# Patent

## Title
Highly flexible near-infrared metamaterials

## Abstract

A novel multilayer flexible metamaterial that can work at near infrared (NIR) regime that can be fabricated on transparent PET substrate using flip chip transfer (FCT) technique. An embodiment of the present invention also provides novel NIR metamaterial device that can be transformed into various shapes by bending the PET substrate, wherein said metamaterial device is tunable via physical manipulation of its flexible substrate with no requirement to change the material composition of the substrate. The flexibility allows the novel NIR metamaterial device to bend and stretch, which can alter the device structure. Since the resonant frequency of each device is a function of the device structure, the resonant frequency can be tuned by the bend and stretch of the substrate. Another embodiment of the present invention further provides novel tunable sensors and emitters using such flexible metamaterials and devices.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to the U.S. Provisional Patent Application No. 61/579,668 filed Dec. 23, 2011; the disclosure of which is incorporated herein by reference in its entirety.


FIELD OF INVENTION
The present invention relates to a novel plasmonic or metamaterial nanostructures. In particular, the present invention relates to a novel tri-layer flexible metamaterials that can work at near infrared (NIR) regime that can be fabricated on transparent polyethylene terephthalate (PET) substrate using flip chip transfer (FCT) technique. FCT technique is solution-free and can also be applied to fabricate other functional nanostructures device on flexible substrate. This invention also provides novel NIR metamaterial device that can be transformed into various shapes by bending the PET substrate. This invention further provides novel tunable sensors and emitters using such flexible metamaterials and devices.
BACKGROUND OF INVENTION
Plasmonic or metamaterial nanostructures are usually fabricated on rigid substrate i.e. glass, silicon. Optical functionality of such kinds of nanostructures is limited by the planar surface and thus sensitive to the incident angle of light. 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 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 and hyperlens fabricated with curved structure were used to meet the modulation of anisotropic refractive index. Metamaterial and plasmonic devices on flexible tape, silk, paper and stretchable PDMS substrate 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. 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 such as focus ion beam (FIB), e-beam Lithography (EBL), nano-imprint lithography (NIL) and soft interference lithography (SIL). Recently, single layer flexible metamaterial 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 by nanometer printing technique. In D. Chanda, K. Shigeta, S. Gupta, T. Cain, A. Carlson, A. Mihi, A. J. Baca, G. R. Bogart, P. Braun, and J. A. Rogers, “Large area flexible 3D optical negative index metamaterial formed by nanotransfer printing,” Nat. Nanotechnol. 6, 402-407 (2011), a stamp is used to transfer nanostructure to target substrate, and the advantage of this method is that the stamp can be reused for many times. The U.S. Patent Application Publication No. 2010/0301971 describes an exemplary metamaterial comprising a flexible dielectric substrate that is tunable using an electrical control signal to adjust its electromagnetic properties or via the utility of a tunable resonant circuit that includes a phase change material, wherein the resonant frequency of said metamaterial is tunable via modification of said phase change material either via an electrical control signal or by adjusting the composition of said phase change material. Furthermore, in the prior art, the flexible metamaterial fabricated is dependent on the original substrate used.
The objective of the present invention is to provide a novel 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 or nanometer printing technique. 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. Thus, the flexible metamaterial can be fabricated independent of the original substrate used. This flexible metamaterial is tunable via physical manipulation of its flexible substrate with no requirement to change the material composition of the substrate. In particular, the present invention further provides novel tunable sensors and emitters using such flexible metamaterials and devices.
Citation or identification of any reference in this section or any other section of this application shall not be construed as an admission that such reference is available as prior art for the present application.
SUMMARY OF INVENTION
Embodiments of the present invention include a novel multilayer flexible metamaterial that can work at near infrared (NIR) regime that can be fabricated on transparent PET substrate using flip chip transfer (FCT) technique. An embodiment of the present invention also provides novel NIR metamaterial device that can be transformed into various shapes by bending the PET substrate, wherein said metamaterial device is tunable via physical manipulation of its flexible substrate with no requirement to change the material composition of the substrate. The flexibility allows the novel NIR metamaterial device to bend and stretch, which can alter the device structure. Since the resonant frequency of each device is a function of the device structure, the resonant frequency can be tuned by the bend and stretch of the substrate. Another embodiment of the present invention is the fabrication method, which allows the transfer of the metamaterial from a rigid substrate such as glass, quartz and metals onto a flexible substrate such as plastic or polymer film. Thus, a flexible metamaterial can be fabricated independent of the original substrate used. Another embodiment of the present invention further provides novel tunable sensors and emitters using such flexible metamaterials and devices.
Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described.
The invention includes all such variation and modifications. The invention also includes all of the steps and features referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.
Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is also noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. patent law; e.g., they can mean “includes”, “included”, “including”, and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the invention.
Furthermore, throughout the specification and claims, unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
Other definitions for selected terms used herein may be found within the detailed description of the invention and apply throughout. Unless otherwise defined, all other technical terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention belongs.
Other aspects and advantages of the invention will be apparent to those skilled in the art from a review of the ensuing description.



