Dipole Radiation-Induced Extraordinary Optical Transmission for

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Dipole Radiation Induced Extraordinary Optical Transmission for Silver Nanorods Covered Silver Nanohole Arrays Steven R Larson, Hoang Luong, Chunyuan Song, and Yiping Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00477 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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The Journal of Physical Chemistry

Dipole Radiation Induced Extraordinary Optical Transmission for Silver Nanorods Covered Silver Nanohole Arrays Steven Larson1, Hoang Luong1, Chunyuan Song1,2*, and Yiping Zhao1* 1 2

Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602

Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, China

Abstract A new extraordinary optical transmission (EOT) mode is discovered when a hexagonal Ag nanohole array are covered by a tilted Ag nanorod array. The resonant wavelength of this EOT mode redshifts with respect to the normal EOT mode predicted by plasmon-grating coupling theory or dynamic scattering theory and increases linearly with the nanorod length. The structures also show a strong polarization dependence, i.e., when the E-field direction of the incident light is perpendicular to the long axis of the nanorods, only normal EOT spectrum is visible; but when the E-field is parallel to nanorod axis, the new mode appears. Our finite-difference-time-domain calculations confirm this finding and show that this new mode is due to the dipole radiation of the nanorods on top of the nanoholes. It is expected that other complicated unite cells of nanohole arrays could generate other new EOT modes and can be used to design new optical devices.

*Corresponding Author: [email protected] and [email protected]

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1. Introduction Since the discovery of the extraordinary optical transmission (EOT) effect on sub-wavelength nanohole arrays by Ebbsen et. al., the peaks and valleys of the transmission spectra have been wellunderstood through either the plasmon-grating coupling or dynamic light scattering theory.1-3 Based on these theories, for a simple hexagonal array of nanoholes, the peak (max, EOT resonant wavelength) and valley (min, the Woods anomaly) positions can be written as,4 𝜆

𝑖, 𝑗

𝜆

√3 2 𝑖 𝑖, 𝑗

𝜀 𝜀

𝑎 𝑖𝑗

√

𝑗

𝜀

𝜀

𝜀 ,

,

1

2

where ao is the lattice spacing of the nanohole array, i and j are integer indexes of the peaks, and εm and εd are the real part of the relative permittivity of the nanohole material and the surrounding environment. In Equation 1, the largest EOT peak position predicted is max(1,0) or max(0,1) = √

. This prediction is based on the fact that the unit cell of the lattice only consist a

simple open hole. However, if the unit cell becomes more complicated, i.e., not a simple open hole, other EOT mode with a resonant wavelength larger than max(1,0) or max(0,1) could appear. Here, we demonstrate experimentally that when the nanoholes (NHs) are covered by tilted nanorods (NRs), a new EOT mode does appear, its resonant wavelength redshifts with respect to max(1,0) or max(0,1) and increases linearly with the nanorod length and show strong polarization dependent. Our finite-difference-time-domain (FDTD) calculations show that this new mode is due to the dipole radiation of the nanorods on top of the nanoholes. 2. Experiments Materials 2 ACS Paragon Plus Environment

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500 nm diameter polystyrene nanospheres (PSNS) (Polyscience, Lot # 679675) were used to form the colloid monolayer onto clean glass slides (Gold Seal, Part# 301) and silicon wafers (University Wafer). Sulfuric acid (Fisher Scientific, 98%), ammonium hydroxide (Fisher Scientific, 98%), and hydrogen peroxide (Fisher Scientific, 30%) were acquired to clean the glass and silicon. Silver and titanium pellets (Plasmaterials, 99.99% and Kurt J. Lesker, 99.995%) were purchased as the evaporation materials. Ethanol, toluene, acetone, and isopropanol were used for the colloid monolayer preparation and to remove residual PSNS from the substrates after the Ag deposition. Methanol, acetone, isopropanol, methyl isobutyl ketone, 1-hexanol, chloroform, carbon tetrachloride, and toluene were obtained for LSPR sensing measurements. Deionized (DI) water (18 MΩ) was used throughout all the experiments. All chemicals and materials were used without further purification. Fabrication of Ag Nanorods on Nanohole (NRonNH) Array Structures The fabrication procedure for Ag nanorods on nanoholes (NRonNH) is a combination of nanosphere lithography (NSL), reactive ion etching (RIE), and oblique angle deposition (OAD) as shown in Figure 1. The polystyrene nanospheres (PSNS ) used in the fabrication had diameter D = 500 nm and the etched nanohole size (diameter) was set to be d = 350 nm. The Ag nanorods fabricated by OAD have a length systemically varied from 75 nm to 600 nm. The fabrication steps are summarized in Figure 1. First, glass slides and silicon wafers were cut into small pieces with dimensions of 1 cm × 2.5 cm and 1 cm × 1 cm, respectively. Glass substrates were heated in piranha solution, 4:1 volume ratio of concentrated sulfuric acid to 30% hydrogen peroxide, for 20 min. Si substrates were cleaned using the first step of the RCA method, boiling in a 5:1:1 volume ratio of DI water, ammonium hydroxide, and hydrogen peroxide for 20 min. All substrates were then thoroughly rinsed in DI water, and dried under a N2 gas flow.



