Out-of-Plane Designed Soft Metasurface for Tunable Surface Plasmon

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Out-of-Plane designed soft metasurface for tunable surface plasmon polariton Xin Liu, Zhao Huang, Chengkai Zhu, Li Wang, and Jianfeng Zang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05190 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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Out-of-plane Designed Soft Metasurface for Tunable Surface Plasmon Polariton Xin Liu, †,‡ Zhao Huang, †,‡ Chengkai Zhu, † Li Wang, † and Jianfeng Zang†,‡,* †

School of Optical and Electronic Information and Wuhan National Laboratory for

Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China ‡

Innovation Institute, Huazhong University of Science and Technology, Wuhan 430074, China *Email: [email protected].

ABSTRACT: Reliable and repeatable tunability gives functional diversity for reconfigurable plasmonics devices. While reversible and large mechanical deformation enabled by soft materials provides a new way for globally or partially regulation of metadevices. Here we demonstrate a soft metasurface with out-of-plane design for tuning the energy of surface plasmon polaritons (SPPs) bloch wave using theory, simulation, and experiments. Our metasurface is composed of two layered gold nanoribbon arrays (2GNRs) on soft substrate. The out-of-plane coupling mechanism is systematically analyzed in terms of separation height effect. Moreover, by harnessing mechanical deformation, continuously tunable plasmonic resonance has been achieved in visible and near infrared range. We further studied the angle dependent reflection spectra of our metastructure. Compared with its planar counterpart, our soft and twolayered metastructure exhibits diverse tunability and significant field enhancement by out-of-

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plane interaction. Our approach in designing soft metasurface with out-of-plane structure can be extended to more complex photonic devices and finds prominent applications such as biosensing, high density plasmonic circuits, surface enhanced luminescence and Raman Scattering.

KEYWORDS tunable metasurface, surface plasmons, out-of-plane coupling, soft materials, gold nanoribbons

Surface plasmon resonance has been particularly attractive over the years due to its strong field confinement and enhancement properties in light-matter interaction, inspiring numerous prominent applications such as biosensors,1-3 plasmonic metasurfaces,4-6 surface enhanced Raman Scattering7-9 (SERS) and luminescence10. Many of these applications require plasmonic resonance to be tunable to realize functional diversity and reconfigurability. For example, tunable plasmonic response is extremely useful for plasmonic biosensors to selectively probe the biomolecules at different frequencies.11 Tremendous efforts have been devoted to achieve tunable plasmonic properties, including changing the plasmonic materials,12 and their geometrical shapes, dimensions, and interval between the structure units. Nanostructures have been extensively attempted for this regard, such as nanoantenna,13,14 nanoshells,15,16 nanodisks17,18 and even more complicated nanostructures.19 Another approach is to put the plasmonic structures in various surrounding media with different relative permittivity to control the resonance frequency.20 A recent innovative method to alter the relative permittivity is demonstrated by temperature induced phase change in vanadium oxide.21 However, most of the methods share the limitations of not being able to tune the plasmonic properties continuously and


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reversibly. By applying external voltage on some electrical sensitive materials such as liquid crystals22 and graphene,23 dynamical tunability has been realized. To achieve tunable plasmonic resonance, mechanical method24-28 provides an alternative while promising approach for reconfigurable plasmonic devices.29,30 Soft elastomeric substrate is capable of reversible and large deformation, which can be used to change the relative position of adjacent resonant units.31,32 However, there are only few studies on mechanical deformation related plasmonics and most of them are mainly based on the in-plane coupling. Here, we present a soft metasurface with out-of-plane coupling design for tuning the energy of surface plasmon polaritons (SPPs) bloch wave using theory, simulation, and experiments. The out-of-plane coupling design offers effective field enhancement, which has not yet been extended to soft material previously.33-35 Our metasurface comprising two layered gold nanoribbon arrays (2GNRs) supports surface plasmon waves that propagate along its surface perpendicular to the nanoribbons. We demonstrate that continuously tunable resonance has been achieved in visible and near infrared range via applying tensile strain. The resonance wavelength can be further adjusted by controlling out-of-plane coupling through changing the separation height between the upper and lower nanoribbons. The out-of-plane coupling mechanism is analyzed. Our approach in designing soft metasurface with out-of-plane structure can be extended to more complex photonic devices and finds prominent applications in high-density plasmonic circuits and optical superabsorbers.36