BRIEF DESCRIPTION OF DRAWINGS
The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, in which:
FIGS. 1(a) to 1(e) show 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, and thickness of Cr: 30 nm; 1(f) is the scanning electron microscope (SEM) image of the two dimensional gold disc-array absorber metamaterials;
FIGS. 2(a) to 2(e) show the schematic diagrams of the flip chip transfer method, the tri-layer absorber metamaterial with an area of 500 μm by 500 μm was transferred to a PET flexible substrate;
FIGS. 3(a) and 3(b) show the flexible NIR absorber metamaterials on a transparent PET substrate; each separated pattern has an area size of 500 μm by 500 μm;
FIG. 4 shows the 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 and was collected by the 15× objective lens; square-dotted line is the experimental result and triangle-dotted line is the simulated reflection spectrum using RCWA method;
FIGS. 5(a) to (d) show: (a) Angle resolved back reflection spectra measured on flexible metamaterial (with curved surface), the light being incident from PET side and the back reflection was collected by NIR detector; (b) transmission spectra measured on the flexible absorber metamaterial, the light being incident from the PMMA side and collected from the PET side; and (c) and (d) are simulated reflection and transmission spectra on flexible absorber metamaterial using RCWA method;
FIG. 6 shows experiment diagram of measuring the reflection spectrum of metamaterial device under different bending condition; the flexible substrate is bent by adjusting the distance between A and B, and the incident angle 90°−ø (varying from 0 to 45 degree) is defined by the slope of PET substrate and direction of incident light;
FIG. 7(a) to (c) shows (a) Cross-section of metamaterial device: gold circular disc/ITO layer/gold slab; (b) and (c) SEM picture of MPC samples: gold disc array/ITO layer/gold slab/Cr layer/quartz; thickness of gold/ITO/gold/Cr is 50 nm/50 nm/50 nm/30 nm wherein (b) Sample A (period: 600 nm and disc diameter 320 nm) and (c) Sample B (period: 910 nm and disc diameter 360 nm);
FIG. 8(a) to (d) shows bright field ((a) and (b)) and dark field ((c) and (d)) imaging of third harmonic generation on sample A. Fundamental wavelength is: (a) 1740 nm; (b) 1800 nm;
FIG. 9 shows the spectra of third harmonic generation on sample A, fundamental wavelength: 1620 nm (line with square symbol); 1640 nm (line with triangle symbol); and
FIG. 10 shows the spectral resolved THG on sample A and B at incident angle of (θ=52−); (a) Experimental (line with circle symbol) and calculated (solid line) THG efficiency of sample A; (b) Experimental (line with circle symbol) and calculated THG efficiency of sample B (solid line).



