1. Introduction

Rapidly expanding terahertz (THz) technology has various potential applications in the areas of chemistry, bio-medical, agricultural imaging, environmental sensing, and communications [1–4]. For more successful and wider applications of THz technology, it is necessary to realize more compact and more energy-efficient THz components, such as sources, detectors, modulators, and filters that can generate, detect, and manipulate THz signals [5–9]. The challenge of controlling terahertz waves requires the advent of new technology. Metamaterials, which are arrays with subwavelength metallic or dielectric resonators, exhibit exotic electromagnetic properties that are not available in natural materials. After the first successful demonstration of electromagnetic (EM) metamaterials in the microwave frequency range [10], these materials have been utilized to realize perfect lenses, absorbers, and antennae [11–13]. Metamaterials have also been applied in the above THz components in recent years, allowing such components to enjoy the exotic electromagnetic properties and sub-wavelength scale compactness of metamaterials [6, 14, 15]. Various metamaterial structural designs have been investigated in attempts to achieve better performance in terms of multiple resonance, high quality factors, and high on-off switching ratios; designs have also been attempted to achieve simpler fabrication processes. Among the various designs, the electric field coupled split-ring resonator (eSRR) design cancels the net effective magnetic response, whereas the conventional split-ring resonator (SRR) design has a magnetic response [16, 17]. The structural symmetry of the eSRR design induces current to flow in the opposite direction, which cancels the magnetic flux. Because the symmetric design used in eSRR eliminates the magnetic resonance, the long and thin wires employed to cancel the magnetic flux in conventional SRR are not required in eSRR structures [18, 19]. The resulting planar and single layer eSRR structures can be attached to curved objects more easily than conventional SRR structures. Furthermore, the reduced unit cell size of the eSRR design results in more compact metamaterials. More recently, multilayer metamaterials have also been realized. By stacking multiple metamaterial structures, higher refractive indices or greater contrast between the stop band and the pass band have been achieved compared to the case of single-layer metamaterial structures [20, 21]. To achieve high performance metamaterial devices, the material properties of the substrate on which metamaterials are realized are as important as the metamaterial design. For electrically controllable metamaterial devices, GaAs substrates have been utilized [8]. Other types of materials, such as polyethylene-terephthalate (PET) [22] and PDMS [23] have been utilized for passive flexible metamaterial devices such as filters and absorbers. Recently, THz filters on 80-μm-thick silk and 280-μm-thick paper were demonstrated for environmentally friendly or edible THz filters that can be used as parts in chemicals and biosensors [24, 25].

For more agile applications of THz metamaterials, flexible and highly transparent substrates are beneficial because flexible substrates allow metamaterial devices to be attached to curved surfaces and because highly transparent substrates result in low insertion loss THz components. Metamaterials on flexible substrates have been demonstrated by using two different fabrication processes: direct fabrication and the substrate transfer technique. The direct fabrication of metamaterials on flexible substrates, which are usually polymer-based materials, involves fewer fabrication steps than the substrate transfer method [26]. However, the chemical influences of solvents such as acetone and methanol during conventional lithography and/or thermal stress during the annealing process may result in substrate deformation and an irregular metallic subwavelength array [27]. Therefore, relying on the conventional lithography process hinders the use of flexible materials despite their thin thickness and excellent THz transmission characteristics. On the other hand, use of the substrate transfer method for the fabrication of metamaterials on different types of substrates via a detaching and transferring process requires a good understanding of the interfacial force between the coated polymer and the rigid substrate. The interfacial force is related to the stability of the metamaterial structures on the polymer substrate and to the high flexibility and robustness of the coated polymer during the detaching process. There have been attempts to peel off metamaterial structures from PDMS [23] and silicon substrates [20]. To improve the transfer efficiency, the detaching process can also be used in conjunction with the laser-induced liftoff method [28], the heat-induced liftoff method [29], and a liftoff method with etching of a sacrificial layer [23, 30]. These liftoff processes also have side effects, such as thermal decomposition during the high-temperature process, residue of the thermal tape, and potential damage during the chemical etching process. To avoid problems due to thermal stress and damage due to the chemical treatment that can occur during direct fabrication processes, and to avoid any possible damage during substrate transfer methods, a chemically stable detaching process that utilizes low-temperature, residue-free steps should be developed. In this paper, flexible bilayer THz metamaterials that can easily be attached to curved surfaces are demonstrated. We utilized a type of Scotch tape (3M^TM Utility Tape #522, 3M Corp.) that is frequently used to transfer graphene onto other rigid substrates [31] but that has not been fully investigated for application as a substrate for THz devices. Its characteristics, such as low cost, thinness (51 μm), and good adhesive properties make it suitable for use as a flexible substrate. This type of Scotch tape was found to have excellent transmittance, exceeding 85% in the range of 0.2 to 3 THz, as can be seen in Fig. 1(a). From the measured transmission and reflection characteristics of the bare substrate, it was possible to determine the dielectric constant and loss characteristics by following a method described in an earlier work [32]. The extracted refractive index of 1.38 at 1 THz is much lower than those of many other substrates [33], which results in very low reflection loss. Normal semiconductor fabrication processes cannot be carried out directly on the top of a flexible substrate because this type of substrate cannot endure either the thermal stress that arises during the metallization process or the common chemical treatments during the photolithography process. To overcome these shortcomings, a pattern transfer process was developed. The original metamaterial, consisting of an array of nested electric split-ring resonator (NeSRR) structures [6], was fabricated on a sapphire substrate covered with an SiO₂ sacrificial layer. By etching off the sacrificial layer, the original metal pattern was released from the rigid substrate and transferred onto the top of the Scotch tape. The same process could be repeated to realize multilayer metamaterials.