RESULTS AND DISCUSSION Our metasurface is composed of two layers of gold nanoribbons array on soft substrate, the cross section of which is a rectangular-like wave (Figure 1a). The separation height ℎ is defined as the


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distance between the bottom of the upper layer and the top of the lower layer of nanoribbons. and denote the widths of the upper layer and lower layer of nanoribbons, respectively. Therefore, the period of the metastructure can be expressed as = + . The duty ratio is defined by =

. We use the replica molding method to fabricate the soft metasurface.

The schematic illustrations of the fabrication procedure are shown in Figure S1. The as-prepared metasurfaces are characterized by scanning electron microscope (SEM) and atomic force microscope (AFM), showing a two layered Au nanoribbon structure with the structure parameters, ℎ, , and , as shown in Figure 1b, 1c, and Figure S2. The mean roughness Ra of the Au layers in Figure 1b and 1c are 10.6 nm and 9.6 nm, which are calculated using AFM images in an area of 1.0 . Surface plasmons (SPs) can be directly excited in our periodic structure without the need of prism coupler37 or extra grating.38 When the light strikes the surface of the structure at normal incidence, incident photons gain momentum from the grating structure itself in integral multiples of the reciprocal lattice vector, = 2⁄ . If the component of incident light parallel to the surface matches the momentum of the surface plasmon, the excitation condition will therefore be satisfied, which conforms to the following relationship39: = sin +

(1)

where , , denote the momentums of the surface plasmon and the incident light, respectively. is the diffraction order. As depicted in Figure 2a and 2b, a dip is observed in visible range and near infrared range in both the simulation and experimental reflection spectra for metastructure with the period of 587 nm and 802 nm. As presented in Figure 2c and 2d, the electric field intensities and the simulated charge distributions of metastructure with period of 802 nm and


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separation height of 23 nm at resonance wavelength are given as examples. The electric field distributions show strong field enhancement of the incident light as a result of out-of-plane coupling. The surface charge distributions indicate the resonance mode is a typical anti-bonding mode. Actually, the separation height between two nanoribbon layers can strongly affect out-ofplane coupling. Figure 2a shows the separation height dependent reflection spectra of the metastructure with the period of 587 nm and the same duty ratio of 0.55. Apparently, the resonance mode in experimental spectra experiences a 26 nm red shift from 660 nm to 686 nm when the separation height increases from 21 nm to 53 nm, which is consistent with the simulations (Figure 2a). For the metastructure with the period of 802 nm and the duty ratio of 0.59, we get similar experimental and simulation reflection spectra in near infrared range (Figure 2b). In experiments, a red shift of 26 nm is observed for resonance wavelength from 861 nm to 887 nm when separation height increases from 23 nm to 55 nm. Since it’s an anti-bonding mode, when the separation height increases, the energy of the system decreases, thus causing the red shift of the resonance wavelength. Compared with the simulation, the modes show a prominent broadening of the line width in experiments. The line width broadening is possibly due to the larger optical losses in experiments. The surface roughness of the gold nanoribbons can add a ∆ to the incident light, which will broaden the SP resonance.40 The separation height dependent resonance mode shift provides an additional degree of freedom for tuning the resonance wavelength. The plasmonic coupling effect usually arisen from the adjacent resonance components in planar structures is critical for design and fabrication of plasmonic circuits. Different from its planar counterpart, out-of-plane coupling may give significantly different effect. Here we


