Wonmi Ahn

Photonic-plasmonic mode coupling in on-chip integrated optoplasmonic molecules.

We investigate photonic-plasmonic mode coupling in a new class of optoplasmonic materials that comprise dielectric microspheres and noble metal nanostructures in a morphologically well-defined on-chip platform. Discrete networks of optoplasmonic elements, referred to as optoplasmonic molecules, were generated through a combination of top-down fabrication and template-guided self-assembly. This approach facilitated a precise and controllable vertical and horizontal positioning of the plasmonic elements relative to the whispering gallery mode (WGM) microspheres. The plasmonic nanostructures were positioned in or close to the equatorial plane of the dielectric microspheres where the fields associated with the plasmonic modes can synergistically interact with the evanescent fields of the WGMs. We characterized the far-field scattering spectra of discrete optoplasmonic molecules that comprised two coupled 2.048 μm diameter polystyrene microspheres each encircled by four 148 nm diameter Au nanoparticles (NPs), through far-field scattering spectroscopy. We observed a broadening of the TE and TM modes in the scattering spectra of the optoplasmonic dimers indicative of an efficient photonic-plasmonic mode coupling between the coupled photonic modes of the WGM resonators and the localized surface plasmon modes of the NPs. Our experimental findings are supported by generalized multiple particle Mie theory simulations, which provide additional information about the spatial distributions of the near fields associated with the photonic-plasmonic hybrid modes in the investigated optoplasmonic molecules. The simulations reveal partial localization of the spectrally sharp hybrid modes outside of the WGM microspheres on the Au NPs where the local E-field intensity is enhanced by approximately 2 orders of magnitude over that of an individual Au NP.

Electroless Gold Island Thin Films: Photoluminescence and Thermal Transformation to Nanoparticle Ensembles

Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 +/- 4.0 to 266 +/- 22 nm at densities from 2.6 x 1011 to 4.3 x 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.

Demonstration of Efficient On-Chip Photon Transfer in Self-Assembled Optoplasmonic Networks

Plasmonic nanoantennas facilitate the manipulation of light fields on deeply sub-diffraction-limited length scales, but high dissipative losses in metals make new approaches for an efficient energy transfer in extended on-chip integrated plasmonic circuits mandatory. We demonstrate in this article efficient photon transfer in discrete optoplasmonic molecules comprising gold nanoparticle (NP) dimer antennas located in the evanescent field of a 2 μm diameter polystyrene bead, which served as an optical microcavity (OM). The optoplasmonic molecules were generated through a guided self-assembly strategy in which the OMs were immobilized in binding sites generated by quartz (SiO2) or silicon posts that contained plasmonic nanoantennas on their tips. Control of the post height facilitated an accurate positioning of the plasmonic antennas into the evanescent field of the whispering gallery modes located in the equatorial plane of the OM. Cy3 and Cy5.5 dyes were tethered to the plasmonic antennas through oligonucleotide spacers to act as on-chip light sources. The intensity of Cy3 was found to be increased relative to that of Cy5.5 in the vicinity of the plasmonic antennas where strongly enhanced electric field intensity and optical density of states selectively increase the excitation and emission rates of Cy3 due to spectral overlap with the plasmon. The fluorescent dyes preferentially emitted into the OM, which efficiently trapped and recirculated the photons. We experimentally determined a relative photon transfer efficiency of 44% in non-optimized self-assembled optoplasmonic molecules in this proof-of-principle study.

Periodic nanotemplating by selective deposition of electroless gold island films on particle-lithographed dimethyldichlorosilane layers.

Uniform hexagonal arrays of diverse nanotemplated metal structures were formed via selective electroless gold plating on particle-lithographed dimethyldichlorosilane layers. Surface-associated water at silica bead interstices was shown to correlate with the formation of silane rings with outer ring diameters ranging from 522.5+/-29.7 to 1116.9+/-52.6 nm and/or spherical gold nanoparticles with diameters from 145.5+/-20.2 to 389.1+/-51.1 nm in the array. Reproducibility and millimeter-size scalability of the array were achieved without the need for expensive and sophisticated lithography or metal deposition equipment. The formation of each structure was explained on the basis of the silanization mechanism and microscopic characterization, as well as dimensional analysis of the nanostructures. This new, facile, and versatile method enables fine fabrication of regular metal nanoparticle array platforms to improve optical and plasmonic features in nanoelectronics and nanophotonic devices.

Enhanced uniformity in arrays of electroless plated spherical gold nanoparticles using tin presensitization.

Gold nanoparticle arrays created with electroless gold plating provide a unique means of transforming nanocylinders usually formed in electron beam lithography to spherical nanoparticles. Alone, electroless gold plating is not selective to the substrate and results in the formation of a gold film on all exposed surfaces of an electron beam patterned sample, including the electron resist. Undesired gold plating occurred near patterned features on the substrate surface, which was reduced by increasing post-spin-coat cure time. When the electron resist is removed, some nanocylinders break off with the gold film, leaving partial cylinders or holes in the patterned elements. By presensitizing the substrate surface with tin, gold cylinders may be selectively deposited to the substrate surface without forming a film on the electron resist. Tin presensitized arrays were produced with 47.1 +/- 7.4 nm radius gold nanoparticles with an interparticle distance of 646.0 +/- 12.4 nm. Defects from sheared, missing, and redeposited Au particles associated with the resist removal were minimized, resulting in enhanced size and shape uniformity of pillars and arrays. Hollow particles were eliminated, and relative standard deviation in particle size was reduced by 7.4% on average, while elongation was reduced 12.3% when astigmatism was eliminated.