DETAILED DESCRIPTION OF INVENTION
The present invention is not to be limited in scope by any of the specific embodiments described herein. The following embodiments are presented for exemplification only.
The present invention provides a novel 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 or nanometer printing technique. 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, which can be transferred onto the adhesive first. Another embodiment of the present invention is the fabrication method, which allows the transfer of the novel metamaterial from a rigid substrate such as glass, quartz and metals onto a flexible substrate such as plastic or polymer film. Thus, a novel flexible metamaterial can be fabricated independent of the original substrate used.
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 a sacrificial layer. Then a gold/ITO (50 nm/50 nm) thin film was deposited onto the Cr surface using thermal evaporation and RF sputtering method respectively. Next, a 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 a two dimensional hole array was obtained on the ZEP520A using 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, a two dimensional gold disc-array nanostructure was formed by removing the resist residue. The area size of 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.
Flip Chip Transfer (FCT) Technique
Transfer process of flexible absorber metamaterial is shown in FIG. 2, double-sided sticky optically clear adhesive (50 μm thick; e.g. a commercially available product manufactured by 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, such that 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 be bended into various shapes. Finally, the metamaterial nanostructure was encapsulated by spin-coating a 300 nm thick PMMA layer on top of the device.
In another embodiment, the present invention provides novel NIR metamaterial device that can be transformed into various shapes by bending the PET substrate.
FIG. 3(a) 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. 3(b)). 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.
Optical Characterization and Simulation
The tri-layer metal/dielectric nanostructure discussed above is an absorber metamaterial device. 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 or magnetic resonance. The absorbing phenomenon discussed here is different from the suppression of transmission effect in metal disc arrays, 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 (Experiment line plot) 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 (Simulation line plot), the real optical constants in E. D. Palik, Handbook of optical constants of solids, Academic Press, New York, 1985 is used; the content of which is incorporated herein by reference in its entirety. At resonant wavelength, the experiment and calculation agree well with each other.
Reflection spectrum of the flexible absorber metamaterial is shown in FIG. 5(a) (0° line plot). 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 the surrounding medium (refractive index of optical adhesive and PET is about 1.44). In FIG. 5(c) and FIG. 5(d), 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 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. 5(c)), 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 degree, reflection efficiency shows an increasing trend as light cannot be efficiently localized under large angle incidence. However, the back reflection efficiency in experiment (FIG. 5(a)) 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. 5(b), 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. 5(d)). 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.
In a further embodiment, the present invention also provides novel tunable sensors and emitters using such flexible metamaterials and devices.
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 the distance between the substrate ends (A and B). The angle for the resolved back-reflection on the absorber device was measured by varying the bending conditions. From FIG. 6, the incident angle (90°−ø) was determined from the bending slope at the position of the metamaterial device. From FIG. 5(a), it is observed that when the incident angle was increased from 0 to 45 degree, the intensity of back reflection becomes weaker and the 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. Devices made from the metamaterials can be made into highly sensitive sensors. This invention provides a novel metamaterial devices on a flexible substrate. The flexibility allows the device to be bended and stretched, which alters the device structure. Since the resonant frequency of each device is a function of the device structure, the resonant frequency can be tuned by the bending and stretching of the substrate. Hence, another embodiment of the present invention is a metamaterial that allows a physical means to change the structure of the material, which leads to a change in its resonant frequency. There is no need to change the material composition. An embodiment of the present invention is application of flexible plasmonic or metamaterial nanostructure device as an electromagnetic wave absorber.
In the aforementioned embodiments of the present invention, it has 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 (e.g. a commercially available product manufactured by 3M). Furthermore, 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. Finally, devices made from such metamaterials can be made into highly sensitive tunable sensors.
Moreover, the solution-free FCT technique described in this invention can also be used to transfer other visible-NIR metal/dielectric multilayer metamaterial onto a 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.
It is herein further provided other embodiments of the present invention wherein the novel metamaterial is applied to various industrial applications.
Application I: Sensing
The reflection spectrum of the flexible absorber metamaterial is shown in FIG. 5(a). The NIR light was normally incident on the sample from the PET layer. Compared to the result in FIG. 4, the absorption dip of the flexible metamaterial has red shifted to  ̃1.8 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). So, the proposed flexible device can be used for environmental sensing application.
Application II: Tunable Light Source Using Near-Infrared Metamaterials
