Achieving Strong Field Enhancement and Light Absorption

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Achieving Strong Field Enhancement and Light Absorption Simultaneously with Plasmonic Nanoantennas Exploiting Film-Coupled Triangular Nanodisks Yang Li, Dezhao Li, Cheng Chi, and Baoling Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03956 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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The Journal of Physical Chemistry

Achieving Strong Field Enhancement and Light Absorption Simultaneously with Plasmonic Nanoantennas Exploiting Film-Coupled Triangular Nanodisks Yang Li†,§, Dezhao Li†,§, Cheng Chi†, and Baoling Huang*,†,‡ †

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Clear Water Bay, Kowloon, Hong Kong. ‡

The Hong Kong University of Science and Technology Shenzhen Research Institute, Shenzhen,

China. §

These authors contributed equally.

ABSTRACT: Plasmonic nanoantennas (PNs) comprised of film-coupled or in-plane coupled nanoparticles have been exploited for light confinement or field enhancement. However, achieving strong local electric field enhancements (|Eloc|/|E0|>100) and near-perfect absorption (>95%) simultaneously remains a challenge, although it will benefit a wide range of applications. Here Ag/Al2O3/Au PNs are proposed by introducing high-density triangular nanodisks into film-coupled systems, which can produce dense “hot spots” with a large |Eloc|/|E0| of 211 and a near-unity absorbance. Due to the combination of the strong lightning rod effect and the

ACS Paragon Plus Environment

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out-of-plane coupling, the MIM structure combined with the triangular nanodisks effectively enhances the coupling strength and thereby the electric field confinements along the x-, y- and z-directions in film-coupled PNs, showing a lateral resolution as small as 4 nm. The highest |Eloc|/|E0| is more than three times higher than for their circular and square counterparts, and more than four times higher than for isolated triangular nanodisks. The near-perfect absorption results from the magnetic resonance induced by plasmonic coupling. Compared to the bowtie-shaped PNs based on the in-plane coupling, the developed PNs here offer more desired advantages including the polarization-independent near-perfect absorption, much larger field enhancements, and greater potential for large-scale production.


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1. INTRODUCTION Plasmonic nanoantennas (PNs),1 which exploit the unique spectral and spatial properties of noble metal nanostructures, are capable of manipulating light and confining electromagnetic field at the nanoscale.2-3 Noble metal nanostructures, such as Au4 and Ag5 nanoparticles, possess these properties as a result of the collective oscillations of their conduction band electrons, known as the localized surface plasmon resonances (LSPR). The local field enhancement and the light absorption are the two most important parameters to evaluate the performance of the PNs.6-7 High light absorption will benefit a wide variety of areas, such as solar thermal applications,8-9 sensors,10-11 thermal emitters,12-13 and photoluminescences.14-15 The enhanced electric fields at resonance can enhance the optical properties of some molecules in the vicinity, thereby improving their light-matter interactions. Particularly, for some applications, including plasmonic sensors,16-17 photocatalytic water splitting,18 surface-enhanced Raman scattering (SERS),19-21 surface-enhanced infrared absorption (SEIRA),22 and nonlinear optics23, it is desirable to employ PNs that lead to not only high-density “hot spots” with a large local field enhancement but also a near-perfect absorbance.

Conventional PNs often adopt isolated or in-plane coupled metal nanoparticles to confine electric field. Among the different nanoparticles, triangular nanoparticles have been proven to be superior building blocks in the PNs because of their strong local field enhancements


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(|Eloc|max~30-50 for isolated nanoparticles) induced by the lightning rod effect.24-27 Compared with the isolated (i.e., uncoupled) nanoparticles, even larger local field enhancements can be attained by adopting in-plane coupled triangular nanoparticles, e.g., bowtie-shaped nanoparticle dimers.28-31 The bowtie-shaped PNs enable a strong electric field localization within the nanogap between two in-plane coupled tip-to-tip triangular nanoparticles, achieving a local field enhancement up to 158.29 However, PNs with the isolated nanoparticles or the in-plane coupled nanoparticles can hardly achieve the near-perfect light absorption due to the absence of magnetic response to the incident light. Moreover, the strength of the in-plane plasmonic coupling between two triangular nanoparticles is mainly determined by the gap size and alignment accuracy, but achieving precise, high-yield sub-10 nm gaps and alignments remains a challenge owing to the fabrication limitations.32-33 Recently, film-coupled nanoparticle systems,32,34-36 also known as the metal-insulator-metal (MIM) structures, have been intensively studied due to their unique out-of-plane near-field coupling between the metal nanoparticles and the metal film (or the nanoparticles’ images in the metal film).37 The coupling strength can be readily tuned by controlling the thickness of the dielectric spacer with any off-the-shelf method, such as the atomic layer deposition (ALD).6,32 There have been significant efforts to leverage the MIM structures to improve either the optical absorption or the local electric field. Because of the simultaneous electric and magnetic


