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In this paper we deal with the macroscopic electromagnetic response of a finite size dispersive dielectric object, in unbounded space, in the framework of quantum electrodynamics using the Heisenberg picture. We apply a Hopfield type scheme to account for the dispersion and dissipation of the matter. We provide a general expression of the polarization density field operator as functions of the initial conditions of the matter field operators and of the electromagnetic field operators. It is a linear functional whose kernel is a linear expression of the impulse response of the dielectric object that we obtain within the framework of classical electrodynamics. The electric field operator is expressed as a function of the polarization density field operator by means of the dyadic Green's function for the free space. The statistical functions of these operators are classical functionals of the statistics of the initial conditions of the matter field operators and of the electromagnetic field operators, whose kernels are linear or multilinear expressions of the impulse response of the dielectric object. We keep the polarization and the electromagnetic field distinct to enable the treatment of the polarization and electromagnetic fluctuations on equal footing. We expand the polarization density field operator in terms of the static longitudinal and transverse modes of the object to diagonalize the Coulomb and Ampere interaction energy terms of the Hamiltonian in the Coulomb gauge. We expand the radiation fields in terms of the transverse plane wave modes of free space. Few static longitudinal and transverse modes are needed to calculate each element of the impulse response matrix for dielectric objects with sizes of the order up to $\min\limits_{\omega}\{c_0/[\omega \sqrt{|{\chi}(\omega)|}]\}$ where ${\chi}(\omega)$ is the susceptibility of the dielectric.

We introduce the longitudinal and transverse static surface modes and use them to solve the full-wave electromagnetic scattering problem from penetrable objects. The longitudinal static modes are the eigenmodes with zero surface curl of the electrostatic integral operator that gives the tangential component of the electric field, as a function of the surface charge density. The transverse static modes are the eigenmodes with zero surface divergence of the magnetostatic integral operator that returns the tangential component of the vector potential, as a function of the surface current distribution. The static modes only depend on the shape of the object, thus, the same static basis can be used regardless of the frequency of operation and of the material constituting the object. We expand the unknown surface currents of the Poggio-Miller-Chang-Harrington-Wu-Tsai surface integral equations in terms of the static surface modes and solve them using the Galerkin-projection scheme. The static modes expansion allows the regularization of the singular integral operators and yields a drastic reduction of the number of unknowns compared to a discretization based on sub-domain basis functions. The introduced expansion significantly reduces the cpu-time required for the numerical solution of the scattering problem from particle arrays.

The problem of finding the optimal current distribution supported by small radiators yielding the minimum quality (Q) factor is a fundamental problem in electromagnetism. Q factor bounds constrain the maximum operational bandwidth of devices including antennas, metamaterials, and nanoresonators, and have been featured in seminal papers in the past decades. Here, we determine the lower bounds of Q factors of small-size plasmonic and high-permittivity dielectric resonators, which are characterized by quasi-electrostatic and quasi-magnetostatic natural modes, respectively. We expand the induced current density field in the resonator in terms of these modes, leading to closed-form analytical expressions for the electric and magnetic polarizability tensors, whose largest eigenvalue is directly linked to the minimum Q factor. Our results allow also to determine in closed form the corresponding optimal current density field. In particular, when the resonator exhibits two orthogonal reflection symmetries the minimum Q factor can be simply obtained from the Q factors of the single current modes with non-vanishing dipole moments aligned along the major axis of the resonator. Overall, our results open exciting opportunities in the context of nano-optics and metamaterials, facilitating the analysis and design of optimally shaped resonators for enhanced and tailored light-matter interactions.

We analyse the resonance conditions of a long and narrow ribbon of finite length whether it is conductive or dielectric. This is accomplished by using a full wave approach based on the material independent modes that naturally discriminates the role of the geometry and of the material. This method effectively allows the design of the material in such a way to obtain the desired resonances. Eventually, as an example, we design two quasi-one dimensional resonators based on a graphene layer and on a silicon thin film.

The electromagnetic scattering from interconnections of high-permittivity dielectric thin wires with sizes smaller than (or almost equal to) the operating wavelength is investigated. A simple lumped element model for the polarization current intensities induced in the wires is proposed. The circuit elements are capacitances and inductances between the wires. An analytical expression for the induced polarization currents in terms of the magneto-quasistatic current modes is obtained. The connection between the spectral properties of the loop inductance matrix and the network's resonances is established. The number of the allowed current modes and resonances is deduced from the topology of the circuit's digraph. The coupling to radiation is also included, and the radiative frequency shifts and the quality factors are derived. The introduced concept and methods may find applications both at the microwaves and in nanophotonics.