 

Fig. 1 (a) Transmittance, reflectance and refractive index of Scotch tape in the terahertz frequency region; (b) SEM image of the fabricated metamaterial on a Scotch tape. Inset: dual-band NeSRR structure with the following geometric parameters: l₁ = 52 μm, l₂ = 36 μm, g = 16 μm, g₁ = 8 μm, g₂ = 4 μm and w = 4 μm. The lattice constant is 60 μm; (c) photograph of a Scotch tape terahertz filter sample; (d) photograph of a THz MM filter attached to a PET bottle.

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2. Metamaterials on Scotch tape

A 10-nm-thick SiO₂ was deposited on the sapphire via plasma-enhanced chemical vapor deposition, and conventional optical lithography was performed to pattern the NeSRR design on the sapphire substrate. A multilayer Ti/Pt/Au (20/10/200 nm) metallization layer was deposited via an e-beam evaporation and was lifted off. Titanium was used for good adhesion with the SiO₂/sapphire substrate and as a sacrificial layer together with SiO₂. The Scotch tape was then attached to the metamaterial on the SiO₂/sapphire substrate. The sample, covered with Scotch tape, was then dipped in diluted hydrogen fluoride (HF) acid to help facilitate the subsequent detaching step. During the HF treatment, both SiO₂ and titanium oxide dissolved, leaving only the Pt/Au metal structure on the sticky side of the tape. The metamaterial structure was released from the sapphire substrate and finally transferred to the Scotch tape. The transferred metamaterial film was then rinsed in deionized water for 5 min. The successfully transferred metamaterial with NeSRR structures without any deformation is shown in Fig. 1(b). The geometric parameters of the NeSRR unit cell are given in the inset of Fig. 1(b).

The fabrication process of the NeSRR metamaterial on Scotch tape can be utilized to realize multilayer metamaterial filters, and these filters can be attached to a variety of curved objects. The microscopic bubbles noted on the sticky part of the Scotch tape popped when pressure was applied, and the tape became attached to the surface of the object. This characteristic shows the possibility of the attachment to a subject with less of an air gap between the filter and object; furthermore, it implies the potential for the creation of multilayer metamaterials. As shown in Fig. 1(c), a multilayer of a Scotch tape filter was fabricated by attaching two single-layer metamaterials fabricated using the method described above. The application of MMs on the curved surface was demonstrated by attaching the metamaterial on the surface of a PET bottle, as shown in Fig. 1(d). Because the metal pattern is attached to the object, it is protected by the Scotch tape, which can increase the filter lifetime in that the filter can be harmed or deformed by physical or chemical processes.

