Synthesis of Bulk Medium with Negative Permeability Using Ring Resonators
 Author: Kim Gunyoung, Lee Bomson
 Publish: Journal of electromagnetic engineering and science Volume 16, Issue2, p67~73, 30 Apr 2016

ABSTRACT
This paper presents simple expressions for the effective permeability of bulk metamaterial consisting of ring resonators (RRs) or split ring resonators (SRRs) based on the convenient geometrical factors of the structure compared with wavelength. The resonant frequency dependence of the medium permeability, including loss effects, is analyzed in detail. Inverting the analysis equations, useful design (or synthesis) equations are derived for a systematic design process with some examples. This paper may particularly be useful for the design of a bulk metamaterial with a specific negative relative permeability at a desired frequency. The loss of metamaterials consisting of RRs (or SRRs) is also analyzed over a wide frequency band from 10 MHz to 10 THz.

KEYWORD
Design Equation , Effective Medium , Metamaterial , Negative Permeability , Ring Resonator

I. INTRODUCTION
Since Pendry et al. [1] introduced the split ring resonator (SRR) to construct an effective medium with negative permeability, a variety of research activities has been conducted. In [2], the electromagnetic fields generated from a source can be focused to a point using a fat lens characterized by an effective
μ_{r} = –1 (realized by SRR) andε_{r} = –1 (realized by thin wires [3]) medium. In the magnetostatic and electrostatic limits, theμ_{r} = –1 slab and theε_{r} = –1 slab can do the same, respectively. Specifically, quasistatic longitudinal magnetic fields from a magnetic point source can be focused by aμ_{r} = –1 slab, and quasistatic longitudinal electric fields from an electric point source can be focused by anε_{r} = –1 slab. The artificial periodic structure consisting of SRRs and thin wires was experimentally verified to have the property of negative refraction among others [4]. Isotropic bulk metamaterials were introduced [5] and developed [6]. A review paper [7] on bulk metamaterials made of resonant rings was also published. Although many applications have been made on transmission linebased metamaterials [8–10], bulk metamaterials have been only applied in cloaking [11], magnetic resonance imaging (MRI) [12–14], and wireless power transfer [15]. The usefulness of the metamaterial slab was demonstrated experimentally by comparing the MRI images with and without it [13]. Despite the many promising features of bulk metamaterials, their narrow band and relatively high loss inherent in SRRs prevent their rapid progress. As presented in [2], although considerable focusing is achieved, the imaginary part of the dielectric function prevents an ideal reconstruction.In this work, we investigate the degree of these limitations in a systematic and quantitative manner. In particular, the capacitively loaded ring resonators (RRs) initially introduced by Schelkunoff and Friis [16] are analyzed in terms of their bandwidth and loss. First, the effective permeability is expressed as a function of structural factors of the resonator relative to the used wavelength. Using this expression, the characteristics of the medium are examined over a wide frequency range, especially in terms of the newly defined munegative bandwidth and losses on RRs. Simple and convenient design equations are then derived by inverting the analysis equations. Some examples are also provided to demonstrate the effectiveness of the formulation.
II. ANALYSIS OF AN EFFECTIVE MEDIUM WITH RANGE OF RELATIVE PERMEABILITY
A natural magnetic material such as ferrite has long been modeled as a bulk structure consisting of many magnetic dipole moments (
m s) [17], which are defined as a product of a spin current and a spin area on an atomic or a molecular scale. Magnetization (M ) is then defined as their vector sum divided by a volume containing them. The effective medium concept, which replaces the particle behaviors on a microscopic scale with a simple effective permeability on a macroscopic scale, is a useful and reliable model when the wavelength is much larger than the particle dimensions. This conventional and familiar procedure can be extended to the problem of artificial effective medium consisting of magnetic particlelike RRs.Fig. 1 shows the unit of the artificial effective medium composed of many RRs. The RR is chosen instead of the SRR for its simple modeling. Similar effects are expected for SRRs. The details of the RR and its orientations with respect to an incident TEM wave are depicted in Fig. 1. The wave travels in the
z direction with the electric and magnetic fields oriented in thex andy directions, respectively. In this work, only the coupling of the magnetic fieldH_{y} leading to a relative effective permeability is considered. The side length of the unit cell isa . The radius of the RR isr , andt is the width of the planartype RR and the diameter of the ringtype RR. The chip capacitor is represented byC . It is inserted for the resonance of the RR, which has an inductance (L ) roughly proportional tor . The total resistance (R ) of the RR is the sum of the ohmic resistance and the radiation resistance. The radiation resistance is negligible whenr is much smaller than the wavelength. The modeling of RRs was already developed in [6,17], but it is briefly outlined here. LetH_{y} =H _{0} be the incident (or applied) magnetic field. Then, the induced phasor voltage to let the current (I ) flow in the reference direction (Fig. 1) is given bywhere
μ _{0} is the free space permeability, andω is the angular frequency of the incident fields. The induced current (I ) in Fig. 1 is determined bywhere
ω _{0} is the resonant angular frequency given by is the reactance slope parameter related to the bandwidth of the induced magnetism. The definition of magnetization (M ) [18], which has long been used for natural magnetic materials, can still be applicable to artificial bulk metamaterials made of RRs when the side length of the unit cell (a ) is much smaller than the wavelength. It is expressed asWhen
R = 0, the magnetization (3) due to the current (2) is in the same direction (paramagnetism) as that of the incident magnetic field whenω <ω _{0}, and the opposite (diamagnetism) is true whenω >ω _{0}. A wellestablished constitutive relation is given bywhere
B is the magnetic flux density andχ_{m} is the magnetic susceptibility. The relative effective permeability (μ_{r} ) is obtained aswhere
ω _{0}L is also the reactance slope parameter.Fig. 2 shows the real and imaginary parts of the relative effective permeability for the RRs presented in Fig. 1 obtained using (5) and an extraction method [19] based on EMsimulated