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investigate the out-of-plane coupling effect on a metastructure with the duty ratio of 0.5 so that the in-plane coupling effect is negligible.41 Figure 3a and 3b presents the schematics of the planar structure and out-of-plane structure and the corresponding near-field distributions of the resonance modes. We conduct the finite element simulations of two structures with parameters setting as: period 800 nm, duty ratio 0.5, thickness of gold nanoribbons 60 nm, and different separation heights of the out-of-plane structure. Figure 3c shows the effect of separation height on the reflectance spectra and the coupling effect. As the separation height decreases from 120 nm to 5 nm, the dip shows a blue shift from 917 nm to 840 nm, which is consistent with the experimental results in Figure 2a and 2b. The observed line width broadening with the separation height in Figure 3c is attributed to the larger optical losses as a result of the increased roughness of the metastructure.40 If we define the coupling strength of the 2GNRs metastructure as the resonance wavelength shift relative to its planar counterpart without out-of-plane coupling, the separation height dependent coupling strength is presented in Figure 3d. The lower separation height, the higher coupling strength. As the coupling strength is stronger, the observed blue shift of the mode is distinctly different from that on in-plane coupling structure, where the resonance wavelength shows a red shift as the in-plane coupling is stronger.41,42 To compare the field enhancement effect, we choose the filling ratio of 1:1 for both the outof-plane metastructure and the planar structure. Either the upper layer or the lower layer of the out-of-plane structure is same as that of the planar structure, with the same width and thickness of Au layer. The electric field distributions for our structures mainly concentrate on the edges of the gold nanoribbons at the resonance frequency. Obviously, the out-of-plane design shows a much stronger field enhancement than the planar structure (Figure 3a and 3b). We define the enhancement factor as =

!

!"#

, where E and Ein denote the electric field amplitudes of the


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resonance mode and incident light, respectively. The maximum enhancement factor in the electric field distribution is denoted as . The out-of-plane structure with h=5 nm can generate resonance field enhancement factor up to 1293 (Figure 3d), which is 31 fold stronger than that in same planar structure ( = 41.7). The lower separation height, the higher field enhancement. The maximum field enhancement factor drops fast with separation height in the range of 5-100 nm, which is within 2 times of the thickness of the Au layer. The enhancement factor decreases gradually to nearly saturation when the separation height is greater than 2 times of the Au thickness (Figure 3d). The resonance wavelength &' of the surface plasmon polariton bloch wave is proportional with the period of the structure, which can be approximately obtained by the following relationship43: (

&' = * )

+,- +.

+,- +.

(2)

Where /01 and /2 are the relative permittivity of gold and surrounding medium respectively. is the period of the metastructure. represents the order of SPP bloch wave. PDMS has excellent stretchability, which can be stretched or compressed to introduce reversible deformation.44 The tensile strain / is defined as / = (4 − 4 )/4 , where 4 is the initial length of the soft substrate and 4 is the current length of the substrate subjected to a tensile strain.45 In order to figure out the strain effect on the lateral changes of the soft metasurface, we use finite element method (FEM) to calculate the deformation of the PDMS substrate. Figure 4a and Figure S3a present the stress distribution in cross section of the out-of-plane metastructure subjected to 5%, 10%, and 15% tensile strain. The deformation of the metastructure under strain is fully described by the geometric parameters marked in Figure S3b. According to the simulation results, the thickness


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and widths of the gold nanoribbons keep nearly constant due to sharp contrast of the shear moduli of the Au layer and soft substrate. The tensile strain results in an increase of the period length and a decrease of the separation height. (Figure S3c). To further clarify the strain effect on the resonance wavelength, reflection spectra of our metasurface at different strains are measured. We demonstrate four situations in experiments: two different periods and each period with two separation heights. After measuring the reflection spectra of the plasmonic device at each strain, we released the soft substrate and conducted the next optical measurement by stretching the device to next strain. The device works well for each measurement during the repeated stretch process. It is obvious that there is a red shift for all the dips of the spectra with increasing strain from a given initial state. For our structure with period of 587 nm and separation height of 21 nm, the resonance mode shifts from 660 nm to 685 nm in the visible range when we increase the tensile strain from 0% to 9.5% (Figure 4b). For the soft structure with period of 587 nm and separation height of 53 nm, a red shift of the dip in visible range from 686 nm to 706 nm is demonstrated when the tensile strain changes from 0% to 15% (Figure 4c). For the structure with period of 802 nm and separation height of 23 nm, the resonance mode shifts from 861 nm to 882 nm in the near infrared range when we increase the tensile strain from 0% to 12.6% (Figure 4d). While for our structure with period of 802 nm and separation height of 55 nm, a red shift of the dip in the near infrared range from 887 nm to 921 nm is demonstrated when the tensile strain changes from 0% to 13.7% (Figure 4e). It turns out that the resonance wavelength shifts to longer wavelength when the period of the metastructure increases. The variation tendency can be qualitatively explained by the Eq. (2). The tensile strain changes not only the period of the metastructure but also the separation height. The resonance wavelength is proportional to the period of the structure &' ∝ . Larger tensile


