Highly Sensitive Plasmonic Sensor Based on Fano Resonance from Silver Nanoparticle Heterodimer Array on a Thin Silver Film

We investigate the optical spectrum of a multilayer metallic slab using multiple-scattering formalism. A thin silver film is attached to a periodic array of heterodimers consisting of two vertically spaced silver nanoparticles of different radii. Depending on the radius of nanoparticles, heterodimer array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of multipolar resonances. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of ～1.5?×?10^?3 RIU/nm.     

Morphology dependent optical response tuning in planar square-shaped array of sodium nanoparticles

Among the plasmonic nanostructures, ordered arrangement of metal nanoparticles with inter-particle gap distances in the nanometer scale is becoming increasingly important due to their ability to confine huge electromagnetic fields and tunable optical properties. Using time dependent density functional theory calculations, we study the optical response evolution in a planar square-shaped array of Sodium nanoparticles via morphology deformation. To this aim, we vary the inter-particle gap distance in the range of 2 to 30 Å separately along one and two directions. We compare and cross-examine the optical response evolution for both deformation process, and we find that the interaction between sodium nanoparticles in an ordered arrangement can be controlled to a large extent by simple deformation process. We believe that our theoretical results will be useful for designing ultra-small and tunable plasmonic devices that utilize quantum effects.     

144 Sculpting light with DNA origami

We used the DNA origami method (Rothemund, 2006) for the fabrication of self-assembled nanoscopic materials (Seeman, 2010). In DNA origami, a virus-based 8?kilobase-long DNA single-strand is folded into shape with the help of ～ 200 synthetic oligonucleotides. The resulting DNA nanostructures can be designed to adopt any three-dimensional shape and can be addressed through DNA hybridization or chemical modification with nanometer precision. We have realized that complex assemblies of nanoparticles, including magnetic, fluorescent, and plasmonic nanoparticles. Such nanoconstructs may exhibit striking optical properties such as strong optical activity in the visible range (Kuzyk et al., 2012). To this end, plasmonic particles were assembled in solution to form helices of controlled handedness. We achieved spatial control over particle placement better than 2?nm and attachment yields of 97% and above. As a collective optical response emerging from our dispersed nanostructures, we detected pronounced circular dichroism (CD) originating from the plasmon–plasmon interactions in the particle helices. In recent experiments, we were able to show that the optical response of chiral biomolecules can be transferred from the UV into the visible region in plasmonic hotspots. Thus, sensitive detection of chiral biomolecules may become feasible in the near future. We also found that the orientation of the helices in respect to the incoming light beam critically influences the resulting CD spectra. Our results can be explained with theoretical models based on plasmonic dipole interaction and demonstrate the potential of DNA origami for the assembly of metafluids with designed optical properties.     

Plasmon-Assisted Optical Curtains

We predict an optical curtain effect, i.e., formation of a spatially invariant light field as light emerges from a set of periodic metallic nano-objects. The underlying physical mechanism of generation of this unique optical curtain can be explained in both the spatial domain and the wave-vector domain. In particular, in each period, we use one metallic nanostrip to equate the amplitudes of lights impinging on the openings of two metallic nanoslits and also shift their phases by π difference. We elaborate the influence on the output effect from some geometrical parameters like the periodicity, the slit height, and so on. By controlling the light illuminated on metallic subwavelength apertures, it is practical to generate optical curtains of arbitrary forms, which may open new routes of plasmonic nanolithography.     

Buried Effects of Surface Plasmon Resonance Modes for Periodic Metal–Dielectric Nanostructures Consisting of Coupled Spherical Metal Nanoparticles with Cylindrical Pore Filled with a Dielectric

We numerically investigate the buried effects of surface plasmon resonance (SPR) modes for the periodic silver-shell nanopearl dimer (PSSND) array and their solid counterparts with different buried depths in a silica substrate by means of finite element method with three-dimensional calculations. The investigated PSSND array is an important novel geometry for plasmonic metal nanoparticles (MNPs), combining the highly attractive nanoscale optical properties of both metallic nanoshell and cylindrical pore filled with a dielectric. Numerical results for SPR modes corresponding to the effects of different illumination wavelengths, absorption spectra, pore–dielectric, electric field components and total field distribution, charge density distribution, and the model of the induced local field or an applied field of the PSSND array are reported as well. It can be found that the buried MNPs with cylindrical pore filled with a dielectric in a substrate exhibit tunable SPR modes corresponding to the bonding and antibonding modes that are not observed for their solid counterparts.     

