Nanowerk Spotlight

SPIE Photonics West 2012

LASE Conference

21 - 26 January 2012, San Francisco, California, USA

NNIN/C Conference - ENCON1
Synergy Between Experiment and Computation in Energy:

Looking to 2030
January 11-13, 2012 - Harvard University

We introduce a new design approach for surface-enhanced Raman spectroscopy (SERS) substrates that is based on molding the optical powerflow through a sequence of coupled nanoscale optical vortices “pinned” to rationally designed plasmonic nanostructures, referred to as Vortex Nanogear Transmissions (VNTs). We fabricated VNTs composed of Au nanodiscs by electron beam lithography on quartz substrates and characterized their near- and far-field responses through combination of computational electromagnetism, and elastic and inelastic scattering spectroscopy. Pronounced dips in the far-field scattering spectra of VNTs provide experimental evidence for an efficient light trapping and circulation within the nanostructures. Furthermore, we demonstrate that VNT integration into periodic arrays of Au nanoparticles facilitates the generation of high E-field enhancements in the VNTs at multiple defined wavelengths. We show that spectrum shaping in nested VNT structures is achieved through an electromagnetic feed-mechanism driven by the coherent multiple scattering in the plasmonic arrays and that this process can be rationally controlled by tuning the array period. The ability to generate high E-field enhancements at predefined locations and frequencies makes nested VNTs interesting substrates for challenging SERS applications.

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.

Efficient delivery of light into nanoscale volumes by converting free photons into localized charge density oscillations (surface plasmons) enables technological innovation in various fields from biosensing to photovoltaics and quantum computing. Conventional plasmonic nanostructures are designed as nanoscale analogs of radioantennas and waveguides. Here, we discuss an alternative approach for plasmonic nanocircuit engineering that is based on molding the optical powerflow through ‘vortex nanogears’ around a landscape of local phase singularities ‘pinned’ to plasmonic nanostructures. We show that coupling of several vortex nanogears into transmission-like structures results in dramatic optical effects, which can be explained by invoking a hydrodynamic analogy of the ‘photon fluid’. The new concept of vortex nanogear transmissions (VNTs) provides new design principles for the development of complex multi-functional phase-operated photonics machinery and, therefore, generates unique opportunities for light generation, harvesting and processing on the nanoscale.

Nanoparticle cluster arrays (NCAs) are a class of electromagnetic materials that comprise chemically defined nanoparticles assembled into clusters of defined size in an extended deterministic arrangement. NCAs are fabricated through integration of chemically synthesized building blocks into predefined patterns using a hybrid top-down/bottom-up fabrication approach that overcomes some of the limitations of conventional top-down fabrication methods with regard to minimum available feature size and structural complexity. NCAs can sustain near-field interactions between nanoparticles within individual clusters as well as between entire neighboring clusters. The availability of near-field interactions on multiple length scales, together with the ability to further enhance the coupled plasmon modes through photonic modes in carefully designed array morphologies, leads to a multiscale cascade electromagnetic field enhancement throughout the array. This feature article introduces the design and fabrication fundamentals of NCAs and characterizes the electromagnetic coupling mechanisms in the arrays. Furthermore, it reviews how the optical properties of NCAs can be tuned through the size and shape of the nanoparticle building blocks and the geometry, size, and separation of the assembled clusters. NCAs have potential applications in many different areas; this feature article focuses on plasmon enhanced biosensing and surface enhanced Raman spectroscopy, in particular.

 Focus Issue of Optics Express on Collective Phenomena in Photonic, Plasmonic and Hybrid Structures that was published online on Mon Oct 24, 2011.

The combination of optical, electronic and mechanical effects occurring in devices and materials that have structure on the nanometer scale are being investigated by researchers around the world. These "collective phenomena" have applications as diverse as the generation of light, optical sensing, and information processing. To highlight the recent progress and trends in physics and applications in this area, the editors of Optics Express published a special focus issue on "Collective Phenomena in Photonic, Plasmonic and Hybrid Structures."

The preface to the Focus Issue that gives a brief introduction to all the invited papers is available here.