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strain leads to longer period, which will shift the resonance wavelength to the lower energy. While the separation height will decrease with strain (Figure S3), generating a blue shift as depicted in Figures 2a, 2b, and 3c. Further FEM analysis of the two competitive factors separately and comprehensively is presented in Figure S4, suggesting the dominant factor is the period effect when tensile strain increases. Thus the resonance modes show red shift as increasing the strain in all of the four metadevices we tested (Figure 4b-4e). We further did the simulation of strain effects of the metastructure with period of 802 nm and separation height of 55 nm as an example based on the deformation parameters in Figure S3. The results are shown in Figure 4f. The stars in Figure 4f denote the experimental results extracted from Figure 4e. We find that the simulation results generally agree well with the experimental results. In order to quantitatively evaluate the tunability of our soft metastructure, we use the normalized resonance sensitivity over strain to describe the strain effect on resonance wavelength shift: ∆:

9=:

;+

(3)

where ∆& is wavelength shift of the metastructure subjected to strain / . & denotes the resonance wavelength without strain. Through linear fitting (Figure S5), the normalized resonance sensitivities of four structures are 0.0040, 0.0019, 0.0019 and 0.0027 respectively according to the sequence in Figure 4b-4e. Our soft out-of-plane designed metastructure exhibits excellent tunability on resonance wavelength with an average resonance sensitivity of 0.0026, which is competitive when compared with the previous researches with normalized resonance sensitivities of 0.0018-0.0065.26,32,46 Actually, the limit of the tunable range of the soft metasurface depends on the mechanical failure strain of the gold nanoribbons due to the large


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modulus mismatch between PDMS and Au. The resistance of gold layer of the metastructure keeps low in the strain range of 0-25%, followed by a quick increase when the strain is larger than 30%, as presented in Figure S6. Moreover, our metastructure demonstrates good stability over one year’s storage time and good repeatability for the metastructure under strain (Figure S7 and Figure S8). In some situations, plasmonic devices experience off-normal incident light and the resonance mode might be sensitive to incident angles. Therefore, we further investigate the angle dependence of the reflected spectra using our plasmonic devices. Angle resolved spectrometer is used to obtain the reflection spectra of the plasmonic device with period 587 nm, separation height of 53 nm and the device with period 802 nm, separation height of 55 nm, as shown in Figure 5. As the incident angle increases in our experiments, resonance mode shifts to lower frequency. For metastructure with period of 587 nm, the resonance mode exhibits an 80 nm red shift as angle of incidence changes from 0 to 14 degrees (Figure 5a and 5b). While for metastructure with period of 802 nm, a 133 nm red shift is observed with the same angle change (Figure 5c and 5d).

CONCLUSIONS In summary, we have demonstrated a tunable soft metasurface for tuning the resonance wavelength of the surface plasmon polaritons bloch wave using theory, simulation and experiments. The operating wavelength varies from visible to near infrared wavelength range, which can be regulated by changing the separation heights or mechanical deformation of the metastructure. The tuning mechanism proposed here is not limited to noble metals, which can be extended to other plasmonic materials such as graphene and some newly emerged two-


