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Microscopic split-ring-resonator (SRR) arrays are fabricated on 100 μm thick polyethylene naphthalate (PEN) films by femtosecond laser micro-lens array (MLA) lithography. The transmission properties of these metamaterials are characterized by THz Time Domain Spectroscopy (THz-TDS). Tunable resonance responses can be achieved by changing SRR structural design parameters. By stacking 2D PEN metamaterial films with different frequency responses together, a broadband THz filter with full width at half maximum (FWHM) of 0.38 THz is constructed. The bandwidth of the resonance response increases up to 4.2 times as compared to the bandwidths of single layer metamaterials. Numerical simulation reveals that SRR layers inside the multi-layer metamaterials are selectively excited towards specific frequencies within the broadband response. Meanwhile, more than one SRR layers respond to the chosen frequencies, resulting in the enhancement of the resonance properties. The multi-layer metamaterials provide a promising way to extend SRR based metamaterial operating region from narrowband to broadband with a tunable feature.
©2011 Optical Society of America
1. Introduction
Metamaterials, which are artificial composite materials being designed to have specific electromagnetic responses, have been receiving increasing research attentions during the last decade. With proper utilization of metamaterials, exotic phenomena, such as negative refractive index [1–3], perfect lens [4] and invisible cloaking [5,6], can be achieved. The unique electromagnetic (EM) response from metamaterials has been proven to be especially valuable in the terahertz (THz) regime, where most naturally occurring materials exhibit weak EM response to the THz wave. The THz regime, usually defined between 0.1 and 10 THz, remains to be the least developed region in the EM spectrum due to the lack of efficient sources, detectors and functional devices. The advent of metamaterials is expected to close the “THz gap”. Metamaterials based on split-ring-resonator (SRR) structures, which exhibit strong magnetic response in designed narrowband frequency region, was first proposed and analyzed by Pendry et al. [7]. The ease of fabrication in the micrometer level makes SRRs applicable and practical for the THz regime. Different design variations based on the SRR structures, such as electric SRR [8], dynamically active SRR [9–11] and structurally reconfigurable SRR [12], were proposed by other researchers and bolstered the development of the THz metamaterial research. Functional THz metamaterial devices, such as switches [10], modulators [11], perfect absorbers [13] and filters [14], were successfully demonstrated.
Due to the resonant nature of the SRR based THz metamaterials, most metamaterial designs up to date exhibit narrowband magnetic and electric responses. This limits their performance for broadband applications. Several planar design variations were carried out to broaden the THz metamaterial responses [15–17]. However, multi-resonance performance of planar metamaterials is usually accompanied by a compromise in resonance strength due to lower resonator densities at a given resonance frequency and the coupling among different resonators [18]. Furthermore, microscopic disorder has the effect of weakening and suppressing the desired response [19]. Meanwhile, most of the current THz metamaterial research works are carried out on planar rigid (semiconductor or glass) substrates, which have shown expected manipulation on the THz wave with these 2D metamaterial designs. Fabricating metamaterial structures on thin plastic substrates/membranes is one possible way to extend THz metamaterials from 2D to three-dimension (3D) [20–24]. By stacking metamaterial layers or utilizing multi-layer processing, it is possible to realize 3D THz metamaterials with enhanced features [25]. In our previous studies, a hybrid SRR design on 2D planar substrates for tunable, broadband and high sensitivity THz sensing was reported [26]. In this paper, we present our recent results to extend SRR based THz metamaterials to a multi-layer configuration in order to broaden the response performance. By stacking 2D plastic metamaterial layers with different resonance responses into a multi-layer structure, a broadband THz filter was built. It shows that multi-layer THz metamaterials provide a promising way to achieve high performance broadband filter with a compact dimension.