Enhanced Optical Absorption and Spectral Photocurrent in a-Si:H by Single- and Double-Layer Silver Plasmonic Interfaces

Single and double plasmonic interfaces consisting of silver nanoparticles embedded in media with different dielectric constants including SiO₂, SiN_x, and Al:ZnO have been fabricated by a self-assembled dewetting technique and integrated to amorphous silicon films. Single plasmonic interfaces exhibit plasmonic resonances whose frequency is red-shifted with increasing particle size and with the thickness of a dielectric spacer layer. Double plasmonic interfaces consisting of two different particle sizes exhibit resonances consisting of double minima in the transmittance spectra. The optical extinction of a-Si:H deposited on these interfaces is broadened into the red indicating higher absorption and/or scattering at wavelengths higher than those typically absorbed by a-Si:H without plasmonic interfaces. While the photocurrent shows an overall decrease for the samples with the interfaces, significant enhancement of photocurrent is observed near the low-energy edge of the bandgap (600–700 nm). These results correlate well with the broadened extinction spectra of the interfaces and are interpreted in terms of enhanced absorption in that region.     

Annealing Process in the Refurbishment of the Plasmonic Photonic Structures Fabricated Using Colloidal Gold Nanoparticles

Solution-processible fabrication techniques have been demonstrated with promising features for realizing different types of plasmonic devices, which combine interference lithography, spin-coating of the colloidal gold nanoparticles, and subsequent annealing process at a temperature of 200–300 °C. However, the resultant device needs to be improved in the following considerations: (1) The photoresist master grating needs to be removed for the applications in optoelectronic or sensor devices and (2) each lattice site of the photonic crystals is still composed of closely contacted gold nanoparticles. Actually, these metallic photonic structures can be refurbished through a further annealing process. Using an annealing temperature above 450 °C, we have successfully removed the remaining photoresist and make the gold nanoparticles join into a solid homogenous unit on each lattice site after being fully molten. Thus, high-quality gold nanostructures with excellent plasmonic response can be obtained. This accomplished an improved recipe for the solution-processible fabrication of plasmonic nanostructures. The corresponding devices with improved optical properties become more suitable for biosensors and optoelectronic devices.     

Non-local Quantum Plasmon Resonance in Ultra-small Silver Nanoparticles

Understanding the mechanisms of light–matter interactions in ultra-small plasmonic nanoparticles (USNP) represents a major challenge because of the importance of size dependence and quantum effects. The plasmon resonance in such small metallic nanoparticles (< 5 nm) exhibits substantial deviation from classical theory predictions, with evident frequency shifts to a higher energy. This is due to the quantum nature of the free charge carriers and the dynamic response of metallic nanoparticle to the self-consistent electromagnetic fields. Such phenomena have so far been poorly understood in experiments while classical theory has mostly focused on nanostructures and sidestepped the size dependence. Here we report a quantum mechanical model of the metal permittivity to describe the USNP behaviour and experimental evidence. The proposed non-local quantum model of the permittivity for the propagation of plasmon waves in quantum-confined silver nanoparticles has no size limitations in the UNSP range.

     

Theoretical Investigation of Plasmonic Properties of Quantum-Sized Silver Nanoparticles

Plasmonic nanoparticles (NPs) like silver (Ag) strongly absorb the incident light and produce enhanced localized electric field at the localized surface plasmon resonance (LSPR) frequency. Enormous theoretical and experimental research has focused on the plasmonic properties of the metallic nanoparticles with sizes greater than 10 nm. However, such studies on smaller sized NPs in the size range of 3 to 10 nm (quantum-sized regime) are sparse. In this size regime, the conduction band of the metal particles discretizes, thus altering plasmon properties of the NPs from classical to the quantum regime. In this study, plasmonic properties of the spherical Ag NPs in size range of 3 to 20 nm were investigated using both quantum and classical modeling to understand the importance of invoking quantum regime to accurately describing their properties in this size regime. Theoretical calculations using standard Mie theory were carried out to monitor the LSPR peak shift and electric field enhancement as a function of the size of the bare plasmonic nanoparticle and the refractive index (RI) of the surrounding medium. Comparisons were made with and without invoking quantum regime. Also, the optical properties of metallic NPs conjugated with a chemical ligand using multi-layered Mie theory were studied, and interesting trends were observed.