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responses to the incident light, MIM nanoantennas have been extensively utilized as perfect metamaterial absorbers (PMAs) in the near-infrared (NIR) and visible regions.10,38-39 To ensure the polarization-independent absorption and fabrication accuracy, the majority of the reported PMAs are composed of nanoparticles with 4-fold geometric symmetry, such as circles,10,40-41 rectangles,42 squares,6,35,43-44 and spheres.19,45 Although near-unity absorbances have been realized in PMAs, the local electric field enhancements are far from satisfactory. By exploiting a lower area-fill factor (density of disks), defined as the ratio of the area of one disk to that of one unit cell,46 PNs can localize the light and field in a smaller mode volume and thereby achieve a larger field enhancement.47-48 Y. Chu et al. proposed an MIM-structured SERS substrate comprised of a circular nanodisk array with an area-fill factor of 2.3%, showing a maximum electric field enhancement of nearly 86.49 T. Seok et al. demonstrated an electric field enhancement of 87 in an MIM optical antenna based on a nanorod dimers array with an area-fill factor of 3.0%.7 However, a low area-fill factor not only results in a low density of the “hot spots” but also inevitably deteriorates the light absorbance (typically less than 60%) due to the dilution effect,46,48 which is not desired. Recently, MIM PNs with sub-3-nm-thick spacers have been developed to achieve strong field localizations. J. Lassiter et al. developed a PN made of film-coupled gratings with a 2.83-nm-thick spacer and achieved a measured absorbance of 90% along with a calculated field enhancement of 57.23 S. Huang et al. compared the MIM plasmonic


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cavities consisting of nanoparticles with various shapes (sphere, rod, and cube) and attained a maximum field enhancement above 100 in the nanorod cavity with a spacer as thin as 2 nm.50 In practice, however, the coupling strengths and field localizations based on such thin spacers are extremely sensitive to the unavoidable surface roughness of the bottom metal films.6,34

In short, despite recent significant advances, developing PNs with both a strong local field enhancement (typically |Eloc|/|E0|>100) and a near-perfect absorbance (>95%) remains a challenge, although it is of great significance for a wide variety of areas. In this study, we overcome this challenge in conventional film-coupled structures by introducing high-density equilateral triangular nanodisks to enhance the out-of-plane plasmon coupling and light absorption

simultaneously.

With

the

obtained

PNs,

we

successfully

achieve

a

polarization-independent and incident-angle-insensitive absorbance of 98% as well as dense “hot spots” with a maximum local electric field enhancement of 211. Compared with the conventional PNs with in-plane coupled (bowtie-shaped) nanodisks, with the identical sizes of disk and gap, the MIM structures with the triangular nanodisks coupled along the out-of-plane direction provide a much stronger field enhancement (around 1.4 times stronger) even with a much higher area-fill factor (~20%). Moreover, the PNs consisting of film-coupled triangular nanodisks show great potential for large-scale production since the out-of-plane plasmon coupling requires no accurate lateral patterning or alignments, and the spacer thickness can be readily controlled using


6

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modern thin film technologies. The recent breakthrough in the chemical synthesis of triangular nanodisks also paves the way for mass production of the proposed PNs.51-52

2. METHODS

2.1. Numerical Simulations

The

numerical

simulations

were

conducted

using

the

three-dimensional

(3D)

finite-difference-time-domain methods (FDTD solutions v8.16, Lumerical). A plane wave with polarization along the x- or y-direction was incident from the top of the metal-insulator-metal structure, as shown in Figure 1a. The 3D simulation region was defined by the periodic boundaries along the x- and y-direction, and the perfectly matching layers (PML) along the z-direction. The mesh sizes along the x-, y-, and z-directions were set to be 2 nm, 2 nm, and 1 nm, respectively, which were much smaller than the incident light wavelengths and the structure dimensions. The optical properties of gold and silver were from Palik’s work.53 Two frequency-domain field profile and power monitors parallel to the x-y plane were set to record the transmission and reflectance. Both 2D plane monitors and point monitors were used to collect information about the electrical field intensity in the structures.