Plasmonic nanostructures with singular geometries can exhibit a broadband scattering response that at first glance appears to violate the lower bounds for the radiation quality (Q) factor of small radiators, known as the Chu limit. Here we explore this apparent contradiction, investigating the Q factor of the resonant modes supported by two nearly touching cylinders, and analyze how their fractional bandwidth fares in relation to the Chu limit. We first derive lower bounds for the radiation Q factors of two-dimensional objects of an arbitrary cross-section. We then discuss the dissipation and radiation Q factors associated with the plasmonic resonances of a cylinder dimer as a function of its gap size. We show that the radiation Q factor is always larger than the minimum Q and, as long as the peaks in the scattering spectrum are well separated, their bandwidth is equal to the inverse of their Q factor. In the limit of touching cylinders, the resonance spectra transition from discrete to a continuum around an accumulation point, yielding a broadband response for any finite level of material loss. Within any given frequency interval, the response is the result of a multitude of plasmon resonances, each individually obeying the Chu limit. Nevertheless, the connection between the Q factor and the overall bandwidth of the scattering response is lost. Our study sheds light onto the exotic resonant phenomena emerging when plasmonic materials are shaped in singular geometries, and outlines their opportunities and limitations for nanophotonics.

A fresh approach to the full wave analysis of time evolution of the polarization induced in the electromagnetic scattering from dispersive non magnetic particles is presented. It is grounded on the combination of the Hopfield model for the polarization field, the expansion of the polarization field in terms of static longitudinal and transverse modes of the particle, the expansion of the radiation field in terms of transverse wave modes of free space, and the principle of least action. The polarization field is linearly coupled to the electromagnetic field. The losses of the matter are provided through a linear coupling of the polarization field to a bath of harmonic oscillators with a continuous range of natural frequencies. The set of linear ordinary differential integral equations of convolution type of the overall system is reduced by eliminating both the radiation degrees of freedoms and the bath degrees of freedom, and the reduced system of equations is studied. The role played by the radiation field in the coupling between the longitudinal and transverse mode amplitudes of the polarization is described. The principal characteristics of the temporal evolution of the mode amplitudes are found as the particle size varies, including the impulse response. Results are presented for the analytically solvable spherical particle. The proposed approach leads to a general method for the analysis of the temporal evolution of the polarization field induced in dispersive particles of any shape, as well as for the computation of transients and steady states.

The stability of spherical gold nanoparticles embedded in polyethylene glycol diacrylate (PEGDA) with varying molecular weights is tested. The refractive index of the chosen PEGDA is measured as function of the wavelength and used as a parameter to predict the extinction spectrum of gold nanoparticles embedded in the polymeric matrix. The experimental absorbance spectrum of AuNPs-containing PEGDA hydrogel is measured via a transmission mode optical setup. The designed device can be used as a platform for biosensing applications and offers promising opportunities for real-time and early diagnosis of diseases.

In this paper we study, in the time domain, the interaction between localized surface plasmons and photons in arbitrarily shaped metal nanoparticles, by using the Hopfield approach to quantize the plasmon modes, where the electron oscillations are represented by a harmonic matter field linearly coupled to the electromagnetic radiation. The plasmon - photon coupling gives rise to dressed plasmon modes. We have found that the radiation does not induce a significant coupling among the different quasi-electrostatic plasmon modes for particles of size up to the plasma wavelength, but causes a frequency shift and an exponential decay in time of the modes. By solving the equations governing the expectation values of the plasmon creation and annihilation operators, we obtain a new closed-form full-wave expression for the decay rate and for the frequency shift of the plasmon modes. It is non-perturbative and it only depends on the surface charge distribution of the quasi-electrostatic plasmon modes. We validate the expression against the Mie theory for a nano-sphere of radius comparable to the plasma wavelength. Eventually, we investigate the decay rate and the frequency shift of the plasmon modes in isolated and interacting nanoparticle of non-canonical shape, as their size increases up to the plasma wavelength.

