A Novel Epsilon Near Zero Tunneling Circuit Using DoubleRidge Rectangular Waveguide
 Author: Kim ByungMun, Son HyeokWoo, Hong JaePyo, Cho YoungKi
 Organization: Kim ByungMun; Son HyeokWoo; Hong JaePyo; Cho YoungKi
 Publish: Journal of electromagnetic engineering and science Volume 14, Issue1, p36~42, 00 March 2014

ABSTRACT
In this paper, an epsilon near zero (ENZ) tunneling circuit using a doubleridge rectangular waveguide (RWG) is proposed for the miniaturization of a waveguide component. The proposed ENZ channel and is located in the middle of the inputoutput RWG (IORWG). The ratio of the height to the width of the channel waveguide is very small compared to the IORWG. By properly adjusting the ridge dimensions, the tunneling frequency of the proposed ENZ channel can be lowered to near the cutoff frequency of the IORWG. For the proposed ENZ tunneling circuit, the approach adopted for extracting the effective permittivity, effective permeability, normalized effective wave impedance, and propagation constant from the simulated scattering parameters was explained. The extracted parameters verified that the proposed channel is an ENZ channel and electromagnetic energy is tunneling through the channel. Simulation and measurement results of the fabricated ENZ channel structure agreed.

KEYWORD
Double Ridge , Epsilon Near Zero , Rectangular Waveguide , Tunneling.