Directed Assembly of Optoplasmonic Hybrid Materials with Tunable Photonic-Plasmonic Properties.

Optoplasmonic materials are metallo-dielectric hybrid structures that combine metallic and dielectric components in defined geometries in which plasmonic and photonic modes synergistically interact. These beneficial interactions can be harnessed by integrating plasmonic nanoantennas into defined photonic environments generated, for instance, by discrete optical resonators or extended systems of diffractively coupled nanoparticles. Optoplasmonic structures facilitate photonic-plasmonic mode coupling and offer degrees of freedom for creating optical fields with predefined amplitude and phase in space and time that are absent in conventional photonic or plasmonic structures. This Perspective reviews the fundamental electromagnetic mechanisms underlying selected optoplasmonic approaches with an emphasis on materials available through template-guided self-assembly strategies.

Template-guided self-assembly of discrete optoplasmonic molecules and extended optoplasmonic arrays

Abstract The integration of metallic and dielectric building blocks into optoplasmonic structures creates new electromagnetic systems in which plasmonic and photonic modes can interact in the near-, intermediate- and farfield. The morphology-dependent electromagnetic coupling between the different building blocks in these hybrid structures provides a multitude of opportunities for controlling electromagnetic fields in both spatial and frequency domain as well as for engineering the phase landscape and the local density of optical states. Control over any of these properties requires, however, rational fabrication approaches for well-defined metal-dielectric hybrid structures. Template-guided self-assembly is a versatile fabrication method capable of integrating metallic and dielectric components into discrete optoplasmonic structures, arrays, or metasurfaces. The structural flexibility provided by the approach is illustrated by two representative implementations of optoplasmonic materials discussed in this review. In optoplasmonic atoms or molecules optical microcavities (OMs) serve as whispering gallery mode resonators that provide a discrete photonic mode spectrum to interact with plasmonic nanostructures contained in the evanescent fields of the OMs. In extended hetero-nanoparticle arrays in-plane scattered light induces geometry-dependent photonic resonances that mix with the localized surface plasmon resonances of the metal nanoparticles.We characterize the fundamental electromagnetic working principles underlying both optoplasmonic approaches and review the fabrication strategies implemented to realize them.

Generation of scalable quasi-3D metallo-dielectric SERS substrates through orthogonal reactive ion etching

We combined two orthogonal reactive ion etching strategies on monolayers of 4 μm diameter polystyrene (PS) microspheres assembled on a planar glass substrate to create geometrically diverse but highly regular quasi-3D nano-structured arrays with centimeter length scales. Reactive ion etching (RIE) with O2/CF4 was used to selectively etch PS and, thus, to adjust the morphology of the PS mask, and CHF3 RIE was then used to etch the SiO2 substrate. The dynamic combination of these two etching procedures facilitated the realization of a wide variety of 3D-corrugated surface morphologies, including pedestals, bowls, honeycombs and blossom bud like arrays. The generated structures were then evaporated with Au films of defined thicknesses to generate metallo-dielectric arrays with tunable surface roughness. Hexagonal diffraction patterns from the fabricated structures confirm the successful realization of extended periodic hexagonal structures and the optical transmission spectra showed an efficient trapping of incident light in these ultra-rough metallo-dielectric arrays. The metallo-dielectric substrates were finally optimized for the detection of the pesticide methyl parathion in a concentration as low as 1 × 10−10 M through surface enhanced Raman spectroscopy (SERS).

Fabrication of regular arrays of gold nanospheres by thermal transformation of electroless-plated films

Rectangular lattices of gold nanospheres have been fabricated by thermally annealing Au nanopillars and nanocylinders deposited via electroless plating onto indium-tin-oxide glass substrates in a novel method. The substrates were patterned using e-beam lithography, and particle size and shape were controlled by adjusting the thickness of the poly(methylmethacrylate) mask, e-beam power, and electroless plating parameters. Nanostructures produced by this electroless plating method exhibited greater coalescence than sputtered gold films. Attachment of electroless-plated structures to indium-tin-oxide substrates was stable to stringent thermal, solvent, and electromagnetic exposures. This facile and versatile method is applicable to the fabrication of regular metal nanoparticle array platforms for improved optical and plasmonic features in sensing and imaging devices.

Low-power light guiding and localization in optoplasmonic chains obtained by directed self-assembly

Optoplasmonic structures contain plasmonic components embedded in a defined photonic environment to create synergistic interactions between photonic and plasmonic components. Here, we show that chains of optical microspheres containing gold nanoparticles in their evanescent field combine the light guiding properties of a microsphere chain with the light localizing properties of a plasmonic nanoantenna. We implement these materials through template guided self-assembly and investigate their fundamental electromagnetic working principles through combination of electromagnetic simulations and experimental characterization. We demonstrate that optoplasmonic chains implemented by directed self-assembly achieve a significant reduction in guiding losses when compared with conventional plasmonic waveguides and, at the same time, retain the light localizing properties of plasmonic antennas at pre-defined locations. The results reinforce the potential of optoplasmonic structures for realizing low-loss optical interconnects with high bandwidth.