2.2. Effective mode volume calculation


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r r r

54-55

The canonical Veff definition for typical cavities

, i.e., Veff

∫ ε (r ) E (r r)

2

d 3r = r r 2 , is max[ε ( r ) E ( r ) ]

unsuitable for the structures containing dispersive and lossy materials such as metals, since the integrand will become negative when the dielectric constants ε 95%. Particularly, as shown in Figure 5b, the absorbance bands of the PNs in P = 200 nm are especially broad owing to the combination of the fundamental mode with the third-order mode. The λres of all the PNs linearly redshift as the disk sizes (or array periods) increase, which coincide well with the simulated results (Figure 5c). The FWHM of PN-T in different periods are larger than those for PN-C and PN-S with the same geometric volume over the whole NIR region (Figure 5d), indicating broader absorption bands.

We quantitatively investigated the |Eloc|/|E0| of PNs with various disk sizes. The calculated maximum values of |Eloc|/|E0| as a function of the disk size (or array period P) are depicted in Figure 6a. The maximum |Eloc|/|E0| values of PN-C and PN-S are around 75 and 65, respectively, showing almost independence from the disk sizes after fixing the f. However, for PN-T, the maximum |Eloc|/|E0| increases from 86 to 141 under TM polarization, and from 122 to 211 under TE polarization as the P decreases from 600 nm to 200 nm. This increase originates from the stronger lightning rod effect when the structure size becomes smaller. Note that the highest |Eloc|/|E0| for PN-T with the P = 300 nm under TE polarization is 161, which is comparable with


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the optimal result (158) for the bowtie-shaped nanodisks of the similar disk and gap size.29 The PN-T has significantly larger |Eloc|/|E0| than its circular and square counterparts in all the periods no matter under TM or TE polarization. Particularly, for the P = 200 nm, the maximum |Eloc|/|E0| (72) of PN-C is lower than the calculated result reported in a previous work (~86) with the similar disk diameter and at the comparable resonant wavelength,49 which results from the higher f (19.6%). However, the PN-T in the P = 200 nm achieves a huge |Eloc|/|E0| value of 211, being 2.45 times larger than for those nanoantennas fabricated by lithographic methods in previous works.7,49 In addition, combined with the roughly eight times higher f, a significantly larger (20 times) diluted field enhancement

| Ediluted | | Eloc | = f is achieved in comparison with the | E0 | | E0 |

previous works.7,49 Moreover, the maximum |Eloc|/|E0| is 3-5 times larger than that seen in the PNs with film-coupled nanospheres,34,67 nanocubes,32,36,50 and nanorods50 prepared by chemical methods with the same spacer thickness (10 nm), and is even larger than that of the PNs with a much smaller spacer thickness of 1-3 nm. Best of all, the huge |Eloc|/|E0| is accompanied with the near-perfect absorption as discussed earlier, both of which are indispensable and desirable for a broad range of applications.

To further understand the huge |Eloc|/|E0|, we calculated the quality factors Q (Figure 6b) and the effective mode volumes Veff (Figure 6c) for all the three kinds of PNs in various periods. The


24

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Figure 6. The calculated (a) maximum local electric field enhancement |Eloc|/|E0|, (b) the quality factor Q, (c) the effective mode volume Veff, and (d) the Purcell factor Fp for all the PNs as a function of the disk size (or array period P). The ratio of P to the disk size is a constant for each PN due to the same area-fill factor f, therefore, here we can use P as the x-axis of these figures. Q values for PNs tend to reduce as the P decrease, whereas the variations among PN-T, PN-C, and PN-S with a given P are so slight that even the smallest Q obtained when P = 200 nm will not remarkably affect the maximum |Eloc|/|E0|. Similar with the case of P = 400 nm, the Q values of PN-T with other P are close to those for PN-C and PN-S except for the P = 200 nm. In contrast, the Veff values for the three kinds of PNs almost shrink by two orders of magnitude as


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the periods decrease from 600 nm to 200 nm. As expected, for PN-T, the smallest Veff is obtained with the setting of P = 200 nm, reaching as small as 2.5×10-6 µm3 and 1.0×10-6 µm3 under the TM and TE polarizations, respectively. This ultrasmall Veff is approximately one order of magnitude smaller than for PN-C (9.5×10-6 µm3) and PN-S (1.7×10-5 µm3), due to its symmetry breaking and sharper features. According to the Equation 3, the lower Veff, the comparable Q, and the perfect absorbance A(λres) discussed above altogether quantitatively explain the larger |Eloc|/|E0| compared with those reported in the literature.

