Multiresonant Composite Optical Nanoantennas by Out-of-plane

Subscriber access provided by - Access paid by the | UCSB Libraries

Communication

Multiresonant Composite Optical Nanoantennas by Out-of-plane Plasmonic Engineering Junyeob Song, and Wei Zhou Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01467 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Multiresonant Composite Optical Nanoantennas by Out-of-plane Plasmonic Engineering Junyeob Song and Wei Zhou* Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States Abstract: Optical nanoantennas can concentrate light and enhance light-matter interactions in subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional optical nanoantennas operating at a single wavelength band are not suitable for multiband applications. Here, we propose and exploit an out-of-plane plasmonic engineering strategy to design and create composite optical nanoantennas that can support multiple nanolocalized modes at different resonant wavelengths. These multiresonant composite nanoantennas are composed of vertically stacked building blocks of metal-insulatormetal loop nanoantennas. Studies of multiresonant composite nanoantennas demonstrate that the number of supported modes depends on the number of vertically stacked building blocks and the resonant wavelengths of individual modes are tunable by controlling the out-of-plane geometries of their building blocks. In addition, numerical studies show that the resonant wavelengths of individual modes in composite nanoantennas can deviate from the optical response of building blocks due to hybridization of magnetic modes in neighboring building blocks. Using Au

ACS Paragon Plus Environment

1

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

nanohole arrays as deposition masks to fabricate arrays of multilayered composite nanoantennas, we experimentally demonstrate their multiresonant optical properties in good agreement with theory predictions. These studies show that out-of-plane engineered multiresonant composite nanoantennas can provide new opportunities for fundamental nanophotonics research and practical applications involving optical multiband operations, such as multiphoton process, broadband solar energy conversion, and wavelength-multiplexed optical system.

KEYWORDS. Plasmonics, composite nanoantennas, multiresonant response, out-of-plane plasmonic engineering

Optical nanoantennas based on bottom-up synthesized or top-down fabricated metal nanostructures have demonstrated unique capabilities to enhance light-matter interactions at nanoscale in physical, chemical, and biological systems.1-19 For example, optical nanoantennas have been used for collection and concentration of light in nanoscale optoelectronics and nonlinear optics devices,2-7 ultra-sensitive detection and analysis of chemical and biological species,8-10 localized photothermal intervention of cells or cellular networks,11-15 and optical generation of hot carriers in photocatalytic processes.16-19 Some of the directions being pursued with optical nanoantennas, such as refractive index based optical sensing and spontaneous photon emission, do not require light-matter interactions at multiple different wavelength regions. Nevertheless, multiresonant optical nanoantennas are favorable for many other applications that involve nonlinear and upconversion multi-photon processes, broadband solar energy harvesting and conversion, or wavelength-multiplexing multifunctional operations.


2

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Previous studies have shown that phase retardation can allow direct optical excitation of higher-order harmonics or multipoles in plasmonic antenna structures of larger dimensions, such as one-dimensional nanorods and two-dimensional nanodisks.20-23 Multipolar plasmon resonances are of fundamental interest for multiband applications, but resonant wavelengths of individual modes cannot be tuned independently by geometric engineering due to their standing wave-like nature. Another approach to enhance local optical interactions at multiple different wavelengths is to incorporate multiple plasmonic resonators of different sizes or geometries into a single composite antenna system. In a planar layout, these composite antenna systems consist of either (1) optically coupled resonators,7, 24-27 or (2) optically uncoupled building blocks.28-34 By in-plane geometric engineering of building blocks of resonators, the resonant wavelengths of individual modes in the composite antenna system can be controlled independently. Despite apparent improvement, in-plane engineered composite optical antennas still face challenges in the reduction of overall footprints, the accurate nanoscale control of in-plane geometries, and the integration with multiple different materials for multiband multifunctional applications. In comparison with planar plasmonic structures, recent studies have demonstrated that vertical nanowires and nanocones with radical metal-dielectric core-shell geometries can show multiresonant optical response by supporting both localized plasmon modes and optical stationary modes.35,

36

However, plasmonic core-shell nanowires/nanocones do not provide

enough geometric degrees of freedom to achieve independent spectral tunability of individual modes. Moreover, it is difficult to incorporate multiple different materials in core-shell nanowires/nanocones for controlled interactions with different resonant modes at specific wavelengths.