S parameters. Two types (planar type and ring type shown in Fig. 1) of typical RRs witha = 12 cm,r = 3.5 cm, andt = 1 cm are considered. They have been designed to haveμ′ = –1 at 13.56 MHz by a trial and error method. The EMbased extracted [19] effective permeability is shown to be in excellent agreement with (5), thus validating the derived expression (5). At the same frequency of 13.56 MHz,μ′′ = 0.2 for the planar type andμ′′ = 0.04 for the ring type.Here, we intend to analyze the bulk metamaterials made of RRs in terms of the effective medium in a more systematic manner. The ringtype RR is considered first. When
a < 0.1λ _{0}, the radiation resistance is much smaller than the ohmic resistance, andR can be approximated as [18].where
σ is the conductivity,R_{s} is the surface resistance per square, and 2r /t is the aspect ratio of the ringtype RR (Fig. 1). The only difference in the planartype RR is that its aspect ratio isπ times larger than that of the ringtype RR. The inductance of the ringtype RR is expresses as [18].The implication here is that if
t becomes relatively large for a fixedr, L and the slope parameterω _{0}L in (5) decrease and result in a larger bandwidth. To analyze (5) in detail, definewhere
λ _{0} is the free space wavelength at the resonant frequency (f _{0}),which must be reasonably greater than 0 to have the effect of diamagnetism and less than 0.5 to prevent a contact with an adjacent RR unit, and
where
t must be smaller than 2(a /2 –r ) but is not recommended to be very small because it increases losses of the medium. The may be called structural reduction factors, andm _{1}m _{2}m _{3} =t /λ _{0}.m _{1} must be very small, i.e., less than 0.1, to apply an effective medium concept.Using (6)–(10), (5) may be, as a function of a normalized frequency
f /f _{0}, expressed aswhere
η _{0} is the intrinsic impedance in free space given by The first term in the denominator may be understood as a lossperturbing factor in a lossless effective medium. As an initial observation, for a medium with lossless RRs (σ →∞), the relative effective permeabilityμ_{r} is reduced towhich is purely real and independent of
m _{1} (=a /λ _{0}). Moreover, the functional behavior of (12) is the same irrespective off _{0} oncem _{2} (=r/a ) andm _{3} (=t/r ) are chosen.In Fig. 3, we plot the real (a) and imaginary (b) parts of (11) and (12) as a function of the normalized frequency
f /f _{0} for the cases of different resonant frequenciesf _{0} = 10 MHz, 10 GHz, and 10 THz, assuming thatm _{1} = 0.1,m _{2} = 0.4, andm _{3} = 0.2. Note that Re[μ_{r} ] = –1 whenf /f _{0} is roughly 1.06 irrespective of the resonant frequency (f _{0}). However, the imaginary parts (b) are shown to become large as the frequency increases as the lossperturbing factor increases depending onIn Fig. 4, we plot the loss tangent 
μ′′ /μ′  of bulk metamaterials made of RRs based on the same assumption as in Fig. 3, whereμ′ is the real part of the relative effective permeability andμ′′ is its imaginary part. The loss tangent is observed to have two peaks. The first one is at the resonant frequency (f _{0}), and the second one is at the magnetic plasma frequency (f_{mp} ), where Re[μ_{r} ] = 0. The loss tangent in the frequency region betweenf _{0} andf_{mp} is shown to be relatively larger than that in other regions. Especially in the case off _{0} = 10 THz, the loss tangent is approximately greater than 0.017.III. SYNTHESIS OF AN EFFECTIVE MEDIUM WITH WIDE RANGE OF RELATIVE PERMEABILITY
If we want to find the resonant frequency (
f _{0}) with which we have a specific relative permeability (μ_{r} ) atf , by inverting (12), we obtainThis formula can be used as a design equation for a bulk metamaterial to have a negative
μ_{r} at any frequencyf .For a desired plasma frequency at
f_{mp} ,f _{0} is obtained byIn the frequency range of
f _{0} tof_{mp} ,μ_{r} is negative. We define the munegative (MNG) fractional bandwidth given byFig. 5 presents the theoretical and EMsimulated real parts of
μ_{r} as a function off /f _{0} for some different values ofm _{2} whenm _{1} = 0.1 andm _{3} = 0.2. The resonant frequency (f _{0}) of 10 GHz is used. Asm _{2} increases,BW_{MNG} also increases as expected in (15). The results in Fig. 5 are summarized in Table 1. When them _{2}’s are 0.35, 0.4, and 0.45, the theoreticalBW_{MNG} are 9.15%, 14.7%, and 23.25%, respectively. The EMsimulatedBW_{MNG} is 9.65% whenm _{2} = 0.35. The theoretical and EMsimulatedBW_{MNG} are shown to be in good agreement.Fig. 6(a) and (b) show the MNG fractional bandwidth (
BW_{MNG} ) and the magnetic loss tangent (μ′′/μ′ ) of the medium made of ringtype RRs (Fig. 1) as a function ofm _{3} (=t/r ) for differentm _{2} (=r/a ) values whenf _{0} = 60 MHz (used for MRI) andm _{1} = 0.01. When them _{2}’s are 0.35, 0.4, and 0.45,m _{3} must be less than 6/7, 1/2, and 2/9, respectively, to prevent contact among the resonators. TheBW_{MNG} for eachm _{2} is shown to considerably increase from 5% to 25% asm _{3} increases in the allowable range.m _{3} (=t/r ) is shown to play an important role in the enhancement ofBW_{MNG} . The reason for this finding is that asm _{3} increases, the inductance (7) and reactance slope parameter in (5) substantially decrease. The loss tangent (μ′′ /μ′ ) of the medium when Re[μ_{r} ] = –1 also decreases because the aspect ratio in (6) decreases asm _{3} increases. Whenm _{2} is 0.35 or 0.4, the loss tangent is shown to be as small as 0.005.The design Eq. (11) is used for a practical example. When an effective medium characterized by
μ_{r} = –1 atf is desired,f _{0} is given byIf a
μ_{r} = –1 medium is required atf (63.87 MHz) for MRI applications with a reasonable choice ofm _{1} = 0.0064,m _{2} = 0.4, andm _{3} = 0.4, (16) and (7) givef _{0} = 58.44 MHz andL = 3.17 nH, respectively. Then, the capacitance of the chip capacitor is determined as 2.34 nF.Let us take more design examples at another resonant frequency. In Table 2, we summarize the characteristics of the media composed of ringtype RR, planartype RR, and broadsidecoupled SRR with their dimensions. The relative effective permeability is –1 at 10 GHz. The
m _{1},m _{2}, andm _{3} are 0.1, 0.4, and 0.4 at 10 GHz, respectively. Using (16), we obtained the resonant frequencies for each case. TheBW_{MNG} of the ringtype RR is wider than that of the others. In addition, the loss of the ringtype RR is smaller than the others. The presented design procedures are scalable to other frequencies.IV. CONCLUSION
Through the provided simple expressions of effective permeability for bulk metamaterials consisting of RRs, their characteristics have been systematically investigated by introducing some convenient geometrical factors. The MNG bandwidth and losses in the medium can be engineered for the best possible performance. The MNG bandwidth has been shown to be enhanced up to 25% by decreasing the reactance slope parameter with the possible widest width of the used RR. The loss tangent when Re[
μ_{r} ] = –1 has been shown to be as small as 0.005. Some convenient synthesis equations have also been derived to obtain a systematic design process with some examples.

