Polarization-selective excitation of plasmonic resonances in silver nanocube random arrays by optical fiber cladding mode evanescent fields

Near-field scanning optical microscopy was used in collection mode to examine the optical field distribution on the surface of tilted fiber Bragg gratings (TFBGs) coated with a layer of randomly spaced silver nanocubes. The nanocubes disturb the periodic pattern of the near field visible light distribution arising from counterpropagating cladding modes excited by the TFBG. Spots with more than two orders of magnitude enhancement of the near field light intensity were observed around the nanocubes, as well as an average enhancement over the whole surface of about an order of magnitude relative to uncoated fibers. The near field speckle pattern associated with nanocubes showed a 180-degree periodicity with respect to the linear polarization of the input excitation light launched in the fiber core. The observed phenomena are explained in terms of the plasmonic properties of silver nanocubes. The enhancement factors measured here explain previously observed improvements in the performance of metal nanoparticle coated TFBG devices in sensing and as sources of light for surface-enhanced spectroscopy

Self-optimized metal coatings for fiber plasmonics by electroless deposition

We present a novel method to prepare optimized metal coatings for infrared Surface Plasmon Resonance (SPR) sensors by electroless plating. We show that Tilted Fiber Bragg grating sensors can be used to monitor in real-time the growth of gold nano-films up to 70 nm in thickness and to stop the deposition of the gold at a thickness that maximizes the SPR (near 55 nm for sensors operating in the near infrared at wavelengths around 1550 nm). The deposited films are highly uniform around the fiber circumference and in spite of some nanoscale roughness (RMS surface roughness of 5.17 nm) the underlying gratings show high quality SPR responses in water

UV resonance Raman spectroscopy probes the localization of tryptophan-containing antimicrobial peptides in lipid vesicles

In this work we employed UV resonance Raman spectroscopy with 229 nm excitation to study two tryptophan-containing antimicrobial peptides with a broad-spectrum activity against Gram-positive and Gram-negative bacteria: lactoferricin B (LfB, RRWQWRMKKLG) and pEM-2 (KKWRWWLKALAKK). UV resonance Raman spectra of both peptides are dominated by tryptophan bands. Raman spectra of LfB and pEM-2 in D2O and 2,2,2-trifluoro ethanol (TFE) have been measured and used to identify the hydrogen-bond strength marker bands W6 and W17. The tryptophan doublet, W7, at 1340 and 1360 cm-1 was used to detect an increase in the hydrophobicity of Trp environment in TFE. The spectra of LfB in complex with model cell membranes composed of zwitterionic dipalmitoylglycero-phosphocholine (DPPC) or anionic dipalmitoyglycero- phosphoglycerol (DPPG) lipid vesicles revealed a more hydrophobic Trp environment in DPPG, suggesting stronger interactions between the cationic peptide and anionic model cell membrane. Copyrigh

Improved refractive-index sensitivity of silver-nanocube monolayers on silicon films

Suitable substrates: An improvement in the refractive-index sensitivity is observed for the quadrupolar plasmonic mode of a monolayer of 60 nm silver nanocubes deposited on a thin silicon film. Dipolar plasmonic band splitting on this high-refractive-index substrate is stronger than on a glass substrate (see picture). As a result, the quadrupolar band is easier to identify, even in high-refractive-index liquids. The work demonstrates the importance of using high-refractive-index substrates. Copyrigh

Spatially inhomogeneous enhancement of fluorescence by a monolayer of silver nanoparticles

Near-field scanning optical microscopy (NSOM) was applied to study the effect of a two-dimensional array of silver nanoparticles on the spatial distribution and magnitude of fluorescence signal enhancement for a monolayer of Rhodamine 6G (Rh6G). Twenty polyelectrolyte monolayers were deposited between the nanoparticles and the dye by a layer-by-layer deposition technique resulting in a 15-20 nm separation cushion, necessary to minimize the fluorescence signal quenching. The fluorescence signal in NSOM images was found to be distributed inhomogeneously as small (100-200 nm in diameter) fluorescent clusters with typically 5-30 times higher fluorescence intensities than a sample without nanoparticles. The position and relative intensity of the clusters was found to be dependent on the excitation wavelength, suggesting that the enhancement originates from the nanoparticle surface plasmon resonance