The electromagnetic scattering resonances and modes of a nonmagnetic object much smaller than the incident wavelength in vacuum can be either described by the electroquasistatic approximation of the Maxwell's equations if its permittivity is negative or by the magnetoquasistatic approximation if its permittivity is positive and sufficiently high. Nevertheless, these two approximations fail to correctly account for the frequency shift and the radiative broadening of the resonances when the size of the object becomes comparable to the wavelength of operation. In this paper, the radiation corrections to the electroquasistatic and magnetoquasistatic resonances and modes of arbitrarily shaped objects are derived, which only depend on the quasistatic current modes. Then, closed-form expressions of the frequency shift and the radiative Q-factor of both plasmonic and dielectric modes of small objects are introduced, where the dependencies on the material and the size of the object are factorized. In particular, it is shown that the radiative Q-factor explicitly depends on the multipolar components of the quasistatic mode and its corrections.

A small dielectric object with positive permittivity may resonate when the free-space wavelength is large in comparison with the object dimensions if the permittivity is sufficiently high. We show that these resonances are described by the magnetoquasistatic approximation of the Maxwell's equations in which the normal component of the displacement current density field vanishes on the surface of the particle. They are associated to values of permittivities and frequencies for which source-free quasistatic magnetic fields exist, which are connected to the eigenvalues of a magnetostatic integral operator. We present the general physical properties of magnetoquasistatic resonances in dielectrics with arbitrary shape. They arise from the interplay between the polarization energy stored in the dielectric and the energy stored in the magnetic field. Our findings improve the understanding of resonances in high-permittivity dielectric objects and provide a powerful tool that greatly simplifies the analysis and design of high index resonators.

We show that infrared radiation impinging onto a one-dimensional array of grooves drilled in the superconductor electrode of a long overlap junction can improve matching between fluxon oscillations at THz frequencies and a spoof plasmon of comparable wavelength. This example proves that metamaterials can be very helpful in integrating superconductive and subwavelength optical circuits with optimized matching, bridging the gap between infrared and microwave radiation.

The automatic inverse design of three-dimensional plasmonic nanoparticles enables scientists and engineers to explore a wide design space and to maximize a device’s performance. However, due to the large uncertainty in the nanofabrication process , we may not be able to obtain a deterministic value of the objective, and the objective may vary dramatically with respect to a small variation in uncertain parameters. Therefore, we take into account the uncertainty in simulations and adopt a classical robust design model for a robust design. In addition, we propose an efficient numerical procedure for the robust design to reduce the computational cost of the process caused by the consideration of the uncertainty. Specifically, we use a global sensitivity analysis method to identify the important random variables and consider the non-important ones as deterministic, and consequently reduce the dimension of the stochastic space. In addition, we apply the generalized polynomial chaos expansion method for constructing computationally cheaper surrogate models to approximate and replace the full simulations. This efficient robust design procedure is performed by varying the particles’ material among the most commonly used plasmonic materials such as gold, silver, and aluminum, to obtain different robust optimal shapes for the best enhancement of electric fields.

Inorganic diatomite nanoparticles (DNPs) have gained increasing interest as drug delivery systems due to their porous structure, long half-life, thermal and chemical stability. Gold nanoparticles (AuNPs) provide DNPs with intriguing optical features that can be engineered and optimized for sensing and drug delivery applications. In this work, we combine DNPs with gelatin stabilized AuNPs for the development of an optical platform for Galunisertib delivery. To improve the DNP loading capacity, the hybrid platform is capped with gelatin shells of increasing thicknesses. Here, for the first time, full optical modeling of the hybrid system is proposed to monitor both the gelatin generation, degradation, and consequent Galunisertib release by simple spectroscopic measurements. Indeed, the shell thickness is optically estimated as a function of the polymer concentration by exploiting the localized surface plasmon resonance shifts of AuNPs. We simultaneously prove the enhancement of the drug loading capacity of DNPs and that the theoretical modeling represents an efficient predictive tool to design polymer-coated nanocarriers.