Ⅰ. INTRODUCTION
Metamaterials, which have been of interest in the fields of physics and engineering for the past several years, are artificial composite materials with negative indexes of permittivity or permeability. Recently, metamaterials with epsilon near zero (ENZ) have been investigated for interesting phenomena, such as supercoupling, transparency, cloaking devices, and highly directive antennas at microwave and optical frequencies [13]. The rapid growth and significant attention paid to ENZ materials is because of their ability to achieve a very long wavelength, allowing for the propagation of a “staticlike” character of the electromagnetic field.
Silveirinha and Engheta [4] investigated the theory of supercoupling, squeezing wave energy, and field confinement in narrow channels and tight bends using ENZ materials. Previous studies [5,6] have experimentally demonstrated ENZ materials that exhibit supercoupling and energy squeezing using a microwave waveguide. These effects can be applied to a waveguide bend, an antenna matching circuit, a microwave filter, and a dielectric sensing application [1,79]. These technologies have a potential advantage for miniaturization because the electromagnetic energy is tunneled at the specific frequency through a narrow waveguide regardless of the total length of the ENZ channel. Radiation loss can be reduced, as compared with other metamaterials, because these technologies are based on a microstrip and waveguide shape. The tunneling frequency is near the cutoff frequency of the dominant mode in the waveguide. In order to minimize the size of a waveguide device, an ENZ channel can be used by a doubleridge rectangular waveguide (RWG) that has a lower cutoff frequency, instead of a general RWG [10,11].
In this paper, we introduce an ENZ tunneling circuit using a doubleridge RWG and we explain the structure of the proposed ENZ channel. The cutoff frequency of the doubleridge RWGs, including a general RWG, is calculated. For the ENZ tunneling circuit using a RWG and a doubleridge RWG, scattering parameters were obtained using the commercial finite element method simulator, Ansoft highfrequency structure simulator (HFSS) software [12]. We discuss in detail two types of transmission resonance phenomena, such as zerothorder resonance (ZOR) and FabryPerot (FP) resonance, from the results obtained in the simulations. We also explain the approach adopted for extracting the constitutive parameters (effective permittivity, permeability, wave impedance, and propagation constant) of the ENZ channel from the scattering parameters using the retrieval method based on an RWG [13].
Using the extracted constitutive parameters, we explain that the proposed ENZ tunneling circuit using a doubleridge RWG is an ENZ channel, and we discuss, in terms of impedance matching, what electromagnetic energy is tunneled at the tunneling frequency. The measured results of the fabricated ENZ tunneling circuit were quite similar to simulated results, with tunneling frequencies being 9.169 GHz (simulated) and 9.172 GHz (measured).
Ⅱ. STRUCTURE OF THE PROPOSED ENZ TUNNELING CIRCUIT AND THE RESONANCE PROPERTIES
1. Description of the Structure of the Proposed ENZ Channel
Fig. 1 shows the geometrical structure of the proposed ENZ tunneling circuit, where the ENZ channel is centrally located in the inputoutput RWG (IORWG, WR90) with a cross section of
a × b (mm^{2}). The ENZ channel is composed of a doubleridge RWG with a ridge widthw_{r} and a ridge gaph_{g} in a rectangular waveguidew_{c} × h_{c} (mm^{2}). The length of the proposed ENZ channel isd .The ratio of the height of the IORWG to that of the proposed ENZ channel is very high, given as follows:
Compared to RWGs, ridge waveguides [10,11] have the advantages of a wide fundamentalmode operation bandwidth, a low cutoff frequency, and low wave impedance. In the ENZ channel, the cutoff frequencies of the RWG and the doubleridge RWG are as follows:
with
and
where
c _{0} is the speed of light in freespace, and ln is the natural logarithm. As mentioned in Eq. (2), the cutoff wavelength of an RWG is equal to twice the waveguide widthw_{c} . We can estimate the cutoff frequency of the doubleridge RWG in Eq. (3). To calculate accurately the cutoff frequency of the doubleridge waveguide, the transverse resonance method (TRM) is used [14]. As seen in Fig. 2, the cutoff frequency is obtained by using the TRM under the initial value of the approximated cutoff frequency from Eq. (3).The cutoff frequency of the ENZ channel without the doubleridge is 10.338 GHz. As the gap of the ridge
h_{g} decreases to 0.1 mm and the width of the ridgew_{r} is maintained at 12.5 mm, the cutoff frequency of the ENZ channel decreases to 5.446 GHz. This indicates that the cutoff frequency becomes low and returns to high when the width of the ridgew_{r} increases to 12.5 mm, keeping the gap of the ridgeh_{g} equal to 0.5 mm. Consequently, because the cutoff frequency of the ENZ channel is reduced to less than the cutoff frequency of the IORWG at 6.557 GHz, we can appropriately change the structure of the ridge according to the purposes of the ENZ channel.The guide wavelength
λ_{g} , propagation constantk_{g} , phase velocityv_{p} , and the characteristic impedanceZ_{r} in the ENZ channel are given as follows [14,15]:where
λ _{0} is the wavelength in free space,λ _{c} is the cutoff wavelength of a waveguide, andZ_{r} (∞) is the characteristic impedance of the ENZ channel at infinite frequency. At the cutoff frequencyλ _{c} =λ _{0}, the guide wavelength is infinite, and the guide wavenumber is zero. This suggests that the electromagnetic fields remain almost unchanged along the longitudinal direction because they are similar to the static electromagnetic fields. Because the characteristic impedance of the ENZ channel is significantly increased around the cutoff frequency, the characteristic impedance of the ENZ channel is equal to the characteristic impedance of the IORWG. This implies that tunneling phenomena are observed to occur when two characteristic impedances are equal. The tunneling frequency is near the cutoff frequency of the ENZ channel.2. Scattering Parameters and Extraction of Constitutive Parameters
Let us consider the reflection and transmission properties of the ENZ channel using an RWG and a doubleridge RWG. First, to obtain scattering parameters, the ENZ channel using an RWG at an operating frequency of 10.370 GHz was created to test tunneling phenomena with a fullwave simulator, based on the finite element method, Ansoft HFSS. The results were processed by deembedding from each port of the IORWG to the interfaces of the ENZ channel. The IORWG uses a standard WR90 RWG. Table 1 lists the dimensions of the standard RWG and the proposed ENZ channel waveguide applied to a resonator of nearfield microwave microscopy. The cutoff frequency