In this embodiment, we demonstrate that third harmonic generation, one kind of radiative nonlinear optical processes, can be spectrally modulated using Fano-resonant magnetic metamaterials. The device is a tri-layer nanostructure, consisting of gold slab/ITO (Indium tin oxide)/two dimensional gold nanodisc. This Fano-resonant metamaterial supports both localized MPR mode and propagating surface plasmon polaritons (SPP). By finely tuning the interference between localized MPR and propagating SPP modes at near infrared wavelength, the surface polarization of pump laser on metamaterial induce Fano-resonant modulation. Spectral resolved third harmonic generation on this metal/dielectric hybrid nanostructure was demonstrated to show a Fano-resonant response. This indicates that third order and other nonlinear optical processes can be tailored by engineering the coupling of MPR and SPP mode, the interference between these two kind of modes provides us a more flexible way to design nonlinear optical nano-devices.
Device Fabrication
The cross-section of nanostructure is shown in FIG. 7(a), and a unit cell being composed of a gold nanodisc is fabricated on top of ITO/gold slab. Two samples (A and B) were fabricated using e-beam lithography, thin film deposition, and metal lift-off process. The SEM pictures of the two samples are shown in FIG. 7(b) and FIG. 7(c). The thicknesses of gold nanodisc, ITO and gold slab are 50 nm thick respectively. The disc diameter of A is 320 nm with period of 600 nm; and for sample B, the diameter of disc is 360 nm with the period of 910 nm.
The nanostructure or glass substrate can be transferred to flexible substrate using the flip-chip transfer technique and thus we can fabricate a flexible plasmonic nanostructure, which can be used to nonlinear optical source. This flexible substrate can be optical fiber or PET, which guide the light to propagate for a long distance in optoelectronic devices.
Nonlinear Optical Experiment
Nonlinear optical property of Fano resonant metamaterial was studied using third harmonic generation process. A femtosecond laser pumped optical parametric amplifier (OPA, TOPAS-C) system was used in this experiment. The output wavelength of OPA can be tuned from 1.61 μm to 1.8 μm with repetition frequency of 1 kHz and pulse duration  ̃100 femtosecond. The horizontally polarized (electric field parallel to incident plane) pump laser was incident at angle of 52−. After being focused by a lens with focal length of 150 mm, the near infrared laser pulse was incident onto the metamaterial device. The laser spot size is about 120 μm by 100 μm, which is less than the total area of the nanostructure. THG signal was collected by silicon based CCD detector and spectrometer (Ocean Optics 4000) after filtering the fundamental wave. The output power was measured by a photodiode (Newport 818-UV). Two linear polarizers were used to control the power and polarization of pump laser, Newport power detector (818-IR) was used to monitor the power variation of pump laser (NIR) from the side beam of beam splitter.
An imaging system was set up to monitor the position of nanostructure and pump laser using a Xenon lamp was illumination light. In bright field mode (FIG. 8(a) and FIG. 8(b)), it can be found that both radiation spot (marked by dashed line) of third harmonic generation and nanostructure can be imaged by CCD camera. The spectrum of third harmonic generation was measured using Ocean Optics spectrometer after filtering the pump laser.
In FIG. 9, the THG spectra on sample A with bandwidth of THG peak of about 9 nm, are measured using fundamental wavelengths of 1620 nm and 1640 nm. The relative efficiency of THG (intensity ratio of THG and pump laser) on sample A and B is given in FIG. 10. For sample A, of which resonant wavelength being at  ̃1.51 μm, the third harmonic generation efficiency decreases when pump wavelength is tuned from 1.61 to 1.8 μm (FIG. 10(a)). However, third harmonic generation on sample B is quite different, in which a Fano-resonant modulation was found (FIG. 10(b)).
In conclusion, we have designed a two-dimensional magnetic metamaterial. Both localized MPR and propagating SPP modes can be excited in such a system. Through the interference between MPR and SPP excitations, linear Fano effect is found from the ellipsometric spectra. The nonlinear optical process on metamaterial device was studied through THG process. By modulating the nonlinear polarization through the interference between MPR and SPP modes, Fano-resonant THG was for the first time observed on magnetic metamaterial device. The integration of Fano resonance into nonlinear optical MPR device will provides many more opportunities for realizing functional optical nano-device. Although we only studied THG here, such kind of nonlinear Fano resonant effect can also be applied to other nonlinear optical processes, such as high harmonic generation, surface enhanced Raman, and four-wave mixing, etc.
Modifications and variations such as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.
INDUSTRIAL APPLICABILITY 
The present invention discloses a novel plasmonic or metamaterial nanostructures. In particular, the present invention relates to a novel tri-layer flexible metamaterials that can work at near infrared (NIR) regime that can be fabricated on transparent polyethylene terephthalate (PET) substrate using flip chip transfer (FCT) technique. FCT technique is solution-free and can also be applied to fabricate other functional nanostructures device on flexible substrate. The present invention also provides a novel NIR metamaterial device that can be transformed into various shapes by bending the flexible substrate, wherein said metamaterial device is tunable via physical manipulation of its flexible substrate with no requirement of changing the material composition of the substrate. The flexibility allows the novel NIR metamaterial device to be bended and stretched, which alters the device structure. Since the resonant frequency of each device is a function of the device structure, the resonant frequency can be tuned by the bending and stretching of the substrate. The present invention also discloses a fabrication method that allows the transfer of the metamaterial from a rigid substrate such as glass, quartz and metals onto a flexible substrate such as plastic or polymer film. Thus, a novel flexible metamaterial can be fabricated independent of the original substrate used. The present invention further provides novel tunable sensors and emitters using such flexible metamaterials and devices.
If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.
While the foregoing invention has been described with respect to various embodiments and examples, it is understood that other embodiments are within the scope of the present invention as expressed in the following claims and their equivalents. Moreover, the above specific examples are to be construed as merely illustrative, and not limitative of the reminder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present invention to its fullest extend. All publications recited herein are hereby incorporated by reference in their entirety.