To form a monolayer of PSNS (Figure 1a), the air-water interface method was used as previously reported. 5 Briefly, a PSNS suspension was first diluted in DI water to a concentration of 0.01 w/v% and then washed several times via centrifugation. Then, the suspension was further diluted with ethanol to a 2:1 volume ratio to modify the surface tension of the solution. The resulting solution was then loaded into a syringe and drops of PSNS suspension were dispensed at a rate of 0.015 mL/min onto the surface of a tilted cleaned glass Petri dish via a syringe pump. This process continued until a monolayer was formed and covered the entire surface of the Petri dish. A Teflon ring was placed gently on the surface of the water to protect the monolayer film and then the water level was raised and lowered in the dish to wash any remaining suspended beads not on the monolayer out of the solution. Then glass and Si substrates were carefully slid below the monolayer and the water level was lowered until dry. The substrates were allowed to dry overnight in air. The PSNS monolayer was then plasma-etched to reduce the size of the nanospheres while keeping their lattice spacing (Figure 1b). The etching was conducted at a pressure of 40 mTorr with a 10 sccm oxygen flow, an ICP power of 25 W, and a RF power of 10 W for 350 s in a Trion Technology Phantom III RIE/ICP system. After etching, the diameter of the PSNS was reduced from 500 nm to ~ 350 nm. The nanohole arrays (NHAs) were then fabricated using a custom-built electron beam deposition system (Figure S1c). In addition to the monolayer covered substrates, clean substrates were also loaded as a control. The chamber was pumped down to a base pressure of < 1× 10-6 Torr. The deposition rate and total thickness of the films were monitored by a quartz crystal microbalance (QCM). 10 nm of Ti was first deposited at a vapor incident angle  = 0° (the angle between the vapor incident direction and the substrate surface normal). Then a layer of 70 nm of Ag was deposited at the same angle. The samples were allowed to cool in vacuum before


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being removed from the chamber. Then the PSNS were removed with the Scotch tape method (Figure 1d). Any residual PS was washed away with toluene, and the samples were rinsed with acetone and then IPA before being dried under a N2 gas flow. Last, the samples were re-loaded into the deposition chamber and Ag nanorods were deposited at  = 86° with a QCM thickness of 75, 150, 250, 300, 350, 400, 500, 600 nm, respectively, as shown in Figure 1e. It was expected that on top of the NHA, tilted Ag nanorod arrays would be formed. Optical and Morphological Characterization Scanning electron microscopy images of the samples were taken with a field emission scanning electron microscope (SEM, FEI Inspect F). Atomic force microscopy (AFM) images of the samples were taken with a Park Systems NX-10 AFM. SEM and AFM images were analyzed with the ImageJ software (NIH). Ellipsometry measurements of the thin films were taken by a spectroscopic ellipsometer (M-2000, J.A Woollam Co., Inc.) at incident angles of 65°, 70°, 75°, and 80°, respectively, over a wavelength range of 370 - 1000 nm. The optical transmission spectra of the NRonNH structures were measured by an ultraviolet-visible spectrophotometer (UV-Vis, Jasco-750). Polarization dependent spectra were measured by adding a matched set of polarizers to the Jasco system via a home built mounting and alignment system. FDTD Calculations A commercial software package (FDTD Solutions, Lumerical Solutions Inc.) was used to calculate the transmission spectra and localized E-field distribution of the nanohole arrays. According to the AFM results, the thickness and the hole diameter were set as 80 nm and 350 nm, respectively. The geometric parameters of nanorod (nanorod length (l), nanorod diameter (d), tilting angle () and position of nanorod on nanohole arrays) were obtained from SEM images. A rectangular unit cell (red box, Figure S1 of the supplementary information) was used with periodic



