We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena related to electron confinement. Surprisingly, the coherent plasmon response is suppressed relative to the hot electron response indicating enhanced plasmon dephasing. In a parallel set of studies we demonstrate nanometer scale localization of the nonlinear optical response of single nanoparticles and aggregates and correlate this with their morphology. Position markers are fabricated on an optical and electron-transparent substrate (Si₃N₄ thin film) that allows optical measurements and transmission electron microscopy (TEM) imaging of the identical nanoparticles or aggregates. The second harmonic (SH) activity optical image of individual Ag nanostructures is registered with the TEM image. Centroid localization of the optical signals allows correlation with better than 25 nm precision. This is sufficient to determine the origin of optical "hot spots" within multi-particle aggregates.

We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods in solution, taking advantage of the longitudinal surface-plasmon resonance to enhance optical forces. Stable trapping is achieved using laser light that is detuned slightly to the long-wavelength side of the resonance; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time depends exponentially on laser power, in agreement with a Kramers escape process. Trapped rods have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. The ability to trap and orient individual metal nanoparticles may find important application in assembly of functional structures, sorting of nanoparticles according to their shape, and development of novel microscopy techniques.

Here we describe the application of single star-shaped gold nanoparticles (nanostars) for localized surface plasmon resonant (LSPR) sensing. The gold nanostars were fabricated by a modified seed-mediated, surfactant-directed nanoparticle synthesis which is known to produce gold nanorods in high yield. Due to the sample heterogeneity, single nanostars were studied by dark-field microspectroscopy. The single particle spectra demonstrate that the plasmon resonances of single gold nanostars are extremely sensitive to the local dielectric environment, yielding sensitivities as high as 1.41 eV photon energy shift per refractive index unit. To test their properties as molecular sensors, single nanostar spectra were monitored upon exposure to alkane thiols (mercaptohexadecanoic acid) and a proteins (bovine serum albumin) known to bind gold surfaces. The observed shifts are consistent with the effects of these molecular layers on the surface plasmon resonances in continuous gold films. The results suggest that LSPR sensing with single nanoparticles is analogous to the well developed field surface plasmon resonance (SPR) sensors, and will push the limits of sensitivity.

Local surface plasmons resonances are widely accepted to be the basis for improving the efficiency of absorption and emission processes through a local electromagnetic field enhancement. Nonlinear processes in gold surfaces such as second harmonic generation or two-photon induced photoluminescence are particularly sensitive to this local effect due to their quadratic dependence on the intensity. Isolated regions of enhanced photoluminescence yield on rough gold surfaces were identified emphasizing the physical similarities with surface enhanced Raman scattering (SERS) substrates. In this vein, we investigated luminescence from individual gold nanorods and found that their emission characteristics closely resemble surface plasmon behavior. In particular, we observed spectral similarities between the scattering spectra of individual nanorods and their photoluminescence emission. We also measured a blue-shift of the photoluminescence peak wavelength with decreasing aspect ratio of the nanorods as well as an optically tuneable shape-dependent spectrum of the photoluminescence. The emission yield of single nanorods strongly depends on the orientation of the incident polarization consistent with the properties of surface plasmons.

The ultrafast dynamics of surface electromagnetic waves photogenerated on the surfaces of an Al film perforated with 2D subwavelength hole array was studied by the pump-probe correlation spectroscopy. The time-resolved differential transmission exhibits a fast rise on subpicosecond time scale followed by a plateau with subsequent slow decay. This dynamics is accompanied by a blue shift in transient spectra of the anomalous transmision band. A theoretical model is developed that explains both time-resolved and spectrally-resolved data.

We simulate the optical fields and optical transmission through nanoarrays of silica rings embedded in thin gold films using the finite-difference-time-domain (FDTD) method. By examining the optical transmission spectra for varying ring geometries we uncover large enhancements in the transmission at wavelengths much longer than the usual cutoffs for cylindrical apertures or where surface plasmons or other periodic effects from the array could play a role. We attribute these enhancements to closely coupled cylindrical surface plasmons on the inner and outer surfaces of the rings, and this coupling is more efficient as the inner and outer ring radii approach each other. We confirm this hypothesis by comparing the transmission peaks of the simulation with those deduced from cylindrical surface plasmon (CSP) dispersion curves obtained from a normal mode analysis. These theoretical peak positions and their dependence on the ring geometry are in close agreement with the simulations. The behavior of the CSP dispersion is such that propagating modes can be sent through the rings for ever longer wavelengths as the ring radii approach, whereas the transmission decreases only in proportion to the ring area.