2. Sample design and fabrication
All the samples are fabricated on heat stabilized 100 μm thick polyethylene naphthalate (PEN) films (Dupont Teijin Films, Teonex Q51). The PEN film is thin enough to offer high flexibility for applications on non-planar surfaces. It can be easily wrapped around a cylinder with a diameter smaller than 1 cm. This feature can be potentially useful for implementing THz devices such as cloaking devices. Patterning is implemented by femtosecond laser micro-lens array (MLA) photolithography, which allows large-area and rapid fabrication of arbitrary unit cell designs for THz metamaterials [27,28]. In the exposure process, the incident laser beam is first expanded to fill out the whole sample area and then split into 10,000 focused tiny light spots by an MLA (pitch: 100 μm, focal length: 100 μm, area: 1 cm × 1 cm), which serves the purpose of exposing the photoresist. The sample is put onto a high resolution precision Z stage which is used to maintain a constant distance between the MLA and the sample during the exposure time. A high precision XY translation nanopositioning stage is used to control the stage movement in the X & Y directions. The light exposure duration is controlled by an optical shutter. The SRR is written on the photoresist by controlling the sample stage movement and the optical shutter through a Computer Numerical Control program. This maskless technique is much more flexible than conventional photolithography as arbitrary unit cell patterns can be written without the need for a photomask. After photoresist development, all the samples are deposited with 15 nm Cr film as the adhesion layer followed by 200 nm Cu film by electron beam evaporation. The sample characterization is carried out by a THz Time Domain Spectroscopy (THz-TDS) system (TPS3000, Teraview Inc.) in transmission mode. All measurements are taken under the pure nitrogen environment. Since the metamaterial samples are treated as integrated devices for filtering applications, the transmission spectra are normalized against the reference spectrum measured without the samples in the same nitrogen environment. The 100 μm MLA pitch size defines the SRR lattice constant p for all the samples. Figure 1 shows the structural parameters for the SRR unit cell and the microscopic images for all the different samples fabricated. For every sample, the length for the gap-bearing side is fixed at b = 50 μm and the gap size is fixed at g = 20 μm, while the length for the side perpendicular to the gap-bearing side is set as a = 40, 50, 60, 70 and 80 μm from SRR1 to SRR5, as seen in Table 1 . SRR metallic line width is defined as w = 6 μm. The incident THz wave is linearly polarized such that the E-field is parallel to the gap-bearing side. The spectra of the samples with varied side length a are shown in Fig. 2(a) . It is observed that the LC resonance frequencies through the bi-anisotropic effect [29] are at 0.74, 0.66, 0.59, 0.54 and 0.49 THz for samples with a = 40, 50, 60, 70 and 80 μm, respectively. A clear red-shift of the LC resonance frequency is observed when a changes from 40 to 80 μm. This is expected since the sub-wavelength SRR essentially acts as an LC resonator circuit with ωLC = (LC)-1/2, where the inductance L is mainly related to the effective enclosed area of the SRR and the capacitance C is largely determined by the gap size and the surrounding medium [30]. The increase in the side length enlarges the enclosed area and increases the inductance of the resonator, thus leading to the resonance frequency red-shift. Meanwhile, there is an overall trend of narrower resonance dip with increasing side length a, which coincides with our simulation results. The full width at half maximum (FWHM) changes from 0.15, 0.14, 0.12, 0.10 to 0.09 THz as a increases from 40 to 80 μm.
Fig. 1 (a) Structural parameters of the SRR unit cell, and (b)–(f) microscope images for SRR1 to SRR5 fabricated by femtosecond laser MLA lithography. Side length a changes from 40 to 80 μm for (b) to (f), while other parameters are kept as constants.
Table 1. SRR Unit Cell Design Parameters and their Resonance Properties
Fig. 2 (a) Transmission spectra of individual samples with different structural design parameters. The resonance frequency changes from 0.49, 0.54, 0.59, 0.66 to 0.74 THz as side length a changes from 80 to 40 μm, and (b) transmission spectra for the 2-layer metamaterials and the corresponding individual single layer metamaterials. The resonance dips for the 2-layer metamaterials match with those from individual single layer metamaterials. The resonance strengths are further enhanced for the 2-layer metamaterials.
