Application of metamaterial concepts to sensors and chipless RFID

Several strategies for the implementation of microwave sensors based on the use of metamaterial-inspired resonators are pointed out, and examples of applications, including sensors for dielectric characterization and sensors for the measurement of spatial variables, are provided. It will be also shown that novel microwave encoders for chipless RFID systems with very high data capacity can be implemented. The fields of applications of the devices discussed in this talk include dielectric characterization of solids and liquids, angular velocity sensors for space applications, and near-field chipless RFID systems for secure paper applications, among others.


Introduction
In this review paper, it is shown that transmission lines loaded with electrically small metamaterialinspired resonators can be applied in many diverse scenarios, including sensing of spatial variables, dielectric characterization of solids/liquids, and chipless radiofrequency identification (chipless-RFID). Transmission line metamaterials based on electrically small resonators (particularly split ring resonators -SRRs) were first proposed in [1], where it was shown that, by loading a coplanar waveguide (CPW) with SRRs and shunt inductive strips, left handed wave propagation was possible. On the basis of impedance and dispersion engineering, transmission line metamaterials based on SRRs and other resonant elements (e.g., complementary split ring resonators -CSRRs [2]) have been applied to the design of multiple devices exhibiting improved performance or novel functionalities (filters [3], dual-band components [4,5], enhanced bandwidth components [6], leaky-wave antennas [7], etc.).
In other applications of transmission lines loaded with SRRs or CSRRs (or related metamaterial particles), the resonance phenomenon, rather than dispersion and impedance engineering is exploited [8]. For example, resonator-loaded lines can be applied to the implementation of notch filters. Moreover, as long as the resonance frequency in such lines (transmission zero) is influenced by the region surrounding the resonant element and by the relative position/orientation between the line and the resonator, sensing of material properties  and spatial variables [35][36][37][38][39][40][41][42][43][44][45][46][47] is possible. It is also possible to use chains of resonant elements printed on a dielectric substrate for the implementation of microwave encoders (tags) useful for applications in chipless-RFID. In such RFID systems, tag reading is achieved by means of a dedicated transmission line, through near-field coupling to the tag [48][49][50][51][52].
This paper is focused on reviewing three applications of transmission lines loaded with metamaterial resonators: (i) sensors for angular displacement and velocity measurements, (ii) sensors 2 1234567890 ''"" for dielectric characterization, and (iii) microwave encoders for chipless RFID. The principle of operation is pointed out for each case, and an illustrative example is provided.

Angular displacement and velocity sensors
The type of angular displacement and velocity sensor reported here is based on a transmission line (CPW) loaded with an electrical-LC (ELC) resonator. Such sensors were first reported in [40], and then implemented in microstrip technology in [41], and by using S-shaped SRRs in [44]. The stator is a CPW circularly shaped, whereas the rotor contains a circular ELC resonator axially attached to it (Fig. 1). The ELC of the rotor is placed on top of the CPW in close proximity and parallel to it, so that coupling between the line and the resonator is possible. The working principle is based on coupling modulation by rotation. Namely, the ELC is a bi-symmetric particle, exhibiting an electric wall and a magnetic wall (orthogonally oriented) at the fundamental resonance. If the magnetic wall of the particle is aligned with the line axis (also a magnetic wall), coupling arises, and the transmission coefficient exhibits a notch with significant depth. Conversely, by rotating the particle 90º, i.e., by aligning the electric wall of the ELC with the line axis, coupling is prevented, and the line exhibits total transmission. Obviously, between these two (extreme) situations, the coupling (and hence the notch depth) is modulated by rotation and the relative angle between the line and the ELC can be determined ( Fig. 2).  Reprinted with permission from [40].
For the measurement of the angular velocity, a harmonic (carrier) signal is injected to the input port of the stator. Through rotor motion, line to resonator coupling is modulated with the result of an amplitude modulated (AM) signal at the output. From the distance between adjacent maxima of the envelope function, the angular velocity can be inferred, provided maximum transmission occurs twice per cycle. The envelope function can be obtained by means of an envelope detector (implemented by a diode and low-pass filter) preceded by an isolator, in order to prevent unwanted reflections from the diode. Figure 3 shows the principle of operation, the experimental set-up is depicted in Fig. 4, whereas Fig. 5 shows the envelope function obtained by an oscilloscope (corresponding to an angular velocity of 3.000 rpm).