3

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

In this letter, we report a detailed study on multiresonant composite optical nanoantennas based on vertically stacked building blocks of metal-insulator-metal (MIM) magnetic loop nanoantennas. Figure 1A illustrates the strategy of out-of-plane plasmonic engineering to control multiple nanolocalized modes in the composite optical nanoantenna. Each MIM loop nanoantenna building block can support a magnetic dipole mode (µn) with a tunable resonant wavelength (λn) depending on its insulator layer thickness (hn). Therefore, we can control the multiresonant optical response of a composite nanoantenna simply by engineering out-of-plane geometric parameters of individual building blocks. Compared to conventional in-plane plasmonic engineering approaches, out-of-plane plasmonic engineering of multiresonant composite nanoantennas provides some unique opportunities. First, the vertical arrangement of MIM building blocks results in a nanoscale footprint and a nanoscale volume of such multiresonant devices, which enables wavelength-multiplexed systems in high-density arrays, and subcellular optical nano-transducers for bio-interface. Second, by sharing the metal layer between neighboring MIM loop nanoantennas in the vertical stack, the out-of-plane arrangement can enable efficient transport of heat or charges between neighboring building blocks triggered by light excitation at multiple different wavelengths. Third, it is much easier to control the outof-plane geometric parameters at nanoscale in terms of thin-film thicknesses than to define inplane geometric features by lithography-based processes. Finally, we can incorporate layers of different functional materials (e.g. phase change materials, luminescent materials, and nonlinear optical materials) in different MIM building blocks in the vertical stack, which opens the opportunities to create multiband multifunctional photonics nanodevices.


4

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. Multiresonant composite optical nanoantennas by out-of-plane plasmonic engineering. (A) Schematic illustration of a multiresonant composite optical nanoantenna composed of MIM loop nanoantenna building blocks in the vertical stack. The composite nanoantenna can support multiple nanolocalized magnetic modes (µn) with individually tunable resonant wavelengths (λn) by controlling dielectric layer thicknesses (hn) of their MIM building blocks. (B) Calculated extinction spectra of MIM building blocks with different dielectric layer thicknesses (h). (C) Extinction spectra of composite nanoantennas with 1, 2, 3, and 9 dielectric gaps of different dielectric layer thicknesses. We first studied the optical properties of a composite nanoantenna in comparison with the optical response of its individual MIM building blocks (Fig. 1B-C). All numerical simulations were performed under linear polarized plane wave excitation at normal incidence by threedimensional finite-difference-time-domain (FDTD) methods using commercial software (FDTD solution, Lumerical Inc.) (see Supporting Information). A uniform mesh size of 3 nm (x, y, and z directions) was used. The multilayered MIM nanoantennas consist of alternating layers of Ag and SiO2, which are on the glass substrate with an air superstrate. The optical constants of Ag are taken from Johnson and Christy in the spectral range from 300 nm to 1600 nm.37 The refractive


5

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

index of glass substrate and SiO2 layers in nanoantennas is 1.5, and the refractive index of the air superstrate is 1. The nanodisk diameter (d) is chosen as 100 nm to induce resonant modes in optical wavelengths, and the Ag nanodisk thickness (t) is chosen as 50 nm comparable to the optical skin depth in Ag. Figure 1B describes the calculated extinction spectra of MIM building blocks with different dielectric layer thicknesses (h) ranging from 0 nm to 30 nm. Without a dielectric gap layer (h = 0 nm), the Ag nanodisk shows a broad resonance around 440 nm due to electric dipole oscillations of localized surface plasmons (LSPs) in the Ag nanodisk. By adding a thin dielectric gap layer (h = 3 nm) between the two Ag nanodisks, the MIM sandwich structure can support two resonant modes corresponding to in-phase and out-of-phase coupling between electric dipole modes of two Ag nanodisks.38, 39 The high-energy mode around 440 nm carries an electric dipole character from the in-phase oscillations of two electric dipoles. The low-energy mode at 1170 nm has a magnetic dipole character induced by an oscillating current loop because of the out-of-phase oscillations of the two electric dipoles. As the thickness of the dielectric layer in the MIM structure increases from 3 nm to 30 nm, the high-energy electric dipole mode does not change its resonant wavelengths while the resonant wavelength of the low-energy magnetic dipole mode continuously shifts from 1170 nm to 580 nm because of a reduced coupling strength. These results confirm that the MIM sandwich structures can serve as tunable loop nanoantenna building blocks by controlling their dielectric layer thicknesses. Figure 1C shows the extinction spectra of composite nanoantennas with 1, 2, 3, and 9 dielectric gaps of different dielectric layer thicknesses. Since the neighboring MIM loop nanoantennas in the vertical stack share the middle metal layer, the number of magnetic dipole resonances equals with the number of MIM building blocks with different dielectric gap thicknesses. Thus, as the number of dielectric gaps increase from 1 to 2, to 3, and to 9, the