3. Multi-layer metamaterials
The performance of a 2-layer configuration is first investigated in order to determine whether the stacking affects the resonance positions and the intensity for each resonance mode. Two plastic samples SRR1 and SRR5 are selected and bonded together. The SRR orientations of the two samples are matched to ensure the incident wave has the same polarization for both layers. It was determined from previous studies that the interlayer interaction is not significant as the 100 μm interlayer distance is greater than the induced electromagnetic field attenuation length from the SRR arrays [31,32]. Therefore, the exact alignment is not required, which significantly reduces the construction complexity for the multi-layer metamaterials. The transmission spectra are shown in Fig. 2(b). It is observed from the 2-layer transmission spectrum that two distinct resonance dips are clearly shown at 0.49 and 0.74 THz, exactly coinciding with the resonance frequencies of the corresponding individual single layer samples. Different from planar multi-resonance structures, no weakening of the resonance strength is observed for the 2-layer metamaterials. Furthermore, the resonance dips for the 2-layer metamaterials appear to be deepened as compared to those of the individual layers. Therefore, multi-layer metamaterials have the potential to broaden the SRR based metamaterial operation region without distortion of individual layer performance.
To fabricate the multi-layer broadband metamaterials, all the five samples with different a values are stacked in sequence from SRR1 to SRR5 with SRR1 on top facing the incident THz wave. Meanwhile, one more bare PEN film is stacked on top of the SRR1 film to keep all the SRR arrays inside the bulk PEN materials for protection purpose as illustrated in Fig. 3(a) . Figure 3(c) is the image of the fabricated broadband THz filter. The size of this THz filter device is 1.8 cm × 1.8 cm with an effective functional area of 1 cm × 0.6 cm. This is suitable to be applied in a compact THz system. The transmission spectra of the combined 5-layer metamaterials and each corresponding single layer metamaterials are shown in Fig. 4(a) . A broadband response with a center frequency of about 0.61 THz is observed, which is clearly the addition of the resonance responses from each single layer metamaterials. The positions of the five resonance dips in the overall frequency response spectrum match with the resonance dips from individual samples. The FWHM of this filter is measured to be 0.38 THz which is about 2.5 times greater than the FWHM of SRR1 (a = 40 μm) and 4.2 times greater than that of SRR5 (a = 80 μm), indicating that a broader response is achieved by sample stacking. Meanwhile, the stopband is suppressed down to −30 dB, which is much lower than that in single layer metamaterials. This feature is adequate for most filtering applications. The filter exhibits a fast roll-off of more than 100 dB/THz on the edge of the stopband. Interestingly, the resonance dip at 0.66 THz is especially strong compared to other resonance dips. Further investigation will be carried out to explore the underlying physics behind this phenomenon. The Fabry–Pérot fringes on the passband are caused by internal reflections within the multi-layer sample with an overall thickness of about 600 μm. This also agrees well with our simulation results. The reason why they are not observed in single layer sample transmission spectrum is due to the different sample thickness between multi-layer and single-layer metamaterials. The much smaller thickness (100 μm) of the single layer metamaterials shifts the internal reflections, and thus the fringes, to frequency regions that are not seen from the plot scale. The broadband filter built by multi-layer SRR based metamaterials can be used in controlling and manipulating THz wave. The bandwidth of the stopband can also be tuned by adding or subtracting metamaterial layers based on the positions of the resonance dips from individual layers.
Fig. 3 (a) Multi-layer metamaterials stacking illustration, (b) photograph of the flexible PEN film, indicating its potential for implanting non-planar THz devices, and (c) photograph of the multi-layer metamaterials.
Fig. 4 (a) Transmission spectra of the overall multi-layer metamaterials and the corresponding single layer metamaterials. A broadband filter with a bandwidth of 0.38 THz is constructed. The resonance dips from the overall transmission spectrum match with those from individual samples, and (b) simulated spectrum of the multi-layer metamaterials. The resonance dips are at 0.41, 0.45, 0.50, 0.56 and 0.62 THz with an overall frequency red-shift compared to the experimental spectrum.
Finite-integration time-domain simulation is carried out using commercial software CST Microwave Studio 2009. For PEN, the real part of the permittivity ε and the loss tangent are set as 2.56 and 0.003 over the frequency of interest. For simplicity, all the SRRs are aligned perfectly as illustrated in Fig. 3(a). The simulated transmission spectrum is plotted in Fig. 4(b). It shows a broadband resonance response with a bandwidth of around 0.30 THz. On the transmission spectrum, five resonance dips corresponding to each metamaterial layer are found to be at 0.41, 0.45, 0.50, 0.56 and 0.62 THz. As compared to the experimental spectrum, the resonance dips in simulation are much sharper. This is due to the ideal assumption of sample materials and structures in simulation. Meanwhile, there is an overall resonance frequency red-shift for the simulated spectrum. The reason behind this result is that in the simulation, the SRRs are assumed to be enclosed in close contact by the surround PEN films, whereas in experiment there is a thin air gap between adjacent layers, resulting from the stacking process. Consequently, the simulated SRRs are in a homogeneous dielectric environment of a higher refractive index than air, leading to a resonance red-shift [33]. Both experiment and simulation reveal that the stacking of the single layer metamaterials with different structural designs provides a promising way to extend the metamaterial frequency response region from narrowband to broadband.