According to quantum theory, the spontaneous emission rate of a molecule (emitter) essentially depends on the optical local density of states (LDOS) at the position of the emitter, which can be enhanced via the Purcell effect. Hence, the Purcell factor Fp =

3 4π

2

(

λres n

)3

Q is Veff

proportional to the ratio of LDOS to the free space DOS.68 Using the obtained Q and Veff, we also calculated the Fp of the PNs and plotted the results in Figure 6d. As the period varies from 600 nm down to 200 nm, the Fp of PN-T rises more rapidly than that of PN-C and PN-S, reaching the order of 104-105 in the period of 200 nm. Under TE polarization, the PN-T possesses a maximum Fp of 1.0×105, which is nearly 14 times larger than that of PN-C (7.3×103) and 18 times larger than that of PN-S (5.8×103). Note that the Fp of PN-C obtained here is comparable to the value reported in the previous work69 with the comparable size, which validates our results.


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Figure 7. The distributions of the electric field in (a) the y-z plane (x = 0) and (b) the x-y plane (1 nm below the Ag/Al2O3 interface) at the fundamental mode for PN-T under TE polarization at the normal incidence. The array period P and the edge length Lt are 200 nm and 135 nm, respectively. The black dashed lines indicate the “hot spots” with |E|>0.5|E|max. Figure 7 shows the local field distributions of the PN-T (P = 200 nm, Lt = 135 nm) under TE polarization. In the y-z plane, one can clearly identify the ultrasmall “hot spot” featured by the strong near-field confinement within a circular area with a radius of only 2 nm, being a few hundredths of the incident wavelength. As discussed above, the strong confinement along z-direction is mainly attributed to the strong out-of-plane plasmonic coupling. In the x-y plane, due to the lightning rod effect, the near-field is strongly localized in the vicinity of the sharp tip, demonstrating lateral field confinements of 7 nm and 4 nm along x- and y-directions, respectively. Such strong local behavior and small lateral resolutions are significant to probe the accurate properties of the single molecules in the vicinity for many surface-enhanced spectroscopies.22 For example, in the single molecular surface enhanced Raman scattering


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(SM-SERS), an SERS factor(~|E|4) on the order of 107 is needed to detect single molecules.70 Using the highest |Eloc|/|E0| of the PN-T, a maximum SERS factor around 2.0×109 is achieved, which is far beyond the baseline (107). Considering that it remains a challenge to match the position of a molecule with the highest |Eloc|/|E0| point in practice, an average value of the |Eloc|/|E0| over the vicinity of the hot spot will be beneficial. To estimate the average value of |Eloc|/|E0| over a specific volume, we first defined the volume to be 7×4×10 nm3 which was obtained by multiplying the surface area of the hot spot (7×4 nm2) shown in Figure 7b by the height of the spacer layer (10 nm). The average value of |Eloc|/|E0| is calculated to be 73, which corresponds to an SERS factor of 2.8×107. Obviously, the SERS factor guarantees the detection of single molecules in the volume. Additionally, the high absorption of PN-T is beneficial for suppressing the parasitic signals created by the reflected and transmitted waves, which hinder the molecule detection in the hot spots.19 Larger local field enhancements and smaller mode volumes can be expected for PN-T with thinner spacers and smaller disks.

In addition, the chemical tarnishing of Ag nanodisks, such as oxidation under ambient conditions, is an issue that has to be taken into consideration in practical applications.71-74 We investigated the effects of oxidation on the optical properties of the PN-T with the P = 200 nm which is the most likely to be oxidized due to the smallest size and the resulted highest surface energy; see details in Supporting Information, Figure S6. The results show that the absorbance of


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PN-T is robust even with a 2-nm-thick Ag2O layer, while the highest |Eloc|/|E0| displays a 13% reduction. The reduced |Eloc|/|E0| of 183 is still much larger compared with their counterparts. Moreover, several effective approaches proposed in previous works can be applied to prevent the Ag nanodisks from oxidation in practical applications.71,73-74