     

Multispectral Broadband Light Transparency of a Seamless Metal Film Coated with Plasmonic Crystals

Broadband light transparency of metallic structures has long been pursued due to the potential applications in the optoelectronic communications, flat panel displays, and clean solar energy. Considerable efforts have been made on the multiband electromagnetic wave transparency of plasmonic metamolecules. However, far less work has been focused on the multispectral light transparency of a seamless metal film. Here, we for the first time propose a seamless metal film structure coated by double conventional plasmonic crystals and demonstrate the observed multispectral broadband light transparency behavior. A maximum transmittance larger than 92 % is achieved. The average transmittance of the whole spectral range from 550 to 1,100 nm is exceeding 45.8 %, suggesting the achievement of an ultra-broadband semi-transparent window. Particularly, the transparency features are highly scalable by tuning the structural parameters. Plasmonic resonances and the metallic particle–film plasmonic interactions are responsible for the observed optical transparency properties. These findings and merits make the proposed structure a good candidate for numerous potential applications, including the optoelectronic components, transparent displayers, and light harvesting.     

Resonant Broadband Field Enhancement in Cylindrical Plasmonic Structure Surrounded by Perovskite Environment

We demonstrate the optical response of metal nanoparticles and their interaction with organic-inorganic perovskite (methyl ammonia lead halide (CH₃NH₃PbI₃)) environment using discrete dipole approximation (DDA) simulation technique. Important optical properties like absorption, scattering, and electric field calculations for metal nanoparticle using different geometry have been analyzed. The metal nanoparticles embedded in the perovskite media strongly support surface plasmon resonances (SPRs). The plasmonic interaction of metal nanoparticles with perovskite matrix is a strong function of MNP’s shape, size, and surrounding environment that can manipulate the optical properties considerably. The cylindrical shape of MNPs embedded in perovskite environment supports the SPR which is highly tunable to subwavelength range of 400–800 nm. Wide range of particle sizes has been selected for Ag, Au, and Al spherical and cylindrical nanostructures surrounded by perovskite matrix for simulation. The chosen hybrid material and anisotropy of structure together make a complex function for resonance shape and width. Among all MNPs, 70-nm spherical silver nanoparticle (NP) and cylindrical Ag NP having diameter of 50 nm and length of 70 nm (aspect ratio 1.4) generate strong electric field intensity that facilitates increased photon absorption. The plasmonic perovskite interaction plays an important role to improve the absorption of photon inside the thin film perovskite environment that may be applicable to photovoltaics and photonics.

     

In-Plane and Out-of-Plane Plasmons in Random Silver Nanoisland Films

Effective permittivity of closely spaced random nanoparticles supported by a substrate has been calculated using a modified Yamaguchi’s model (MYM) which involves the exact expression of a local field outside a metal nanoparticle (NP) along with the effective-medium approach. Pulsed laser deposition has been used to deposit silver nanoisland films on SiO₂ substrates. In-plane and out-of-plane plasmonic responses have been calculated using MYM for various filling fractions and the results are compared with those obtained from spectroscopic ellipsometry. Distinct features of out-of-plane and in-plane plasmons are observed with an spectroscopic ellipsometer and their behavior is supported by the present theoretical investigation. The comparison of the effective dielectric constants of the films obtained from ellipsometry data with those calculated using MYM shows uniaxial optical anisotropy in our case. The calculated morphological parameters (filling fraction, aspect ratio, and average particle size) using MYM are also found to be consistent with those obtained from FESEM images.     

Utilization of Plasmonic and Photonic Crystal Nanostructures for Enhanced Micro- and Nanoparticle Manipulation

