ICTON 2014 Tu.A6.4

978-1-4799-5601-2/14/$31.00 ©2014 IEEE 1

Photonic-Plasmonic Mode Engineering in Metallo-Dielectric Nanoparticle Arrays

Yan Hong, Wonmi Ahn, and Björn M. Reinhard Department of Chemistry and The Photonics Center, Boston University, Boston, MA 02215, United States

e-mail: bmr@bu.edu

ABSTRACT Nanoparticle arrays containing metallic and dielectric nanoparticles at defined locations provide unique opportunities for controlling light field distributions in both spatial and frequency domain as well as for engineering intricate phase landscapes. We will introduce in this paper novel template guided self-assembly approaches that facilitate the integration of electromagnetically strongly coupled metallic nanoparticles and dielectric (or semiconductor) nanoparticles at separate, pre-defined locations. The assembly approach facilitates the creation of sub-5 nm gaps between plasmonic nanoparticles and, at the same time, allows the realization of tens to hundreds of nanometer separations between metallic and dielectric components. The electromagnetic working principles of the resulting optoplasmonic hetero-nanoparticle arrays and potential application fields of these multiscaled electromagnetic materials will be discussed. Keywords: plasmonics, nanoparticles, metasurface, hybrid material, nanophotonics. self-assembly.

1. INTRODUCTION

Discrete optoplasmonic molecules [1, 2] contain a well-defined number of metallic and dielectric nanoparticles (NPs) arranged in a defined geometry. This design approach has been shown to generate new electromagnetic materials with promise for overcoming some of the intrinsic limitations of the individual building blocks [1, 3, 4]. In this paper we will explore a new concept of an optoplasmonic material in which metallic and dielectric building blocks are integrated into a lateral array at pre-defined locations [5]. The idea of our design approach is to position metal nanoparticle clusters as nanolenses at location of enhanced E-field intensity in a 2-dimensional self-assembled grating structure to generate a cascaded electromagnetic E-field enhancement [6-8].

At ICTON 2013 we reported about a versatile template guided self-assembly approach for integrating metallic and dielectric NPs into clusters with subdiffraction limit dimensions based on a versatile template guided self-assembly approach [4]. In this paper we will report about an advancement of this approach that makes it possible to position dielectric nanoparticles and metallic nanoparticle clusters at separate, pre-defined locations in an array. We will characterize the morphology-dependent response of the resulting hybrid materials through electromagnetic simulations as well as through elastic and inelastic scattering spectroscopy. The fabrication approaches introduced in this paper are important since they provide new opportunities for generating metasurfaces and 2-D photonic-plasmonic crystals with new properties by controlling the lateral composition of spatially extended electromagnetic structures in a controlled fashion.

Figure 1: Template guided self-assembly of optoplasmonic arrays. a) A mask consisting of regular arrays of wells is created via electron beam lithography (EBL). The mask creates binding sites with diameters D2 > D1 on the quartz substrate. b) The center-to-center separation of the wells, Λ, as well as the morphology of the array is precisely controlled through EBL. c) TiO2 NPs are bound to the D2 binding sites, resulting d) in an array of TiO2 NPs. e) Smaller Au NPs are then assembled onto the D1 binding sites. f) The complete optoplasmonic array is obtained after lift-off of the mask. Reprinted with permission from Hong et al., Adv. Funct. Mat. 24, 739 (2014). Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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2. RESULTS AND DISCUSSION

Our fabrication strategy is shown in Figure 1 [5]. A mask with two different assembly sites is created in a 300 nm thin poly(methyl methacrylate) (PMMA) film spincoated on top of a quartz substrate. The created binding sites have two different diameters D2 < D1. This patterned mask is then incubated with TiO2 NPs with a diameter d1, where D2 < d1 < D1. Due to geometric constraints, the TiO2 NPs can, consequently, only bind to the assembly sites of diameter D1. The TiO2 NPs are subsequently removed and smaller noble metal NPs with diameter d2, where d2 < D2, are bound to the vacant binding sites with diameter D2.

Figure 2 summarizes representative scanning electron microscopy (SEM) micrographs of the resulting arrays. The images confirm that the template guided assembly approach facilitates the preferential localization of the two different types of NPs at separate pre-defined locations in the array. We emphasize that the template guided self-assembly approach does not only provide control over nanoparticle separations by choice of the location of the different binding sites, but also provides some means to control separations between NPs on much shorter length scales within the individual binding sites. We have demonstrated recently that the average separation between NPs within individual NP clusters can be controlled on nanometer length scales through choice of the ligands on the NP surface and the assembly conditions [9].