4. CONCLUSION

In summary, we proposed and systematically investigated high area-fill factor MIM plasmonic nanoantennas comprised of equilateral triangular Ag disks array, Al2O3 spacer and Au film. A maximum local electric field enhancement |Eloc|/|E0| of 144 is achieved in these film-coupled triangular nanodisks while maintaining a high absorbance of 98%. We attributed this huge |Eloc|/|E0| to the synergism of the large in-plane accumulation of charges at tips and the strong out-of-plane coupling between two metallic patterns. By reducing the array period to 200 nm while keeping the same area-fill factor, a maximum |Eloc|/|E0| of 211 is achieved for the PNs with triangular nanodisks, which is more than three times larger than for their circular and square counterparts. The huge |Eloc|/|E0| also gives rise to a decent Purcell factor as high as ~105, which is of great significance for a lot of applications. The oxidation effects on the performances of PNs are found to be negligible. The huge |Eloc|/|E0| and ultrasmall lateral resolution are accompanied with a near-perfect absorbance over a relatively broad band, rendering the MIM


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PNs with film-coupled triangular nanodisks quite suitable for the applications of surface-enhanced spectroscopies, Purcell-enhanced light emission, and enhanced light absorption. Moreover, in comparison with film-coupled nanospheres, such strong local behavior of film-coupled

triangular

nanodisks

offers

greater potential

for

the development

of

super-“picocavities” after considering atomic-scale features.75-76

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge.

Geometric parameters optimization, absorbance spectra in visible and NIR regions, field enhancement in isolated triangular nanodisk and bowtie-shaped nanodisk, and the oxidation effects on the performances of PNs (PDF).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Phone: 852-23587181.

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Hong Kong General Research Fund under Grant Nos. 16213015 and 16245516.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are thankful for the financial support from the Hong Kong General Research Fund under Grant Nos. 16213015 and 16245516.

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TOC Graphic:


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Figure 1. Schematic of the Ag/Al2O3/Au plasmonic nanoantennas: (a) PN-T, (b) PN-C, and (c) PN-S. The SEM images of fabricated (d) PN-T, (e) PN-C, and (f) PN-S. P = 400 nm, Lt = 269 nm, Dc = 200 nm, and Ls =177 nm. 106x66mm (300 x 300 DPI)



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Figure 2. (a) The simulated and (b) measured absorbance spectra of the PNs with nanodisks of different shapes at the normal incidence. Magnetic field distributions at the fundamental modes for PN-T under (c) TM and (d) TE polarizations, and (e) PN-C and (f) PN-S under TM polarization. The color bars indicate the field intensities. 184x412mm (300 x 300 DPI)


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Figure 3. (a) The distribution (in the y-z plane, x = 0) of the electric field at the fundamental mode for PN-T under TE polarization at the normal incidence. (b) The electric field enhancements in the y-z plane for the three kinds of PNs under TM and TE polarizations. Distributions (in the x-y plane, 1 nm below the Ag/Al2O3 interface) of the electric field at the fundamental mode for the PNs made of triangular disks under (c) TM polarization and (d) TE polarization, and those of (e) circular disks and (f) square disks under TM polarization at the normal incidence. The color bars show the field intensities. 86x50mm (300 x 300 DPI)



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Figure 4. The schematic illustrating the interaction between an Ag nanodisk and an Au film. (a) The surface charge distribution of an Ag triangular nanodisk under the plane wave with TE polarization. (b) The out-ofplane near-field plasmon coupling between an Ag triangular nanodisk and its image dipole in the Au film. The red arrows indicate the directions of the electric field. 70x32mm (300 x 300 DPI)


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Figure 5. (a) The measured absorbance spectra of PN-T with the edge length Lt =135 nm, 202 nm, 269 nm, 337 nm, and 404 nm under TM polarization at the normal incidence. The two insets are the SEM images of the two samples with Lt = 135 and 202 nm. (b) The measured absorbance spectra of the PNs in P = 200 nm at the normal incidence. (c) The measured resonant wavelength and (d) the FWHM of all the three kinds of PNs as a function of the array period P (or disk size). The area-fill factors f were 19.6% for all the samples. 119x96mm (300 x 300 DPI)



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Figure 6. The calculated (a) maximum local electric field enhancement |Eloc|/|E0|, (b) the quality factor Q, (c) the effective mode volume Veff, and (d) the Purcell factor Fp for all the PNs as a function of the disk size (or array period P). The ratio of P to the disk size is a constant for each PN due to the same area-fill factor f, therefore, here we can use P as the x-axis of these figures. 124x107mm (300 x 300 DPI)


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Figure 7. The distributions of the electric field in (a) the y-z plane (x = 0) and (b) the x-y plane (1 nm below the Ag/Al2O3 interface) at the fundamental mode for PN-T under TE polarization at the normal incidence. The array period P and edge length Lt are 200 nm and 135 nm, respectively. The black dashed lines indicate the “hot spot” with |E|>0.5|E|max. 61x28mm (600 x 600 DPI)


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