boundary conditions in two dimensions, and perfectly matched layer boundary conditions were used on the top and bottom surfaces of the calculation domain. A linear polarized light source was used to excite the structure. The angle between the polarization plane and nanorods () was varied from 0 to 180. Two monitors of “frequency domain field profile” and “frequency-domain field and power” were set up to calculate the localized E-field distributions and the transmission spectra under the continuous wave excitation, respectively. The obtained EM fields were normalized to the magnitude of the incident E-fields. To ensure the convergence of the calculations, the mesh size of 2.5 nm  2.5 nm  2.5 nm was chosen. The dielectric constant spectra of the Ag materials were taken from Johnson et al. 6 The electromagnetic (EM) wave propagation movies at P3 are presented as Movies M1-M4: NRonNH (M1), NR-on-Ag-film (M2), NR-on-glass (M3), and NH structure (M4). The movies were generated from the complex E-field data, which can be extracted from a frequency domain field profile monitor. The detail of movie calculation can be found in Ref. 7 .For the reliability of comparison between different cases, NR arrangements in NR on Ag film and NR-on-glass calculations are identical to the ones in the calculation of NRonNH, and the hole parameters in NH structure calculation are identical to the ones the calculation of NRonNH.

3. Results and Discussion Figure 2 shows some representative SEM images of the NRonNH structures for various NR deposition lengths (h) from 0 to 600 nm (The cross-section images of corresponding structures are shown in Figure S2 of SI). For h ≤ 400 nm, the nanoholes are visible beneath the nanorod structure, and are consistent with other NH fabrications with the combination of NSL and RIE.8-9 The holes


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have an average diameter d = 339 ± 9 nm with a lattice spacing D = 500 ± 10 nm. As the h increases (h > 75 nm), the length of the Ag NRs increases noticeably. For larger h, the number density n of nanorods increases first to 74 ± 8 NR/μm2 and then decreases after h = 150 nm. These trends are shown in Figure S3a of the SI. The decrease in NR density is due to small NRs being obscured by larger ones during the shadowed growth. The cross-section SEM images show that the bottom of the NHs were completely shadowed during the deposition and no nanorods were formed inside nanoholes. This geometric shadowing effect is typical for nanorods deposited by the OAD process on patterned substrates.10-11 The tilting angle β of the nanorods (defined in Figure S2) is also summarized in Figure S3b of the SI, and constantly stay around 72o ± 2, which is consistent with our previous reports.12-13 Figures 3a-d show the polarization dependent UV-Vis-NIR transmission spectra of the NRonNH samples for h = 0, 150, 250, and 300 nm, respectively. Additional polarization dependent transmission spectra for h = 75, 350, 400, 500, and 600 nm are shown in Figure S4 of the SI. For each sample, 16 spectra were taken from the polarization angle  = 0° to 180°, where  is the angle of the E-field of the incident light with respect to the long axis direction of the NRs, as shown in the inset of Figure 3d for  = 0°. For h = 0 nm, the spectra show no polarization dependence, which is consistent with the structural symmetry of the bare NH. The spectra show four characteristic peak/valley features, which are marked as V1, P1, V2, and P2 in Figure 3a. Based on Equations 1 and 2 for subwavelength plasmonic NHs,8, 14-15 the peak P1 (P1 ~ 540 nm) and P2 (P2 ~ 840 nm) can be assigned to the (1,0) Ag/glass and Ag/air resonance peaks, while the valley V1 (V1 ~ 430 nm) and V2 (V2 ~ 660 nm) are the (1,0) Ag/glass and Ag/air transmission minima of Wood’s anomaly. When h = 75 nm, though the size and the coverage of NRs are small, the spectra only change slightly in peak/valley magnitudes, and the overall features of the transmission 7 ACS Paragon Plus Environment


