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Electromechanically Tunable Plasmonic Nanowires Operating in Visible Wavelengths Masashi Miyata, Akira Kaijima, Yusuke Nagasaki, and Junichi Takahara ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00691 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Electromechanically Tunable Plasmonic Nanowires Operating in Visible Wavelengths Masashi Miyata,†,§ Akira Kaijima,†,§ Yusuke Nagasaki,† and Junichi Takahara*,†,‡ †

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871,

Japan ‡

Photonics Advanced Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-

0871, Japan KEYWORDS: tunable plasmon resonance, nanoelectromechanical system, optical antenna, metallic nanowire, gap plasmon, pick-and-place method

ACS Paragon Plus Environment

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ABSTRACT: We present an experimental demonstration of electrically controlled plasmonic resonators in visible wavelengths. This is accomplished by employing a suspended plasmonic nanowire (NW) placed on a metallic film. By incorporating nano-electromechanics into the plasmonic NW, we electromechanically tune the gap between the NW and the metallic film. This electromechanical tuning dramatically changes the optical resonant properties of the NW, enabling dynamic modulation of the plasmon resonances. The proposed approach provides a promising route for achieving electrical control of plasmonic building blocks operating at visible wavelengths.


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Plasmonic nanostructures offer the unique optical property to resonantly produce highly confined optical fields at nanometer length scales.1 This ability has propelled their use as optical concentrators and antennas in many emerging applications, such as ultra-compact optoelectric devices,2−4 ultra-thin optical elements,5−9 and chemical/biological sensors.10,11 For these applications, it is highly desirable to have the capability to electrically tune the optical properties of plasmonic structures over a wide spectral range to achieve dynamic functionalities. Over the past decade, researchers have proposed a wealth of novel plasmonic nanostructures combined with materials that exhibit changes in the optical properties in response to external electrical stimuli, including electro-refractive polymers,12 electro-chemical materials,13 thermo-responsive materials,14,15

liquid

crystals,16,17

semiconductors,18,19

epsilon-near-zero

materials,20

graphene,21−23 and phase-change media,24,25 for controlling resonant (localized) and/or propagating surface plasmons. Recently, electromechanically reconfigurable plasmonic resonators and metamaterials have been proposed as an alternative pathway for achieving tunable plasmonic characteristics at subwavelength scales.26−28 When high-speed modulation is not the prime objective, this approach is indeed promising because of a high degree of resonance tunability across a wide range of optical frequencies, which arises from a strong dependence of the plasmonic properties on the structural changes. However, it is still challenging to electromechanically modulate the plasmonic response at the visible wavelengths where plasmonic effects are most pronounced. One key reason for this is the technical difficulties to further shrink the existing structural design to enter the visible spectral range. It has been recently reported that an electromechanically actuated plasmonic dimer antenna can operate in wavelengths of 700–750 nm;29 however, challenges remain in inducing strong resonance modification over the visible spectral range.


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Here, we demonstrate strong electromechanically-induced resonance modification that is capable of dynamically tuning the resonant properties, and is even capable of inducing color changes in the visible range. This is accomplished by employing a suspended plasmonic nanowire (NW) placed on a metallic film30,31 shown in Fig. 1. This nanostructure has two key features: (i) the suspended NW supports the plasmon resonance driven by strongly confined near-fields in the gap between the NW and the film, and its optical properties have an extremely strong dependence on the gap size; (ii) the suspended geometry provides room for mechanical motion in a nanoscale regime. These features allow us to introduce the concept of nanoelectromechanics32 to structurally control the plasmonic resonant nanostructure. As illustrated in Figs. 1a,b, an attractive electrostatic force arising from the voltage application moves the NW to the film surface, closing the gap between them. This electromechanical response dramatically changes the optical resonant properties of the NW, enabling dynamic tuning of the plasmon resonances. Our study demonstrates this approach in experiments and shows that it can be a new route to electrically control the plasmonic building blocks in the visible wavelengths. The suspended NW was fabricated using a probe-based pick-and-place method30,31 capable of arranging NWs on a substrate. The details of the fabrication process are available in Supporting Information S1. Figure 1c shows a scanning electron microscope (SEM) image of a 500-nm-wide and 150-nm-thick gold NW suspended over an aluminum oxide-coated (20 nm thick) gold film (80 nm thick) with an initial air gap of ~250 nm. A gold NW was chosen for the first demonstration because (i) both the plasmonic response30 and the mechanical (elastic) property33−35 have been previously investigated in details and (ii) the gold NWs have been widely used as NEMS actuators so far.36 These points aid in understanding and designing the first electromechanically actuated plasmonic NWs. Other plasmonic NWs, such as silver NWs,