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[Fig. 1.] Unit of resonant ring, its typical crosssection types, and various structures of resonant rings. RR = ring resonator, BCSRR = broadsidecoupled SRR.

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[Fig. 2.] Relative effective permeability of the ring resonator in Fig. 1 (a = 12 cm, r = 3.5 cm, and t = 1 cm).

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[Fig. 3.] Relative effective permeability of the bulk ring resonators in Fig. 1 (m1 = 0.1, m2 = 0.4, and m3 = 0.2). Lossless RR and lossy RRs are made of PEC (σ→∞) and copper (σ = 5.8×107 S/m). (a) Real part and (b) imaginary part.

[Fig. 4.] Loss tangent μ′′/μ′ of bulk ring resonators (m1 = 0.1, m2 = 0.4, and m3 = 0.2) as a function of f/f0. Lossless RR and lossy RRs are made of PEC (σ→∞) and copper (σ = 5.8×107 S/m).

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[Fig. 5.] Real part of relative effective permeability as a function of f/f0 for different m2’s (f0 = 10 GHz, m1 = 0.1, and m3 = 0.2).

[Table 1.] Theoretical and EMsimulated BWMNG (f0 = 10 GHz, m1 = 0.1, and m3 = 0.2)

[Fig. 6.] (a) Munegative fractional bandwidth (BWMNG) and (b) magnetic loss tangent of the ringtype ring resonators (μ′′/μ′ when Re[μr] = ？1). Fixed f0 = 60 MHz and m1 = 0.01.

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[Table 2.] Theoretical BWMNG (f = 10 GHz, μr = ？1, m1 = 0.1, m2 = 0.4, and m3 = 0.4 at 10 GHz)