Evren Ekmekci

Performance enhancement of terahertz metamaterials on ultrathin substrates for sensing applications

We design, fabricate, and characterize split-ring resonator (SRR) based planar terahertz metamaterials (MMs) on ultrathin silicon nitride substrates for biosensing applications. Proof-of-principle demonstration of increased sensitivity in thin substrate SRR-MMs is shown by detection of doped and undoped protein thin films (silk fibroin) of various thicknesses and by monitoring transmission changes using terahertz time-domain spectroscopy. SRR-MMs fabricated on thin film substrates show significantly better performance than identical SRR-MMs fabricated on bulk silicon substrates paving the way for improved biological and chemical sensing applications.

A tunable multi-band metamaterial design using micro-split SRR structures

This paper presents the results of a feasibility study for the design of multi-band tunable metamaterials based on the use of micro-split SRR (MSSRR) structures. In this study, we have designed and constructed a conventional split-ring resonator (SRR) unit cell (type A) and two modified SRR unit cells having the same design parameters except that they contain two (type B) or four (type C) additional micro-splits on the outer square ring, along the arm having the main split. Transmission characteristics of the resulting MSSRR cells are obtained both numerically and experimentally and compared to those of the ordinary SRR unit cell. It is observed that the presence of the additional micro-splits leads to the increase of resonance frequency by substantial amounts due to the series capacitance effect. Next, we have designed and constructed 2 x 2 homogeneous arrays of magnetic resonators which consist of the same type of cells (either A, or B, or C). Such MSSRR blocks are found to provide only a single frequency band of operation around the magnetic resonance frequency of the related unit cell structure. Finally, we have designed and constructed 2 x 2 and 3 x 2 inhomogeneous arrays which contain columns of different types of metamaterial unit cells. We have shown that these inhomogeneous arrays provide two or three different frequency bands of operations due to the use of different magnetic resonators together. The number of additional micro-splits in a given MSSRR cell can be interactively controlled by various switching technologies to modify the overall metamaterial topology for the purpose of activating different sets of multiple resonance frequencies. In this context, use of electrostatically actuated RF MEMS switches is discussed, and their implementation is suggested as a future work, to control the states of micro-splits in large MSSRR arrays to realize tunable multi-band metamaterials.

COMPARATIVE INVESTIGATION OF RESONANCE CHARACTERISTICS AND ELECTRICAL SIZE OF THE DOUBLE-SIDED SRR, BC-SRR AND CONVENTIONAL SRR TYPE METAMATERIALS FOR VARYING SUBSTRATE PARAMETERS

This paper introduces a planar µ-negative (MNG) metamaterial structure, called double-sided split ring resonator (DSRR), which combines the features of a conventional SRR and a broadside-coupled SRR (BC-SRR) to obtain much better miniaturization at microwave frequencies for a given physical cell size. In this study, electromagnetic transmission characteristics of DSRR, BC-SRR and conventional SRR are investigated in a comparative manner for varying values of substrate parameters which are thickness, the real part of relative permittivity and dielectric loss tangent. Simulation results have shown that magnetic resonance patterns of all these three structures are affected in a similar way from variations in permittivity and in loss tangent. However, changes in substrate thickness affect their resonance characteristics quite differently: In response to decreasing substrate thickness, resonance frequency of the SRR increases slowly while the bandwidth and the depth of its resonance curve do not change much. For the DSRR and BC- SRR structures, on the other hand, resonance frequency, half power bandwidth and the depth of resonance curve strongly decrease with decreasing substrate thickness. Among these three structures, all having the same unit cell dimensions, the newly suggested DSRR is found to reach the lowest resonance frequency, hence the smallest electrical size, which is a highly desired property not only for more effective medium approximation but also for miniaturization in RF design. The BC-SRR, on the other hand, provides the largest

Frequency tunable terahertz metamaterials using broadside coupled split-ring resonators

We present frequency tunable metamaterial designs at terahertz (THz) frequencies using broadside-coupled split ring resonator (BC-SRR) arrays. Frequency tuning, arising from changes in near field coupling, is obtained by in-plane horizontal or vertical displacements of the two SRR layers. For electrical excitation, the resonance frequency continuously redshifts as a function of displacement. The maximum frequency shift occurs for displacement of half a unit cell, with vertical displacement resulting in a shift of 663 GHz (51% of f0) and horizontal displacement yielding a shift of 270 GHz (20% of f0). We also discuss the significant differences in tuning that arise for electrical excitation in comparison to magnetic excitation of BC-SRRs.