6

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

number of magnetic dipole resonances above 500 nm increases accordingly from 1 to 2, to 3, and to 9. For the composite nanoantennas with 2 and 3 dielectric gaps, the exact peak wavelengths (λ6 nm, λ9 nm, λ21 nm) of their magnetic dipole resonances are slightly different from their MIM loop nanoantenna building blocks with the same dielectric thicknesses. Moreover, as illustrated in Figure 1C, the magnetic dipole resonances in a 9-dielectric-gap MIM nanoantenna significantly deviate in peak wavelengths from the MIM building blocks. These observations suggest that strong optical interactions can occur between magnetic resonances in individual MIM building blocks to form new hybridized modes with shifted resonant wavelengths. Figure 2 presents FDTD-calculated far-field and near-field optical properties of a representative MIMIMIM composite nanoantenna with 3 dielectric layers. This MIMIMIM nanoantenna consists of three vertically stacked MIM loop nanoantennas with 3 dielectric layers at different thicknesses of 9 nm, 21 nm, and 6 nm from bottom to top. Figure 2A shows that the MIMIMIM nanoantenna exhibits four major peaks. The high energy peak at λ1 = 439 nm is associated with the electric dipole mode in Ag nanodisks, and the other three peaks at λ2 = 587 nm, λ3 = 762 nm, and λ4 = 884 nm originate from magnetic dipole resonances in three individual MIM loop nanoantennas. Between the three magnetic dipole modes, the mode at a longer resonant wavelength shows a smaller normalized extinction cross-section value σext/σ0 due to a thinner dielectric gap and consequently a smaller current loop cross-section.


7

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 29

Figure 2. Far-field and near-field optical properties of multiresonant composite nanoantennas. FDTD-calculated spectra of (A) normalized extinction cross-section σext/σ0, (B) normalized scattering cross-section σscat/σ0, (C) normalized absorption cross-section σabs/σ0, and (D) normalized mode volume Vm/λ3 for a MIMIMIM composite nanoantenna with 3 dielectric layers under linear polarized plane wave excitation at normal incidence angle. (E-H) FDTDcalculated distribution maps of electric field intensity |E|2, phase of in-plane electric field φ(Ex), magnetic field intensity |H|2, and phase of magnetic field φ(Hy) for MIMIMIM composite nanoantenna at resonant wavelengths of (E) 439 nm, (F) 587 nm, (G) 762 nm, and (H) 884 nm. By decomposing the extinction spectrum into the scattering (Fig. 2B) and the absorption (Fig. 2C) contributions, we find that the three magnetic modes show different scattering and absorption behaviors. While the magnetic mode with a larger resonant wavelength shows a


8

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

reduced scattering cross-section, the absorption cross-section first increases and then decreases. In the short wavelength range between 300 nm and 500 nm, the resonant peak of electric dipole mode at λ1 = 439 nm dominates in the scattering spectrum as expected. The absorption spectrum, however, shows complicated features with multiple resonant peaks. Our near-field calculations reveal that these absorption peaks are associated with a series of high order surface plasmon modes intrinsic in the composite multilayered MIM nanostructures, which can be excited via the near-field coupling to bright electric dipole mode in the same spectral region. Our FDTD calculations have also demonstrated that the multiresonant optical response of a MIMIMIM composite nanoantenna has little dependence on the incidence angle and the polarization of the excitation light (see Figure S1 in Supporting Information). The independence of optical properties on the incidence angle and light polarization arises from the nondispersive nature of localized plasmon modes in composite nanoantennas. To quantify the subwavelength field concentration of different modes in a composite nanoantenna, we have used FDTD simulations to calculate the spectrum of mode volume Vm defined by 40, 41

, where , is the electromagnetic energy density at position r and frequency ω in the simulation system (see Supporting Information). Figure 2D shows that the mode volume of three magnetic dipole modes is about 3-4 orders smaller than the diffraction limit (Vm/λ3 < 10-3) due to the tight confinement of optical fields in the dielectric gap region of MIM loop nanoantennas. In the shorter wavelength range between 350 nm and 550 nm where the electric dipole mode overlaps with different high order surface plasmon modes, the spectrum of the normalized mode volume shows multiple dips with values ranging from 10-3 to 10-5. Thus, by supporting multiple