## Claims

1. A metamaterial comprising:
a multi-layer comprising at least:
a first conductor layer,
an insulator layer,
a second conductor layer; and

a flexible substrate;
wherein the multi-layer is transferred from a rigid substrate to the flexible substrate using a solution-free flip chip transfer (FCT) technique;
wherein the solution-free FCT technique comprising:
attaching a double-sided optically clear adhesive on the flexible substrate,
placing the multi-layer in intimate contact with the optically clear adhesive, and
peeling the multi-layer off from the rigid substrate; and

wherein the metamaterial having a range of operating frequencies including near infrared.

2. A metamaterial according to claim 1, wherein the flexible substrate being permeable to electromagnetic radiation of frequencies including visible light.

3. A metamaterial according to claim 1, wherein the range of frequencies of the metamaterial can be adjusted by physically manipulating the flexible substrate.

4. A metamaterial according to claim 1, wherein the metamaterial can be transformed into arbitrary shaped devices by physical manipulation of the flexible substrate.

5. A tunable receiver device made from the metamaterial according to claim 1, wherein the tunable receiver being one or more of:
electromagnetic wave sensors,
electromagnetic wave absorbers, and
electromagnetic wave receivers.

6. A tunable transmitter device made from the metamaterial according to claim 1, wherein the transmitter device being one or more of:
electromagnetic wave emitters,
electromagnetic wave transmitters, and
electromagnetic wave reflectors.

7. The metamaterial according to claim 1, wherein the rigid substrate is coated with a thin native oxide film which allows the multi-layer being peeled off from the thin native oxide film coated rigid substrate and possessing sufficient flexibility to be bended into various shapes when the multi-layer is transferred to the flexible substrate.

8. The metamaterial according to claim 7, wherein the thin native oxide film being chromium thin film and exposed to air to become oxidized.

9. The metamaterial according to claim 1, wherein the multi-layer is sandwiched between the optically clear adhesive and the rigid substrate prior to completing the transfer to the flexible substrate.

10. The metamaterial according to claim 1, wherein the rigid substrate being glass, quartz, metal, or any combination thereof.

11. The metamaterial according to claim 1, wherein said flexible substrate being plastic, polymer film, polyethylene terephthalate, or any combination thereof.

12. The metamaterial according to claim 1, wherein the metamaterial and optical adhesive are encapsulated by spin-coating a polymethyl methacrylate (PMMA) layer after transferring said metamaterial to said flexible substrate.

13. The metamaterial according to claim 1, wherein the multi-layer comprising:
a gold layer,
an ITO layer, and
a plurality of gold discs;
wherein the ITO layer is located on said gold layer, the plurality of gold discs are located on said ITO layer; and
wherein each of the plurality of the gold discs has a diameter of approximately 320 to 360 nm and said plurality of gold discs has a disc period of approximately 600 to 910 nm.

14. The metamaterial of claim 13, wherein each of said plurality of the gold discs has a thickness of approximately 50 nm; said ITO layer has a thickness of approximately 50 nm; said gold layer has a thickness of approximately 50 nm.