THz time-domain spectroscopy (THz-TDS) was used to measure the transmission and reflection characteristics of fabricated devices with an area of 1.5 × 1.5 cm². The fabricated samples were mounted on a sample holder with a 5-mm-diameter hole at its center for the THz transmission and reflection measurement to ensure the flat sample surface. The measurements were carried out in an N₂ atmosphere to avoid the effect of water vapor in air. The generation of terahertz pulsed electromagnetic radiation was accomplished using a photoconductive antenna. The emitted terahertz pulse was inherently broadband and the frequency response of the devices could be measured up to 3 THz. The transmission characteristics of the filters with NeSRR structures realized on various substrates are compared in Fig. 2. Conventional optical lithography was carried out on 443-μm-thick sapphire and 100-μm-thick PET substrates (DAESUNG Film CO., Clear PET film) and the NeSRR patterns were realized directly on top of these substrates by using liftoff process. Even though PET substrates exhibited quite strong thermal and chemical resistance, the conventional liftoff process on PET was not easy compared with that on other rigid substrates. More careful optimization should be carried out to achieve the higher yield process on PET substrates. The filters on the 51-μm-thick Scotch tape were fabricated via the detaching and transferring process described above. The dual-band characteristics of the NeSRR exhibit resonant reflection at two frequency bands, where the first resonant reflection is due to the outer SRR structure and the second resonant reflection is due to the inner SRR structures. As shown in Figs. 2(a) and 2(b), surface current is induced along the outer SRR at the first resonance frequency, and it is induced along the inner SRR structure at the second resonance frequency. The first resonance frequencies were 0.51, 0.79, and 0.95 THz and the insertion losses were 3.28, 1.40, and 0.60 dB at the pass band for the filters on the sapphire, PET and Scotch tape, respectively. The Scotch tape, exhibiting the lowest dielectric constant, resulted in the lowest reflection in the pass band and the highest reflection at the resonance frequency. With an increase in the frequency, as shown in Figs. 2(c) and 2(d), the sapphire and PET samples show highly attenuated transmission spectra that can be ascribed to the higher absorption in the substrates considering that the absorption coefficient of the substrates are 12.6, 8.9, and 3.5 cm⁻¹ for sapphire, PET, and Scotch tape at 1 THz. Among these three filters, the NeSRR filter on the Scotch tape exhibits a higher transmission at high frequency, which shows that the Scotch tape has much higher transparency in the THz frequency range. The NeSRR on PET has a least 3-dB higher attenuation at 3 THz compared with that on the Scotch tape. The NeSRR on sapphire displays very high attenuation at a frequency as high as 3 THz; sapphire substrate with the same thickness is not suitable for filter application at this frequency.

 

Fig. 2 Surface current distribution on a NeSRR pattern at the first resonating frequency (a) and at the second resonating frequency (b), the THz transmission characteristics of NeSRR filters on different substrates in linear (c) and log scale (d).

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The NeSRR filter on the Scotch tape also demonstrated excellent band rejection characteristics. The insertion loss (IL) of 0.60 dB at 1.59 THz is the lowest reported as yet from a metamaterial-based THz filters. The attenuation (ATT) at resonance frequencies of 0.95 and 2.44 THz are 17.26 and 20.76 dB, respectively. The band rejection ratio (BRR), defined by ATT-IL, are 17 and 20 dB at 0.95 and 2.44 THz, respectively. This BRR is also the highest ever reported from a dual-band THz metamaterial filter. Excellent filter characteristics could be achieved by the low loss characteristics of the Scotch tape substrate and the optimum design of the NeSRR structures, as shown in Table 1.

Table 1. Performance comparison of the fabricated multi-layer MMs

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3. Bilayer metamaterials on Scotch tape

By repeating the above detaching and transferring method, a bilayer NeSRR filter was fabricated. An identical NeSRR pattern was utilized twice for the bilayer metamaterial structures. As shown in Fig. 3(a), the two NeSRR layers could be aligned with maximum error of less than 2.5μm. The small alignment error could be achieved by using optical mask aligner for the repeated pattern transfer process. Figure 3(b) shows the impulse responses of the single- and bi-layer metamaterial filters. The single-layer filter exhibited one main pulse, whereas the bilayer filter exhibited two main pulses due to the multiple reflections at the NeSRR layers. Although multiple reflections also take place in the single-layer NeSRR structure, the reflection from the back surface of the substrate is so low that the impulse response resulted in a single main peak. The calculated time difference between the first arriving pulse and the second arriving pulse in the impulse response of the bilayer filter was 0.5 ps, which is identical to the measured time difference. The transmission and reflection spectra of the single-layer filter and the bilayer filter are compared in Fig. 3(c). Both filters exhibit resonant stop-band characteristics around 0.9 THz. The bilayer filter exhibits a steeper band transition and much broader band rejection characteristics, which originate from the high-order filter characteristics.