9

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 29

magnetic dipole modes as well as different high order surface plasmon modes with deep subwavelength optical confinement, composite nanoantennas have the unique capability to enhance the optical interactions with materials in different gap regions at multiple different wavelengths. A near-field optical picture can provide a microscopic understanding of physics behind the different far-field spectral features in composite nanoantennas. We have calculated near-field distribution maps for the intensity of electric and magnetic fields (|E|2, and |H|2) and the phase of electric and magnetic fields (φ(Ex), φ(Hy)) at different resonant wavelengths. The resonance at λ1 = 439 nm reveals combined near-field optical features from the radiative electric dipole mode and high-order surface plasmon modes (Fig. 2E). The electric dipole mode induces in-phase oscillations of in-plane electric fields (Ex) extending from vertically stacked MIM nanodisks into the air, while the high-order surface plasmon modes result in alternating patterns of electrical field intensity and phase at a high spatial frequency (> 50 µm-1). In contrast, the other three resonances at λ2 = 587 nm (Fig. 2F), λ3 = 762 nm (Fig. 2G), and λ4 = 884 nm (Fig. 2H) are magnetic modes showing an intense dipolar magnetic field Hy with a constant phase φ(Hy) in MIM loop nanoantennas. These magnetic modes are associated with the oscillating current loop in MIM nanostructures, which consists of the conduction current driven by in-plane electric field (Ex) in metal disks and the displacement current driven by out-of-plane electric field (Ez) in the dielectric gap. At resonant wavelengths of these magnetic dipole modes, the large charge accumulation as well as the tight field confinement in the MIM loop nanoantenna structure can result in very large enhancement factors of both local electric and magnetic fields (Fig. 2F-H), where |Eloc|2/|Eo|2 > 104 and |Hloc|2/|Ho|2 > 103 can be achieved. Interestingly, by comparing between |E|2 and |H|2 distribution maps for different magnetic resonances, we find that the


10

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

magnetic field always has a larger penetration depth into the metal layer than the electric field in MIM loop nanoantennas. The strength of optical interactions between magnetic modes in neighboring MIM loop nanoantennas depends on their spectral and spatial overlaps. Figure 3A shows a 2D map of calculated extinction spectra for MIMIMIM composite nanoantennas with different thicknesses of the middle dielectric layer (h2: 0 nm - 30 nm). In the calculation, the thickness of top dielectric layer is fixed as 6 nm (h3= 6 nm), and the combined thickness of the middle and the bottom dielectric layers is fixed as 30 nm (h1+h2 = 30 nm). The 2D extinction map shows four branches of resonant features associated with the electric dipole mode (p) of metal nanodisks, and three magnetic dipole modes (µ1, µ2, and µ3) associated with the three MIM loop nanoantennas in the vertical stack. Among them, the resonant wavelengths of the electric dipolar mode (p) in individual Ag nanodisks and the magnetic dipolar mode (µ3) in the top MIM loop nanoantenna show no dependence on h2. As h2 increases and accordingly h1 decreases, the magnetic dipolar modes (µ2) in the middle MIM loop nanoantenna shifts to the blue and the magnetic dipolar modes (µ1) in the bottom MIM loop nanoantenna shifts to the red, which agrees with the calculations for single MIM building blocks (Fig. 1B).


11

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

Figure 3. Optical interactions between magnetic modes. (A) FDTD-calculated 2D map of extinction spectra for MIMIMIM composite nanoantennas with different thicknesses of the middle dielectric layer (h2). (B-C) FDTD-calculated normal incidence scattering and absorption spectra (B), and FDTD-calculated distribution maps of |H|2 and φ(Hy) at 884 nm (C) for a MIMIMIM composite nanoantenna with dielectric layer thicknesses of 6 nm, 24 nm, and 6 nm from bottom to top. (D-F) FDTD-calculated scattering and absorption spectra (D), and FDTDcalculated distribution maps of |H|2 and φ(Hy) at 840 nm (E) and 910 nm (F) for a MIMIMIM composite nanoantenna with dielectric layer thicknesses of 24 nm, 6 nm, and 6 nm from bottom to top. When the bottom and the top MIM loop nanoantennas have the same dielectric layer thickness (h1 = h3 = 6 nm), they can support magnetic dipole modes (µ1 and µ3) of the same resonant wavelength. As illustrated in the dashed circle (1) in Figure 3A, the two magnetic dipole modes


12

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

directly cross each other at the same resonant wavelength λ1= 884 nm, which suggests their weak optical interactions due to the separation by the middle dielectric layer (h2 = 24 nm). Figure 3B shows that the far-field optical response of the two largely uncoupled MIM loop nanoantennas at λ1= 884 nm is mainly from the absorption rather than the scattering, and the values of their total absorption and scattering cross-sections double compared to those of an individual MIM loop nanoantenna of the same geometry. From the near-field calculations (Fig. 3C), we find that the magnetic field intensity |H|2 in the top MIM loop nanoantenna is about 3 times larger than in the bottom one due to the geometric shadowing of the incident light. Moreover, the calculated phase difference (δφ = 0.35π) of magnetic field Hy between the top and the bottom MIM loop nanoantennas is slightly larger than the phase retardation (0.295π) of free space light between them, which indicates the existence of weak optical interactions via surface plasmon fields that carry a wavevector larger than the free-space light. When the middle and the top dielectric layers have the same thickness (h2 = h3 = 6 nm), we see anti-crossing behavior between magnetic dipole modes (µ2 and µ3) in the middle and the top MIM loop nanoantennas, as illustrated in the dashed circle (2) in Figure 3A. Therefore, magnetic dipole modes of the same resonant wavelengths in neighboring MIM loop nanoantennas can hybridize to form new bonding and anti-bonding modes with an energy splitting. From the calculated far-field spectra in Figure 3D, we can see that the low-energy bonding mode at λ2B= 910 nm is a dark resonance that barely interacts with free-space light and only has a small absorption cross-section (σabs/σ0 ~ 1). In contrast, the high-energy anti-bonding mode at λ2A= 840 nm is much brighter, whose extinction consists of similar contributions from both absorption (σabs/σ0 ~ 10) and scattering (σscat/σ0 ~ 7) processes. Figure 3E illustrates the near-field characteristics of the anti-bonding mode at λ2A= 840 nm, where the two coupled


