Letter Cite This: ACS Photonics 2019, 6, 322−326
pubs.acs.org/journal/apchd5
Tunable Lattice Plasmon Resonances in 1D Nanogratings Yi Hua,†,‡ Ahmad K. Fumani,§,‡ and Teri W. Odom*,†,§ †
Department of Materials Science and Engineering and §Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
ACS Photonics 2019.6:322-326. Downloaded from pubs.acs.org by LANCASTER UNIV on 04/13/19. For personal use only.
S Supporting Information *
ABSTRACT: Lattice plasmon resonances or surface lattice resonances (SLRs) supported in two-dimensional (2D) metal nanoparticle arrays have extremely narrow line widths and highly localized electric field enhancements, which are key properties for realizing plasmon lasers and hybrid solid-state lighting devices. This paper reports lattice plasmons in one-dimensional (1D) metal nanogratings with broadband tunability (over 400 nm) far beyond their 2D counterparts at visible wavelengths. The large wavelength tunabilities of 1D or line-SLRs are from the lower symmetry of the structures compared to 2D arrays based on nanoparticles. We demonstrate that line-SLRs exhibit a Fano-like character based on coupling between an out-of-plane plasmon excitation and 1D Bragg diffraction modes. We show how the height and periodicity of the grating determine the spectral properties of the line-SLRs. By adjusting the line height, we achieved high-quality lattice resonances, even in index-mismatched environments. KEYWORDS: lattice plasmon resonance, surface lattice resonance, out-of-plane charge oscillations, nanograting, Fano resonance
E
Manipulating the geometry of plasmonic unit cells is a simple yet effective means to take advantage of different charge oscillation types in designing a desired SLR response. Here we show that 1D gold nanogratings can support narrow line-SLRs that are tunable over a wide spectral range. Unlike 2D arrays, 1D nanogratings can maintain high-quality lattice modes from visible to near-infrared wavelengths (580− 1000 nm) by changing the azimuthal and incident angles. We tailored line shape and wavelength of line-SLRs by varying the resonance wavelength of individual gold lines and the grating diffraction mode. In addition, we achieved line-SLR resonances in nonuniform refractive index environments by adjusting grating height. Figure 1a depicts 1D gold nanogratings that can be fabricated using the soft nanofabrication procedure PEEL (photolithography, etching, electron-beam deposition, and liftoff) over cm2 areas.21 First, a grating pattern was formed in photoresist (Shipley 1805) on a silicon (100) wafer by soft interference lithography using a poly(dimethylsiloxane) (PDMS) mask.21 After deposition of chromium (8 nm) and lift-off of the photoresist layer, deep reactive ion etching (DRIE) was used to create trenches with steep sidewalls (depth >200 nm) in the silicon substrate. The plasmon gratings were then produced by thermal deposition of gold (up to 200 nm) on the template and removing the chromium sacrificial layer by chemical etching. Finally, the nanogratings were stripped from the silicon template using a layer of UV-
xploiting the collective behavior of 2D nanoparticle arrays can result in plasmonic resonances hundreds of times narrower than the localized surface plasmons (LSPs) of individual particles.1−4 Diffractive coupling between neighboring particles at the lattice resonance suppresses the radiative loss and produces orders of magnitude larger localized electric fields compared to single particles. High-quality surface lattice resonances (SLRs) from particle arrays have enabled nanoscale lasing and nonlinear optical phenomena, including higherorder harmonic generation.5−8 SLR resonance characteristics are determined by the particle and lattice geometry and are fixed at the time of fabrication. One method to vary SLR wavelength reversibly is to embed nanoparticle arrays in a stretchable matrix and to mechanically change the particle spacing.9,10 High-quality surface lattice resonances from 2D particle arrays require stringent structural and refractive-index conditions.11 SLRs support the narrowest resonance line width at the optical band edge, and their dispersive behavior shows lower quality resonances at off-normal incident angles.9 Asymmetric dielectric environments inhibit long-range coupling between particles and suppress lattice resonances.12 With the appropriate particle size13 and lattice constant,14 SLRs can be excited in asymmetric environments but with relatively poor quality.11 In 2D arrays, particle dimensions determine whether the type of LSP coupled to the diffraction mode is dipolar or quadrupolar and whether the excitation is in-plane or out-ofplane.15−19 Moreover, the interplay between in-plane and outof-plane charge oscillations can result in hybridized excitations with reduced net dipole moments on the nanoparticles.10,16,20 © 2019 American Chemical Society
