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Abstract
This article presents a dual-band tunable metamaterial sensor in the terahertz region, which has a high sensitivity and can work on curved surface with good flexibility. The proposed metamaterial sensor is composed of omega-ring arrays with two types of omega-shape structures fabricated on parylene-C thin substrates. An omega-I structure is developed from unit omega cells arranged along the same direction. Meanwhile, in order to investigate the effect of the unit cell direction on THz transmission, each unit cell is rotated by 90° in sequence to configure omega-II structure. Compared to the omega-I structure, the omega-II structure achieves a 31.1% intensity change when only 4% length is changed and the strain change is around 1.3 × 10−4. Moreover, the omega-II structure is insensitive to the polarization angles of incident waves, which will be beneficial for the potential application of the new flexible THz sensing technology.
© 2017 Optical Society of America
Metamaterials are commonly composed of artificial engineered arrays of structures at the subwavelength scale, which have been paid more attention in the past several years [1]. Metamaterials made up of man-made ‘atoms’ have some unique properties which are not obtained through natural conventional materials, such as negative refractive index and analogue of electromagnetically induced transparency (EIT) [2–6]. Among the entire electromagnetic (EM) spectrum, terahertz (THz) is an especially potential region with plenty of practical applications [7–13] in electromagnetic information system. However, many applications of THz metamaterials are limited by the lack of tuning capabilities due to the resonance nature of the sub-wavelength structures [14]. Some progress in metamaterial design has led to the realization of frequency or intensity tunability in metamaterials.
There are some methods to tune interactions between the metamaterials and the THz incident waves, such as optical [15], electrical [16] and thermal [17] methods. Usually tunable metamaterials depend on structural reconfiguration, such as changing the dimension of unit cell [18-19], reshaping the structural elements [20], rotating the unit cell [21] or bending the substrate or lattice [22-23]. These variations in geometry of the basic metamaterial structure are often resulted from mechanical movement or deformation of the metamaterials [24]. Ho et al. [25] presented an omega-ring terahertz metamaterial actuated by electrothermal method with the frequency tuning range of 0.3 THz. However, this structure with an extra circuit for electrical current heating was fabricated with a complicated process. Meanwhile, it was formed on rigid substrate with a dependent polarization, and these certain characteristics would severely limit its broad application on curved surface. Shen et al [26] proposed a tunable metamaterial embedded with photoconductive semiconductor, which could realize broad frequency shift and strength modulation by optical control. However, optical modulation demands a pump beam to alter the property of medium of the device which might influence incident wave. To facilitate the fabrication and measurement process, tunable metamaterials controlled by loading pressure become an alternative. Li et al. [27] proposed I-shaped structure and the capped dipole sensors on polydimethylsiloxane (PDMS) substrate controlled by compression, which are suitable for single-axis and dual-axis stain sensing, respectively. But it only could generate a slight frequency change and a small amplitude modulation with less than 15%. Zheng et al. [28] demonstrated two types of flexible terahertz metamaterials on polyethylene naphthalate (PEN) substrates, which achieved relative intensity change in transmission around 23.19% when the strain was 6.41‰. In order to increase the ability of amplitude modulation, a complementary omega-ring shaped configuration was proposed in this letter. It can generate a large deformation under a small strain due to its low stiffness with large proof mass. As a result, a high sensitive amplitude modulation with about 31.1% was obtained. Moreover, the further rotated and optimized structure could successfully realize a perfect polarization independent characteristic. Overall, this developed device with great tunable ability and good polarization independence can be applied as flexible detector or modulator.
Figure 1(a) depicts the schematic view of omega-shaped structure (Omega-I) with each unit cell along the same direction, and its dimension is with period of p = 200 μm, gap of w = 25 μm, the inner and outer radii of the hollow part r1 = 55 μm and r2 = 80 μm, respectively, width of the hollow part g = 25 μm. Figure 1(b) shows the optical image of proposed Omega-I structure taken by VHX digital microscope after peeling off from the silicon substrate. In order to investigate the effect of each unit cell direction on THz transmission, each unit cell is rotated by 90° in sequence to form the Omega-II structure, as shown in Fig. 1(c). Figure 1(d) is the optical image of proposed Omega-II structure. With flexible free-standing substrate, the samples can easily be curved, just as illustrated in Fig. 1(e). After releasing the double-layer devices from silicon wafer, the circle inside the hole would curve up due to the residual stress, which can be partly seen from the shadow over the circles.
Fig. 1 (a) Schematic view of omega-shape structure (Omega-I) with each unit cell along the same direction, and its dimension of r1 = 55 μm, r2 = 80 μm, w = g = 30 μm and p = 200 μm, (b) Optical images of Omega-I structure after peeling off from the substrate taken by VHX-5000 digital microscope. (c) Schematic view of omega-shape structure (Omega-II) with each unit cell rotating by 90° in sequence. (d) Optical images of Omega-II structure after peeling off. (e) Photograph of the flexible metamaterial fabricated on parylene-C substrate.