13

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

magnetic dipole modes in the top and the middle MIM loop nanoantennas oscillate in-phase (δφ = 0) and generate intense magnetic fields extending across the shared metal layer of the two MIM nanoantennas. From an intuitive picture, the new anti-bonding magnetic mode consists of two in-phase magnetic dipole modes in neighboring MIM components. Thus, the anti-bonding mode has a larger effective area of current loop and accordingly a larger optical cross-section than the magnetic dipole mode in the individual MIM nanoantenna. In contrast, the low-energy bonding mode at λ2B= 910 nm shows a node with zero magnetic field intensity in the center plane of the shared metal layer for the two neighboring MIM nanoantennas (Fig. 3F). Furthermore, the bonding mode shows an out-of-phase oscillation of magnetic field Hy between the neighboring two MIM loop nanoantennas.

Such a magnetic quadrupole nature of the

bonding mode explains its dark behavior with a much smaller optical cross-section than the bright anti-bonding mode of a magnetic dipole nature.

Figure 4. Nanofabrication of multiresonant composite nanoantennas in large arrays. (A) Scheme for fabricating MIMIMIM composite nanoantennas with three dielectric layers. (B)


14

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Scanning electron microscopy (SEM) images of gold nanohole arrays. (C) Tilted and (D) crosssectional view of SEM images of fabricated MIMIMIM composite nanoantennas. For fabrication of composite nanoantenna arrays on glass, we used a freestanding film of Au nanohole arrays as a physical deposition mask to create a periodic array of multilayered MIM nanodisks (Fig. 4A). Briefly, freestanding films of Au nanohole arrays were fabricated by a soft nanofabrication procedure (see Supporting Information), which involves steps such as photoresist patterning on Si substrates by phase-shifting lithography with a PDMS mask, Cr deposition, photoresist removal, reactive ion etching (RIE), Au deposition, Cr etching, and liftoff of freestanding films of Au nanohole arrays. Here, we used Cr layer as a RIE etching mask to generate nanowell arrays on the Si substrate. Next, we transferred periodic patterns of photoresist posts into freestanding Au films of periodic nanohole arrays using the PEEL technique (photolithography, etching, electron-beam deposition, and lift-off).42

Finally, we deposited

alternating layers of Ag and SiO2 through the Au nanohole array mask on glass substrates and then peeled off the Au nanohole array masks by the scotch tape to expose composite nanoantenna arrays on the glass substrates. Generation of vertically stacked composite nanoantenna arrays by electron-beam deposition of materials through Au nanohole array masks has three unique advantages: (1) high throughput and uniform fabrication of nanoplasmonic devices over cm2 areas enabled by the PDMS-mask based nanolithography; (2) large selection of different materials (including metals, dielectrics, and semiconductors) in multilayered structures simply by switching source materials during the physical vapor deposition process; and (3) accurate nanoscale control of out-of-plane geometries (thicknesses) in multilayered structures. Figure 4B shows the SEM image of the freestanding film of Au nanohole arrays that served as a deposition mask to create composite nanoantenna arrays. The films of Au nanohole arrays have


15

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

a thickness of about 150 nm, a nanohole diameter of around 130 nm, and a nanohole array periodicity of 400 nm. Figure 4C and Figure 4D show tilted-view and cross-sectional scanning electron microscopy (SEM) images of Ag-SiO2-Ag-SiO2-Ag-SiO2-Ag composite nanoantennas with three dielectric layers, where each Ag layer has the same thickness of 30 nm, and the SiO2 gap layers have different thicknesses of 3 nm, 8 nm, and 12 nm from bottom to top. The multilayered Ag-SiO2-Ag composite nanoantennas show a truncated-cone shape because the shadowing effect in the electron-beam deposition procedure can gradually cause the size reduction of nanohole openings in the Au nanohole arrays.43 The cross-sectional SEM image (Fig. 4D) clearly reveals the interface roughness between alternating Ag and SiO2 layers in composite nanoantennas, which can cause additional optical losses in metal due to surface scattering. In addition, we can observe noticeable geometric variations between individual nanoantennas from SEM images (Fig. 4C & 4D) because of the geometric imperfection of the Au nanohole array mask in terms of nanohole diameter and shape (Fig. 4B).