spectra do not change. However, when h ≥ 150 nm, an additional peak (P3), at a larger wavelength compared to P2, appears, especially in the  = 90° spectra. This can be clearly seen in Figures 3bd. More specifically, for h = 150 nm and when  increases from 0° to 90°, a shoulder to the right of peak P2 gradually appears (Figure 3b) and it magnitude becomes larger and larger, while the magnitude and location of P2 do not seem to change significantly. The final shoulder peak P3 is found at P3 = 940 nm. Based on the corresponding SEM image shown in Figure 2, the NRs only slightly cover the NHs. In fact, the open area (areas not covered by Ag from top view SEM images) coverage NH decreases from ~ 40% (h = 0 nm) to ~ 36% (h = 150 nm). The red dashed lines in Figures 3a-d mark open area coverage values for different samples. Based on previous reports,1-2 both peaks P2 and P3 can be viewed as the EOT peak. For h = 250 nm sample, the  = 0° and  = 90° spectra are significantly different: both peaks P1 and P2 dramatically decrease while the peak P3 (P3 = 1354 nm) dominates the  = 90° spectrum. The -dependent spectra also reveal that the peak/valley locations of V1, P1, V2, and P2 remain almost the same for different . In this case,

NH is about 27%. For longer NR samples, similar trends are observed, the location of P3 keeps on red-shifting, and the magnitudes of all peaks in the spectra keep on decreasing. However, when h (≥ 300 nm) increases, the maximum transmission peak becomes significantly larger than NH, i.e., a significant dynamic diffraction effect is revealed.16-18 It is interesting to see how P3 changes with h. Figure 3e plots both experimentally determined P2 and P3 as a function of h (the plots of the locations of V1, P1, V2, and P2 versus h at  = 0o are shown in Figure S5 of the SI, and all of them do not show significant change with h). Clearly P2 is almost a constant for different h while

P3 increases almost linearly with h. The appearance of peak P3 in NRonNH samples is very intriguing. Even though the OAD grown Ag nanorods introduce randomness into the NH lattice, for small h samples, as revealed by 8 ACS Paragon Plus Environment

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the SEM images and the transmission spectra, the long range lattice order is still maintained, i.e. the spectral features of EOT effect are preserved. According to surface plasmon and grating coupling condition (Equation 1), the maximum EOT peak wavelength should be fixed at P2 location, i.e., around 840 nm, regardless of NR length (see Figure S5 of the SI), and any peak with wavelength larger than P2 is forbidden. Thus, the peak P3 is not due to the usual EOT mechanisms reported in the literature. 1-3 Since the Ag NRs can be treated as another plasmonic layer onto the NH, it might introduce additional EOT effect. We have also investigated the transmission spectra of Ag NR arrays on bare glass substrates prepared simultaneously with the NH substrates. The polarization dependent transmission spectra are shown in Figure S6 of the SI. The polarization dependence spectra are consistent with what we observed before:13 when h is small, the aspect ratio of the NR is close to 1, the -dependence spectra show no significant difference (h = 75 nm) and only one LSPR peak is revealed; however, when h (≥ 250 nm) increases, all  = 0o spectra show a sharp valley at  ~ 354 nm, which is corresponding to the transverse mode of the LSPR of NRs; while  = 90o spectra show a broadband low transmission peak, which is due to both the transvers and longitudinal LSPR modes of NRs and the randomness of the NR arrays. Clearly the NR arrays alone cannot contribute to the appearance of the P3 peak. To understand the origin of peak P3, we have performed the FDTD calculations on the structural models based on SEM images. Figure 4 shows the calculated  = 0o and  = 90o transmission spectra and the corresponding experimental spectra for h = 0, 150, 250, and 400 nm, respectively. Except for the peak/valley magnitudes and exact locations, the main spectral features of the NRonNH samples have been captured by the FDTD calculations, i.e., when h is small (< 150 nm), the calculated  = 0o and  = 90o transmission spectra do not show significant difference. 9 ACS Paragon Plus Environment