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dimensional materials. The out-of-plane interaction enriches the mechanism for tuning the plasmonic properties by adjusting the coupling effect, which holds great promise for revolution of current plasmonic devices. MATERIALS AND METHOD Theoretical Calculations. All the simulations were performed using the finite element method (FEM). To improve the efficiency, the 2D FEM simulation is carried out in one single period and periodic condition is applied to the boundaries. The optical constants of the gold are adopted from Johnson and Christy.47 Sample Fabrication. We choose the polydimethylsiloxane (PDMS) as the soft substrate for its good elasticity. Electron-beam lithography (EBL) is employed to create the template pattern using the positive resist (ZEP 520A). The thickness of the resist is about 340 nm. Inductively coupled plasma (ICP) etching is employed to etch away the exposed part of the silicon with a mixture of sulfur hexafluoride ( ) and octafluorocyclobutane (?@ =A ) gas.48 The remaining parts masked by resist form the periodic rectangular pattern on silicon. After ultrasonic cleaning in isopropyl alcohol solution and being blown dry with nitrogen gun, the silicon mold is covered with uncured PDMS to form the soft substrate with a positive surface structure. In order to completely duplicate the complementary surface structure of the silicon template, the template along with the covering PDMS should be placed in a vacuum tank for removing the air bubbles for 24 hours to guarantee that the PDMS fits the template tightly and compliantly. The above steps are operated on optical table to assure both the flatness and horizontality of the topside of the PDMS. Peeled off from the silicon mold, the PDMS substrate with periodic pattern is


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deposited with 60 nm thick of Au by electron-beam evaporation (EBE) with 5 nm thick Cr as an adhesion layer. Sample characterization. The SEM images were obtained using Helios G3CX. The AFM images were measured with SPM9700. Angle resolved spectroscopy. Reflection spectra are measured using angle resolved spectrometer, which has a light source with a spot diameter of about 1 mm. The fabricated pattern area of our structure is 2 mm × 2 mm, which is larger than the beam diameter of the optical source to ensure the optical source can be always in the working area. The measurement wavelength range of the instrument is from 320 nm to 1110 nm, covering the visible and near infrared range. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. Funding Sources National Natural Science Foundation of China No. 51572096 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China No. 51572096, and the National 1000 Talents Program of China tenable in HUST. We also thank Dr. Qingzhong


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Huang in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in EBL, Laboratory of Electronics Manufacturing for carrying out the field emission scanning microscope measurements, and the Technology Analytical and Testing Center in HUST for AFM test. ASSOCIATED CONTENT Supporting Information. Supporting Information Available: < Eight additional figures (Figures S1-S8) (PDF).> This material is available free of charge via the Internet at http://pubs.acs.org. ABBREVIATIONS SPPs, surface plasmon polaritons; 2GNRs, two layered gold nanoribbon arrays; SERS, surface enhanced Raman Scattering; SPs, surface plasmons; PDMS, Polydimethylsiloxane.