5 μm thick parylene-C layer was first deposited on a clean silicon wafer by Chemical Vapor Deposition (CVD) process at around 170 °C. Then, 10 nm thick Cr thin film was deposited as an adhesive layer using electron beam evaporation (EBE), and 200 nm thick copper thin film was deposited on it. The bilayer materials of the copper and parylene-C layers were patterned and etched by ion beam etching (IBE) and plasma etching, respectively to form. After removing photoresist, the bilayer materials can be easily peeled off from the silicon substrate to form free-standing omega-shaped structure. Figure 2 (a) and (b) show the three-dimensional optical images of Omega-I and Omega-II structures formed by VHX-5000 digital microscope, respectively. We collected more than a dozen of data of the outlines of the structures. As shown in Fig. 2 (c), the initial angle of the curvature θ is calculated around 4.2° due to the residual stress between Cr/Cu metal layer and parylene-C layer.
Fig. 2 Three-dimensional composite images of Omega-I structure (a) and Omega-II structure (b), respectively. (c) Several data of the outlines of the structures after appropriate data processing, from which we can roughly measure and calculate that the initial angle θ of the curvature is around 4.2°
With the flexible parylene-C substrate, we can experimentally explore the effects of curvature strain on the metamaterials. Generally, when the parylene-C is thin enough and almost transparent to THz waves, the curvature of parylene-C alone has extremely scarce influence on transmission spectra. With the increasing of the bending strain, the omega-shaped part will curve up obviously along out-of-plane direction, thus it will adjust the transmission spectra. Our sample was mounted on a home-made test jig, and one side was fixed and the other movable part can be precisely controlled by electrically moving component. The sample on the test jig was placed in the chamber of the THz time domain spectroscopy (THz-TDS), which was under a nitrogen environment to eliminate water vapor.
The initial length (L) of the sample, which means the distance between the fixed end and the movable one, is 25 mm. The compression distance (Δl) varies from 0 to 1 mm with a discrete step of 0.2 mm. The relative maximum length change (Δl/L × 100%) reached 4% of the initial sample length. We simulated the transmissions of both structures at their initial status (Δl = 0 mm). The parylene-C substrate was modeled as a lossy dielectric medium with the relative permittivity of 2.9. The copper layer was deployed as lossy metal with electric conductivity of 5.8 × 107 S/m. The simulated and measured transmissions of two structures are shown in Fig. 3(a) and 3(b), respectively. There is a good agreement between the simulation and the testing results. The spectra are normalized with transmission through bare nitrogen background. When the bending strain increases, the transmission amplitude of Omega-I structure does not change much, at the constant resonant frequency of 0.842 THz, as shown in Fig. 3(c). The reason for this phenomenon is that the bending direction of omega-shape structure is perpendicular to the symmetry axis of the structure, and the omega-shaped part will hardly curve up along its central axis, as shown in inserted schematic figure of Fig. 3(c). However, because Omega-II structure is rotationally symmetric one, the unit cells of 2 and 3, whose symmetry axis are along the curving direction, will easily deform out of the plane, as shown in inset of Fig. 3(d), while the unit cells of 1 and 4, just like the unit of Omega-I, will only generate negligible deformation. The deformations from the unit cells of 2 and 3 lead to consecutive change at the second peak of the transmission spectra while unit cells of 1 and 4 do not deform and result in almost the same transmission amplitude at the first peak. When the Δl increases step by step up to 1 mm, all the first resonant frequency is at 0.293 THz with the intensity of around 0.7, while the amplitude at the second resonant frequency of around 0.75 THz is gradually depressed from 0.9405 to 0.9328, 0.9284, 0.8756, 0.7546, 0.6295, contributing to the total intensity change in transmission up to 31.1%. Then we further investigated the relationship between the compression length (Δl) and the magnitude of the resonant peaks, which is plotted in Fig. 3(e). The function can be described as T = 0.96898-0.01792 × exp(2.96076Δl), with coefficient of determination of 0.97529, which indicated a reasonable exponential function fitting. According to [26], the curvature radius R at the ridge of the bending metamaterial can be expressed as:
where W is the thickness of the samples (5.2μm). Thus we can calculate the maximum stain at the top of the bending metamaterial by:Fig. 3 The simulated and measured results of Omega-I (a) and Omega-II (b) in flat. The measured results of the bend testing of these two structures (c) and (d), above them are their corresponding deformation diagrams. (e) The exponential function fitting of compression distance Δl and the intensity of resonant peaks. T = 0.96898-0.01792 × exp(2.96076Δl). (f) The exponential function fitting of bending strain ε ( × 10−5) and the intensity of resonant peaks. T = 0.94926-0.000828475 × exp(0.4573ε).
