This paper is devoted to study one of the most important applications of metamaterials based on Split Ring Resonator (SRR) which is thin-film sensing and sensing liquids properties. We provide a broad overview of thin film deposition methods, depositing the substance material in the gap, under the rings or on the rings. The sensor allows to extract various information about the dielectric properties and the loss from the resonant frequency. The analyses process consists of comparing the resonant shift to characterize the liquid type and the resonant dip to examine the impurities induced in the liquid. Split rings accommodate a wide range of individual preferences and sensing abilities. We demonstrate sensing features of rectangular split rings designed to resonate in the frequency range of 8-12 GHz.
© 2011 OSA
Currently, there is a wide need to improve the sensors performance and take advantage of a new technology; nevertheless, passive and active devices operating at high frequencies are a big challenge. Metamaterials hold great promises for such devices and for many applications like chemical and biological detection. It has already been shown that left-handed materials have many peculiarities which can be utilized to enhance the sensors applications.
Split Ring Resonators (SRRs) are subwavelength magnetic resonant structures, they provide a negative permeability. Using SRR as a probe offers a significant advantages as we transform detection of the dielectric layers from a transmission-amplitude level measurement to a resonant frequency position measurement, which is often more accurate. The capacitance of the SRR alters by adding a sample substance to the SRR. Moreover, the high intensity of the electric field E in the gap region makes the resonant frequency sensitive to small changes in the dielectric ε. This can shift the resonant frequency by an easily measurable scenario.
In this paper we will explore the use of metamaterials to sense thin films at X-band frequencies  and their application in sensing liquids properties in the second part.
In the first part, three coverage approaches for thin film sensing are proposed, and illustrated. Moreover, the shift in the magnetic resonant excited by the electric field is compared for each case. Metamaterials that are synthesized of Split Ring Resonator structures or their various counter parts have been found to exhibit highly lossy properties [2,3]. It is therefore important to study the factors that may influence the optimization of SRR designs . Such kind of information could help to minimize the loss in designed split open ring resonators. In SRR structure, there are two sources of loss, radiation loss and materiel loss. This investigation opens the door for a new application of left-handed material in sensing thin layers.
In the second part of this paper, the SRR consists of a metallic ring acts as an inductor. The dielectric substrate and gap behaves as a capacitance. Therefore, the ring resonates at f0 = 1/(LC)1/2. It has been observed that an electric field parallel to the gap of SRR can couple to the magnetic resonance and excite the LC-resonance. As a sensor application, sensitivity of SRR has been demonstrated for different kinds of materials, DNA , silicon and benzocyclobutane (BCB) , B2O3 , photoresist AZ9260  and SU-8 photoresist . The process allows sensing small quantities of liquids and analysis their containments. However, both the shift and strength of the resonant frequency reflect the dielectric properties of the induced layer. The features and limitation of the sensor has been also studied.
2. Thin-film sensing using SRR
Split rings accommodate a wide range of individual preferences and sensing abilities. The capacitance of the SRRs alters by adding a dielectric material to the structure. In this work, we demonstrate sensing features of rectangular split rings designed to resonate in the frequency range of 8-12 GHz. We consider them exclusively here for simplicity and familiarity. As it is mentioned before, there are three different methods, shown in Fig. 1 , of coverage: depositing the substance material in the gap, under the rings or on the rings.
These methods will be studies. Moreover, the shift in the magnetic resonant excited by the electric field is compared for each case. The various dimensions of the squared SRR for a functioning in X-band [8,2 GHz; 12,4 GHz] are given onto the Fig. 2 . Figure 2 shows the square for the ring SRR, dielectric used is the Rogers R04003 with ε = 3.38 and tanδ = 0.0027.
2.1 Film deposited in the gap
The electric field of SRR at resonant frequency is shown in Fig. 3 . In this figure one can readily observe that the high intensity of the electric field is in the gap region. This makes the resonant frequency sensitive to small changes in the dielectric permittivity.
Figure 4(a) shows the transmissions frequency shifts of SRR only and with a thin film of FR4_epoxy and silicon deposited in the gap. The resonance frequency and a shift are listed in the Table 1 . The simulation data presented in Table 1 have revealed that the Silicon film gives higher shift (about 270 MHz) than low resistivity FR4_epoxy (190 MHz).
2.2 Film deposited on SRR plane
Alternatively, the thin film is spun on the whole surface. This method is more practical because it offers several advantages, so in the fabrication process is much easier than depositing the film in the gap or under the rings. The simulation transmission of both material (epoxy and silicon) are depicted in Fig. 4(b) and compared with the transmission SRR only as it can be observed, there are significant shifts in the resonant frequencies of both samples. Alternatively, as shown in Fig. 4(b) and listed in Table 1, high resistivity epoxy film gives lower shift than silicon film, as in the case of depositing the film in the gap region.
2.3 Film deposited under rings
Thin film can be deposited under the rings as an intermediate layer. In the simulation, a significant shift in the resonant frequency is observed. The simulation transmission of both material (epoxy and silicon) are depicted in Fig. 4(c) and compared with the transmission SRR only as it can be observed, there are significant shifts in the resonant frequencies of both samples.
