’s sides that are parallel to the electric component of incident wave can strong couple with the electric field and supply an independent electric dipole response, and the surface charge oscillates along the sides driven by the external electric field (Figs. 5(a)–5(c)). The magnetic component of the incident wave penetrates between the top and bottom layers and generates antiparallel surface current on the CRRs and the ground metal plane, leading to the magnetic coupling and the response (Figs. 5(d)–5(i)). Figure 5 indicates that the absorption originate from the dipole electric response of the top rings and the magnetic response between the two layers. The first, second, and third absorption associated with the EM resonance of the outer, middle, and inner ring, respectively. The resonant frequencies are inversely proportional to the side length L i (i = 1, 2, 3), [15,16], which indicates that the longer the ring’s side is the smaller the resonant frequency will be.
As aforementioned there exists blueshifts of the triple-band absorption peaks relative to those of the outer, middle, and inner rings along, the mechanism can also be induced from the surface current distributions. Figures 5(d)–5(f) indicate that each resonance absorption originates from not only the dipole resonance with the external electric field but also a coupled-mode pair. There are anti-parallel surface current oscillations in the neighboring rings, which results from the interaction between the neighboring rings. The resonance coupling between the neighboring metallic sides depends greatly on the distance between them; the greater the distance, the weaker the coupling . The induced currents on the neighboring sides will excite the resonant magnetic response of the triple rings. Compared with each ring’ s EM response with external electric field and the magnetic response between the top ring and bottom metal plane, the electromagnet response between neighboring rings is weak. They may be identified as first and second order of EM response. Compared with the single ring, the interaction among these three rings amounts for the blueshifts of the resonant absorption frequency, but there is almost no influence to the strength of absorption. The blueshift can be decreased by increasing the distance between neighboring rings.
We further investigated the origin of the loss to understand the contributions of each part of the MM absorber. Figure 6 shows the absorption spectra of the absorber under four different loss conditions of metal layers and dielectric spacer: (i) PEC and lossless FR4 (green curve), (ii) PEC and loss FR4 (black curve), (iii) copper and lossless FR4 (red curve), (iv) copper and loss FR4 (blue curve). As can be seen from the absorption spectra, the latter three composites exhibit three absorptions at the same frequencies with different absorptivity. Without Ohmic loss, the sole dielectric loss can also realize near unity absorption (black). However, it’s not the same with loss metal and lossless dielectric composite (red); the lower the frequency is, the lower the absorption will be. Therefore we can’t judge how much energy is consumed in dielectric and in metal just by comparing under different conditions of material’s property in our design. To our multiband absorber design, the absorption arises from both the dielectric losses and Ohmic losses, but the Ohmic losses of the higher frequency absorption take more proportion compared with the lowest frequency absorption. This is different with results in [2,10,18]. To our knowledge, such an absorption mechanism has not been reported in single-band absorber [8,10,18].
In conclusion, we have designed, fabricated, and characterized a triple-band microwave absorber, with experimental absorptivity of 0.99, 0.93, and 0.95 at three separated frequencies respectively. The designed MM absorber is polarization insensitive and could achieve wide-angle absorption. Experimental results show that the triple absorptions remain over 0.9 as the incident angle ranging from 5° to 50°. Using the CRR as building block, the absorber has a very simple geometry structure, and it’s easy to realize single, dual, triple, even more band resonance absorption in a compact single particle unit cell. Other multiband absorbing devices lack this flexibility. With geometrical scalability, this multiband MM absorber may operate at other frequency regimes with near unity absorption (a terahertz triple-band absorber will be presented in another paper). These advantages aforementioned make it a good candidate to design a high-performance absorber used in explosives detection, bolometer, thermal detector, spectroscopic imaging, etc.
This work was supported in part by the National Science Foundation of China (NSFC) under Grants Nos. 60990320, 60990321, 60990324, 60871016, and 60901011 and in part by the 111 Project under Grant No. 111-2-05. X. P. Shen acknowledges support in part from the Science Foundation of China University of Mining and Technology under Grant No. 2007A031 and in part from the Graduate Innovation Program of Jiangsu Province under Grant No. CX09B_045Z.
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