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Plasmonic Biosensing with Graphene. Science 2015, 349, 165-168. (12) Boltasseva, A.; Atwater, H. A. Low-Loss Plasmonic Metamaterials. Science 2011, 331, 290-291. (13) Neubrech, F.; Pucci, A. Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection. Phys. Rev. Lett. 2008, 101, 157403. (14) Fischer, H.; Martin, O. J. F. Engineering the Optical Response of Plasmonic Nanoantennas. Opt. Express 2008, 16, 9144-9154. (15) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. Metallic Nanoparticle Arrays: A Common Substrate for Both Surface-Enhanced Raman Scattering and SurfaceEnhanced Infrared Absorption. ACS Nano 2008, 2, 707-718. (16) Gobin, A. M.; Lee, M. H.; Halas, N. J.; James, W. D.; Drezek, R. A.; West, J. L. Near-Infrared Resonant Nanoshells for Combined Optical Imaging and Photothermal Cancer Therapy. Nano Lett. 2007, 7, 1929-1934. (17) Fang, Z.; Thongrattanasiri, S.; Schlather, A.; Liu, Z.; Ma, L.; Wang, Y.; Ajayan, P. M.; Nordlander, P.; Halas, N. J.; Garc A De Abajo, F. J. Gated Tunability and Hybridization of Localized Plasmons in Nanostructured Graphene. ACS Nano 2013, 7, 2388-2395. (18) Knight, M. W.; King, N. S.; Liu, L.; Everitt, H. O.; Nordlander, P.; Halas, N. J. Aluminum for Plasmonics. ACS Nano 2014, 8, 834-840. (19) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302, 419-422. (20) Moran, C. E.; Steele, J. M.; Halas, N. J. Chemical and Dielectric Manipulation of the Plasmonic Band Gap of Metallodielectric Arrays. Nano Lett. 2004, 4, 1497-1500. (21) Dicken, M. J.; Aydin, K.; Pryce, I. M.; Sweatlock, L. A.; Boyd, E. M.; Walavalkar, S.; Ma, J.; Atwater, H. A. Frequency Tunable Near-Infrared Metamaterials Based On VO2 Phase Transition. Opt. Express 2009, 17, 1833018339. (22) Kossyrev, P. A.; Yin, A.; Cloutier, S. G.; Cardimona, D. A.; Huang, D.; Alsing, P. M.; Xu, J. M. Electric Field Tuning of Plasmonic Response of Nanodot Array in Liquid Crystal Matrix. Nano Lett. 2005, 5, 1978-1981. (23) Grigorenko, A. N.; Polini, M.; Novoselov, K. S. Graphene Plasmonics. Nat. Photonics 2012, 6, 749-758. (24) Huang, F.; Baumberg, J. J. Actively Tuned Plasmons On Elastomerically Driven Au Nanoparticle Dimers. Nano Lett. 2010, 10, 1787-1792. (25) Wang, L.; Liu, X.; Zang, J. Mechanically Tunable Terahertz Graphene Plasmonics Using Soft Metasurface. 2D Materials 2016, 3, 41007. (26) Gao, L.; Zhang, Y.; Zhang, H.; Doshay, S.; Xie, X.; Luo, H.; Shah, D.; Shi, Y.; Xu, S.; Fang, H. et al. Optics and Nonlinear Buckling Mechanics in Large-Area, Highly Stretchable Arrays of Plasmonic Nanostructures. ACS Nano 2015, 9, 5968-5975. (27) Aksu, S.; Huang, M.; Artar, A.; Yanik, A. A.; Selvarasah, S.; Dokmeci, M. R.; Altug, H. Flexible Plasmonics On Unconventional and Nonplanar Substrates. Adv. Mater. 2011, 23, 4422-4430. (28) Kamali, S. M.; Arbabi, E.; Arbabi, A.; Horie, Y.; Faraon, A. Highly Tunable Elastic Dielectric Metasurface Lenses. Laser Photonics Rev. 2016, 10, 1002-1008. (29) Chu, C. H.; Tseng, M. L.; Chen, J.; Wu, P. C.; Chen, Y.; Wang, H.; Chen, T.; Hsieh, W. T.; Wu, H. J.; Sun, G. et al. Active Dielectric Metasurface Based On Phase-Change Medium. Laser Photonics Rev. 2016, 10, 986-994. (30) Zheludev, N. I.; Kivshar, Y. S. From Metamaterials to Metadevices. Nat. Mater. 2012, 11, 917-924. (31) Ee, H.; Agarwal, R. Tunable Metasurface and Flat Optical Zoom Lens on a Stretchable Substrate. Nano Lett. 2016, 16, 2818-2823. (32) Pryce, I. M.; Aydin, K.; Kelaita, Y. A.; Briggs, R. M.; Atwater, H. A. Highly Strained Compliant Optical Metamaterials with Large Frequency Tunability. Nano Lett. 2010, 10, 4222-4227. (33) G Ng R, K. V.; Nal, E.; Demir, H. V. Nanoplasmonic Surfaces Enabling Strong Surface-Normal Electric Field Enhancement. Opt. Express 2013, 21, 23097. (34) Clausen, J. S.; H Jlund-Nielsen, E.; Christiansen, A. B.; Yazdi, S.; Grajower, M.; Taha, H.; Levy, U.; Kristensen, A.; Mortensen, N. A. Plasmonic Metasurfaces for Coloration of Plastic Consumer Products. Nano Lett. 2014, 14, 4499-4504. (35) Joung, D.; Nemilentsau, A.; Agarwal, K.; Dai, C.; Liu, C.; Su, Q.; Li, J.; Low, T.; Koester, S. J.; Cho, J. SelfAssembled Three-Dimensional Graphene-Based Polyhedrons Inducing Volumetric Light Confinement. Nano Lett. 2017, 17, 1987-1994. (36) Huang, L.; Li, G.; Gurarslan, A.; Yu, Y.; Kirste, R.; Guo, W.; Zhao, J.; Collazo, R.; Sitar, Z.; Parsons, G. N. et al. Atomically Thin MoS2 Narrowband and Broadband Light Superabsorbers. ACS Nano 2016, 10, 74937499. (37) Ruan, Z.; Wu, H.; Qiu, M.; Fan, S. Spatial Control of Surface Plasmon Polariton Excitation at Planar Metal