3. Sensing liquids properties
The proposed structure is presented in Fig. 5 . It consists of aluminum circular split rings fabricated on quartz substrate of 300 µm thick . The geometric parameters are induced in the caption of the figure. The simulations are carried out using CST Microwave studio software. Moreover, the boundary conditions are set with normal incident and the electric field is parallel to the gap. This allows the coupling between the electric field and the magnetic capacitance induced in gap. The structure exhibits left-handed behavior at 143.2 GHz.
3.2 Results and discussion
The liquids that we have studies are pure water, seawater, oil and essence. The dielectric properties are listed in Table 1. The insertion loss is depicted in Fig. 6 . The coupling capacitance between the rings and the gap capacitance both change after adding a dielectric layer on the ring. These changes are proportional to the permittivity of the dielectric layer. Since the resonant frequency of the SRR is inversely proportional to the capacitance. Thus adding a layer will shift down the resonant frequency. The amount of the shift depends on the permittivity of the dielectric layer. Since oil has a permittivity of 2.33, whereas essence has a permittivity of 1.948. So it is expected that the produced shift of the same thickness (amount) of both liquids causes a different shift. Indeed, after adding oil layer of 10 µm thick the resonant frequency is shifted by 7.4 GHz while, essence produces a shift of about 5.6 GHz. On the other hand, the complex permittivity of water is exceptionally high due to the polar molecular structure. Hence, water produces higher shift than that of oil and essence. In the simulations we consider the permittivity of water as 81 and the shift of f0 is 87.8 GHz.
3.3 Water containments
There is now an extensive literature on the interaction between mm-waves and different materials including water. Hence, it would be interesting to use SRR for monitoring the water containments. Therefore, our aim is to check the sensitivity of SRR, described in Section I, after adding layers of water and seawater. The dielectric behavior of pure water at mm-wave frequency can be described using Debye equation:Eqs. (1) and (2), one can find out that the real part of the complex permittivity of water is not dependence on the salinity. In order to explain that we simulated the insertion loss of SRR upon adding pure water and seawater. On the other hand, the dissolved salts in seawater (salinity up to 35% by weight) increase the ionic conductivity. Consequently, the dielectric loss factor of seawater is higher than that of pure water. However, it is interesting to analyze the sensitivity of the strength of the resonant frequency to the loss of the liquid layer. In this work the loss of water and seawater is 0.0013 and 0.502, respectively. Meanwhile, the permittivity is kept same (81). Figure 7 shows the simulated insertion loss of both structures. The thickness of both layers is 10 µm. The simulated resonant frequency of both liquids is exactly same, it is 55.4 GHz. That is obvious, because both liquids have same permittivity and the resonant frequency shift is function of the permittivity. Furthermore, the insertion loss of f0 of pure and seawater are −33.75 and −26.4 dB, respectively. The difference is attributed to the increase of the loss in the case of the seawater. This result indicates that an increase of the loss by 0.26% causes a reduction in the strength of the resonant frequency by 7.35 dB or in other words 21% of f0.
The sensitivity of SRR is also dependent on the thickness of the layer. The shift of the resonant frequency is highly dependent on the thickness of the dielectric layer. However, this in fact brings to mind the idea of using SRR to sense the thickness of a dielectric layer from the resonant frequency. An extensive study has been performed using asymmetric SRR to demonstrate the sensing effects to small thickness of PMMA (30 nm) . The idea is also applicable for liquids. SRR is sensitive to small thicknesses. In order to illustrate that, we have simulated the same liquids with same dielectric properties that are listed in Table 2 , but with smaller thickness, which is 5 μm.
As a comparison with the results obtained for 10 μm thick samples, which are listed in Table 2, one can see the highest shift associated with thin samples. Furthermore, the sensitivity to the loss in the liquids is slightly decreased. The reduction is about 5.6 dB or 18% of f0. At mm-wave frequency, the thickness of the sample should be at large enough in order to contain enough of the wavelength of interest to be measurable and to allow good characterization. Therefore, compensation should be done by choosing the optimal thickness. Moreover, Fig. 8 shows the sensitivity limitation of our design for water. It is clear from the figure that the shift of f0 drops down gradually as the thickness of the liquid increased. The shift is saturated at 20 μm.
In this paper, we have studied different sensing methods. The most striking result emerge is that the resonant frequency response of SRRs is tunable by small quantity of a dielectric material. Depositing a film on the rings is interesting because it offers possibility to sense variant types of materials including liquids. On the other hand, for cost reduction, it is sufficient to deposit small spots of expensive materials in the gap area instead of covering the complete surface. We have also compared between three different coverage approaches. Depositing the film on the rings is the most practical method and with the measurement of frequency shift we can identify our film. And we have demonstrated a liquid characterization process using circular split rings. The characterization procedure has been performed for the type and the containment of the liquid. Water, seawater, oil and essence liquids are studied. A notable shift in the resonant frequency has been observed, which was about 87 GHz. Moreover, the containment of the water is reflected by the amount of the strength of the resonant frequency. In this work, 21% change in the strength was perceived between pure water and seawater. The study showed also that the sensitivity of the suggested design is limited by the thickness of the liquid.
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