We present a study of optical transmission in the visible and near-infrared regimes through subwavelength apertures in gold films. Samples consisting of single, ~100 nm wide, 50 micron long, linear apertures, centered between two finite grating structures, were prepared using electron-beam lithography with subsequent broad-beam argon-ion milling. The period and number of the corrugations that make up the grating structures was constant, while the distance between the gratings on each side of the aperture was varied. Spectrally resolved far-field transmission measurements were obtained for normal incidence with a spectrometer-coupled optical microscope configured for transmission measurements. Transmission through these structures was significantly enhanced relative to an isolated aperture at resonant wavelengths for transverse magnetic polarized incident light, in agreement with the literature. Wavelengths where the transmission was suppressed relative to an isolated aperture were also observed. The wavelengths of maximum transmission and of suppression were found to depend on the spacing between the grating arrays and the aperture. Measured spectra were consistent with modeled results and can be interpreted in terms of the interference between the incident light and surface plasmon polaritons (SPP) as well as cavity resonances of the SPPs.

This study develops a coupled waveguide-surface plasmon resonance (CWSPR) biosensor with a sub-wavelength grating structure for the real-time analysis of biomolecular interactions. In the proposed optical metrology system, normally incident white light is coupled into the waveguide layer through the sub-wavelength grating structure thereby enhancing the wave vector which excites the surface plasmons on the metal sensing surface. The proposed CWSPR biosensor not only retains the same sensing sensitivity as that of a conventional surface plasmon resonance device, but also yields a sharper dip in the reflectivity spectrum and therefore provides an improved measurement precision. Moreover, the metrology setup overcomes the limitations of the conventional Kretschmann attenuated total reflection approach and is less sensitive to slight variations in the angle of the incident light. The experimental results confirm that the current CWSPR biosensor provides a straightforward yet powerful technique for real-time biomolecular interaction analysis.

Fluorescence from a layer of Rhodamine 6G (R6G) is observed to be enhanced strongly if a dielectric grating deposited onto a gold film is used as a substrate. The fluorescence enhancement has been studied as a function of the grating periodicity and the angle of incidence of the excitation light. The enhancement mechanism is consistent with excitation of surface-plasmon-polaritons on the metal film surface. The observed phenomenon may be promising in sensing applications.

To realize the optical devices required by future systems, we have proposed nanometer-scale photonic integrated circuits (i.e., nanophotonic ICs). These devices consist of nanometer-scale dots, and an optical near field is used as the signal carrier. Since an optical near field is free from the diffraction of light due to its size-dependent localization and resonance features, nanophotonics enables the fabrication, operation, and integration of nanometric devices. To drive a nanophotonic device with an external conventional diffraction-limited photonic device, a far/near-field conversion device is required. Here, we review the use of a nanometer-scale waveguide as such a conversion device for nanophotonic ICs. Furthermore, the fabrication of a nanophotonic device using an optical near-field is introduced.

The science of surface plasmon polaritons (SPPs) has attracted a lot of attention in the last years. In this contribution, we study applications of two-photon absorption of femtosecond laser radiation for the fabrication of dielectric and metallic SPP-structures, which can be used for localization, guiding, and manipulation of SPPs. Dielectric SPP components, e.g. waveguides, bends and splitters are fabricated on gold films. SPP properties are investigated by scanning optical near-field microscopy (SNOM), indicating guiding and reflection of SPPs by polymer lines. SPP excitation on dielectric line and point structures is observed by far-field microscopy. Results on plasmon focussing and on the fabrication and characterization of metallic SPP-structures and components on dielectric substrates will be presented and discussed.

The theory of scattering eigenstates or resonances of a monochromatic electromagnetic (EM) field is briefly reviewed, and is then used to expand the physical field which is present in a two-constituent composite medium when an external or incident field is applied to the system. Special attention is devoted to the case of a composite which has the form of a finite volume of p-constituent (usually metallic, with electric permittivity that has a large negative real part and a small imaginary part, both of them frequency dependent) embedded in an infinite volume of h-constituent (usually a conventional dielectric, with electric permittivity that is nearly real, positive, and frequency independent). Focusing on the case where the frequency is such that the system is close to one of those resonances, we develop a calculation of the shape of the physical field in the composite medium, as well as of the energy content and rate of dissipation of the physical field, and of its lifetime. Consequences for the shape of the physical field are that it is very similar in shape to the eigenfield, and its magnitude is very much amplified when that eigenstate is very localized in space. The size of this localization region is unrelated to the wavelength or any other EM length such as skin depth. It is determined only by the microstructure, whenever the micro-geometric features of the p-constituent are much smaller than any of the EM lengths. This is important in any attempt to use such a resonance in order to implement a nanolens, i.e., to focus an incident EM plane wave or other information-carrying field into a sub-wavelength-sized region.^{1, 2} Consequences for the lifetime of the physical field are discussed-those are important for any attempt to implement a SPASER^3-5 (Surface Plasmon Aplification by Stimulated Emission of Radiation) device.