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can potentially be used as well. The gold NW with a suspended length of 13.4 µm is connected to gold electrodes both mechanically and electrically. The thin aluminum oxide layer coated on the underlying gold film serves to plasmonically and electrically separate the gold NW and the gold film. To electrically separate the top electrodes and the bottom film, an insulator layer (100nm-thick aluminum oxide) is inserted between them. In this study, we analyze the mechanical and optical properties of this device while applying a voltage. To begin with, we use numerical simulations to analyze the electromechanical response of the NW. The simulations were performed with beam theory in the three-dimensional finite element method platform (COMSOL Multiphysics). Here, the mechanical properties for bulk gold (Young's modulus: 77 GPa, Poisson's ratio: 0.42)37 were used because the elastic properties of gold NWs with a sub-micron thickness are generally identical to those of bulk gold.33−35 The dielectric constant of aluminum oxide is set to 9.5.38 A fringing effect for the electric fields39 was included in the simulations. To more closely approximate the cross-sectional geometry of the fabricated NW, a trapezoid cross section with an upper base of 430 nm and a lower base of 500 nm was employed and the corners were rounded to have a radius of 20 nm in all simulations (a cross-sectional SEM image of the NW is presented in Supporting Information S2). Figure 2a shows a simulated mechanical map that illustrates the dependence of a gap size g on an applied voltage Vapp. The map shows that the gap size at the central point of the NW gradually reduces with increasing applied voltage. This structural change occurs for driving voltages below 14.5 V. This is the regime where the NW is pulled down by the attractive electrostatic force balanced with the mechanical restoration force of the NW39,40 (Fig. 2b, (i)–(ii)). Reversible tuning of the NW geometry as well as the optical properties can thus be achieved for applied voltages below this threshold (Vpull-in). Above 14.5 V, the NW shows a step-like mechanical response; the NW


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abruptly comes in contact with the substrate. This response is known as “pull-in” in which the electrostatic force can no longer be balanced by the restoration force of the bent NW39,40 (Fig. 2b, (iii)). As this mechanical motion is irreversible, exceeding the pull-in voltage Vpull-in results in non-volatile switching of the optical properties. In the following, we will focus on both regimes below and above the pull-in voltage. It is noteworthy that the metallic gap size is position-dependent; the gap size is gradually decreased along the NW and the minimum point is obtained at the center of the NW (Fig. 2a, inset). In this study, we investigate the optical properties at the central point of the NW, which exhibits the largest structural changes. Next, we explore the resonant optical properties of the NW with varying gap size by numerically analyzing the light scattering properties. We used full-field simulations with finite element method calculations (COMSOL Multiphysics), in which the scattered-field formulation was employed to specify the background field. The scattered light was collected in the solid angle with NA = 0.9 under transverse magnetic (TM) illumination with an incident angle of 72° from the surface normal. The dielectric constant of gold taken from the experimental data41 was used for the simulations. Figure 3a shows a simulated scattering spectral map that quantifies the predicted resonances as a function of both the wavelength and gap size. Here, a scattering efficiency Qsca is defined as the scattering cross section normalized with respect to the NW width. The two red bands in the map show that strong light scattering occurs at short/long wavelengths. From the left band, it is clear that strong scattering occurs at a wavelength of ~560 nm for all gap sizes above ~50 nm and its efficiency is gradually decreased with decreasing gap sizes. The right band shows that an additional scattering peak emerges for the gap sizes below ~200 nm at longer wavelengths, which features a red-shift with decreasing gap sizes. The electric field distributions