But when h is large (> 150 nm), a red-shifted broad peak appears in  = 90o spectra, which is peak P3 observed in experiment. When the calculated peak P3 location, P3, is plotted as a function of h, as shown in Figure 3e, it also follows a linear relation. The slope of the liner fit, 3.9, is very close to the slop obtained from the experiment data, 3.5. Overall, the FDTD models reflect the experiment results very well. To understand the origin of P3 mode, the local electric field distributions at the wavelengths of the valleys V1 and V2, and the peaks P1, P2, and P3 with  = 90o for h = 250 nm sample were calculated and presented in Figure 5. At a glance, one can notice the strong local electric fields near rim of the hole at Ag/air (in V1 and P1 maps) and Ag/glass (in V2 and P2 maps). These strong local electric fields at Ag/air interface are due to (1,0) Ag/air resonance peaks at P1 and (1,0) Ag/air transmission minima of Wood’s anomaly at V1, while the strong local electric fields at Ag/glass interface are due to (1,0) Ag/glass resonance peaks at P2 and Ag/glass transmission minima of Wood’s anomaly at V2. In addition, at P3, the electric field map shows a very strong local resonance at the tip of nanorod, which is due to a dipole resonance. In order to quantitatively compare the local field strength, the maximum electric field intensity (|E/E0|max) and the volume-average electric field intensity (|E/E0|average) are extracted. The map at P3 shows the largest |E/E0|max and |E/E0|average of 36.1 and 2.2, respectively, in comparison to corresponding values of P2 (24.0 and 1.5), V2 (15.1 and 0.9), P2 (7.5 and 1.0), and P2 (9.0 and 1.1). Furthermore, wave propagation calculations were further performed for the sample with h = 250 nm, assuming four different structures, NRonNH, NR-on-Ag-film, NR-on-glass substrate, and NH only. In all the NR related structures, the distribution and topology of the NRs were exactly the same. The electromagnetic (EM) wave propagation (FDTD calculated |E(t)/E0|2, where E0 is the E-field amplitude of the incident wave) movies at P3 are presented as Movies M1-M4 in the SI. When the plane EM wave at P3 propagates through the NRonNH (Movie M1), a very strong localized 10 ACS Paragon Plus Environment

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electric field (E-field) appears at the tip of the nanorods directly above the nanoholes, while strong E-field coupling also occurs between adjacent nanorods. The EM wave also propagates through the NH as well. For the NR-on-Ag-film structure (Movie M2), strong local E-fields appear around all the NRs and only reflected EM wave is obtained. For the NR-on-glass substrate (Movie M3), strong E-fields are observed between adjacent nanorods, showing the local coupling effect, and it is very hard to identify propagating EM wave through the glass NH. For the NH only structure (Movie M4), since the P3 is far away from the EOT resonant wavelength P2, no strong local Efield is observed, and no visible propagating EM wave is identified. To further quantify above observation, we placed four point-monitors at similar locations of different structures to quantitatively record the time-dependent local E-fields and the propagating waves (see cartoons in Figure 5 and Figure S1): Location R, the tips of NRs over NH (about 10 nm away from the tip, along the long axis of the NR); Location B, the center of hole (top); Location G, the center of hole (bottom); and Location O, the rim edge below the bottom of the nanohole (about 5 nm below and 5 nm to the edge of the NH). Figures 5a-d plot the normalized field intensity (|E/E0|2) versus normalized time (t/T, where T is the period of the EM wave) at identical locations for different structures. Generally, regardless of the position of point-monitor, a larger oscillation E-field is observed in NRonNH compared to other structures. More specific, at the tip of the NRs or the NHs (Figure 5a), the maximum |E/E0|2 is about 160 for NRonNH, even larger than that (~ 140) of the NR-on-Ag-film structure, indicating a strong local E-field enhancement. The |E/E0|2 amplitudes at Locations G and O (Figures 5c-d) have a similar value, ~ 1.6 for NRonNH, indicating an enhanced transmission. This transmitted wave does not damp while it propagates through the NHs. However, for the NR-on-glass and NH structures, the |E/E0|2 amplitude changes from ~ 22 for NR-on-glass (1.3 for NH), to 4 (0.8), to 0.2 (0.3), and finally to 0.2 (0.3), when the wave propagates from



