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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012
Design and Characterization of Tunable Terahertz Metamaterials With Broad Bandwidth and Low Loss Yong-Jun Huang, Student Member, IEEE, Guang-Jun Wen, Senior Member, IEEE, Tian-Qian Li, Joshua Le-Wei Li, Fellow, IEEE, and Kang Xie
Abstract—Achievable tunable left-handed metamaterials are physically designed and numerically characterized at terahertz (THz) frequency in this letter. The Lu Bi Fe O (LuBiIG) garnet films prepared by liquid phase epitaxy (LPE) method on a gadolinium gallium garnet (GGG) substrate are used to achieve negative permeability, while the silver films are used to achieve negative permittivity. Both the LuBiIG garnet films and silver films are made physically available using the present techniques. The transmission and tunability characteristics of such metamaterials at THz frequency are numerically investigated, and the effective refractive index is retrieved in terms of the simulated transmission parameters. The numerical results obtained demonstrate that such metamaterials have a negative passband centered at 0.1415 THz. The passband can also be shifted by changing the applied dc magnetic field. These results depict a new way of designing low-loss THz transmission media and the resulted waveguides. Index Terms—Materials processing, materials science and technology, metamaterial, numerical analysis, terahertz radiation.
I. INTRODUCTION
metallic structures with metallic plasma resonance properties. For instance, metallic wires are used to achieve effective negative permittivity, and split-ring resonators (SRRs) are utilized to provide effective negative permeability [2], [7], [8]. The cut-wire pairs [9], dendritic cells [10], [11], symmetrical -shaped units [12], and double fishnet structures [13], [14] have been also employed to achieve negative magnetic response. The terahertz (THz) and optical metamaterials attracted some attention in recent years [15]–[18]. In various design methods of THz and optical metamaterials, however, the resonant cells should be made much smaller than the operation wavelength. Therefore, difficulties in fabrication of such small resonant elements and reduction in material loss turn out to be big challenges in reality. Moreover, the above metamaterials practically designed and physically fabricated are restricted by narrow bandwidths, so tunability of operating frequencies becomes an important issue in the metamaterials research, which motivates the present work. II. PROPOSED THZ METAMATERIALS AND TUNABILITY
I
N THE past 10 years, left-handed materials (LHMs), or metamaterials [1], or negative index materials (NIMs) [including double-negative (DNG) materials] with simultaneously negative permittivity and permeability , have attracted considerable attention since Smith et al. [2] experimentally demonstrated the existence of equivalent negative index materials. Much of the fascination arises from the unusual electromagnetic properties such as reversals of both Doppler shift and Cherenkov radiation [1], enhancement of evanescent waves [3], subwavelength resolution imaging [4], and broadening of impedance bandwidth of microstrip antenna [5], [6]. To date, most metamaterials are physically realized by artificial Manuscript received October 15, 2011; revised December 17, 2011; accepted February 22, 2012. Date of publication February 29, 2012; date of current version March 19, 2012. This work was supported in part by the National Natural Science Foundation of China (NSFC) under Grants 60571024, 60771046, 60588502, and 61171046; the State Key Laboratory of Millimeter Waves, Southeast University, Nanjing, China, under Project No. K201104; and the AFOSR under Projects AOARD-07-4024 and AOARD-09-4069, having continued from the earlier research work. Y.-J. Huang, G.-J. Wen, and T.-Q. Li are with School of Communication and Information Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China. J. L.-W. Li is with Institute of Electromagnetics and School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail:
[email protected]). K. Xie is with School of Opto-Electronic Information, University of Electronic Science and Technology of China, Chengdu 611731, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2012.2189090