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for the NW on points A and B on these two bands (A and B in Fig. 3c) provide insight into the observed spectral features. For point A on the left band, the electric fields are strongly localized on the top of the NW. In the fields, there are three anti-nodes on the NW surface, indicative of the excitation of the second-order standing-wave resonance for surface plasmons.42 For point B on the right band, the fields are strongly confined in the metallic gap and show three strong antinodes. This implies the excitation of the second-order standing-wave resonance for gap surface plasmons whose optical properties are strongly dependent on the gap size.30,43–45 In order to verify these facts, we calculate the numerical solutions for the standing-wave (Fabry–Pérot) resonances for surface plasmons. In this calculation, we employed the previously established procedure30,42 to quantify the resonant condition (detail parameters for the model calculation are available in Supporting Information S3). The optical scattering at a wavelength of ~560 nm is well matched with the analytical standing-wave resonant condition on the top face of the NW (the analytical resonant wavelength is 576 nm); therefore, the gap never contributes to the resonant wavelength. This resonant peak also corresponds to the resonant mode of the gold NW not coupled to the substrate (the simulated resonant wavelength is 564 nm). Furthermore, the analytical curves for gap plasmon resonances correlate well with the strong optical scattering features at longer wavelengths (see white dashed lines in Fig. 3a). It is important to note that even orders of the standing-wave resonance are dominant for scattering in our case because of the oblique incidence. This well-fitted standing-wave resonance model aids in understanding the spectral red-shift behavior; the decrease of the gap size induces a high value of the mode index of gap plasmons and therefore increases the net cavity length for the standing-wave, resulting in the strong red-shift of the resonant wavelength.


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Above a gap size of 200 nm, the scattering peak in the left band diverges and exhibits a spectral shift. For these gap sizes, a vertical cavity mode, in which light travels back and forth between the NW bottom and the substrate surface, is pronounced and can give rise to spectral modulation. The black dashed line in Fig. 3a indicates an analytical curve of this cavity mode described as 2π/λ0 × 2g + ϕsub + ϕwire = 4π (ϕsub is the phase pickup at the substrate surface, ϕwire is the phase pickup at the NW bottom), which closely tracks the spectral shift. This fact is also shown in the field distribution on point C on the left band (C in Fig. 3c), which illustrates that the dominant faction of the scattered fields is localized in space between the NW and the substrate, forming the optical cavity mode. From these analyses, it becomes clear that the spectral features are gap-dependent, and they can be well explained by the fact of the standing-wave resonances of surface plasmons as well as the vertical cavity mode. Further, we find that the gap-size range of the optical map where the two scattering bands are dominant partially matches that of the mechanical map below the pull-in voltage (see horizontal dotted lines in the Fig. 3a). This indicates that the electrical and reversible modulation of optical resonant properties can be attained in the considered system by applying voltages below the pull-in voltage. At small gap sizes in the optical map, the resonant properties of the NW show narrow features exhibiting notable scattering. This area is magnified in Fig. 3b. High-order modes of the standing-wave resonance in the gap are observed and highlighted by the analytical curves (see white dashed lines in Fig. 3b). These facts are also confirmed in the field distributions on resonance (D and E in Fig. 3c), showing the excitation of the sixth- and fourth-order modes. At a 0 nm gap size that corresponds to the pull-in state, these two resonance modes are dominant in


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determining the optical properties. Exceeding the pull-in voltage thus results in an abrupt change of the optical properties of the NW. To demonstrate the proposed approach in experiments, we measured scattered light from the NW under dark-field white light illumination while applying a voltage. The illumination was polarized by a linear polarizer to be in TM polarization defined in the inset of Fig. 3a. The optical setup details are available in Supporting Information S4. Figure 4a–d shows dark-field images of the NW while varying the applied voltage. As shown, the scattering color in the NW center is gradually changed to a more reddish color with increasing voltage (Fig. 4a–c). Such changes in scattering behavior are reversible for voltages below ~13.0 V. However, the scattering image is dramatically changed at 13.1 V (Fig. 4d); uniform red/green color scattering along the wire-length-axis is observed. This corresponds to the pull-in response in which the NW is suddenly pulled down by electrostatic force and touches the substrate. The pull-in voltage Vpullin

in this device was measured to be ~13.1 V, which is in good quantitative agreement with the

theoretical prediction. A slight difference in the pull-in voltages is possibly due to the small discrepancies in the structural shape and size between the experiments and simulations. A residual stress inside the fabricated NW as well as a photothermal effect may also contribute to this subtle difference. To quantify the optical properties of the NW, we performed spectral analysis by confocally detecting the scattered light from the central point of the NW. Figure 4e shows the experimentally observed spectral evolution while varying the applied voltage. These spectra provide clear evidence of electrical modulation of the optical properties. At the initial state (Vapp = 0 V), two scattering peaks are observed at wavelengths of 580 (blue dots) and 650 nm (orange dots), which can be ascribed to the excitation of the standing-wave resonance on the NW top