Great progress has been achieved in fabricating arbitrary metal nanoparticle shapes and geometries in order to control their linear optical properties. However, their nonlinear optical properties, particularly their second-order response, are frequently overlooked. Exploiting the nonlinear responses of metal nanoparticles opens another exciting avenue for developing nanoscale photonics applications. Second-harmonic generation (SHG) from metal nanoparticles is typically attributed to electric dipole excitations at their surfaces, but nonlinearities involving higher multipole effects, such as magnetic dipole interactions, electric quadrupoles, etc., may also be significant due to strong nanoscale gradients in the local material properties and fields. The nanoscale nonlinear optical processes in metal nanoparticles are not well-understood at present, and determining the sources of the SHG response can be arduous. In order to study the role of higher multipoles in the second-order response of gold nanoparticle arrays, we propose SHG measurements employed in both transmission and reflection geometries. Due to different radiative properties of the various multipoles in the forward and backward directions, the presence of multipoles should lead to opposing interference effects in the two directions. Strong polarization dependence of the response can modify the relative strengths of the interfering terms, thereby allowing electric-dipole and higher-multipole contributions to the overall SHG response to be distinguished. Analysis of the measured polarization dependencies would thus provide further knowledge of the mechanisms underlying the nanoscale SHG process in gold nanoparticles.

Closely spaced pairs or "dimers" of elongated gold nanoparticles may be expected to exhibit electric field hotspots. We investigate the possible influence of hotspots on second harmonic generation. Preliminary results show that arrays of nanoparticle dimers exhibit reduced second-harmonic generation compared with arrays of single nanoparticles having similar extinction spectra, contradicting a simple model of second-harmonic generation (varying as the fourth power of the local fundamental field) if hotspots can be shown to exist in such gaps.

In this study, we use the finite-difference time-domain (FDTD) method, the attenuated-total-reflection (ATR) fluorescent and the near field scanning optical microscopy (NSOM) to investigate into the enhancement of near electro-magnetic field via plasmonic effects. In order to enhance the near electro-magnetic (EM) field on the sensing surface, a metal particle layer is added under the Kretschmann configuration of the conventional surface plasmon resonance (SPR) sensor based on the attenuated-total-reflection method. The affiliation by the simulation and experimental results can help us to understand the mechanisms of surface plasmons and particle plasmons on the sensor surface, and the effects of the EM field enhancement are classified as the surface plasmon effect, particle plasmon effect, interparticle coupling effect, and gap mode effect. With the helps of the both techniques, we can understand more about the plasmonic effects in order to deign a novel ultrahigh-resolution plasmonic biosensor.

Using the plasmon hybridization method, we investigate the plasmonic properties of nanoparticles and structures of reduced symmetry. We derive analytical expressions for prolate spheroidal particles including core-shell structures. We also explore the properties of the non-concentric spherical nanoshell geometry present in nanoeggs. Finally, we investigate the plasmonic properties of small nanoparticle aggregates such as trimers and quadrumers and show that group theory can be used to analyze their plasmonic structure.

We predict that nanoparticles of octupolar symmetry (nano-triangles and nano-tetrahedra), whose orientation cannot be affected by means of linear optics, subjected to a coherent mixture of fundamental and second harmonic fields will rotate and orient controlled by the relative phase between these fields. This is due to the generation of the second-harmonic polarization and its interaction with the second-harmonic field. We have described this effect quantitatively for triangular and tetrahedral clusters of metal nanospheres where it can be observed experimentally and used in applications.

Collective oscillations of valence electrons in metallic materials determine their optical response. The energy and strength of these surface oscillations are a function of the shape, size and coupling of the nanoparticles. With the use of a boundary element method (BEM), we solve Maxwell's equations to calculate light scattering and surface modes in nanorods that are commonly used as hosts and/or samples in different field-enhanced scanning-probe microscopies and spectroscopies. We calculate the near-field and far-field response of nanorods and show that different geometrical terminations of the rods give different optical response in the far field for short rod lengths. For longer lengths, the response of rods with different terminations becomes more similar. The near field features of the ends become most evident close to the rod structural features that define the end capping. We identify four regimes for the separation between nanorod pairs that provide different coupling between nanorods. We also show that the size dependence of the nanorod response is characterized by a rod radius that gives a minimum wavelength for the dipolar response. For thicker and thinner rods, the response redshifts.