 

Fig. 3 (a) SEM image of a bilayer filter, (b) time-domain signals, (c) transmittance and reflectance of a single layer and bilayer filters.

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To study the effect of misalignment in multilayer MMs, the frequency responses of the multilayer MM filters were simulated at various alignment errors. A three-dimensional electromagnetic simulation was carried out using periodic boundary conditions because the filter under investigation has thousands of unit cells periodically repeated in the x and y directions with a period of 60 μm. For comparison, multilayer MM filters shifted on the x- and y-axis by 5 μm or tilted at angles of 5° and 10° were simulated as shown in Fig. 4. The spectral response was mostly invariant to longitudinal or vertical shift error up to 5 μm. However, it exhibited strong dependence on the tilted angle between the top and bottom NeSRR layers. When the tilted angle exceeded 10°, the transmission and reflection characteristics changed significantly, though the stop-band characteristics were almost preserved. The angle dependence of its spectral response is due to the polarization-dependent characteristics inherent to the properties of NeSRR structures. The fabricated bilayer NeSRR filter in Fig. 3(a) exhibits horizontal and vertical shift of less than 2.5 μm and a tilted angle of less than 1°. The measured transmission spectra exhibited similar response with the simulated characteristics when the top and the bottom NeSRR structures were tilted by 1° as shown in Figs. 4(c) and 4(d). The induced surface current in the misaligned NeSRRs of the bilayer filters is shown in Figs. 4(e) and 4(f). The surface currents in the two horizontally or vertically shifted NeSRRs exhibit similar magnitudes, but the two tilted NeSRRs exhibit a notable reduction in the surface currents of the bottom NeSRR. As the polarization of the incident wave is aligned with the gap of the top NeSRR, the resonance at the tilted NeSRR at the bottom is much weaker than that at the top NeSRR.

 

Fig. 4 (a) Simulated transmittance of the bilayer filters with no misalignment (solid line) and 5-μm-shift misalignment in x- and y-axis (dashed line). Simulated reflectance of the bilayer filters with no misalignment (red squares) and 5-μm-shift misalignment in x- and y-axis (red circles), (b) Simulated transmittance of the bilayer filters with no misalignment (solid line), 5þ tilted (black diamonds), and 10þ tilted (black triangles) angle misalignment. Simulated reflectance of the bilayer filters with no misalignment (red squares), 5þ tilted (red circles), and 10þ tilted (red triangles) angle misalignment. (c) Comparison of the measured and simulated transmittance of the bilayer filter and (d) comparison of the measured and simulated reflectance of the bilayer filters: no misalignment (red triangles), 5-μm-shift misalignment in x- and y-axis (black squares), 1þ tilted (blue circles) angle misalignment, measured result (green diamonds). Surface current densities of the bilayer filter with 5 þ tilted angle misalignment (e) and 5-μm-shift misalignment in x- and y-axis (f).