Guest Editors:

Svetlana V. Boriskina, Boston University, USA

Michelle Povinelli, University of Southern California, USA

Vasily N. Astratov, University of North Carolina at Charlotte, USA

Anatoly Zayats, King's College London, UK

Viktor A. Podolskiy, University of Massachusetts Lowell, USA

A major challenge for plasmonics as an enabling technology for quantum information processing is the realization of active spatio-temporal control of light on the nanoscale. The use of phase-shaped pulses or beams enforces specific requirements for on-chip integration and imposes strict design limitations. We introduce here an alternative approach, which is based on exploiting the strong sub-wavelength spatial phase modulation in the near-field of resonantly-excited high-Q optical microcavities integrated into plasmonic nanocircuits. Our theoretical analysis reveals the formation of areas of circulating powerflow (optical vortices) in the near-fields of optical microcavities, whose positions and mutual coupling can be controlled by tuning the microcavities parameters and the excitation wavelength. We show that optical powerflow though nanoscale plasmonic structures can be dynamically molded by engineering interactions of microcavity-induced optical vortices with noble-metal nanoparticles. The proposed strategy of re-configuring plasmonic nanocircuits via locally-addressable photonic elements opens the way to develop chip-integrated optoplasmonic switching architectures, which is crucial for implementation of quantum information nanocircuits.

We propose and demonstrate a novel photonic-plasmonic antenna capable of confining electromagnetic radiation at several mid-infrared wavelengths to a single sub-wavelength spot. The structure relies on the coupling between the localized surface plasmon resonance of a bow-tie nanoantenna with the photonic modes of surrounding multi-periodic particle arrays. Far-field measurements of the transmission through the central bow-tie demonstrate the presence of Fano-like interference effects resulting from the interaction of the bow-tie antenna with the surrounding nanoparticle arrays. The near-field of the multi-wavelength antenna is imaged using an aperture-less near-field scanning optical microscope. This antenna is relevant for the development of near-field probes for nanoimaging, spectroscopy and biosensing.

Research on hybrid optoplasmonic sensors and quantum-optical network elements highlighted in the August 2011 issue of NanoTimes magazine

IEEE Photonics 2011 Conference (IPC11)

(formally Photonics Society Annual Meeting)

Arlington, Virginia | 9 - 13 October 2011

We demonstrated a biosensing approach which, for the first time, combines the high sensitivity of whispering gallery modes (WGMs) with a metallic nanoparticle-based assay. We provided a computational model based on generalized Mie theory to explain the higher sensitivity of protein detection. We quantitatively analyzed the binding of a model protein (i.e., Bovine Serum Albumin) to gold nanoparticles from high-Q WGM resonance frequency shifts, and fit the results to an adsorption isotherm, which agrees with the theoretical predictions of a two-component adsorption model.

Filopodia have been hypothesized to act as remote sensors of the cell environment, but many details of the sensor function remain unclear. We investigated the distribution of the epidermal growth factor (EGF) receptor (EGFR) density on filopodia and on the dorsal cell membrane of A431 human epidermoid carcinoma cells using a nanoplasmonic enabled imaging tool. We targeted cell surface EGFR with 40 nm diameter Au nanoparticles (NPs) using a high affinity multivalent labeling strategy and determined relative NP binding affinities spatially resolved through plasmon coupling. Distance-dependent near-field interactions between the labels generated a NP density (ρ)-dependent spectral response that facilitated a spatial mapping of the EGFR density distribution on subcellular length scales in an optical microscope in solution. The measured ρ values were significantly higher on filopodia than on the cellular surface, which is indicative of an enrichment of EGFR on filopodia. A detailed characterization of the spatial distribution of the NP immunolabels through scanning electron microscopy (SEM) confirmed the findings of the all-optical plasmon coupling studies and provided additional structural details. The NPs exhibited a preferential association with the sides of the filopodia. We calibrated the ρ-dependent spectral response of the Au immunolabels through correlation of optical spectroscopy and SEM. The experimental dependence of the measured plasmon resonance wavelength (λ_res) of the interacting immunolabels on ρ was well described by the fit λ_res = 595.0 nm – 46.36 nm exp(−ρ/51.48) for ρ ≤ 476 NPs/μm². The performed correlated spectroscopic/SEM studies pave the way toward quantitative immunolabeling studies of EGFR and other important cell surface receptors in an optical microscope.