Location R to O, and a significant damping in |E/E0|2 is evident, indicating a heavily absorbed wave for these two structures at P3. Additional FDTD calculations have also been carried out by scanning the wavelength  around P3, and the amplitude of |E/E0|2 at Location O for the same NRonNH structure is plotted as a function of  as shown in Figure 4e. The maximum amplitude of |E/E0|2 occurs at P3. Clearly, the resonance wavelength P3 belong to a new plasmonic mode, it generates a very large local electric field at the tip of the NRs over NHs and also produces an enhanced transmission propagating wave. To have a better picture on this plasmonic mode, the movie of the current density on the NRonNH at P3 is also obtained from FDTD calculations and is presented as Movie M5 in the SI. A clear dipole oscillation appears on the NR right above the NH. Based on these results, we believe that the NRs on NHs act as a nano-antenna (a dipole antenna) and the length of the NR determines the resonance radiation wavelength. At the resonance (P3), the incident light can be absorbed strongly by the NRs, and the electrons oscillations on the NRs induce a strong local field on the tips of the rods while re-radiate EM waves through the subwavelength holes (diffraction) to the other side of the NHs. Clearly, it would be very interesting to look at how the nanorod tilting angle would affect the plasmonic property of the structures. One simplest strategy is to change the vapor incident angle

 for OAD Ag NR deposition. According to the geometry of the nanohole fabricated in our experiments, the critical vapor incident angle c (i.e., the smallest ) to form nanorods on nanohole is determined by the diameter d (~ 340 nm) and the thickness t (~ 80 nm) of the nanoholes, tanc = t/d, i.e., c ~ 77o. Considering the experimental variation in the hole size and thickness, the valid deposition angle is from 80o to 87o. Based on our previous detailed deposition study of the formation of Ag NRs at different deposition angles (Fig. 2 in Ref.19), the quality of Ag NRs formed at  < 84o were not good (the nanorods were interconnected). Therefore, there is only a 12 ACS Paragon Plus Environment

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very limited range of θ angles (84o – 87o) one can chose experimentally. In addition, according to the characteristics of OAD deposition, at different θ, not only the nanorod tilting angle would change, but also the nanorod diameter and density, i.e., experimentally one can not only change the tilting angle while keeping the Ag NR diameter and separation the same. Thus, with current fabrication technique, it is very difficult to systematically examine how the nanorod tilting angle affect the plasmonic property. However, theoretically we can predict the tilting angle effect by FDTD. We have conducted calculations of optical transmission for NRonNH (h = 250 nm,  = 90o) with tilting angle of nanorod () from 60o to 80o. All other geometrical parameters (including nanorod positions) were kept identical. The results are summarized in Figure 7. P3 redshifts when

 increases, while the transmission TP3 at P3 decreases slightly. This observation can be simply explained as the projection length of nanorod to nanohole plane is longer when  decreases, leading to the redshift of dipole resonance peak P3; while the effective open area coverage decreases, which makes TP3 decrease. In addition, we also explored the hole size (diameter d) effect using the FDTD calculations (Figure 8). For this calculation, we use NRonNH (h = 250 nm,  = 90o) with all parameters were fixed except hole diameter (d). The larger hole induces a stronger optical transmission TP3 and redshifts peak P3 since the effective open area coverage increases with d. Clearly, compared to the changes induced by nanorod length h, both the  and d have a relative less effect on the change of the optical property.

4. Conclusions Such a unique plasmonic mode with high local E-fields and propagating wave, provides a unique way to tune the EOT resonance wavelength of a fixed NH arrays. For example, by adding 13 ACS Paragon Plus Environment


planner nanorods inside the nanoholes, one could red-shift the EOT resonant wavelength continuously from visible to IR region, based on the length of the rods. If multiple rods can be fabricated inside the nanoholes, broadband EOT or multiple EOT comb like spectra can be achieved. If more complicated nanorod patterns can be designed inside the NHs, the structure could show interesting metasurface properties, such as chiral response, zero- behavior, etc. and could find applications in designing different optical filters in a wide wavelength range. Also the greatly enhanced local E-field and associated EOT effect in current samples make the structure good candidate for surface enhanced Raman scattering and index sensor. The NRonNH structures can be easily integrated into microfluidic systems for practical applications with added functionality such as filtration, separation, or targeted capturing.


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Associated Content Supporting Information: The Supporting Information is available free of charge on the publication website at DOI: XXXXXXXXXX. FDTD simulation model, cross sectional SEM images, statistical NR parameters extracted from SEM including NR length, density, and tilting angle as a function of NR deposition thickness, polarization dependent transmission spectrum measurements for other NR lengths and NR arrays without NHs, tracked spectral features transmission vs NR length at 0, 30, 60, 90°, tracked spectral feature location vs NR length at 0°, additional polar plots of spectral features transmission as a function of polarization angle, and additional cross section FDTD calculated local E-field for P1 and V1. Movies M1-M4: FDTD calculated EM propagation (|E/E0|2) of a plane wave at P3 through four different structures: NRonNH (M1), NR-on-Ag-film (M2), NR-on-glass (M3), and NH structure (M4). The structure parameters were adopted from the h = 250 nm sample. Movie M5 is the corresponding current density movie for the NRonNH structure.