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face as well as the vertical cavity mode (see Fig. 3a). Notable spectral change is observable at 10 V; the peak arising from the vertical cavity mode disappears and a new peak emerges at a ~700 nm wavelength (red dots). This peak becomes dominant and slightly red-shifts with further increasing voltage (Vapp = 10.0–13.0 V). This additional peak will correspond to the excitation of the plasmonic resonant mode supported by the metallic gap (see Fig. 3a). In this voltage range, we confirmed that the observed spectral evolution is repeatable at least 4 cycles (see Supporting Information S5). With a further increase of the voltage, a drastic spectral change can be seen (see the spectrum at Vapp = 13.1 V), which corresponds to the pull-in response. Two scattering peaks appear at wavelengths of 550 (a blue dot) and 710 nm (a green dot), which matches our expectations for the optical properties in the pull-in state (Fig. 3b). These voltage-dependent spectral features qualitatively agree with the simulated results (Fig. 4f) obtained from the electromechanical and optical properties in Figs. 2a and 3a, further supporting our understanding. Minor differences between the experiments and simulations can be attributed to small discrepancies in the simulated shape, size, or material dielectric constants from those in the experiments. The resolution (~2 µm) of the confocal setup can also explain subtle differences between the experimental and simulated spectra at longer wavelengths. Owing to the resolution limitation, the experimental spectra illustrate spectral information on scattered light from not only the central point of the NW but also its vicinity where the resonance peak is slightly blueshifted due to the gap size increase along the NW. This directly leads to the broader spectral shape with the blue-shifted peak compared with the simulated one. In our electromechanical system, the gap size where the pull-in response appears (the pullin gap size) is expected to be approximately 100 nm; however, the highest reversible tunability can be attained bellow a gap size of ~100 nm (see the scattering map in Figs. 3a and 3b). As the


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pull-in gap size is fundamentally determined by the initial gap size of the NW (a simple beam theory predicts that the pull-in gap size is two-thirds of the initial gap size), the stronger continuous modification of the plasmonic characteristics will be achieved by making the initial gap size smaller. Further optimization of the fabrication process may allow us to fabricate such a nanostructure and may offer a higher degree of resonance tunability. It is worth noting that the observed step-like switching is irreversible because the NW remains in the pull-in state even after removing the voltage. In Supporting Information S6, we present an SEM image of the suspended NW after removing the pull-in voltage and provide electromechanical analysis for this NW. We find that the observed irreversible switching response can be attributed to a “sticking” effect46 in which adhesion forces (such as van der Waals forces and capillary forces) stronger than the restoring elastic forces make the NW stick to the substrate. As this sticking effect can be eliminated by employing a NW shorter than the critical length determined by the adhesion energy,47 the proposed approach potentially allows both “pull-in” and “pull-out” phenomena within an individual suspended NW and provides reversible hysteresis-loop-like switching of the NW geometry. This could lead to the realization of unique switchable operation in plasmonic optical functions. We also investigated the tunability in local field enhancements according to the electromechanical response. The simulations present that the enhancement factors are greatly dependent on the applied voltage, showing potential for manipulating the plasmonic field enhancements in situ (Supporting Information S7). To investigate the potential performance of the device, we estimated the modulation speed that is linked with the suspended beam's fundamental flexural resonant frequency:32,40


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݂଴ = 1.028

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‫ܧ ݐ‬ ඨ , ‫ܮ‬ଶ ߩ

where t is the thickness of the NW, L is the length of the NW, E is Young's modulus, and ρ is the mass density. We obtained f0 ~ 1.72 MHz, showing that the device supports continuous tuning of the optical properties at up to megahertz rates. As such, the speed of dynamic responses, including the modulation speed, transit response, and recovery time, is fundamentally limited by the mechanical characteristics, and this feature inherently limits the applicability of the device (for reference, actuation times of recent NEMS devices are in the nanosecond range or higher).36 Future work needs to be done to further boost the operation speed. The use of shorter NWs consisting of stiffer materials would be a pathway for this, but higher operation voltages would be required. In addition, we investigated the energy required to induce step-like switching in the device. The energy can be estimated as 1/2CV2pull-in, where C is the capacitance of the structure and Vpullin

represents the pull-in voltage. Simplifying the nanostructure as two parallel electrodes, the

capacitance can be estimated analytically as C = 235 aF. Given Vpull-in of 13.1 V, the energy required for step-like switching is estimated to be ~20 fJ. As future chip-scale integrated devices require extremely low power consumption, ideally on the order of ~10 fJ,48 our nanostructure shows potential for such highly power-efficient devices. The energy consumption can further be reduced by shrinking the NW width for reducing the capacitance. In summary, we demonstrated the electrical tuning of plasmon resonant properties at visible wavelengths by utilizing an electrically reconfigurable plasmonic NW. The plasmonic NW can be electromechanically actuated by electrostatic forces arising from the voltage application.