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4. Discussion

The fabricated single-layer and bilayer filters were attached onto a surface of a PET bottle (500 ml, thickness of 580 μm, diameter of 6 cm) and a metal plate coated with gold to determine their potential for use in THz identification (ID) applications. The reflectance spectra of the bare PET bottle and the single-layer and bilayer filters attached onto a curved surface of a PET bottle are shown in Fig. 5(a). The reflectance of the bare PET bottle exhibits clear Fabry-Perot resonance with a free spectral range of around 0.3 THz. The reflectance spectra of the single-layer and bilayer filters attached to the curved PET surface exhibit characteristics similar to those of a free-standing filter, shown in Fig. 3(c). The differences due to the effect of the PET are the reduced reflectance level and the lowered resonance frequency. The reflectance was reduced because the PET surface was curved such that the incident THz beam is no longer normal to the surface at the edge of the filter. The resonance frequencies of the free-standing filter, which was 0.95 THz for both the single-layer and bilayer filter, were shifted to 0.63 THz and 0.72 THz for the single-layer and bilayer MM, respectively. The Q-factors increased from 1.19 to 1.59 and from 2.17 to 2.75 for the single-layer and bilayer filters, respectively. The higher effective refractive index of the PET bottle led to a red shift in the local maxima of the reflectance and a higher Q-factor. As shown in Fig. 5(b), the primary frequency stop-band attenuation of the PET bottle decreased from 17 dB to 16 dB and from 30 dB to 26 dB for the single-layer and the bilayer samples compared with the free-standing filter due to the curved PET surface. Although there is some change in the reflection spectra due to the effect of the PET substrate, the resonant reflection characteristics of the single-layer and bilayer filter are still apparent. If some information is coded in types of metamaterial structures, the information stored in the MM can be retrieved from a back-scattered THz wave whose frequency is aligned with the resonance frequency of the metamaterial using a strategy similar to that used in radio-frequency identification (RFID) system [36].

 

Fig. 5 Spectral characteristics of the filters attached onto a PET bottle and a metallic object: (a) reflectance and (b) transmittance of a single layer filter and a bilayer filter on a PET bottle, (c) reflectance of single layer (solid line) and a bilayer (dashed line) filter attached to a gold mirror.

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There is an issue in RFID systems in which the back-scattered wave is highly dependent on the object onto which the RFID tag is attached. The worst case occurs when the RFID tag is attached to a metallic object. The antenna impedance of the RFID tag changes dramatically due to the effect of the backing metal such that the back-scattering function of the RFID tag does not work. Similar phenomena were also observed in the THz MM filter. As shown in Fig. 5(c), the reflection spectra of the single-layer filter attached to the top of the metal plate shows complete reflection at a low-frequency band. The reflection from the MM and back metal plate cannot be differentiated. In fact, the single-layer MM filter on the gold metal plate exhibits the characteristics of a MM absorber [37]. At frequencies of 1.4 and 1.95 THz, the wave reflected from the front MM and the wave reflected from the back reflector destructively interfere, causing the reflection minima to appear. This destructive interference can only take place when the MM filter without back reflector is partially transparent at 1.4 and 1.95 THz, as shown in Fig. 2(a), indicating that the frequencies at the reflection minima of the MM filter on metal and those of the free-standing filter are completely different. When the bilayer filter attached onto metal is considered, the resonance reflection at 0.85 THz is still obvious. Thus, back-scattering from the bilayer MM can be detected. The reflection band is mostly invariant when compared to the free-standing bilayer filter because the primary reflection takes place at the front side of the MM structures. The second layer of the MM structure effectively isolates the effect of the backside metal plate, allowing the primary reflection band to be preserved. At the neighboring frequency band, a couple of reflection minima were observed, as shown in Fig. 5(c). These reflection minima can also be ascribed to the destructive interference between the THz wave reflected at the surface of the top MM layer and the wave reflected from the second MM layer and the backside metal layer.

5. Summary

In summary, single-layer and bilayer metamaterial filters based on the NeSRR structure were fabricated on top of low-cost flexible Scotch tape which has advantages in terms of flexibility, easy attachment, and high transparency in THz frequency range. The single-layer filter demonstrated record low insertion loss of 0.6 dB and a very high band rejection ratio of 17 dB. The bilayer filter exhibited high-order filter characteristics because it was a cascade of the two single-layer filters, giving it the advantages of steep transition from the pass band to the stop band and a higher band rejection ratio compared to the single-layer filter. The advantages of the bilayer filter are more pronounced when these types of filters are attached to numerous objects. Both the single-layer and bilayer filters exhibit resonant reflection when they are attached to a dielectric substrate such as PET. However, the single-layer filter lost its resonant reflection characteristics when it was attached onto the top of a metallic object. In contrast, the bilayer filter preserves its resonant reflection at the primary reflection band because the second MM layer effectively isolates the top MM layer from the backside metal. With the future development and advances in THz components and systems associated with THz identification applications, bilayer MM structures on flexible substrates can be utilized as parts in THz ID tags which can be attached to a range of objects, including those made of metal.