Author Information Corresponding Author Prof. Yiping Zhao E-mail: [email protected] Department of Physics and Astronomy University of Georgia Athens, Georgia 30602 Phone: 706/542-7792 Fax: 706/542-2492 Prof. Chunyuan Song Email: [email protected] Institute of Advanced Materials (IAM) Nanjing University of Posts & Telecommunications Nanjing 210023, China 15 ACS Paragon Plus Environment


Phone: 86-025-85866332 Fax: 86-025-85866396 Notes: These authors declare no competing financial interests.

Acknowledgments SL, HL, and YZ were supported by the National Science Foundation under Grant no. CMMI1435309 and ECCS-1609815, and CS was supported by the National Natural Science Foundation of China under Grant no. 61871236.


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References: 1. Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F.; Thio, T.; Wolff, P. A., Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998, 391, 667. 2. Treacy, M. M. J., Dynamical diffraction explanation of the anomalous transmission of light through metallic gratings. Phys. Rev. B 2002, 66, 195105. 3. García de Abajo, F. J., Colloquium: Light scattering by particle and hole arrays. Reviews of Modern Physics 2007, 79 (4), 1267-1290. 4. Li, Y., Plasmonic optics: theory and applications. SPIE Press: 2017. 5. Larson, S.; Zhao, Y., Localized Surface Plasmonic Resonance and Sensing Properties of Ag–MgF2 Composite Nanotriangles. The Journal of Physical Chemistry C 2018, 122 (13), 73747381. 6. Johnson, P. B.; Christy, R.-W., Optical constants of the noble metals. Physical review B 1972, 6 (12), 4370. 7. FDTD Solutions. https://www.lumerical.com/ (accessed 12 March). 8. Ai, B.; Basnet, P.; Larson, S.; Ingram, W.; Zhao, Y., Plasmonic sensor with high figure of merit based on differential polarization spectra of elliptical nanohole array. Nanoscale 2017, 9 (38), 14710-14721. 9. Murray-Methot, M.-P.; Menegazzo, N.; Masson, J.-F., Analytical and physical optimization of nanohole-array sensors prepared by modified nanosphere lithography. Analyst 2008, 133 (12), 1714-1721. 10. Song, C. Y.; Larsen, G. K.; Zhao, Y. P., Anisotropic resistivity of tilted silver nanorod arrays: Experiments and modeling. Appl. Phys. Lett. 2013, 102 (23), 4. 11. He, Y. Z.; Fu, J. X.; Zhao, Y. P., Oblique angle deposition and its applications in plasmonics. Front. Phys. 2014, 9 (1), 47-59. 12. Chaney, S. B.; Shanmukh, S.; Dluhy, R. A.; Zhao, Y. P., Aligned silver nanorod arrays produce high sensitivity surface-enhanced Raman spectroscopy substrates. Appl. Phys. Lett. 2005, 87 (3), 031908. 13. Zhao, Y. P.; Chaney, S. B.; Zhang, Z. Y., Absorbance spectra of aligned Ag nanorod arrays prepared by oblique angle deposition. Journal of Applied Physics 2006, 100 (6), 063527. 14. Murray-Methot, M. P.; Ratel, M.; Masson, J. F., Optical properties of Au, Ag, and bimetallic Au on Ag nanohole arrays. J. Phys. Chem. C 2010, 114 (18), 8268-8275. 15. Zhang, X. M.; Li, Z. B.; Ye, S. S.; Wu, S.; Zhang, J. H.; Cui, L. Y.; Li, A. R.; Wang, T. Q.; Li, S. Z.; Yang, B., Elevated Ag nanohole arrays for high performance plasmonic sensors based on extraordinary optical transmission. J. Mater. Chem. 2012, 22 (18), 8903-8910. 16. Treacy, M. M. J., Dynamical diffraction explanation of the anomalous transmis. Physical Review B 2002, 66, 195105. 17. Gordon, R., Bethe ’ s aperture theory for arrays. Phys. Rev. A 2007, 76, 053806. 18. Garcia de Abajo, F. J., Colloquium: Light scattering by particle and hole arrays. Rev. Moden Phys. 2007, 79, 1267. 19. Liu, Y. J.; Chu, H. Y.; Zhao, Y. P., Silver Nanorod Array Substrates Fabricated by Oblique Angle Deposition: Morphological, Optical, and SERS Characterizations. The Journal of Physical Chemistry C 2010, 114 (18), 8176-8183.