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological research. Perhaps the most widely used physical force to achieve noninvasive manipulation of small particles has been dielectrophoresis(DEP).¹ However, DEP on its own lacks the versatility and precision that are desired when manipulating cells since it is traditionally done with stationary electrodes. Optical tweezers, which utilize a three dimensional electromagnetic field gradient to exert forces on small particles, achieve this desired versatility and precision.² However, a major drawback of this approach is the high radiation intensity required to achieve the necessary force to trap a particle which can damage biological samples.³ A solution that allows trapping and sorting with lower optical intensities are optoelectronic tweezers (OET) but OET''s have limitations with fine manipulation of small particles; being DEP-based technology also puts constraint on the property of the solution.^4,5This video article will describe two methods that decrease the intensity of the radiation needed for optical manipulation of living cells and also describe a method for orientation control. The first method is plasmonic tweezers which use a random gold nanoparticle (AuNP) array as a substrate for the sample as shown in Figure 1. The AuNP array converts the incident photons into localized surface plasmons (LSP) which consist of resonant dipole moments that radiate and generate a patterned radiation field with a large gradient in the cell solution. Initial work on surface plasmon enhanced trapping by Righini et al and our own modeling have shown the fields generated by the plasmonic substrate reduce the initial intensity required by enhancing the gradient field that traps the particle.^6,7,8 The plasmonic approach allows for fine orientation control of ellipsoidal particles and cells with low optical intensities because of more efficient optical energy conversion into mechanical energy and a dipole-dependent radiation field. These fields are shown in figure 2 and the low trapping intensities are detailed in figures 4 and 5. The main problems with plasmonic tweezers are that the LSP''s generate a considerable amount of heat and the trapping is only two dimensional. This heat generates convective flows and thermophoresis which can be powerful enough to expel submicron particles from the trap.^9,10 The second approach that we will describe is utilizing periodic dielectric nanostructures to scatter incident light very efficiently into diffraction modes, as shown in figure 6.¹¹ Ideally, one would make this structure out of a dielectric material to avoid the same heating problems experienced with the plasmonic tweezers but in our approach an aluminum-coated diffraction grating is used as a one-dimensional periodic dielectric nanostructure. Although it is not a semiconductor, it did not experience significant heating and effectively trapped small particles with low trapping intensities, as shown in figure 7. Alignment of particles with the grating substrate conceptually validates the proposition that a 2-D photonic crystal could allow precise rotation of non-spherical micron sized particles.¹⁰ The efficiencies of these optical traps are increased due to the enhanced fields produced by the nanostructures described in this paper.Download video file.^{(57M, mov)}     

Study of Broadband Tunable Properties of Surface Plasmon Resonances of Noble Metal Nanoparticles Using Mie Scattering Theory: Plasmonic Perovskite Interaction

We suggest semi-analytical approach to study the optical properties of noble metal nanoparticles and their interaction to the perovskite material (methyl ammonia lead halide: CH₃NH₃PbI₃). Metal nanoparticles embedded in perovskite matrix exhibits broadband surface plasmon resonances, and the tunability of these plasmonic resonances is highly sensitive to particle size. The calculation of optical cross section have been done using Mie scattering theory which is applicable to arbitrary size and spherical-shape metal nanoparticles. We have taken five different radii ranging from 15 to 100 nm to understand the plasmonic resonances and its spectral width in the wavelength range 300 to 800 nm. Out of these noble metal nanoparticles, silver have highest scattering efficiency nearly of the order of 18 for the case of 15 nm radii at resonance wavelength 613 nm. Our finding reveals a new concept to understand the applications of plasmonic resonances in order to enhance the photon absorption inside the thin film of perovskite.     

Tip-Enhanced Raman Imaging and Nanospectroscopy: Sensitivity, Symmetry, and Selection Rules

The fundamental mechanisms of tip-enhanced Raman spectroscopy (TERS) have been investigated, including the role of the plasmonic excitation of the metallic tips, the nature of the optical tip–sample coupling, and the resulting local-field enhancement and confinement responsible for ultrahigh resolution imaging down to just several nanometers. Criteria for the distinction of near-field signature from far-field imaging artifacts are addressed. TERS results of molecules are presented. With enhancement factors as high as 10⁹, single-molecule spectroscopy is demonstrated. Spatially resolved vibrational mapping of crystalline nanostructures and determination of crystallographic orientation and domains is discussed making use of the symmetry properties of the tip scattering response and the intrinsic Raman selection rules.     

Broadening of Plasmonic Resonance Due to Electron Collisions with Nanoparticle Boundary: а Quantum Mechanical Consideration

We present a quantum mechanical approach to calculate broadening of plasmonic resonances in metallic nanostructures due to collisions of electrons with the surface of the structure. The approach is applicable if the characteristic size of the structure is much larger than the de Broglie electron wavelength in the metal. The approach can be used in studies of plasmonic properties of both single nanoparticles and arrays of nanoparticles. Energy conservation is insured by a self-consistent solution of Maxwell's equations and our model for the photon absorption at the metal boundaries. Consequences of the model are illustrated for the case of spheroid nanoparticles, and results are in good agreement with earlier theories. In particular, we show that the boundary-collision broadening of the plasmonic resonance in spheroid nanoparticles can depend strongly on the polarization of the impinging light.     