This review focuses on the optical properties and device applications of deterministic aperiodic media generated by mathematical rules with spectral features that interpolate in a tunable fashion between periodic crystals and disordered random media. These structures are called Deterministic Aperiodic Nano Structures (DANS) and can be implemented in different materials (linear and nonlinear) and physical systems as diverse as dielectric multilayers, optical gratings, photonic waveguides and nanoparticle arrays. Among their distinctive optical properties are the formation of multi-fractal bandgaps and characteristic optical resonances, called critical modes, with unusual localization, scaling and transport properties. The goal of the paper is to provide a detailed review of the conceptual foundation and the physical mechanisms governing the complex optical response of DANS in relation to the engineering of novel devices and functionalities. The discussion will mostly focus on passive and active planar structures with enhanced light-matter coupling for photonics and plasmonics technologies.

We demonstrate lasing in two-dimensional Thue–Morse structures fabricated in a semiconductor membrane. By changing the relative size of two scatterers that correspond to the building blocks A and B, we gradually vary structural aperiodicity and find an optimal degree of aperiodicity where light confinement is maximal and lasing is the strongest. At various degrees of aperiodicity, different types of modes acquire the highest quality factors and may be selected for lasing. This work opens a way of controlling lasing characteristic via structural aperiodicity.

Special session on Photonic Atoms & Molecules at ICTON 2011
Stockholm, Sweden, June 26 - 30, 2011

Special Session on Photonic Atoms & Molecules will be traditionally held as a part of the International Conference on Transparent Optical Networks (ICTON’11), this year, in Stockholm, Sweden.

Optimizing Gold Nanoparticle Cluster Configurations (n ≤ 7) for Array Applications

Bo Yan, Svetlana V. Boriskina, and Bjoern M. Reinhard

J. Phys. Chem. C, 2011, 115 (11), pp 4578–4583

Nanoparticle cluster arrays (NCAs) are novel electromagnetic materials whose properties depend on the size and shape of the constituent nanoparticle clusters. A rational design of NCAs with defined optical properties requires a thorough understanding of the geometry-dependent optical response of the building blocks. Herein, we systematically investigate the near- and far-field responses of clusters of closely packed 60 nm gold nanoparticles (n ≤ 7) as a function of size and cluster geometry through a combination of experimental spectroscopy and generalized Mie theory calculations. From all of the investigated cluster configurations, nanoparticle trimers with D_3h geometry and heptamers in D_6h geometry stand out due to their polarization-insensitive responses and high electric (E) field intensity enhancements, making them building blocks of choice in this size range. The near-field intensity maximum of the D_6h heptamer is red-shifted with regard to the D_3h trimer by 125 nm, which confirms the possibility of a rational tuning of the near-field response in NCAs through the choice of the constituent nanoparticle clusters. For the nanoparticle trimer we investigate the influence of the cluster geometry on the optical response in detail and map near- and far-field spectra associated with the transition of the cluster configuration from D_3h into D_∞h.

Spectroscopic Ultra-Trace Detection of Nitroaromatic Gas Vapor on Rationally Designed Two-Dimensional Nanoparticle Cluster Arrays

Jing Wang, Linglu Yang, Svetlana V. Boriskina, Bo Yan, and Bjoern M. Reinhard

Anal. Chem., 2011, 83 (6), pp 2243–2249

Nanoparticle cluster arrays (NCAs) are engineered two-dimensional plasmonic arrays that provide high signal enhancements for critical sensing applications using surface enhanced Raman spectroscopy (SERS). In this work we demonstrate that rationally designed NCAs are capable of detecting ultra-traces of 2,4-dinitrotoluene (DNT) vapor. NCAs functionalized with a thin film of an aqueous NaOH solution facilitated the detection of DNT vapor at a concentration of at least 10 ppt, even in the presence of an excess of potential interferents, including Diesel fuel, fertilizers, and pesticides. Both in the presence and in the absence of this complex background the SERS signal intensity of the NO₂ stretching mode showed a continuous, concentration dependent response over the entire monitored concentration range (10 ppt−100 ppb). The small size, superb sensitivity, and selectivity, as well as the fast response time of <5 min, make NCAs a valuable photonic sensor platform for ultra-trace nitroaromatic gas vapor detection with potential applications in landmine removal and homeland security.

Spectrally and Spatially Configurable Superlenses for Optoplasmonic Nanocircuits

S.V. Boriskina and B.M. Reinhard

Proc. Natl. Acad. Sci. USA, 2011, vol. 108, no. 8, pp. 3147-3151.

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