Substrate-induced effects on the plasmonic properties of strongly coupled silver nanocubes

The behaviour of the plasmonic modes of supported strongly coupled silver nanocubes is studied. Silver nanocube monolayers with controlled particle density were fabricated via the Langmuir-Blodgett technique and deposited on substrates with varying refractive indices. Substrates include glass, thin films of silicon, and titanium oxide on glass. The dipolar and bonded dipolar modes are red shifted with increasing refractive index of the substrate. Surface-enhanced Raman spectroscopy (SERS) is used as a tool to probe the electric field enhancements of the silver nanocube monolayers. SERS enhancement of silver nanocube monolayers is found to be highly substrate dependant, typically decreasing with increasing refractive index of the underlying substrate. This work aims to find the source of this enhancement decrease, and distinguishes between effects related electromagnetic enhancement and effects caused by the optics of the Raman spectroscopy system itself

Reflection and absorption spectra of silver nanocubes on a dielectric substrate: Anisotropy, angle, and polarization dependencies

Angle and polarization dependent UV-visible re fl ection and transmission spectra were measured for monolayers of weakly interacting silver nanocubes (of 40 and 80 nm edge lengths) supported by thin films of titanium oxide on glass utilizing both front and rear geometries with respect to the incident radiation. The supporting substrate mediates the hybridization of dipolar and quadrupolar plasmon resonances. From the spectra absolute extinction and absorption c

Interfacial Diffusion of Silver Nanocrystals into Polymer Surfaces Monitored by Hybrid Plasmon Modes

Functional response of polymer-nanoparticle nanocomposites depend strongly on physical organization and interactions between constituting components. In this work, we study thermally induced embedding of silver nanocrystals (nanocubes, AgNC) into different polymer films to probe the interfacial nanoparticle diffusion dynamics and to determine temperature of embedding (Te) into the polymer substrates. The spatial sensitivity of the AgNCs hybrid plasmonic modes allowed the detection of a thin layer on top of the polymer film with increased polymer segmental motion and Te lower than the glass transition temperature Tg of the bulk polymer. Utilizing supported AgNCs as a nanoruler, diffusion constants of the nanocubes embedding into thin films of polystyrene (PS), poly(methyl methacrylate) (PMMA), and polyvinyl chloride (PVC) were determined, and an Arrhenius temperature dependence on the diffusion constants was observed. The activation energies for the AgNCs embedding into the polymers were found to be 220 ± 51, 84 ± 18, and 239 ± 31 kJ mol-1 for PS, PMMA, and PVC, respectively. This study presents important information for rational design of nanoparticle-polymer composites

Optical properties of strongly interacting supported silver nanocube monolayers

Plasmonic properties of monolayers of strongly interacting silver nanocubes (AgNC) with controlled interparticle spacing are investigated. Uniform monolayers with controlled particle densities are made using the Langmuir-Blodgett technique with passive phospholipid spacers, such as dioleoyl phosphatidylcholine (DOPC). Both extinction intensity and wavelength of dipole-dipole coupling modes are tuned via particle spacing. The refractive indices of the substrates are used to tune dipolar and interparticle coupling modes via deposition onto thin films of silicon (0 - 25nm). By varying silicon film thickness it is possible to shift and control peak widths and position for both the dipole and interparticle dipole-dipole coupling modes. Control of plasmon shifts and interparticle spacing is applied towards the optimization of SERS substrates. SERS substrates using a Rhodamine B label are tuned at different excitation wavelengths which are in resonance with either the plasmon dipole, fluorescent dye, or interparticle coupling mode. Substrates display reproducible enhancement across multiple sites. This work presents methodology to design and optimize uniform silver nanocube SERS substrates through tuning of plasmon shifts and particle spacing

Plasmonic properties of silver nanocube monolayers deposited on thin metal films

The present work investigates the plasmonic properties and behaviour of silver nanocube monolayers deposited on thin gold films. Monolayers were deposited via the Langmuir-Blodgett metho