Figure 4(f) depicts the magnitude of the second frequency resonant peaks change as a function of the strain ( × 10−5). Exponential fitting function is also used to fit the curves and evaluate the sensitivity of the sensor, which can be described by T = 0.94926-0.000828475 × exp(0.4573ε) with coefficient of determination of 0.98044. From mentioned above, we can infer that the single unit can achieve the function of direction selection, which means, it behaves differently as the direction of unit cell changes. The rotationally combined structure acts like a delicate strain sensor, with only 4% length change (strain change around 1.3 × 10−4) leading to 31.1% in intensity change exponentially, which can find potential applications in the flexible THz modulation technology and high sensitive sensor.
Fig. 4 (a)-(c) show the electric and magnetic field magnitudes and surface current distributions at the transmission peaks of the Omega-II structure at 0.31THz. (d)-(f) show the electric and magnetic field magnitudes and surface current distributions at the transmission peaks of the Omega-II structure 0.74THz.
In order to further analyze the modulation mechanism of Omega-II structure, finite-difference time-domain (FDTD) simulations are carried out to calculate the electromagnetic field and surface current distributions at the resonance peaks. The confinement of the surface plasmon which is also known for the localized surface plasmon resonance (LSPR) mode significantly enables the coupled plasmon polariton. The detailed interpretation of the complementary metamaterial can also be applied with the Babinet’s principle that the E-field of the complementary Omega-shaped void is interchangeable with the H-field of the normal Omega ring. At 0.31THz as shown in Fig. 4(a) and 4(b), the enhancement of electric and magnetic fields are mainly occurred at the unit cells of 1 and 4. In Fig. 4(c), the surface current is observed oscillating along the Omega-ring edge and resonating at the gap, analogous to the LC resonance mode in the split-ring resonators. When the device is bending as in Fig. 3(d) the unit cells of 1 and 4 is not curved up, thus the transmission peak at 0.31THz remains unchanged. However, for the transmission peak at 0.74THz, the contribution is mostly made by the field enhancement from the plasmon resonance in the unit cells of 2 and 3 as seen in Fig. 4(d) and 4(e). Fig. 4(f) shows the dipole resonance along the outline of the Omega ring. Therefore, as the strain is increased during testing, the unit cells of 2 and 3 are curved up higher, resulting in larger angle θ and longer distance d between the top of omega-shaped part and the horizontal direction of base shown in Fig. 2(c). The larger distance d will weaken the electromagnetic coupling strength and decrease the transmission amplitude.
We also investigated the effect of the polarization angle of the THz waves on the spectra. The normalized simulated result and measured one of two structures are represented in Fig. 5(a)-5(b) and Fig. 5(c)-5(d), respectively. Overall, the testing results are in decent agreement with the simulated results. Observing from Fig. 5(a) and 5(c), when the sample rotates and the polarization angle changes from 0° to 90°, one original transmission peak of Omega-I structure is dampened and split into two sharp ones, and the magnitudes of the newly emerged peaks are enhanced at the fixed frequency. Thus, Omega-I structure can be further applied as dual-band amplitude modulation device. As presented in Fig. 5(b) and 5(d), Omega-II structure is insensitive to polarization angle, which is a very useful characteristic for the flexible metamaterial to be covered any surfaces with arbitrary angles.
Fig. 5 The normalized simulated results of polarization investigation of Omega-I structure (a) and Omega-II structure (b), respectively.Measured results of polarization investigation of Omega-I structure (c) and Omega-II structure (d), respectively. The polarization angle of 0° and 90°.
We have reported an omega-shaped metamaterial THz flexible sensor which could realize a large amplitude modulation based on free-standing parylene-C substrate. Due to the residual stress of parylene-C and metal layers, the omega-shaped structure curves up after releasing from the supporting silicon wafer. The omega-shaped structure (Omega-I) consists of identical unit cells along the same direction which cannot change the transmission amplitude at the resonant frequencies with a dependent polarization characteristic. Then each unit cell in Omega-I is rotated by 90° in sequence to form Omega-II structure. As a result, this structure can achieve 31.1% amplitude modulation when a slight strain with a 1.3 × 10−4 change is applied. Finite-difference time-domain (FDTD) numerical simulations were further conducted to calculate the electromagnetic field as well as surface current distributions at the resonant frequencies to reveal the principle behind the bending results. Omega-II structure is insensitivity to the polarization angle of incident waves due to its symmetry structure, which would be applied in modulation and sensing applications.
Funding
National Natural Science Foundation of China (61674104).
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