Composite au nanostructures for fluorescence studies in visible light

Ordered Array of Metal Particles on Semishell Separated with Ultrathin Oxide: Fabrication and SERS Properties

Metal particles in gap cavities provide an interesting system to achieve hybrid local surface plasmon modes for local field enhancement. Here, we demonstrate a relatively simple method to fabricate Ag nanoparticles positioned on Ag semishells separated by a thin (~5 nm) dielectric layer. The obtained structure can provide strong local electric field enhancement for surface-enhanced Raman scattering (SERS). The fabrication of the ordered array structure was realized by nanosphere self-assembly, atomic layer deposition, and metal thin-film dewetting. Numerical simulation proved that, compared to the conventional metal semishell arrays, the additional Ag particles introduce extra hot spots particularly in the valley regions between adjacent Ag semishells. As a result, the SERS enhancement factor of the metal semishell-based plasmonic structure could be further improved by an order of magnitude. The developed novel plasmonic structure also shows good potential for application in plasmon-enhanced solar water-splitting devices.

Plasmonic nanostructures for surface enhanced spectroscopic methods

The development within the last five years in the field of surface enhanced spectroscopy methods was comprehensively reviewed.

An enzyme-responsive metal-enhanced near-infrared fluorescence sensor based on functionalized gold nanoparticles

Near-infrared (NIR) fluorescence imaging is promising due to the high penetration depths and minimal levels of autofluorescence in living systems. However, it suffers from low fluorescent quantum yield, and metal-enhanced fluorescence (MEF) is considered to be a promising technique to overcome this. Stimuli-responsive NIR fluorescence enhancement shows remarkable potential for applications in medical imaging and diagnosis. Herein, we successfully fabricated an enzyme-responsive near-infrared sensor based on MEF by functionalizing gold nanoparticles with NIR fluorophores and enzyme-responsive self-aggregation moieties. The NIR fluorescence of fluorophores on the gold nanoparticles was significantly enhanced due to increases both in the light scattering intensity and in the radiative decay rate (kr) of the NIR fluorophores, along with relatively small variation in the nonradiative decay rate. This novel strategy for NIR fluorescent sensors should be particularly promising for NIR fluorescence imaging of enzyme activities and early diagnosis based on rationally designed nanomaterials.

Towards integrated nanoplasmonic logic circuitry

Surface plasmon polaritons (SPPs) may serve as ultimate data processing expedients in future nanophotonic applications. SPPs combine the high localization of electrons with the bandwidth, frequency and propagation properties of photons, thus supplying nature with the best of two worlds. However, although plasmonics have recently gained constantly growing scientific attention, logic devices that operate on SPPs on a deep nanometer scale are yet to be demonstrated. Here, we design, fabricate and experimentally verify the smallest, first ever reported all optical nanoplasmonic XOR logic gate. The introduced XOR device is based on a novel engineerable interferometry scheme with extremely compact dimensions of λ(3)/15,500, which can be used to realize a variety of plasmonic logic functionalities. We use frequency modulated Kelvin probe microscopy to provide evidence of binary XOR functionality performed directly on SPPs with λ(3)/80,000 mode volumes. An extinction ratio of 10 dB is achieved for a device length of 150 nm, increasing up to 30 dB for a device length of 280 nm. Our findings confirm plasmonics as the favorite data carriers in integrated all optical logic devices operating on the deep nanoscale, and pave the way to the development of future ultrafast information processing technologies based on SPPs.

Highly ordered arrays of particle-in-bowl plasmonic nanostructures for surface-enhanced raman scattering

A highly ordered particle-in-bowl (PIB) nanostructure array is designed and fabricated to achieve large field enhancement for the surface-enhanced Raman scattering (SERS) application. This new type of PIB structure is composed of an Ag particle located at the bottom of an Au bowl, and the two are separated by a precisely controlled nanoscale dielectric layer. The fabrication of the PIB structure is based on the self-assembly of polystyrene spheres and atomic layer deposition (ALD), which allows good control of the metal particle size and gap distance, as well as large-scale ordering. Numerical simulation reveals a high enhancement of the local field at the nanogaps. The SERS performance of the PIB arrays, and the effects of the Ag particle size and the ALD dielectric layer thickness are characterized, results of which are in reasonable agreement with simulation. With Rhodmaine 6G as the probe molecule, the spatially averaged SERS enhancement factor is on the order of 3.8 × 10(7) and the local field enhancement from simulation can be up to 10(8) .

Surface-enhanced emission from single semiconductor nanoribbons

Emission enhancement from single semiconductor CdSe nanoribbons by introduction of surface plasmon polaritons (SPPs) via Au contacts is studied. Scanning confocal microscopy is employed to investigate the emission enhancement behavior via photoluminescence measurements. Large enhancement factors of 77-130 at a peak emission of CdSe of ∼710 nm are obtained, which are ascribed to the gain-assisted propagation of the short-range mode of SPPs. Our findings open the exciting possibilities for high-efficiency SPP-enhanced light-emitting devices based on luminous bodies with finite lateral dimensions.

Enhancing single molecule imaging in optofluidics and microfluidics

Microfluidics and optofluidics have revolutionized high-throughput analysis and chemical synthesis over the past decade. Single molecule imaging has witnessed similar growth, due to its capacity to reveal heterogeneities at high spatial and temporal resolutions. However, both resolution types are dependent on the signal to noise ratio (SNR) of the image. In this paper, we review how the SNR can be enhanced in optofluidics and microfluidics. Starting with optofluidics, we outline integrated photonic structures that increase the signal emitted by single chromophores and minimize the excitation volume. Turning then to microfluidics, we review the compatible functionalization strategies that reduce noise stemming from non-specific interactions and architectures that minimize bleaching and blinking.

Cascaded optical field enhancement in composite plasmonic nanostructures

We present composite plasmonic nanostructures designed to achieve cascaded enhancement of electromagnetic fields at optical frequencies. Our structures were made with the help of electron-beam lithography and comprise a set of metallic nanodisks placed one above another. The optical properties of reproducible arrays of these structures were studied by using scanning confocal Raman spectroscopy. We show that our composite nanostructures robustly demonstrate dramatic enhancement of the Raman signals when compared to those measured from constituent elements.

Fluorescence enhancement by a two-dimensional dielectric annular Bragg resonant cavity

We show that photons can be efficiently extracted from fluorescent molecules, utilizing the strongly enhanced local field of a two-dimensional dielectric annular Bragg resonant cavity. Due to the diffraction and constructive interference together with the annular focusing, the periodic ring structure converts the normal incident light into planar guided modes and forms a hot spot at the center of the structure. Theoretically, the field can be enhanced more than 40 times, which leads to the averaged 20-fold enhancement of the fluorescence signal observed in experiments. Compared with fluorescence enhancement by plasmonic structures, this dielectric approach does not suffer from pronounced quenching that often occurs near metallic structures. These results not only can be applied as ultrasensitive sensors for various biological systems, but also have broad potential applications, such as optical trapping and fluorescent microscopy.

Plasmonics: an emerging field fostered by Nano Letters

While studies of surface plasmons on metals have been pursued for decades, the more recent appearance of nanoscience has created a revolution in this field with "Plasmonics" emerging as a major area of research. The direct optical excitation of surface plasmons on metallic nanostructures provides numerous ways to control and manipulate light at nanoscale dimensions. This has stimulated the development of novel optical materials, deeper theoretical insight, innovative new devices, and applications with potential for significant technological and societal impact. Nano Letters has been instrumental in the emergence of plasmonics, providing its readership with rapid advances in this dynamic field.