16

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. Experimental demonstration of multiresonant optical response. (A) Schematic illustrations of composite nanoantennas with different numbers of dielectric layers (Sample A-D) and cross-sectional SEM image of fabricated Ag-SiO2-Ag-SiO2-Ag-SiO2-Ag (Sample D) with 3 dielectric layers. (B-C) Measurements and FDTD simulations of normal incidence extinction spectra of Sample A-D. (D-H) FDTD-calculated electric and magnetic field intensity distribution maps for Sample D at resonant wavelengths of (D) 425 nm, (E) 600 nm, (F) 640 nm, (G) 820 nm, and (H) 1185 nm. To demonstrate experimentally the multiresonant optical response of composite nanoantennas, we used the above explained fabrication method (Fig. 4A) to produce four samples (A, B, C, and D) consisting of different numbers of Ag-SiO2-Ag loop nanoantennas in the vertical stack. Figure 5A illustrates the detailed geometric specifics of individual nanoantennas in these samples. We measured their normal incident extinction spectra using a high performance UVVis-NIR spectrophotometer (Cary 5000 from Agilent). Figure 5B shows the measured extinction spectra from different samples in the range between 350 nm and 1500 nm. Figure 5C shows that the FDTD-calculated extinction spectra of these samples are in good agreement with experimental results in terms of resonant wavelengths of different modes. In contrast to the calculations, however, the measured spectra show resonant peaks with much broader linewidths, which arises from inhomogeneous broadening effects due to geometric variations between individual nanoantennas in the array (Fig. 4C) as well as from homogenous broadening effects due to increased metal losses associated with the interface roughness between metal and dielectric layers (Fig. 4D). Among different samples, Sample A, containing arrays of Ag nanodisks, shows a major extinction peak at λA1 = 675 nm associated with the electric dipole mode in individual Ag


17

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

nanodisks. Sample B has arrays of Ag-SiO2-Ag nanostructures with one MIM loop nanoantenna in the vertical stack. In addition to the major electrical dipole peak at λB1 = 650 nm, the measured extinction spectrum of Sample B reveals two new resonant features: one at λB2 = 450 nm from high order surface plasmon modes, and another one at λB3 = 1100 nm from magnetic dipole mode in the MIM loop nanoantennas with a dielectric gap thickness of h1 = 3 nm. Sample C holds arrays of Ag-SiO2-Ag-SiO2-Ag nanostructures with two MIM loop nanoantennas in the vertical stack. Compared to Sample B, Sample C reveals a new shoulder feature at λC4 = 820 nm near the major electric dipole peak at λC1 = 620 nm, which is attributed to the magnetic mode of the added new MIM loop nanoantennas (h2 = 8 nm) in the vertical stack. Interestingly, as the samples (A, B, and C) have an increased number of MIM building blocks, their major extinction peaks keep blue shifting from λA1 = 675 nm, to λB1 = 650 nm, to λC1 = 620 nm. This observation is in agreement with FDTD calculations (Fig. 5C), and can be explained by the fact that the average diameter of all Ag nanodisks in multilayered MIM nanostructures with a truncated-cone shape decreases as the number of layer increases. The measured extinction spectrum for Sample D, which contains arrays of Ag-SiO2-Ag-SiO2Ag-SiO2-Ag nanostructures with three MIM loop nanoantennas in the vertical stack, shows five resonant features respectively at λD2 = 450 nm, λD5 = 570 nm, λD1 = 640 nm, λD4 = 860 nm, and λD3 = 1160 nm. Compared to Sample C, the major extinction peak in Sample D red shifts from λC1 = 620 nm to λD1 = 640 nm, and a new shoulder feature emerges at λD5 = 570 nm partially overlapping the major extinction peak. The calculated extinction spectrum (Fig. 5C) for Sample D also shows five peaks at similar wavelengths compared to the measurement. To reveal the microscopic nature of different peaks, we calculated their near-field distribution maps (Fig. 5DH). As shown in Figure 5D, the highest energy resonant feature at λ’D2 = 425 nm originates


