1. Introduction

Over the last ten years, the understanding and broader application implication of metamaterial has been greatly extended. In fact, metamaterial has been proposed for optical cloak, illusion, absorber, negative index materials [1–4], etc., in which the electromagnetic response could be engineered by scaling the size parameter of the artificial structures. Furthermore, the shape of the metamaterial device is also an important parameter for manipulating the light scattering. For example, optical cloak [5–7] and hyperlens [8] fabricated with curved structure was used to meet the modulation of anisotropic refractive index. Metamaterial and plasmonic devices on flexible tape, silk, paper [9–15] and stretchable PDMS substrate [16] have been demonstrated to show unusual optical response. However, most of the reported flexible metamaterial or plasmonic devices work in the gigahertz, terahertz, or far-infrared frequency [9–16]. For NIR and visible wavelength applications, the feature size of each unit cell has to be scaled down to tens of nanometer. Most of the current optical metamaterial nanostructures were fabricated on rigid substrate such as glass, silicon and they are fabricated using fabrication techniques [17–23] such as focus ion beam (FIB), e-beam lithography (EBL), nano-imprint lithography (NIL) and soft interference lithography (SIL). Recently, single layer flexible metamaterial [24] working at visible-NIR wavelength was directly fabricated on PET substrate using EBL. However, the chemical solution used in metal lift-off process needs to be carefully chosen to avoid chemical damages on the flexible substrate. Besides, the curved surface of the PET substrate brings additional difficulty for the focusing of electron in EBL process. Another important progress in this area is realizing large area 3D flexible metamaterial [25] by nanometer printing technique. In Ref. [25], a stamp is used to transfer nanostructure to target substrate, and the advantage of this method is that the stamp can be reused many times.

In this work, we demonstrated that multilayer flexible metamaterial can be fabricated using flip chip transfer (FCT) technique. This technique is different from other similar techniques such as metal lift off process which fabricates the nanostructures directly onto the flexible substrate [24] or nanometer printing technique [25]. It is a solution-free FCT technique using double-side optical adhesive as the intermediate transfer layer and a tri-layer metamaterial nanostructures on a rigid substrate can be transferred onto adhesive first. Then, the thin optical adhesive and the nanostructure can be conformably coated onto flexible substrates, such as the bent PET substrate, paper, etc.

2. Device fabrication

A schematic fabrication process of multilayer metamaterials is shown in Fig. 1 . First, the multilayer plasmonic or metamaterial device was fabricated on chromium (Cr) coated quartz using conventional EBL process. The 30 nm thick Cr layer was used as sacrificial layer. Then gold/ITO (50 nm/50 nm) thin film was deposited onto the Cr surface using thermal evaporation and RF sputtering method respectively. Next, ZEP520A (positive e-beam resist) thin film with thickness of about 300 nm was spun on top of the ITO/gold/Cr/quartz substrate and two dimensional hole array was obtained the ZEP520A using the EBL process. To obtain the gold nanostructure (disc pattern), a second 50 nm thick gold thin film was coated onto the e-beam patterned resist. Finally, two dimensional gold disc-array nanostructures was formed by removing the resist residue. The area size of the each metamaterial pattern is 500 µm by 500 µm, and the period of the disc-array is 600 nm with disc diameter of ~365 nm.

 

Fig. 1 (a) to (e) are the EBL steps to fabricate the absorber metamaterials, period of the disc-array device is 600 nm, disc diameter: 365 nm; thickness of gold: 50 nm; thickness of Cr: 30 nm; (f) is the scanning electron microscope (SEM) image of the two dimensional gold disc-array absorber metamaterials.

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Transfer process of flexible absorber metamaterial is shown in Fig. 2 , double-sided sticky optically clear adhesive (50 µm thick, from 3M) was attached to the PET substrate (70 µm thick). Thus the tri-layer metamaterial device was placed in intimate contact with optical adhesive and sandwiched between the rigid substrate and the optical adhesive. Note that the Cr thin film on quartz substrate was exposed to the air for several hours after the RF sputtering process, so there is a thin native oxide film on the Cr surface. Hence the surface adhesion between Cr and gold is much weaker than that of gold/ITO/gold disc/optical adhesive bounding. This allows the tri-layer metamaterial nanostructure to be peeled off from the Cr coated quartz substrate. Once the metamaterial nanostructure was transferred onto the PET substrate, it possesses sufficient flexibility to bend into various shapes. Finally, the metamaterial nanostructure was encapsulated by spin-coating a 300 nm thick PMMA layer on top of the device.

 

Fig. 2 (a) to (e) is schematic diagram of flip chip transfer method, the tri-layer absorber metamaterial with an area of 500 µm by 500 µm was transferred to PET flexible substrate.

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Figure 3a shows the flexible absorber metamaterial sandwiched by the transparent PET and PMMA thin film. Several absorber metamaterial nanostructures with area size of 500 µm by 500 µm were fabricated on flexible substrate. In fact, using the flexibility property of the PET layer, the absorber metamaterial device can be conformed into many shape e.g. cylindrical shape (Fig. 3b). The minimum radius of the cylindrical substrate is about 3 mm, not obvious defect on the metamaterial device can be observed after 10 times of repeatable bending tests.

 

Fig. 3 (a) and (b) Flexible NIR absorber metamaterials on transparent PET substrate. Each separated pattern has an area size of 500 µm by 500 µm.