The plasmon hybridization theory is based on a quasi-electrostatic approximation of the Maxwell's equations. It does not take into account magnetic interactions, retardation effects, and radiation losses. Magnetic interactions play a dominant role in the scattering from dielectric nanoparticles. The retardation effects play a fundamental role in the coupling of the modes with the incident radiation and in determining their radiative strength; their exclusion may lead to erroneous predictions of the excited modes and of the scattered power spectra. Radiation losses may lead to a significant broadening of the scattering resonances. We propose a hybridization theory for non-hermitian composite systems based on the full-Maxwell equations that, overcoming all the limitations of the plasmon hybridization theory, unlocks the description of dielectric dimers. As an example, we decompose the scattered field from silicon and silver dimers, under different excitation conditions and gap-sizes, in terms of dimer modes, pinpointing the hybridizing isolated-sphere modes behind them., Comment: Supplemental material available upon request

The plasmon hybridization theory is based on the quasi-electrostatic approximation of the Maxwell equations. It neglects radiation effects and magnetic interactions. As a consequence of the absence of magnetic interactions, the magnetic modes, which play a dominant role in the scattering from dielectric nanoparticles, are completely disregarded. We introduce a hybridization theory for non-hermitian composite systems, based on the full-wave Maxwell equations that overcomes all these limitations and returns the well-known plasmon hybridization model when applied to deep subwavelength metal dimers. Specifically, we investigate the modes and resonances of the electromagnetic scattering by a dimer of spheres, using the concept of material-independent-modes [1], [2]. Each dimer mode is naturally expanded in terms of the hybridization of the modes of the two constituent spheres (assumed to be isolated), and we provide the quantitative expressions of the hybridization weights. The scattered field is then represented by an expansions in terms of the dimer-modes, where the contributions of the geometry and of the materials are disentangled. By using this theoretical framework, we analyse the resonant scattering from metallic and dielectric dimers under different excitation conditions. We rigorously identify the dimer-modes behind each peak of the scattered power spectra.

In the last few decades, plasmon nanoparticles (NP) have been proposed as a suitable platform to control and enhance the light-matter interaction. Nowadays, modern technologies are pushing this interaction to the extreme: single quantum emitters can be isolated, few-nanometer particles can be fabricated, single-photon state can be measured. For a complete analysis of the coupling phenomena involving plasmon nanoparticles, a proper quantum description is needed.

A 3D plasmonic sensing platform that combines the properties of citrate gold nanoparticles (AuNPs) and poly-(ethylene glycol) diacrylate (PEGDA) hydrogels is proposed as a nanocomposite hybrid material for biosensing applications, whose optical properties and sensitivity can be tuned by varying the particle mean diameter as also predicted by the Mie theory. It is found that AuNPs embedded in the hydrogel network are more stable when compared to the colloidal aqueous solutions. PEGDA hydrogel physically retains the gold nanoparticles even after a full swelling process during immersion in liquids. Such a property is confirmed by exposing the AuNPs-containing PEGDA hydrogels to organic solvents and buffers that would usually cause the aggregation of the nanoparticles in solution. Moreover, biotin, as a small molecule model, has been captured, and optically detected with a transmission mode customized setup, by a cysteamine modified AuNPs-containing PEGDA hydrogel layer to achieve a biorecognition hybrid device.

The current praxis of designing plasmonic devices by hand, mainly guided by qualitative arguments, often derived from simplified semianalytical theories, significantly limits the accessible design space and, consequently, the achievable performances. In the present work, we propose a rigorous inverse design method to engineer three-dimensional metal nanoparticles according to a preassigned objective function, coupling an efficient global optimization algorithm to a full-retarded, electromagnetic solver based on the surface integral equation method. Thus, we use the proposed strategy to design the morphology of metal nanoparticles, maximizing the electric field average on their surface. We performed the optimization by varying the exciting wavelength in the ultraviolet and visible spectral ranges and the particle’s material among the most used plasmonic materials, namely, gold, silver, and aluminum, obtaining different corresponding optimal shapes. General design criteria of nanoparticle’s shape and size f...

We introduce a representation of the electromagnetic field scattered by a homogeneous sphere in terms of a set of full-retarded modes independent of its permittivity. Within this framework, we introduce the orthogonality properties of the modes, their resonance conditions, and their classification into narrow and broad modes. We also discuss the role played by the material properties in determining the resonant width of a given mode and in enabling or preventing the multimode interference. We use this theory to unveil the origin of Fano lineshapes in the scattering efficiency of a spherical nanoparticle, by identifying the interfering modes responsible for peaks and dips. Eventually, by using the introduced theoretical approach, we design the permittivity of a homogeneous sphere of size comparable to the incident wavelength to cancel its backscattering through directional multimode interference.