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Following this electromechanical response, the optical resonant properties of the NW were dynamically modified in the visible spectral range. This opens opportunities to electrically tune the plasmonic resonant properties for applications where the resonances are used to enhance light-matter interactions. Continuous modulation operating at up to megahertz rates and lowenergy consumption would also be useful for applications in optoelectronic systems. The proposed approach is general and can be applied to tunable optical metasurfaces by placing the electrically reconfigurable NWs at subwavelength spacing. For such emerging applications, the scalability and speed of the device need to be improved, which remains for a future study. Overall, this study provides a new route for achieving electrical control of plasmonic building blocks operating at visible wavelengths.


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FIGURES

Figure

1.

Electromechanically

tunable

plasmonic

NWs.

(a,b)

Schematics

of

an

electromechanically controlled NW with (a) and without the application of a voltage (b). An electromechanical response dramatically modulates the optical resonant properties of the NW. (c) Oblique-view SEM image of a fabricated gold NW suspended on a dielectric-coated gold film with an air gap of ~250 nm. The NW width and thickness is 500 and 150 nm, respectively.


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Figure 2. Electromechanical properties of a suspended plasmonic NW. (a) Simulated mechanical response for a suspended gold NW over an aluminum oxide/gold substrate with different applied voltages. The red line represents the gap size between the NW and the substrate at the central point of the NW. The inset shows the simulated position-dependence of the gap size at 14.4 V. (b) Schematics illustrating the NW location with applied voltage. (i) Vapp = 0, (ii) 0 < Vapp < Vpullin,

(iii) Vapp > Vpull-in.


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Figure 3. Optical properties of a suspended plasmonic NW. (a) Simulated scattering spectral map for the NW at different gaps under dark-field illumination. Overlaid white dashed lines are the predictions from a standing-wave resonance model. The black dashed line indicates the analytical prediction of a vertical cavity mode. Horizontal white dotted lines represent the controllable range of the gap size by electromechanical motion. Inset: schematic of a suspended NW cross section above a gold film under dark-field illumination. (b) Enlarged map of a 0–50 nm gap-size range in (a). (c) Scattered electric field distributions |Esca| for the NWs on the points A–E in (a) and (b). The electric field is normalized to the incident field |E0|.


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Figure 4. Evolution of plasmon resonances with an applied voltage. (a–d) Dark-field optical images of the NW with different applied voltages of 0 V (a), 11.5 V (b), 13.0 V (c), and 13.1 V (d) under TM illumination. Vertical dashed white lines represent the gold edges. Scale bar, 2 µm. (e) Experimental scattering spectra measured at the central point of the NW with different applied voltages. (f) Simulated scattering spectra obtained from the results of Figs. 2a and 3a. The color dots in (e) and (f) indicate the locations of the tunable scattering peaks; the blue dots correspond to the second-order resonance on the NW top face, the orange dots show the vertical cavity mode, and the red and green dots correspond to the second- and sixth-order resonances in the gap, respectively.


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ASSOCIATED CONTENT Supporting Information. Fabrication details, a cross-sectional SEM image of a NW, calculation details of a standing-wave resonance model, experimental setup details, tuning repeatability, electromechanical analysis for a sticking effect, and simulated results of local field enhancement factors. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions M.M. conceived the idea of an electromechanically tunable plasmonic NW. J.T. supervised the project. M.M. and A.K. performed the numerical simulations. A.K. and Y.N. fabricated the samples. M.M., A.K. and Y.N. performed the optical experiments. M.M. wrote the initial draft of the manuscript. All the authors analyzed and discussed the results and contributed to the writing of the manuscript. §These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We would like to thank Prof. Mark L. Brongersma and Aaron Holsteen (Stanford University) for helpful discussions. This work was supported by Grant-in-Aid for Scientific Research B (25286007) from the Ministry of Education, Culture, Sports, Science and Technology, Japan


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(MEXT). One of the authors (M. Miyata) is supported by Research Fellowships of Japan Society for the Promotion of Science (JSPS) for Young Scientists. A part of this work was supported by “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of Ministry of Education, Culture, Sports, Science and Technology, Japan [No.: F-15-OS-0015].


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