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3. Optical characterization and simulation

The tri-layer metal/dielectric nanostructure discussed above is an absorber metamaterial device [26]. The design of the device is such that the energy of incident light is strongly localized in ITO layer. The absorbing effects of the NIR tri-layer metamaterial architecture could be interpreted as localized surface plasmon resonance [27] or magnetic resonance [28]. The absorbing phenomenon discussed here is different from the suppressed of transmission effect in metal disc arrays [29], in which the incident light is strongly absorbed due to resonance anomaly of the ultrathin metal nanostructure. To characterize the optical property of gold disc/ITO/gold absorber metamaterial, Fourier transform infrared spectrometer (FTIR) was used to measure the reflection spectrum of the absorber metamaterial. By combining the infrared microscope with the FTIR spectrometer, transmission and reflection spectra from micro-area nanophotonic device can be measured. In Fig. 4 , the reflection spectrum (blue line) from air/metamaterial interface was measured with sampling area of 100 µm by 100 µm. At the absorption peak with wavelength of ~1690 nm, reflection efficiency is about 14%, i.e. the absorber metamaterial works at this wavelength. In RCWA simulation (red line), the real optical constants in Ref. [30] is used. At resonant wavelength, the experiment and calculation agree well with each other.

 

Fig. 4 Relative reflection spectrum of the absorber metamaterials on quartz substrate (gold disc/ITO/gold/Cr/quartz), NIR light was normally focused on the device and the reflection signal was collected by the 15X objective lens; blue line is experimental result and red line is simulated reflection spectrum using RCWA method.

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Reflection spectrum of the flexible absorber metamaterial is shown in Fig. 5a (black line). Compared to FTIR result in Fig. 4, the absorption dip of the flexible metamaterial has red shifted to ~1.81 µm. This red shift is mainly due to the refractive index change of surrounding medium (refractive index of optical adhesive and PET is about 1.44). In Fig. 5c and Fig. 5d, three dimensional rigorous coupled wave analysis (RCWA) method is employed to calculate the reflection and transmission spectra on the absorber metamaterial, and experimentally confirmed parameters of materials of gold, ITO, Cr, SiO₂ [30] and PET were used. Resonant absorption at wavelength of ~1.81 µm can also be observed in theoretical simulations. However, there are two resonant dips around 1.2 µm in the measured reflection spectrum. In the RCWA calculation (Fig. 5c), the double dips are reproduced and ascribed to two localized resonant modes, as they are not very sensitive to incident angles. For the angle dependent calculation, TE polarized light is used (electric field is perpendicular to incident plane) to fit the experimental result. While the incident angle is changed from 0 to 45 degrees, reflection efficiency shows an increasing trend as light cannot be efficiently localized under large angle incidence. However, the back reflection efficiency in experiment (Fig. 5a) decreases obviously. This is because our current experimental setup (discussed in next section) only allow us to collect the back-reflection signal (incident and collection direction are same as each other), and the collection efficiency is very low for large incident angles. In Fig. 5b, transmission spectrum of the flexible metamaterial was measured using the same FTIR setup, the main difference is light was incident from the air/PMMA interface. A Fano-type transmission peak is observed at wavelength ~1.85 µm. At resonant wavelength, the transmission efficiency from experiment is higher than that in the theoretical simulation (Fig. 5d). This could be due to defects on gold planar film and the two dimensional disc arrays, which enhances the efficiency of leakage radiation and thus contribute to the higher transmission efficiency in the measured results.

 

Fig. 5 (a) Angle resolved back reflection spectra measured on flexible metamaterial (with curved surface). The light is incident from PET side and the back reflection was collected by NIR detector; (b) Transmission spectra measured on the flexible absorber metamaterial, light was incident from the PMMA side and collected from the PET side. (c) and (d) are simulated reflection and transmission spectra on flexible absorber metamaterial using RCWA method.

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As shown in Fig. 6 , bending PET substrate allows us to measure the optical response of absorber metamaterial under different curving shape. The shape of the bent PET substrate was controlled by adjusting distance of substrate ends (A and B). Angle resolved back-reflection on the absorber device was measured by varying the bending conditions. From Fig. 6, incident angle (90°−φ) was determined from the bending slope at position of metamaterial device. From Fig. 5a, it is observed that when the incident angle was increased from 0 to 45 degrees, the intensity of back reflection becomes weaker and absorption dip becomes shallower. Nevertheless, it shows that the resonant absorption wavelength of the flexible absorber metamaterial is not sensitive to the incident angle of light.

 

Fig. 6 Experiment diagram of measuring the reflection spectrum of metamaterial device under different bending condition. The flexible substrate is bent by adjusting the distance of A and B, and the incident angle 90° – φ (varying from 0 to 45 degrees) is defined by the slope of PET substrate and direction of incident light.

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4. Conclusion

In conclusion, we reported a highly flexible tri-layer absorber metamaterial device working at NIR wavelength. By using FCT method, the tri-layer gold disc/ITO/gold absorber metamaterial was transferred from quartz substrate to a transparent PET substrate using optically clear adhesive (3M). Finally, the tri-layer absorber metamaterial was encapsulated by PMMA thin film and optical adhesive layer to form a flexible device. FTIR experiment showed that the absorber metamaterial works well on both the quartz substrate and the highly flexible PET substrate. Besides, angle insensitive absorbing effects and Fano-type transmission resonance were observed on this flexible metamaterial.

The solution-free FCT technique described in this work can also be used to transfer other visible-NIR metal/dielectric multilayer metamaterial onto flexible substrate. The flexible metamaterial working at visible-NIR regime will show more advantages in manipulation of light in three dimensional space, especially, when the metamaterial architecture is designed on curved surfaces.