Received: November 6, 2018 Published: January 25, 2019 322
DOI: 10.1021/acsphotonics.8b01541 ACS Photonics 2019, 6, 322−326
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nanograting excited out-of-plane charge oscillations on the gold lines. Since the perpendicular component was negligible, no resonance was observed at small incident angles (θ < 10°). The line shape of the line-SLR was prominent in reflection spectra (Figure S1) and arose from Fano interference between the single-line plasmon and diffraction mode of the grating. Under transverse electric (TE) polarized light, when the electric field is parallel to the line direction, the sharp resonance disappeared. TE-polarized light did not couple to single-line plasmons, and hence, line-SLRs were not excited (Figure S2). The wavelength and line shape of line-SLRs were determined by the spectral position of the Bragg diffraction mode and the single-line plasmon. Although the resonant wavelength of the single-line plasmon wavelength is fixed by width and height of the lines, the effective diffraction wavelength and, therefore, line-SLRs can be tuned by changing the incident angle θ. Additionally, since 1D gratings have lower symmetry compared to 2D arrays, changing the azimuthal angle provides a second parameter to tune the SLR. For example, the same grating structure generated plasmonic resonances over different wavelength ranges at azimuthal angles ϕ = 45° and 90° (Figure 2). At ϕ = 45°, the red-shift in the resonance over the same θ range was smaller compared to ϕ = 0° (Figure 2a). In contrast, at ϕ = 90°, the resonance blueshifted and the line width broadened as θ increased (Figure 2b). Notably, by tuning the azimuthal angle, the resonance wavelength could be tuned over 400 nm for a given incident angle. Finite-difference, time-domain (FDTD) simulations were in excellent agreement (Figures S3 and S4). Figure 2c,d illustrates that the electric dipole was in the z-direction, which confirms the out-of-plane nature of the charge oscillations at two different resonances (θ = 40°, ϕ = 45° and 90°). To explore Fano interference between line-plasmons and different diffractive modes, we varied the period of the gratings. Figure 3a,b shows how changing a0 while keeping the line dimensions constant (w = 80 nm and h = 100 nm) influences the line-lattice resonance. Periodicities a0 = 400, 485, and 570
Figure 1. One-dimensional nanograting exhibits a continuously tunable Fano-shaped resonance under TM polarized excitation. (a) SEM image of 400 nm period 1D Au nanogratings on Si template. The inset shows a schematic diagram of the nanogratings and the optical measurement. (b) Transmittance spectra of Au nanograting with a0 = 400 nm and h = 100 nm at ϕ = 0° at different incident angles. The spectra are shifted vertically for clear demonstration.
curable polyurethane and transferred to a glass substrate (Supporting Information, Methods). Figure 1b shows how the transmission spectrum of a nanograting (h = 100 nm and a0 = 400 nm) evolves with incident angle θ and azimuthal angle ϕ = 0° in a uniform refractive index (n = 1.52). Under transverse magnetic (TM) polarized light, a narrow dispersive feature emerged at 698 nm around θ = 10° with 3.7 nm full width at half-maximum. The resonance red-shifted and narrowed at increased θ. The electric field component of the TM-polarized light perpendicular to the
Figure 2. Dispersive property of the out-of-plane lattice plasmon at two different azimuthal angles. (a, b) Transmittance spectra of Au nanograting with h = 100 nm and a0 = 400 nm at different incident angles for ϕ = 45° and 90°, respectively. (c, d) Simulated electric field vectors in the x−z plane corresponding to resonances at θ = 40° for the azimuthal angle ϕ = 45° and 90°, respectively. 323
DOI: 10.1021/acsphotonics.8b01541 ACS Photonics 2019, 6, 322−326
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Figure 3. The dispersion property of the lattice resonance can be tuned by controlling the diffraction mode. (a, b) Dispersion diagram of nanograting with a0 = 400, 485, and 570 nm in transmission for azimuthal angle ϕ = 0° and 90°. The gray scale depicts the transmission intensity and the dashed lines show different diffraction modes.
Figure 4. Lattice plasmon resonance excited in nonuniform dielectric environments. (a) Dispersion diagram for nanogratings with a0 = 400 nm and h = 100 nm with water n = 1.33 superstrate. The lattice resonance is absent. (b, c) Dispersion diagrams for a nanograting with a0 = 400 nm and h = 200 nm with water n = 1.33 and index-matching oil n = 1.52 superstrates, respectively. The line-SLR follows the (−1,0) diffraction mode.