The electromagnetic modes and the resonances of homogeneous, finite size, two-dimensional bodies are examined in the frequency domain by a rigorous full wave approach based on an integro-differential formulation of the electromagnetic scattering problem. Using a modal expansion for the current density that disentangles the geometric and material properties of the body the integro-differential equation for the induced surface (free or polarization) current density field is solved. The current modes and the corresponding resonant values of the surface conductivity (eigenconductivities) are evaluated by solving a linear eigenvalue problem with a non-Hermitian operator. They are inherent properties of the body geometry and do not depend on the body material. The material only determines the coefficients of the modal expansion and hence the frequencies at which their amplitudes are maximum (resonance frequencies). The eigenconductivities and the current modes are studied in detail as the frequency, and the shape and the size of the body vary. Open and closed surfaces are considered. The presence of vortex current modes, in addition to the source-sink current modes (no whirling modes), which characterize plasmonic oscillations, is shown. Important topological features of the current modes, such as the number of sources and sinks, the number of vortices, and the direction of the vortices are preserved as the size of the body and the frequency vary. Unlike the source-sink current modes, in open surfaces the vortex current modes can be resonantly excited only in materials with a positive imaginary part of the surface conductivity. Eventually, as examples, the scattering by two-dimensional bodies with either a positive or negative imaginary part of the surface conductivity is analyzed and the contributions of the different modes are examined.

We demonstrate the directional scattering cancellation for a dielectric sphere of radius up to ten times the incident wavelength, by coating it with a surface of finite conductivity. Specifically, the problem of determining the values of the surface conductivity that guarantees destructive interference among hundreds of multipolar scattering orders at the prescribed angular direction is reduced to the determination of the zeros of a polynomial, whose coefficients are analytically known.

In the frame of volume integral equation methods, we introduce an alternative representation of the electromagnetic field scattered by a homogeneous object of arbitrary shape at a given frequency, in terms of a set of modes independent of its permittivity. This is accomplished by introducing an auxiliary eigenvalue problem, based on a volume integral operator. With this modal basis the expansion coefficients of the scattered field are simple rational functions of the permittivity of the scatterer. We show, by studying the electromagnetic scattering from a sphere and a cylinder of dimensions comparable to the incident wavelength, that only a moderate number of modes is needed to accurately describe the scattered far field. This method can be used to investigate resonant scattering phenomena, including plasmonic and photonic resonances, and to design the permittivity of the object to pursue a prescribed tailoring of the scattered field. Moreover, the presented modal expansion is computationally advantageous compared to direct solution of the volume integral equation when the scattered field has to be computed for many different values of the dielectric permittivity, given the size and shape of the dielectric body.

We introduce a volume integral equation method for the numerical solution of the electromagnetic scattering problem from electrically anisotropic inhomogeneous objects. The problem’s unknown, that is the sum of the conduction and polarization current densities, is decomposed in terms of its loop, star and facet components. Suitable strategies have been implemented to enforce the uniqueness of this discrete representation. Furthermore, through a convenient scaling of the unknowns, we also make the present method immune to the low-frequency breakdown problem, showing its stability regardless of the operating frequency. This approach has been extensively validated against the null-field method by analyzing the scattering from a uniaxial dielectric sphere and a uniaxial dielectric slab and comparing the scattered electric field in both the near and far zones. The condition number of the scattering problem consisting of a slab with an assigned conductivity tensor is also monitored as a function of the exciting frequency.

In this paper, we combine experimental measurements and numerical techniques to investigate the electromagnetic scattering of a solution of hybrid polyethylene glycol-gold (PEG-Au NPs) nanoparticles before and after bio-conjugation with an enzyme, the Arsenate reductase, in brief ArsC. In particular, we unveil the origin of the broadening of the measured absorption spectrum induced by the interaction of the PEG-Au NPs with the enzyme by using multiparticle Mie theory. We numerically investigated the effect of the near field interaction between particles, and thus we attributed the broadening to the scattering from multiple Au clusters with different inter-particles separation. This conclusion is also supported by TEM image of PEG-AuNPs after ArsC adsorption.

In this paper, we recast the problem of interaction between a classical electromagnetic field and a pair of quantum dots coupled through dipoledipole interaction in terms of an equivalent circuit model. Then, we use the introduced circuit to investigate the electromagnetic scattering from a pair of interacting quantum dots. Equivalent circuits for complex systems of quantum dots greatly simplify the analysis and the design of light-matter interaction processes at the nanoscale.