Recently, novel microwave metamaterials consisting of ferrite and wire arrays were proposed theoretically, characterized numerically, and fabricated experimentally, respectively, by measuring the transmission, tunability, and refraction characteristics [19]–[24]. In this letter, based on previously developed ferrite techniques, THz frequency tunable metamaterials are proposed and designed. Special ferrite films [such as the Lu Bi Fe O (LuBiIG) garnet film [25] prepared by liquid phase epitaxy (LPE) method on a gadolinium gallium garnet (GGG) substrate] are used to achieve negative permeability. The silver films deposited on polyimide (Kapton 500HN, Krempel, Vaihingen/Enz, Germany) substrates using inkjet printing techniques [16] are utilized to achieve negative permittivity. The substrate polyimide, with a relative permittivity of , decouand a dielectric loss tangent of ples the interactions between the ferrite films and silver films. The finite element method is used to model the problem and optimize physical parameters so as to achieve broad bandwidth and low loss transmission at THz frequency. The simulated transmission characteristics and the retrieved effective parameters validate the designed metamaterials in the letter. The LuBiIG film is deposited on GGG substrate by using the LPE method from lead-free flux, as shown in Fig. 1 [25]. Fig. 1(a) depicts the image produced by scanning electron microscope (SEM) DS-130 C, while Fig. 1(b) illustrates the image from SEIKO SPA-300HV atomic force microscopy (AFM). Both of the pictures indicate that the epitaxy grown LuBiIG film possesses a pure single-crystal garnet phase and perfect
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HUANG et al.: TUNABLE TERAHERTZ METAMATERIALS WITH BROAD BANDWIDTH AND LOW LOSS
Fig. 1. Surface pattern images of the LuBiIG thin film [25]. (a) SEM image. (b) AFM image.
microstructure. The absorbance measured as a function of frequency within THz frequency range shows the loss smaller than 0.3 cm (in the range of 0.1–1 THz) [25]. For the film materials used, the resonance beamwidth is about 1 Oe, the saturation magnetization is about 1750 Gs, and the relative permittivity is about 13. Based on the above parameters of the LuBiIG film, the single negative permeability with very low loss can be obtained at THz frequency. Assume a TEM wave normal incidence on the ferrite film and an applied dc magnetic field acting on the ferrite film along the polarization direction of incident TEM wave. The effective permeability of ferrite was defined [26]. At a frequency above the ferromagnetic resonance (FMR) frequency and below the ferromagnetic antiresonance (FMAR) frequency, the effective permeability was shown to be negative [19]. For an actual LuBiIG film, the damping describing the loss in the film should be considered. Therefore, the FMR frequency is changed to , where denotes the damping of the ferromagnetic precession. Thus, the specific effective permeability becomes (1)
is the FMR frequency, denotes the gyrowhere magnetic ratio, stands for the characteristic frequency of ferrite film, represents the angular frequency, and indicates the applied magnetic field. The higher-order infinitesimal due to the very low loss in LuBiIG film is usually ignored in (1). Fig. 2(a) shows the effective permeability calculated using the above parameters of LuBiIG film under different magnetic fields as defined in (1). It
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Fig. 2. (a) Real and imaginary parts of effective permeability versus frequency calculated from (1) by using the parameters of LuBiIG thin film and different applied magnetic field values. (b) Schematic representation of tunable metamaterials consisting of LuBiIG and silver materials.
can be seen that the effective permeability exhibits typical resonant characteristics, as we expect. When the applied magnetic field is 50 kOe, the real part of effective permeability is negative in the frequency range from 0.142 to 0.145 THz. The imaginary part, which represents the loss of the medium, is much smaller than the real part and it thus negligible. Noticeably, the resonant frequencies of effective permeability increase from 0.1397 to 0.1453 THz as is increased from 49 to 51 kOe. On the other hand, the silver can be simply deposited on the substrate by inkjet printing technique or other conventional photolithography and etching techniques on thick polytetrafluoroethene (PTFE) substrate from Rogers (RT/duroid 5880). Fig. 2(b) shows the schematic element of LuBiIG–silver–LuBiIG structure tunable metamaterials. The silver strip is deposited on one of the two polyimide substrates, and the two substrates connect to the two LuBiIG films. The electromagnetic wave propagates in the metamaterials along the -direction with the electric field along the -direction and the magnetic field along the -direction. The applied dc magnetic field acts on the LuBiIG film along the -direction. The schematic element of such metamaterials may be the same model as those in [21]–[23], however the new method for achieving tunable metamaterials at THz frequency is proposed for the first time herein. After the physical parameters of LuBiIG film, substrate, and silver are determined above, we now optimize the size of each film using commercially available simulator HFSS ver. 13.0 (Ansoft). The initial theoretical analysis results show that the effective permeability of LuBiIG film under 50 kOe has a negative value in the range of 0.142–0.145 THz, and the plasma frequency of silver is about 1.5 THz when the periodicity is about 100 m and the radius of wire is about 2 m. Based on
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012
Fig. 3. Simulated transmission and reflection characteristics of LuBiIG–silver–LuBiIG structure metamaterials. (a) Magnitude. (b) Phase.