response was more noticeable when ϕ = 90° (Figure 3b) and results from the change in coupling between the lineplasmon and diffraction mode. At the largest periodicity a0 = 570 nm, at ϕ = 0°, the interaction between the (−1,0) diffraction mode and the line-plasmon was too weak to form a lattice resonance; however, the (−2,0) mode was located closer to the line-plasmon and could generate a line-SLR. To access lattice modes in a nonuniform index environment, we studied how changing the superstrate and the height of the lines affected the line-SLR. When n of the superstrate decreased relative to the PU substrate, the lattice resonance disappeared (Figure 4a). Decreasing n blue-shifted the lineplasmon, but the diffraction mode dominated by the PU layer did not change. Due to this blue-shift, the single-line plasmon could no longer couple to the diffraction mode, and the lineSLR vanished. To excite lattice plasmons in a mismatched refractive index environment, we tuned the energy of the line
nm were selected to shift the diffraction mode relative to the single-line plasmon wavelength. Increasing the periodicity shifted the (±1,0) diffraction modes to longer wavelengths, and SLRs appeared as narrow dark features on the lower energy side of the photonic mode. The diffraction lines were labeled with two order numbers (nx,ny) to differentiate the cases with ϕ = 0° and 90°. To determine the energy of the line plasmon, we carried out FDTD simulations of the optical response of a single gold line (Figure S5). The extinction spectrum for a line with h = 100 nm and w = 80 nm had a resonance at 2.2 eV; increasing the height shifted the lineplasmon to lower energies. Line-SLRs vanished when the outof-plane dipolar oscillations in individual gold lines could not be excited at k∥ = 0 since the incident light does not have outof-plane electric field components. As grating periodicity increased (Figure 3), line-SLRs approached photonic modes (dashed white lines). This 324
DOI: 10.1021/acsphotonics.8b01541 ACS Photonics 2019, 6, 322−326
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plasmon by increasing the height of the lines (Figure 4b). Increasing h to 200 nm decreased the line plasmon energy, and the line-SLR reappeared along the (−1,0) diffraction mode. Increasing the superstrate n to match the substrate shifted the line-SLR to lower energies (Figure 4c). Compared to the structure with h = 100 nm (Figure 3a), the resonance in indexmatched 200 nm tall gratings occurred at lower energies with a larger separation from the diffraction mode. We observed about a 50 nm shift in the resonance wavelength by changing the refractive index from n = 1.52 to 1.33. Line-SLRs are distinct from surface plasmon polaritons (SPPs) supported in 1D gratings that are usually considered analogues of 2D nanohole arrays.22−25 Although SLRs and SPPs share some similar properties, including strong angledependent optical responses (Supporting Information), there are a few key differences: (1) SLRs correspond to reduced transmission and SPPs lead to enhanced transmission.26 (2) SLRs result from Fano interference between a single-line plasmon and Bragg diffraction, and therefore, an appropriate grating height is needed to observe SLRs. SPPs are confined to the surface and do not require minimum grating height for excitation. (3) SLRs in 1D nanogratings disappear at k∥ = 0 because the out-of-plane dipolar oscillations cannot be excited by the incident electric field. In contrast, SPPs can be excited at k∥ = 0 because electron oscillations of SPPs are along the surface of the grating. (4) Asymmetric dielectric environments suppress the quality of SLRs. SPPs are confined to metal− dielectric interfaces, and if the indices of superstrate and substrate were different, the SPP resonance would only shift in wavelength. As grating width increases, however, 1D nanogratings will approach the structure of nanoslit arrays.27 We increased the grating width w = 80 nm to w = 320 nm (a0 = 400 nm fixed; Figure S6) and found that SPPs and SLRs can both be excited (Figure S7). FDTD simulations demonstrated that SPPs are more prominent on gratings with large enough widths (Figure S8). Thus, 1D nanogratings can function as an intermediate structure between 2D nanohole arrays that support SPPs and 2D particle arrays that can show SLRs. In summary, we realized line-SLRs from 1D metallic nanogratings that can be excited in nonuniform dielectric environments. The lower symmetry of 1D gratings compared to 2D nanoparticle arrays allowed line-SLRs to be tuned via changing the azimuthal angle. Also, we showed that the line shape and quality factor of the lattice resonances were determined by Fano interference between the line-plasmon and different diffraction modes. The geometry of 1D nanogratings along with their unique tunable spectral properties make them suitable for interfacing photoluminescent materials for light emission modification and plasmon lasing. Also, a line-SLRs can be potentially used for modifying the light wavefront and beam steering.
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nm) under TM polarized light at ϕ = 0°. FDTD simulated transmission spectra of 1D gold nanograting arrays (h = 100 nm) under TM polarized light at ϕ = 45° and 90°. FDTD simulated extinction spectra of a single gold nano-line. Nanofabrication technique for producing 1D nanograting with variable line width. Dispersion diagram of nanograting with w = 320 nm, a0 = 400 nm. FDTD simulated transmission spectra in relation to nanograting width. Angle dependence of SLRs and SPPs (PDF).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Teri W. Odom: 0000-0002-8490-292X Author Contributions ‡
These authors contributed equally to this manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Vannevar Bush Faculty Fellowship from DOD under Grant No. N00014-17-1-3023. This work used Northwestern University Micro/Nano Fabrication Facility (NUFAB), which is partially supported by Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the Materials Research Science and Engineering Center (DMR-1720139), the State of Illinois, and Northwestern University. This research was supported in part through the computational resources and staff contributions provided for the Quest high performance computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology. The authors are thankful to Jingtian Hu for thoughtful suggestions on the manuscript.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01541. Reflection spectra of gold 1D nanograting arrays under TM-polarized excitation. Transmission and reflection spectra of gold 1D nanograting arrays under TEpolarized excitation. FDTD simulated transmission and reflection spectra of 1D gold nanograting arrays (h = 100 325
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