f_{cr} and the cutoff wavelengthλ_{cr} of this ENZ channel are 10.338 GHz and 29.0 mm, respectively. To reduce the tunneling frequency in the rectangular waveguide, a doubleridge RWG was used, as shown in Fig. 1. Fig. 3 shows the simulation results of the ENZ channel using the RWG and the doubleridge RWG for a frequency range of 9.0 to 13.0 GHz. These results imply that the two types of transmission resonances, ZOR and FP, have been observed in the proposed ENZ channel. The tunneling frequency of the ENZ channel using the RWG and the doubleridge RWG are 10.370 and 9.169 GHz, respectively.Because the frequencies are near the cutoff frequency of each ENZ channel (10.336 and 9.152 GHz), the guide propagation constant is close to zero, implying that the length of the ENZ channel is insensitive because the guide wavelength is very long. These resonances are of the ZOR type. The ZOR mainly depends on the transverse crosssectional shape, which is related to the cutoff frequency of an ENZ channel. Other resonant frequencies of the ENZ channel using the RWG and the doubleridge RWG are (
f_{r2} = 10.977 GHz,f_{r3} = 12.645 GHz) and (f_{d2} = 9.886 GHz,f_{d3} = 11.750 GHz). These resonances are of the FP resonance type. The FP resonances occur when the length of the ENZ channel is around an integer number of the halfguide wave length,( n = 1, 2, 3, ...). The wavelengths of the channel using the RWG and the doubleridge RWG are 81.2mm (at
f_{r2} = 10.977 GHz) and 80.2 mm (atf_{d2} = 9.886 GHz), respectively. The length of the proposed ENZ channeld is 40 mm. The length of the ENZ channel at the resonance frequencies does not exactly agree with, because of the shunt susceptance for the stored electromagnetic energy at the interface of the IORWG and the ENZ channel.
The effective permittivity
ε_{eff} , permeabilityμ_{eff} , wave impedanceZ_{eff} , and propagation constantγ_{gd} can be extracted using the retrieval method from the simulated scattering parameters [13,16]. The scattering parameters can be expressed as functions of the reflection coefficientΓ and the transmission coefficientT as follows:Eqs. (10) and (11) can be solved for
with
and
In Eq. (12), the sign of the square root has to be chosen such that
Γ ≤ 1. The propagation constantsγ_{gr} andγ_{gd} of the IORWG and the ENZ channel using the doubleridge RWG can be calculated as follows:where
λ_{cr} is the cutoff wavelength of the IORWG. The effective permittivityε_{eff} and permeabilityμ_{eff} can be determined using Eqs. (12), (15), and (16) as follows:The effective wave impedance of the ENZ channel is where
Z_{0r} is the wave impedance of the IORWG. Then from Eqs. (19) and (16), the reflection coefficient and transmission coefficient are finally determined asand
At the tunneling frequency
Z_{eff} = Z_{0r} , the reflection coefficient and transmission coefficient have to be "0" and "1", respectively.The extracted effective permittivity
ε_{eff} , permeabilityμ_{eff} , normalized wave impedancez_{eff} , and propagation constantγ_{gd} are shown in Figs. 4 and 5.In Fig. 4(a), we note that the proposed structure is the ENZ channel, because the real value of the effective permittivity
ε_{eff} is 0.0264 at the tunneling frequency (9.169 GHz). The real value of the effective permeability is 0.0297 in Fig. 4(b). The normalized wave impedancez_{eff} , which is normalized with the wave impedanceZ_{0r} of the IORWG, is 1.06 in Fig. 5(a). The effective wave impedance of the proposed ENZ channel using the doubleridge RWG is typically lower than that of the IORWG, but that of the former was significantly increased around the cutoff frequency. Then, the effective wave impedance was equal to the wave impedance of the IORWG at the tunneling frequency. Thus, good impedance matching was observed, as with [17], implying that the tunneling phenomena of electromagnetic energy occurred in the ENZ channel. The propagation constantγ_{gd} of the ENZ channel as shown in Fig. 4 (b)is zero at the cutoff frequency of 5.3741 (rad/m) and the guide wavelength is 1.1691 m at the tunneling frequency (9.169 GHz). The phase variation in the electric fields in the ENZ channel using a doubleridge waveguide is approximately 0.2150 rad (12.3186°). It is clear that the effective permittivity has a value of zero near the tunneling frequency, and the propagation constant is near zero, which satisfies the conditions for ENZ behavior.Fig. 6 shows the field distribution along the channel at the tunneling frequency, which exhibits little phase shift along the channel.
Ⅲ. FABRICATION AND MEASUREMENTS OF THE ENZ CHANNEL
Aluminum was used for the ENZ channel circuit. The ENZ channel using the doubleridge RWG was manufactured using a wire electrodischarge machine, as shown in Fig. 7. The IORWGs, which were the size of a WR90 standard waveguide, were connected to the ENZ channel using separated screws. The ENZ tunneling circuit for characteristic measurements was connected to a network analyzer (Anritsu 37397C) through a coaxial waveguide adapter, which differs from the waveguide port used in the simulation to compute the input reflection coefficient (
S _{11}).This causes small differences between the measured and simulated input reflection coefficients, as shown in Fig. 8. The measured tunneling frequency
f_{m} is 9.172 GHz and has a tolerance of 3 MHz in comparison with the theoretical value (f_{d1} =9.169 GHz).Ⅳ. CONCLUSIONS
In this paper, we proposed an ENZ tunneling circuit using a doubleridge waveguide for the miniaturization and field confinement of waveguide circuits and the characteristics of that system were analyzed. The proposed ENZ channel which was centrally located in a standard RWG, used a narrowed doubleridge RWG. The tunneling frequency of the proposed ENZ channel can be reduced to near the cutoff frequency of the IORWG. From the extracted parameters, the proposed doubleridge waveguide channel must become the ENZ channel because the effective permittivity is zero at the cutoff frequency and close to zero at the tunneling frequency. The normalized effective wave impedance is almost "1" at the tunneling frequency, implying that we have verified that the electromagnetic energy was tunneled. The measured scattering parameters of the proposed ENZ channel agreed with the simulation results, which had tunneling frequencies of 9.169 GHz (simulated) and 9.172 GHz (measured). In the future, we will apply an ENZ channel using a doubleridge waveguide to the probe antenna of a nearfield microwave microscope.

[Fig. 1.] Geometrical parameters of the proposed epsilon near zero (ENZ) tunneling circuit. (a) Lateral side view and (b) crosssectional view. RWG = rectangular waveguide.

[Fig. 2.] Cutoff frequency against ridge variation wc = 14.5 mm, hg = 1 mm).

[Table 1.] Dimensions of the proposed epsilon near zero (ENZ) channel waveguide

[Fig. 3.] Simulation results of the proposed epsilon near zero (ENZ) channel. W/G= waveguide.

[Fig. 4.] Effective permittivity and permeability for the proposed epsilon near zero (ENZ) channel. (a) Effective permittivity εeff and (b) effective permeability μeff.

[Fig. 5.] Normalized wave impedance zeff and propagation constant γgd. ENZ= epsilon near zero.

[Fig. 6.] Electric field distribution along the proposed epsilon near zero (ENZ) channel.

[Fig. 7.] Photo of the fabricated epsilon near zero (ENZ) channel using the doubleridge rectangular waveguide (RWG).

[Fig. 8.] Simulated and measured scattering parameters (S11) of the proposed epsilon near zero (ENZ) tunneling circuit using a doubleridge rectangular waveguide.