18

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

from high-order surface plasmon modes with high-spatial-frequency oscillating patterns of |E|2 and |H|2 at metal surface. For the spectral features at λ’D5 = 600 nm (Fig. 5E) and λ’D1 = 640 nm (Fig. 5F), they share not only a signature of magnetic dipole mode with strongly concentrated magnetic fields in the top MIM loop nanoantenna, but also a characteristic of electrical dipole mode of the whole multilayered MIM nanostructures with in-plane electric fields oscillating inphase. These behaviors indicate that they are hybridized dark and bright modes from the destructive and constructive interference between the electrical dipole mode of the whole MIM structure and the magnetic dipole mode in the top MIM building block. From calculations of their near-field mode profiles (Fig. 5E-H), we find that while the four spectral features at λ’D5, λ’D1, λ’D4, and λ’D3 are respectively associated with one magnetic dipole mode in one of the MIM building blocks, they can induce significant electrical and magnetic field enhancement in the neighboring building blocks across the share metal layer. In summary, we have demonstrated that composite nanoantennas based on vertically stacked building blocks of MIM loop nanoantennas can support multiple nanolocalized modes at individually tunable resonant wavelengths by controlling thicknesses of different dielectric layers. Compared to in-plane engineered multiresonant composite nanoantennas, out-of-plane engineered multiresonant composite nanoantennas have several advantages: 1) reduced device footprint by vertical stacking, 2) reduced device volume by sharing the metal layer between neighboring building blocks, 3) accurate nanoscale control of out-of-plane geometries, or layer thicknesses, 4) easier integration with multiple different functional materials in different gap layers of high fields, and 5) efficient transport of heat or charges between neighboring building blocks. Therefore, we envision that out-of-plane engineered multiresonant composite nanoantennas will be ideal to achieve optical multiband applications at nanoscale, such as


19

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nonlinear

or

upconversion

multi-photon

optical

Page 20 of 29

nanodevices,

wavelength-multiplexed

biomedical nanotransducers, and broadband solar energy nano-convertors. ASSOCIATED CONTENT Supporting Information FDTD simulation conditions; far-field optical properties of multiresonant composite nanoantennas with different incident angles; method of mode volume calculation; and additional fabrication details are described in the supporting information; This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS. WZ acknowledges support of this study by startup funds from Virginia Tech. JS received research assistantship from Institute for Critical Technology and Applied Science (ICTAS), Virginia Tech.


20

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

REFERENCES 1. Novotny, L.; van Hulst, N. Nat Photonics 2011, 5, (2), 83-90. 2. Schuller, J. A.; Barnard, E. S.; Cai, W. S.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Nat Mater 2010, 9, (3), 193-204. 3. Liu, Y.; Cheng, R.; Liao, L.; Zhou, H. L.; Bai, J. W.; Liu, G.; Liu, L. X.; Huang, Y.; Duan, X. F. Nat Commun 2011, 2. 4. Kinkhabwala, A.; Yu, Z. F.; Fan, S. H.; Avlasevich, Y.; Mullen, K.; Moerner, W. E. Nat Photonics 2009, 3, (11), 654-657. 5. Kauranen, M.; Zayats, A. V. Nat Photonics 2012, 6, (11), 737-748. 6. Suh, J. Y.; Kim, C. H.; Zhou, W.; Huntington, M. D.; Co, D. T.; Wasielewski, M. R.; Odom, T. W. Nano Lett 2012, 12, (11), 5769-5774. 7. Celebrano, M.; Wu, X. F.; Baselli, M.; Grossmann, S.; Biagioni, P.; Locatelli, A.; De Angelis, C.; Cerullo, G.; Osellame, R.; Hecht, B.; Duo, L.; Ciccacci, F.; Finazzi, M. Nat Nanotechnol 2015, 10, (5), 412-417. 8. Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat Mater 2008, 7, (6), 442-453. 9. Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem Rev 2008, 108, (2), 494-521. 10. Ding, S. Y.; Yi, J.; Li, J. F.; Ren, B.; Wu, D. Y.; Panneerselvam, R.; Tian, Z. Q. Nat Rev Mater 2016, 1, (6). 11. Lal, S.; Clare, S. E.; Halas, N. J. Accounts Chem Res 2008, 41, (12), 1842-1851. 12. Delcea, M.; Sternberg, N.; Yashchenok, A. M.; Georgieva, R.; Baumler, H.; Mohwald, H.; Skirtach, A. G. Acs Nano 2012, 6, (5), 4169-4180. 13. Boulais, E.; Lachaine, R.; Hatef, A.; Meunier, M. J Photoch Photobio C 2013, 17, 26-49. 14. Xiong, R. H.; Raemdonck, K.; Peynshaert, K.; Lentacker, I.; De Cock, I.; Demeester, J.; De Smedt, S. C.; Skirtach, A. G.; Braeckmans, K. Acs Nano 2014, 8, (6), 6288-6296. 15. Carvalho-de-Souza, J. L.; Treger, J. S.; Dang, B.; Kent, S. B. H.; Pepperberg, D. R.; Bezanilla, F. Neuron 2015, 86, (1), 207-217. 16. Christopher, P.; Xin, H. L.; Linic, S. Nat Chem 2011, 3, (6), 467-472. 17. Christopher, P.; Xin, H. L.; Marimuthu, A.; Linic, S. Nat Mater 2012, 11, (12), 10441050. 18. Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano Lett 2013, 13, (1), 240-247. 19. Clavero, C. Nat Photonics 2014, 8, (2), 95-103. 20. Payne, E. K.; Shuford, K. L.; Park, S.; Schatz, G. C.; Mirkin, C. A. J Phys Chem B 2006, 110, (5), 2150-2154. 21. Kuttge, M.; de Abajo, F. J. G.; Polman, A. Nano Lett 2010, 10, (5), 1537-1541. 22. You, E. A.; Zhou, W.; Suh, J. Y.; Huntington, M. D.; Odom, T. W. Acs Nano 2012, 6, (2), 1786-1794. 23. Alaee, R.; Menzel, C.; Huebner, U.; Pshenay-Severin, E.; Bin Hasan, S.; Pertsch, T.; Rockstuhl, C.; Lederer, F. Nano Lett 2013, 13, (8), 3482-3486. 24. Harutyunyan, H.; Volpe, G.; Quidant, R.; Novotny, L. Phys Rev Lett 2012, 108, (21). 25. Thyagarajan, K.; Rivier, S.; Lovera, A.; Martin, O. J. F. Opt Express 2012, 20, (12), 12860-12865. 26. Thyagarajan, K.; Butet, J.; Martin, O. J. F. Nano Lett 2013, 13, (4), 1847-1851.