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Figure 1. Out-of-plane designed soft metasurface. a, Schematic diagram of the soft metasurface. b, SEM and AFM images of our structure with period of 587 nm and separation height of 53 nm for visible wavelength. The width of the upper layer and the lower layer nanoribbons are 320 nm and 267 nm respectively. c, SEM and AFM images of our structure with period of 802 nm and


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separation height of 55 nm for near infrared wavelength. The width of the upper layer and the lower layer nanoribbons are 470 nm and 332 nm respectively. All the scale bars are 1 µm.

Figure 2. The effect of the separation height on the reflection spectra. a, Simulated and experimental results of the reflection spectra of the structure with period of 587 nm at two different separation heights of 21 nm and 53 nm. The widths of the upper layer and the lower layer nanoribbons are 320 nm and 267 nm respectively. b, Simulated and experimental results of


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the reflection spectra of the structure with period of 802 nm at two different separation heights of 23 nm and 55 nm. The width of the upper layer and the lower layer nanoribbons are 470 nm and 332 nm respectively. c, Simulated electric field intensity of resonance dip in Fig 2b at separation height of 23 nm. d, Simulated surface charge distributions of resonance dip in Fig 2b at separation height of 23 nm.

Figure 3. Comparison between the planar and out-of-plane structure and the out-of-plane coupling. a, Simulated field intensity of the planar structure with period of 800 nm and duty ratio of 0.5 at the resonance wavelength of 1064 nm. b, Simulated field intensity of the out-of-plane


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structure with period of 800 nm and separation height of 20 nm with the same duty ratio as the planar structure at the resonance wavelength of 849 nm. c, Simulated reflection spectra of the out-of-plane structure at different separation heights. d, Maximum enhancement factor and coupling strength of the out-of-plane structure as a function of the separation height.

Figure 4. The effect of the tensile strain on the reflection spectra. a, Simulated stress distribution of the metastructure with 60 nm Au layers, period of 802 nm, and separation height of 55 nm subjected to tensile strain of 15%. In the scale bar, we use the dimensionless stress scaling with stress normalized by the modulus of PDMS. b-e, Strain dependent experimentally measured reflection spectra of four different structures in visible and near infrared wavelength range. The


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structures of b, c with the period of 587 nm have the same duty ratio of 0.55. The structures of d, e with the period of 802 nm have the same duty ratio of 0.59. f, Simulated reflection spectra of the metastructure with period of 802 nm and separation height of 55 nm at different strains based on our model. The stars denote the experimentally measured resonance wavelength extracted from e.


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Figure 5. Angle dependence of the reflected spectra. a-b Experimental results of the angle resolved spectra of the metastructure with period of 587 nm and separation height of 53 nm. c-d Experimental results of the angle resolved spectra of the metastructure with period of 802 nm and separation height of 55 nm. Angle resolved spectra were measured from 0 to 15 degrees. TOC


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Figure 1. Out-of-plane designed soft metasurface. 34x14mm (300 x 300 DPI)


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Figure 2. The effect of the separation height on the reflection spectra. 79x73mm (300 x 300 DPI)


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Figure 3. Comparison between the planar and out-of-plane structure and the out-of-plane coupling. 71x59mm (300 x 300 DPI)


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Figure 4. The effect of the tensile strain on the reflection spectra. 67x54mm (300 x 300 DPI)


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Figure 5. Angle dependence of the reflected spectra. 97x112mm (300 x 300 DPI)


Out-of-Plane Designed Soft Metasurface for Tunable Surface Plasmon

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