Metamaterial sensor applications based on broadside-coupled SRR and V-Shaped resonator structures

In this study, the use of broadside-coupled SRR (BC-SRR) metamaterial topology is suggested for pressure, temperature, humidity and concentration sensor applications. Also, the use of V-shaped resonator topology is suggested for pressure sensor application. The feasibility of such sensors are demonstrated by numerical simulations for microwave region under magnetic excitation.

Miniaturization of U-shaped multi-band metamaterial structures

In this study, transmission characteristics of single-sided and double-sided (in broadside-coupled configuration) U-shaped multiple ring resonators (UMRR) are investigated on a comparative basis for the purpose of miniaturization. Transmission spectra (i.e. |S21| versus frequency curves) of both single and double sided UMRR topologies are computed by CST Microwave Studio for the special cases of unit cells with single ring and double concentric rings. Although all these unit cells have exactly the same physical size, simulation results have revealed that broadside-coupled UMRR topologies provide much smaller resonance frequencies (hence considerably smaller electrical sizes) as compared to their single-sided counterparts.

Reducing the electrical size of magnetic metamaterial resonators by geometrical modifications: a comparative study for single-sided and double-sided multiple SRR, spiral and U-spiral resonators

In this study, this paper presented the simulation results that both increasing N and doubling the resonator structure provide additional capacitive effects and hence reduces the resonance frequency and electrical size, f0 and u, noticeably. For example, increasing N from 2 to 6 for the MSRR, SR, USR, DMSRR, DSR, and DUSR structures caused their u and f0 values changed to the %75, %38, %52, %76, %27, and %31 of their initial values, respectively. Besides, doubling a six-turn MSRR, a six-turn SR, and a six-turn USR reduced their u and f0 values to 76%, 52%, and 35% of their initial values, respectively, where it is important to note that all these structures have the same unit cell size. Among all structures investigated in this work, the electrically largest resonator is the two-turn MSRR with u=lambda0/12 and f0=2.24 GHz while the electrically smallest one is the six-turn DUSR with u=lambda0/133 and f0= 0.20 GHz. In other words, the double-sided U-type spiral resonator (DUSR) with N=6 turns has an electrical size which is only 9 percent of the electrical size of the multi Split Ring Resonator (MSRR) with N=2 turns, although both structures have the same physical cell size of l=8 mm.

A feasibility study for tunable metamaterial design using multi-Split SRR structures and RF MEMS switching

We investigated in this study, both numerically and experimentally, the effects of additional splits on the magnetic resonance frequency of the conventional SRR. Additional splits placed in the close vicinity of the main split of the outer SRR ring provide additional capacitance effects in series leading to a lower effective capacitance term. Thus, the resulting magnetic resonance frequency increases. When the experiments are repeated for the 2×2 array structures, similar amounts of shifts were observed in the resonance frequencies with wider stop bands and deeper transmission minimums as compared to the results obtained for the unit cell structures. As a future experimental verification, states of the additional narrow slits will be controlled by RF MEMS switches to provide an adaptive tuning of the magnetic resonance frequency.

Parametric investigation of a new multi-band metamaterial design: U-shaped multiple ring magnetic resonators

In this study, a new multi-band metamaterial structure is proposed based on a unit cell that has multiple U-shaped metal rings printed one inside the other. The number of distinct magnetic resonances is determined by the number of U-shaped rings placed in the unit cell. Using three rings in this paper, three distinct resonance frequencies can be shifted to desired values simply by changing the design parameters which are the metal width w, ring to ring separation s, arm length along y axis for each ring (hin, hmid, hout) as well as the material properties and dimension of the substrate. Transmission and reflection spectra of the proposed structure are obtained by using the CST Microwave Studio. Effects of the design parameters w, s, hin and hmid on the resonance behavior of the proposed structure are investigated in a comparative manner.

Dual mode antenna design using split ring resonator array

In this study an array structure formed by four split ring resonators (SRR) has been used as the radiating part of the antenna. Due to the coupling between resonators placed in proper distances along the propagation direction, two mode of radiation has been shown by numerical calculations. Changing the distance between SRR couples shifts the resonance frequencies and affects return loss values associated with resonance frequencies. Calculated surface current distributions shows that each radiation mode has been caused by different SRR structures on the array structure.