We successfully demonstrate the plasmonic coupling between metal nanoantennas and individual GaAs nanowires (NWs). In particular, by using dark-field scattering and second harmonic excitation spectroscopy in partnership with analytical and full-vector FDTD modeling, we demonstrate controlled electromagnetic coupling between individual NWs and plasmonic nanoantennas with gap sizes varied between 90 and 500 nm. The significant electric field enhancement values (up to 20×) achieved inside the NW-nanoantennas gap regions allowed us to tailor the nonlinear optical response of NWs by engineering the plasmonic near-field coupling regime. These findings represent an initial step toward the development of coupled metal???semiconductor resonant nanostructures for the realization of next generation solar cells, detectors, and nonlinear optical devices with reduced footprints and energy consumption.

We investigate the fluorescence rate of a dipolar emitter coupled to Al nanoparticles of varying shapes and sizes and to dimer nanoantennas in the deep-ultraviolet (UV) spectral range, using the surface integral equation method. In particular, we show that the shape of the Al nanostructures plays a fundamental role in controlling the complex interplay between the excitation rate and the quantum yield in radiative plasmonic systems. In addition, we also investigate the role of the near-field interaction of two coupled Al nanoparticles in enhancing the fluorescence rate of the dipole. This study is important for the engineering of more efficient light-emitting nanostructures in the UV spectral range, such as Al-based material systems or light-emitting fluorophores for biodetection.

In this Rapid Communication, we introduce a representation of the electromagnetic field for the analysis and synthesis of the full-wave scattering by a homogeneous dielectric object of arbitrary shape in terms of a set of eigenmodes independent of its permittivity. The expansion coefficients are rational functions of the permittivity. This approach naturally highlights the role of plasmonic and photonic modes in any scattering process and suggests a straightforward methodology to design the permittivity of the object to pursue a prescribed tailoring of the scattered field. We discuss in depth the application of the proposed approach to the analysis and design of the scattering properties of a dielectric sphere.

Summary Block-copolymers (BCPs) self-assembling provides a unique tool for realizing large-area ordered metamaterials with desired optical properties due to the rich range of tunable nano-morphologies and to the chemical properties of the block polymers that can lead to the selective inclusion of functionalized nanoparticles (NPs) in specific nanodomains, generating periodic arrays of NPs according to the geometry of the BCP acting as template. In this work, we investigate nanostructured thin films of polystyrene-block-poly(methylmethacrylate) (PS-PMMA) BCP filled with functionalized metallic (Au) nanoparticles. The BCP is characterized by an hexagonal array of PS cylinders in the PMMA matrix and the Au NPs are selectively included in the PS cylindrical domains. The optical properties of such nano-structures are strongly affected by localized surface plasmons (LSPs) in the NPs. We theoretically demonstrate that the optical properties of such nano-structures can be tuned by varying the NPs loading concentration and the NPs radius. Then, we investigate the effects of the hexagonal geometry on the collective plasmonic oscillations. We fabricated the PS-PMMA/Au NPs composite and we experimentally demonstrate the existence of collective plasmonic resonances of gold nanoparticles after an electromagnetic stimulation, arising from the ordered arrangement of AuNPs in the periodic array of hexagonally packed cylinders created by BCP self-assembly. This approach allows a fine modulation of the NPs optical properties and can be used as an intriguing and versatile tool to build useful devices for Optics & Photonics applications, with significant benefits for both fundamental and applied investigations.

In the present Letter, we demonstrate how the design of metallic nanoparticle arrays with large electric field enhancement can be performed using the basic paradigm of engineering, namely the optimization of a well-defined objective function. Such optimization is carried out by coupling a genetic algorithm with the analytical multiparticle Mie theory. General design criteria for best enhancement of electric fields are obtained, unveiling the fundamental interplay between the near-field plasmonic and radiative photonic coupling. Our optimization approach is experimentally validated by surface-enhanced Raman scattering measurements, which demonstrate how genetically optimized arrays, fabricated using electron beam lithography, lead to order of ten improvement of Raman enhancement over nanoparticle dimer antennas, and order of one hundred improvement over optimal nanoparticle gratings. A rigorous design of nanoparticle arrays with optimal field enhancement is essential to the engineering of numerous nanoscale optical devices such as plasmon-enhanced biosensors, photodetectors, light sources and more efficient nonlinear optical elements for on chip integration.