Figure Captions Figure 1 Fabrication diagram: (a) polystyrene nanosphere monolayer with a diameter of 500 nm were formed on a clean glass slide, (b) samples were then O2 plasma etched to a diameter of 350 nm, (c) thin films were then deposited, 10 nm of Ti followed by 70 nm of Ag, (d) polystyrene nanospheres were removed by the Scotch tape method, (e) and Ag nanorods were deposited at an oblique angle of 86°. Figure 2 Representative SEM images of the NRonNH samples with h = 0, 75, 150, 250, 300, 350, 400, 500, and 600 nm, respectively Figure 3 (a)-(d) Selected polarization dependent UV-Vis-NIR transmission spectra of NRonNH samples with h = 0, 150, 250, and 300 nm, respectively. (e) The plots of the resonant wavelengths P3 and P2 extracted from experiments and FDTD calculations. The error bars are within the size of the symbols. The solid and dashed lines are linear fitting results. Figure 4 The FDTD calculated UV-Vis-NIR transmission spectra (solid curves) at  = 0o and 90° for NRonNH samples with h = 0, 150, 250, and 400 nm, respectively. The corresponding experimental spectra are shown as dashed curves. Figure 5 Electric field |E/E0| distributions at wavelengths of valleys V1 and V2, and the peaks P1, P2, and P3 with  = 90o for h = 250 nm sample. Figure 6 (a)-(d) The FDTD calculated |E(t)/E0|2 versus normalized propagation time t/T extracted at indicated positions (R, B, G, and O) in the cartoon for four different structures, NRonNH, NR-on-Ag-film, NR-on-glass, and NH structure, when a plane EM wave is incident on the structures with a wavelength of P3. The model is based on the h = 250 nm NRonNH samples. (e) The plot of the oscillation amplitude of |E(t)/E0|2 at Location O as a function of the incident wavelength of the plan EM waves for the NRonNH sample. 18 ACS Paragon Plus Environment

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Figure 7 (a) FDTD calculated optical transmission spectra of the h = 250 nm NRonNH sample with different nanorod tilting angle . (b) The plot of P3 peak position P3 and the peak transmission T P3 (%) as a function of the tilting angle . Figure 8 (a) FDTD calculated optical transmission spectra of the h = 250 nm NRonNH sample with different nanohole diameter d. (b) The plot of P3 peak position P3 and the peak transmission T P3 (%) as a function of the hole diameter d.



Figure 1


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Figure 2



Polarization Angle () 0 10 20 30 40 50 60 70 80 90 100 110 120 140 160 180

(a) h = 0 nm P2

P1

35 30 25 20 15

V2

10 5 0 40

(b) h = 150 nm P3

2000

P2

V1

𝐸

(d) h = 300 nm

(c) h = 250 nm

35 30

P2

25

P3

20

𝑘

P3

P2

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Transmission (%)

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Transmission (%)

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

(e)

1800 1600 1400 P2 (experiment) P3 (experiment) P3 (FDTD)

1200 1000 800 600

15

0

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NR Length h (nm)

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Wavelength  (nm)

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Figure 3


600

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(b) h = 150 nm

(a) h = 0 nm

90° 0°

(d) h = 400 nm

(c) h = 250 nm

Figure 4



Figure 5


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Figure 6



20

Tilting angle  (deg) 60 65 70 75 80

(b)

P3

19

1500

10

18 1400 17 1300

0 400

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60

65

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 (deg)

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Figure 7


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80

16

TP3 (%)

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T (%)

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

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20

(b)

d (nm) 300 320 340 360 380

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1400 15

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d (nm)

Figure 8


360

380

TP3 (%)

(a)

P (nm)

30

T (%)

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


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Dipole Radiation-Induced Extraordinary Optical Transmission for

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