Fig. 4. (a) Effective refractive index retrieved from -parameters. (b) Tunability characteristics of metamaterials numerically demonstrated.
these results, the optimized transmission results are depicted in Fig. 3. The optimized parameters are found to be the following: m, m, m, m, and m. To demonstrate more clearly the transmission characteristics in THz frequency, the simulated frequency interval is set as 50 MHz per step. It can be seen that the metamaterials designed here have a passband centered at 0.1413 THz. The bandwidth, within which the magnitude above 8 dB, is about 2.4 GHz, and the transmission peak is about 0.5 dB at 0.1407 THz. The bandwidth of such metamaterials is primarily determined by the negative permeability band of ferrite, namely
due to the additional dielectric substrate. At a frequency below 0.1401 THz and above 0.1434 THz, the real part of effective refractive index has a fixed value close to 10, and the imaginary part has a large value close to 5. Therefore, the composite medium [28] in the frequency band turns out to be a conventional material with a positive refractive index. The tunability characteristic is demonstrated earlier by tuning the applied dc magnetic field. As shown in Fig. 4(b), the transmission passband shifts from 0.1385 to 0.1442 THz, while the applied magnetic field is increased from 49 to 51 kOe. The tunability of metamaterials also depends on the tunability of negative permeability in the ferrite film [21]. The silver periodic structure has a negative permittivity within a broad frequency region below the plasma frequency. Thus, the passband of metamaterials can be shifted in a broad THz frequency region. Moreover, the shifted bandwidth and transmission peak characteristics do not vary when the applied magnetic field is changed. This is due to the regular saturation magnetization of ferrite film. Therefore, this important property can be used practically in various applications.
Therefore, to obtain broader negative transmission bandwidth, one should choose the ferrite film that possesses big saturation magnetization. III. BROAD BANDWIDTH AND LOW LOSS The effective refractive index values of the LHM are retrieved from the simulated -parameters (shown in Fig. 3) by using the retrieval method [27] and are shown in Fig. 4(a). It is seen that the metamaterials exhibit a negative refractive index in a range of 0.1401–0.1434 THz with a very low loss characteristic. Comparing the frequency bands of negative refractive index [Fig. 4(a)] and negative permeability [Fig. 2(a)] at the same applied dc magnetic field of 50 kOe, some frequency shifts appear
IV. CONCLUSION In conclusion, tunable metamaterials at terahertz frequency are initially proposed, practically designed, and numerically characterized. It is evident that such tunable metamaterials can be achieved by using different material samples and also various applied external magnetic fields. Practically, it is shown that the hybrid of LuBiIG film and silver film exhibits
HUANG et al.: TUNABLE TERAHERTZ METAMATERIALS WITH BROAD BANDWIDTH AND LOW LOSS
a negative passband in THz frequency—for instance, a 8-dB bandwidth of 2.4 GHz with a transmission peak of 0.5 dB. The bandwidth can be further broadened by choosing a big saturation magnetization ferrite film. Moreover, the passband can be tuned from 0.1385 to 0.1442 THz by changing the applied magnetic field from 49 kOe via 50 kOe to 51 kOe. These results are very encouraging for designing low-loss THz transmission media and the resultant waveguides. It is expected that the proposed metamaterials can be used in various future submillimeter-wave components and also THz antenna designs.
ACKNOWLEDGMENT The authors are grateful to H. W. Zhang and Q. H. Yang with the State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China, for their help in choosing the parameters of LuBiIG film.
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