We propose a novel approach to compute the field scattered by a particle in the near-zone, in the framework of the transition matrix formulation. This method is based on the expansion of the total near-field in terms of discrete sources vector spherical wave functions and turns out to be particularly effective to model strongly elongated or flattened particles with high permittivity. The performances are evaluated on gold spheroids with several aspect ratios. This method results very useful to understand the complex behavior of enhanced plasmon fields in resonant metallic nanostructures.

In carbon nanotubes (CNTs) with large radii, either metallic or semiconducting, several subbands contribute to the electrical conduction, while in metallic nonarmchair nanotubes with small radii the wall curvature induces a large energy gap. In this paper, we propose a model for the signal propagation along single wall CNTs (SWCNTs) of arbitrary chirality, at microwave through terahertz frequencies, which takes into account both these characteristics in a self-consistent way. We first study an SWCNT, disregarding the wall curvature, in the frame of a semiclassical treatment based on the Boltzmann equation in the momentum-independent relaxation time approximation. It allows expressing the longitudinal dynamic conductivity in terms of the number of effective conducting channels. Next, we study the behavior of this number as the nanotube radius varies and its relation with the kinetic inductance and quantum capacitance. Furthermore, we show that the effects of the spatial dispersion are negligible in the collision dominated regimes, whereas they may be important in the collisionless regimes, giving rise to sound waves propagating with the Fermi velocity. Then, we study the effects on the electron transport of the terahertz quantum transition induced by the wall curvature by using a quantum kinetic approach. The nanotube curvature modifies the kinetic inductance and gives arise to an additional RLC branch in the equivalent circuit, related to the terahertz quantum transition. The proposed model can be used effectively for analyzing the signal propagation in complex structures composed of SWCNTs with different chirality, such as bundles of SWCNTs and multiwall CNTs, providing that the tunneling between adjacent shells may be disregarded.

In this paper, using the rigorous Surface Integral Equation (SIE) method, we study light scattering by Au nano-helices with geometrical dimensions comparable to the wavelength of visible light and we demonstrate that they behave as highly directional nano-antennas with largely controllable radiation and polarization characteristics in the optical regime. In particular, we systematically investigate the radiation properties of helical nano-antennas with realistic Au dispersion parameters in the visible spectral range, and we establish general design rules that enable the engineering of directional scattering with elliptical or circular polarization. Given the realistic material and geometric parameters used in this work, our findings provide novel opportunities for the engineering of chiral sensors, filters, and components for nano-scale antennas with unprecedented beam forming and polarization capabilities. (C) 2015 Optical Society of America

We experimentally and theoretically investigated the second-harmonic (SH) radiation from spherical gold nanoparticles as function of the particle radius in the range from 40 to 100 nm. By fitting the measured SH intensity to an analytical model, we demonstrated size-dependent second-order susceptibility tensor components, which significantly increase by reducing the size of the particles. Finally, we found new SH polarization diagrams for large particle radii arising from the interference among the different multipolar terms.

We present a full-wave analytical solution for the problem of second-harmonic generation from spherical nanoparticles. The sources of the second-harmonic radiation are represented through an effective nonlinear polarization. The solution is derived in the framework of the Mie theory by expanding the pump field, the nonlinear sources and the second-harmonic fields in series of spherical vector wave functions. We use the proposed solution for studying the second-harmonic radiation generated from gold nanospheres as function of the pump wavelength and the particle size, in the framework of the Rudnick-Stern model. We demonstrate the importance of high-order multipolar contributions to the second-harmonic radiated power. Moreover, we investigate the p- and s- components of the SH radiation as the Rudnick-Stern parameters change, finding a strong variation. This approach provides a rigorous methodology to understand second-order optical processes in metal nanoparticles, and to design novel nanoplasmonic devices in the nonlinear regime., Comment: 16 pages, 10 figures

We present a theory for the cloaking of arbitrarily-shaped objects and demonstrate electromagnetic scattering-cancellation through designed homogeneous coatings. First, in the small-particle limit, we expand the dipole moment of a coated object in terms of its resonant modes. By zeroing the numerator of the resulting rational function, we accurately predict the permittivity values of the coating layer that abates the total scattered power. Then, we extend the applicability of the method beyond the small-particle limit, deriving the radiation corrections of the scattering-cancellation permittivity within a perturbation approach. Our method permits the design of invisibility cloaks for irregularly-shaped devices such as complex sensors and detectors.