We have investigated the spectral response of assembled hybrid arrays containing Au NP clusters and TiO2 NPs under white light excitation in a dark field geometry. The obtained spectra of the fabricated arrays (TiO2 only, Au NPs only, heteronanoparticle array), all show geometry-dependent features that can be assigned to radiating diffraction modes. The observation of these tunable modes confirms a high level of structural regularity of the self-assembled arrays.

The hetero-nanoparticle arrays allow for an interesting synergistic interaction between delocalized modes in the array and the strongly localized modes in the junctions and crevices of the nanoparticle clusters. This is illustrated in Figure 3. The simulated E-field intensity enhancement spectra for TiO2 NP arrays show resonances associated with delocalized array modes (Figure 3a). The resonance wavelengths of these collective modes depend on the array morphology and can be systematically tuned through choice of the interparticle separation, Λ. The resonance red-shifts with increasing Λ. Figure 3a also contains the E-field intensity enhancement spectrum for an individual Au NP cluster. From the simulations, it is immediately apparent that the collective resonances of the array can be tuned to overlap with the plasmon resonance. In Figure 3b we show the E-field intensity enhancement spectra for an array of Au NP trimer clusters. The spectra are dominated by the cluster resonance and fairly weak changes are observed as function of Λ. The additional enhancement of the cluster plasmon resonance through array modes (at Λ = 800 nm) is weak. This changes in the hybrid array in Figure 3c. Here, the resonant overlap of the array mode and the plasmon resonance in the cluster results in a strong enhancement of the peak E-field

Figure 2: SEM images of self-assembled opto-plasmonic arrays. Reprinted with permission from Hong et al., Adv. Funct. Mat. 24, 739 (2014). Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 3: Simulated E-field intensity enhancement spectra at the hottest electromagnetic spot in a) an array of 20×20 TiO2 NPs, b) an array of 20×20 Au NP clusters, and c) a combined optoplasmonic array for different grating periods, Λ. Reprinted with permission from Hong et al., Adv. Funct. Mat. 24, 739 (2014). Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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intensity located in the noble metal NP clusters. The comparison of the different arrays in Figure 3 illustrate the advantages for boosting peak E-field intensities that results from a combination of metallic and dielectric NPs into one array. Interestingly, maps of the E-field intensity enhancement reveal that the integration of dielectric NPs into the plasmonic NP cluster array does not only boost the E-field intensity in and around the plasmonic metal NP clusters but also (albeit to a lesser extent) at locations away from the metal [5]. New concepts of engineering homogeneous field distribution in optoplasmonic array will be discussed.

3. CONCLUSIONS

Template guided self-assembly strategies make it possible to integrate dielectric nanoparticles and metallic nanoparticles into hybrid arrays. Electromagnetic interactions between the different building blocks can be controlled through the composition of the materials as well as the morphology of the array. This tunability creates entirely new opportunities for controlling light localization in spatial an frequency domains as well as for controlling the phase landscape in rationally designed metallo-dielectric heteronanopartice arrays and metasurfaces.

REFERENCES

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[3] W. Ahn, Y. Hong, S.V. Boriskina, and B.M. Reinhard: Demonstration of efficient on-chip photon transfer in self-assembled optoplasmonic networks, ACS Nano 7, 4470-4478 (2013).

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[5] Y. Hong, Y. Qiu, T. Chen, and B.M. Reinhard: Rational assembly of optoplasmonic hetero-nanoparticle arrays with tunable photonic-plasmonic resonances, Adv. Funct. Mater. 24, 739-746 (2014).

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[8] L. Yang et al.: Engineering nanoparticle cluster arrays for bacterial biosensing: The role of the building block in multiscale SERS substrates, Advanced Functional Materials 20, 2619-2628 (2010).

[9] T. Chen, M. Pourmand, A. Feizpour, B. Cushman, and B.M. Reinhard: Tailoring plasmon coupling in self-assembled one-dimensional Au Nanoparticle chains through simultaneous control of size and gap separation, J. Phys. Chem. Lett. 4, 2147-2152 (2013).