Fano Resonance-Assisted Plasmonic Trapping of Nanoparticles

Plasmonic optical trapping is widely applied in the field of bioscience, microfluidics, and quantum optics. It can play a vital role to extend optical manipulation tools from micrometer to nanometer scale level. Currently, it is a challenge to obtain the highly stable optical trapping with low power and less damage. In this paper, we propose Fano resonance-assisted self-induced back-action (FASIBA) method, through which a single 40-nm gold particle can be trapped in hole-slit nano-aperture milled on metallic film. It is used to achieve ultra-accurate positioning of nanoparticle, metallic nanostructures at wide infrared wavelength range, quite effectively and evidently. The stable plasmonic trapping is achieved by tuning the transmission wavelengths and modifications of nanoslit, indicating that the depth of potential well can be increased from minus 8KT to 12KT, with the input power of 10⁹ W/m². This can be attributed to great modifications in Fano resonance transmissions according to self-induced back-action (SIBA) theory. The results are basically helpful to facilitate the trapping with lower power and less damage to the objects, which enables new scenario for the treatment of undesirable spread of a single nanoscale creature, such as virus.     

Plasmonic Properties of β-Sn Nanoparticles in Ordered and Disordered Arrangements

This work investigates the localized surface plasmon resonance (LSPR) of β-Sn also known as white tin. Recently, studies on arrays of β-Sn nanoparticles have shown that these arrays possess strong optical features caused by diffractive effects in the particle grating (Johansen et al., Phys Rev B 84:113405–113408, 2011). In the presence of the grating, the LSPR could not clearly be distinguished in the spectra. To get a better understanding of the plasmonic properties of the particles, we have now eliminated the diffractive effects by placing the particles in a random distribution. The particles were fabricated by electron beam lithography on a fused silica substrate and investigated by optical transmission measurements. In the random configuration, a clear LSPR is observed at 530 nm for particles with a diameter of 155 nm and a height of 50 nm.     

Study of the Plasmon Talbot Effect of Metallic Nanolenses Induced by Linearly Polarized Illumination

The plasmon Talbot effect of metallic nanolenses was studied theoretically and experimentally for the linearly polarized incident beam case. To demonstrate this self-imaging-based focusing property of the metallic nanolenses, a plasmonic nanolens with five periodic concentric through rings on Al film supported on quartz substrate was numerically studied firstly by the use of rigorous finite-difference and time-domain algorithm. To further demonstrate its working performance experimentally, it was fabricated by means of a focused ion beam direct milling technique. A near-field scanning optical microscope (NSOM) was then employed for the optical characterization of its focusing property. The experimental results indicate that the NSOM probing-based results are in agreement with the theoretical calculation results in general.     

Optical Absorption in Overcoats of Nanoparticle Arrays on a Metallic Substrate

Surface plasma oscillations in metallic particles as well as in thin metallic films have been studied extensively in the past decades. New features regarding surface plasma excitations are, however, constantly discovered, leading, for example, to surface-enhanced Raman scattering studies and enhanced optical transmission though metal films with nanohole arrays. In the present work, the role of a metallic substrate is examined in two cases, one involving an overcoat of dielectric nanoparticles and the other an overcoat of metallic nanoparticles. Theoretical results are obtained by modeling the nanoparticles as forming a two-dimensional, hexagonal lattice of spheres. The scattered electromagnetic field is then calculated using a variant of the Green function method. Comparison with experimental results is made for nanoparticles of tungsten oxide and tin oxide deposited on either gold or silver substrates, giving qualitative agreement on the extra absorption observed when the dielectric nanoparticles are added to the metallic surfaces. Such absorption would be attributed to the mirror image effects between the particles and the substrate. On the other hand, calculations of the optical properties of silver or gold nanoparticle arrays on a gold or a silver substrate demonstrate very interesting features in the spectral region from 400 to 1,000 nm. Interactions between the nanoparticle arrays surface plasmons and their images in the metallic substrate would be responsible for the red shift observed in the absorption resonance. Moreover, effects of particle size and ambient index of refraction are studied, showing a great potential for applications in biosensing with structures consisting of metallic nanoparticle arrays on metallic substrates.