21

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

27. Zhang, Y.; Wen, F.; Zhen, Y. R.; Nordlander, P.; Halas, N. J. P Natl Acad Sci USA 2013, 110, (23), 9215-9219. 28. Aydin, K.; Ferry, V. E.; Briggs, R. M.; Atwater, H. A. Nat Commun 2011, 2. 29. Unlu, E. S.; Tok, R. U.; Sendur, K. Opt Express 2011, 19, (2), 1000-1006. 30. Aouani, H.; Navarro-Cia, M.; Rahmani, M.; Sidiropoulos, T. P. H.; Hong, M. H.; Oulton, R. F.; Maier, S. A. Nano Lett 2012, 12, (9), 4997-5002. 31. Navarro-Cia, M.; Maier, S. A. Acs Nano 2012, 6, (4), 3537-3544. 32. Zhang, B. Y.; Hendrickson, J.; Guo, J. P. J Opt Soc Am B 2013, 30, (3), 656-662. 33. Bossard, J. A.; Lin, L.; Yun, S.; Liu, L.; Werner, D. H.; Mayer, T. S. Acs Nano 2014, 8, (2), 1517-1524. 34. Wallace, G. Q.; Tabatabaei, M.; Hou, R. J.; Coady, M. J.; Norton, P. R.; Simpson, T. S.; Rosendahl, S. M.; Merlen, A.; Lagugne-Labarthet, F. Acs Photonics 2016, 3, (9), 1723-1732. 35. De Angelis, F.; Malerba, M.; Patrini, M.; Miele, E.; Das, G.; Toma, A.; Zaccaria, R. P.; Di Fabrizio, E. Nano Lett 2013, 13, (8), 3553-3558. 36. Zhou, L.; Yu, X. Q.; Zhu, J. Nano Lett 2014, 14, (2), 1093-1098. 37. Johnson, P. B.; Christy, R. W. Phys Rev B 1972, 6, (12), 4370-4379. 38. Dmitriev, A.; Pakizeh, T.; Kall, M.; Sutherland, D. S. Small 2007, 3, (2), 294-299. 39. Ekinci, Y.; Christ, A.; Agio, M.; Martin, O. J. F.; Solak, H. H.; Loffler, J. F. Opt Express 2008, 16, (17), 13287-13295. 40. Chikkaraddy, R.; de Nijs, B.; Benz, F.; Barrow, S. J.; Scherman, O. A.; Rosta, E.; Demetriadou, A.; Fox, P.; Hess, O.; Baumberg, J. J. Nature 2016, 535, (7610), 127-130. 41. Sauvan, C.; Hugonin, J. P.; Maksymov, I. S.; Lalanne, P. Phys Rev Lett 2013, 110, (23). 42. Henzie, J.; Lee, M. H.; Odom, T. W. Nat Nanotechnol 2007, 2, (9), 549-554. 43. Zhou, W.; Odom, T. W. Nat Nanotechnol 2011, 6, (7), 423-427.


22

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Table of Contents Graphic


23

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC 73x43mm (300 x 300 DPI)


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. Multiresonant composite optical nanoantennas by out-of-plane plasmonic engineering. 79x67mm (300 x 300 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Far-field and near-field optical properties of multiresonant composite nanoantennas. 158x113mm (300 x 300 DPI)


Page 26 of 29

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3. Optical interactions between magnetic modes. 148x105mm (300 x 300 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Nanofabrication of multiresonant composite nanoantennas in large arrays. 77x90mm (300 x 300 DPI)


Page 28 of 29

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. Experimental demonstration of multiresonant optical response. 154x99mm (300 x 300 DPI)


Multiresonant Composite Optical Nanoantennas by Out-of-plane

Recommend Documents