Sensors for dielectric characterization
There are many types of sensors for dielectric characterization based on resonator-loaded lines. Here we report sensors based on frequency splitting [27,[29][30][31][32]. By symmetrically loading a line with two identical resonators coupled to it, a single notch at the fundamental resonance of the resonant elements arises. However, by truncating symmetry, e.g., by means of an asymmetric dielectric loading, two notches arise, and the frequency separation between them as well as the difference in the notch depths depends on the level of asymmetry. Therefore, the determination of material properties, specifically the permittivity, of a sample under test (SUT), as compared to a reference sample, can be inferred. In this paper we report a sensor based on a splitter/combiner configuration [31,32,34], where the splitter/combiner is loaded with a pair of SRRs (Fig. 6) [34]. The sensor is equipped with a pair of microfluidic channels, in order to be able to measure the complex dielectric constant of liquids, particularly, mixtures of deionized (DI) water and ethanol, considering DI water as reference sample (with well known dielectric properties). Fig. 7 depicts the response of the structure for various combination of DI water and ethanol for the SUT, and Fig. 8 shows the variation of the real and imaginary parts of the dielectric constant, inferred from the response of Fig. 7 according to the method detailed in [34].

Microwave encoders for chipless RFID
Typically, RFID systems, used for identification and tracking, are based on the use of tags equipped with chips, where the information relative to the object or item is stored. Chipped tags are relatively expensive for many applications involving low-cost items. An alternative to RFID systems based on chipped tags is chipless-RFID [54]. In such systems, the chip is replaced with a planar passive encoder, which contains the identification (ID) code. There are many types of encoders, working in time domain [55][56][57][58][59][60][61][62][63], frequency domain [48][49][50][51][52][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80], or even hybrid encoders [77][78][79][80], the latter exploiting several domains simultaneously. The main drawback of chipless-RFID systems is the limited data storage capability of the tags (encoders) as well as encoder size. Here we report a chipless-RFID system, where the number of bits is only limited by tag size. The approach, first reported in [51], and subsequently improved in various works [52,81,82], is based on near-field coupling between the reader and the tag and sequential bit reading. The tag is a chain of identical resonant elements etched or printed at equidistant and predefined positions on a dielectric substrate. The presence or absence of resonant elements in such positions determines the ID code. An alternative to tag absence in a certain position is resonator detuning by short-circuiting it, or by any other mean to drastically shift the resonance frequency (i.e., laser ablation, etc.). Tag reading is achieved by means of a transmission line fed with a harmonic signal tuned to the resonance frequency of the tag resonators. In a reading operation, the tag should be displaced above the transmission line of the reader, in close proximity to it, so that the resonant elements sequentially cross the line axis. By this means each time a resonator crosses the line, line-to-resonator coupling prevents the harmonic signal to be transmitted, thus generating an amplitude modulated (AM) signal that contains the ID code of the tag. As compared to other chipless-RFID systems based on multiple resonators, each one tuned to a different frequency (spectral signature barcodes), the proposed tags exhibit much larger data capacity, only limited by tag size. This is due to the fact that the feeding signal is a single tone signal, contrary to the multi-frequency (sweeping) signal required to read spectral signature barcodes, forcing the tag to exhibit a limited bandwidth and hence number of bits. Figure 9 sketches the proposed chipless-RFID system, whereas Fig. 10 depicts the transmission line of the reader, as well as a 10-bit tag with all resonators (S-SRRs) present at the predefined positions (corresponding to the code '1111111111'). It is interesting to mention that the line is loaded with an S-SRR as well (rotated 180º with regard to the S-SRRs of the tag). This is done in order to avoid inter-resonator coupling in the tag as well as coupling between the line and multiple tag resonators, as discussed in [51]. Figure 11 shows the response of various tags with the indicated code, pointing out the validity of the proposed system. This near-field chipless-RFID system with sequential bit reading is especially useful in security paper applications, e.g. to avoid unauthorized copies, counterfeiting, etc. of corporate and official documents, ballots, certificates,… Note also that the reported tags have only 10 bits, but this number can be increased at convenience, with the unique limitation of tag size.

Conclusions
In conclusion, it has been shown that transmission lines loaded with metamaterial resonant elements are useful for many diverse purposes, including sensing of material properties, sensing of spatial variables and velocities, and implementation of high data capacity chipless-RFID systems based on near-field coupling.