In order to further understand the performance of each individual layer in the 5-layer metamaterials, we investigated the cross-sectional E-field intensity through numerical simulation. Five specific frequencies at 0.62, 0.56, 0.50, 0.45 and 0.41 THz, which correspond to the five transmission dips in the simulated multi-layer metamaterials transmission spectrum, are selected. Each chosen frequency corresponds to the LC resonance frequency for one of the SRR layers. For example, SRR1 has its LC resonance frequency at 0.62 THz while SRR5 has its LC resonance frequency at 0.41 THz. We define resonant SRR layer as the SRR layer with the chosen frequency as its LC resonance frequency. The cross-sectional plane is along the y-z plane and cuts through the center of all the SRR gaps. The E-field intensity distributions at the five chosen frequencies in this cross-sectional plane are presented in Figure 5(a) to 5(e). It is observed that at each chosen frequency, the SRR layers are selectively excited with a strong E-field concentration in the gap due to charge accumulation at the ends of the SRR. As the selected frequency changes from 0.62 to 0.41 THz, the resonant SRR layer, which is the excited SRR layer with the largest z position value, shifts from SRR1 to SRR5 correspondingly. This indicates a selective LC resonance excitation from the SRR layers in response to the chosen frequency. Furthermore, it can also be seen that more than one SRR layers are excited strongly at each given frequency since the LC resonance frequencies of adjacent layers are close to each other. This multi-layer excitation further enhances the resonance intensity of the overall transmission performance due to more excited SRR layers, resulting in much deeper transmission dips in the multi-layer metamaterials transmission spectrum as compared to transmission dips of the individual single layer metamaterials. Meanwhile, the number of excited SRR layers at each chosen frequency is limited up to three, since SRR does not respond to frequency ranges that are too far away from its LC resonance frequency. For f = 0.56, 0.50, 0.45 and 0.41 THz, the localized E-field intensity of the excited SRR layer to the left of the resonant SRR layer is comparable or even stronger than the localized E-field intensity of the resonant SRR layer, as can be seen in Figure 5(b) to 5(e). On the contrary, it can also be observed that SRR layers to the right of the resonant SRR layer are generally not excited by the selected frequency. This may be due to the reason of the asymmetric line shape of the SRR resonance curve.
Fig. 5 (a)–(e) Cross sectional E-field intensity distribution at the gaps of the multi-layer metamaterials under selected frequencies. The frequencies are chosen to coincide with the five resonance dips from the overall transmission spectrum. The localized E-field intensity enhancement at the gaps indicates a strong resonance response towards the selected frequency. The magnitude of the localized E-field enhancement is up to 10 times larger than the magnitude of the incident E-field.
4. Conclusions
In conclusion, we report a broadband frequency response from multi-layer metamaterials. SRR arrays with LC resonance frequencies of 0.49, 0.54, 0.59, 0.66 and 0.74 THz are fabricated on 100 μm thick flexible PEN films by femtosecond laser MLA lithography. A broadband filter with a bandwidth of 0.38 THz is constructed by stacking individual 2D metamaterials together. Simulation results reveal that SRR layers inside the multi-layer metamaterials are selectively excited towards specific frequencies within the broadband response. Meanwhile, more than one SRR layers respond to the chosen frequencies with a localized E-field intensity enhancement, which strengthens the resonance properties of the overall performance. Multi-layer metamaterials have potential use in building functional broadband THz devices.
Acknowledgments
The author would like to acknowledge support by National University of Singapore Start-up Grant (Project No. R-263-000-515-133) and ASTAR SERC Terahertz Program (Project: Terahertz Spectroscopy, Project No. 082 141 0039).
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