We report on the development of a surface micromachined process for the fabrication of coaxial apertures surrounded by periodic grooves. The process uses a combination of copper electroforming and the negative epoxy based resist, SU8, as a thin flexible substrate. The device dimensions are suitable for the implementation of filters at THz frequencies, and measurements show a pass band centred around 1.5 THz. These devices could form the basis of the next generation of THz biosensors.

We observed resonantly enhanced (or anomalous transmission) terahertz transmission through two-dimensional (2D) periodic arrays of subwavelength apertures with various periodicities fabricated on metallic organic conducting polymer films of polypyrrole heavily doped with PF₆ molecules [PPy(PF6)]. The anomalous transmission spectra are in good agreement with a model involving surface plasmon polariton excitations on the film surfaces. We also found that the resonantly enhanced transmission peaks are broader in the exotic metallic PPy(PF6) films compared to those formed in 2D aperture array in regular metallic films such as silver, indicating that the surface plasmon polaritons on the PPy(PF6) film surfaces have higher attenuation.

We discuss various designs of two-dimensional dielectric optical elements, which enable efficient coupling of external light to surface plasmon polaritons, and allow us to guide and redistribute surface plasmon energy in the plane of propagation. Examples of these 2D dielectric optical elements include lenses, mirrors, waveguides, 2D plasmonic crystals, etc. The effective refractive index of the plasmonic crystals may be either positive or negative. These simple elements enable us to create compound 2D optical devices, such as microscopes and waveguide couplers. These devices exhibit diffraction-limited resolution, which is considerably better than the resolution of usual three-dimensional light optics.

Localized surface plasmon resonance (LSPR) is one of the signature optical properties of noble metal nanoparticles. Since the LSPR wavelength λ_max is extremely sensitive to the local environment, it allows us to develop nanoparticle-based LSPR chemical and biological sensors. In this work, we tuned the LSPR peaks of Ag nanotriangles and explored the wavelength-dependent LSPR shift upon the adsorption of some resonant molecules. The induced LSPR peak shifts (Δλ_max) vary with wavelength and the line shape of the LSPR shift is closely related to the absorption features of the resonant molecules. When the LSPR of the nanoparticles directly overlaps with the molecular resonance, a very small LSPR shift was observed. An amplified LSPR shift is found when LSPR of the nanoparticles is at a slightly longer wavelength than the molecular resonance of the adsorbates. Furthermore, we apply the "amplified" LSPR shift to detect the substrate binding of camphor to the heme-containing cytochrome P450cam protiens (CYP101). CYP101 absorb light in the visible region. When a small substrate molecule binds to CYP101, the spin state of the molecule is converted to its low spin state. By fabricating nanoparticles with the LSPR close to the molecular resonance of CYP101 proteins, the LSPR response as large as ~60 nm caused by the binding of small substrate has been demonstrated.

Plasmon-enhanced vibrational Raman spectroscopy enables us to learn about the conformations of single molecules. Here, we report surface-enhanced Raman studies of single chains of the widely studied model conjugated polymer, MEH-PPV. We will provide evidence that field localization on nanotextured silver surfaces is so strong that, remarkably, scattering from a single chromophore out of hundreds dominates the Raman spectrum. We observe wide variation of spectra from chain to chain and strong temporal fluctuations in the Raman spectrum and intensity for individual chains of MEH-PPV. In some cases, we are able to clearly associate these fluctuations with photochemistry and polaron formation. In most cases, however, we believe that local conformational changes in the polymer are responsible for the fluctuations.

Recent theoretical and experimental studies have shown that imaging with resolution well beyond the diffraction limit can be obtained with so-called superlenses. Images formed by such superlenses are, however, in the near field only, or a fraction of wavelength away from the lens. In this paper, we propose a far-field superlens (FSL) device which is composed of a planar superlens with periodical corrugation. We show in theory that when an object is placed in close proximity of such a FSL, a unique image can be formed in far-field. As an example, we demonstrate numerically that images of 40 nm lines with a 30 nm gap can be obtained from far-field data with properly designed FSL working at 376nm wavelength.

Surface plasmon polaritons, sometimes referred to as Surface Plasmons (SPs) have brought us great opportunities to work in nanoscale at optical frequencies. The SPs at the two surfaces of a thin metal film interact with each other, hence generate new modes which are either symmetric or anti-symmetric. For anti-symmetric modes, the dispersion curve turns to be of negative slope at large wave vectors, so two different anti-symmetric modes can be excited at the same frequency. These two modes can form beats with novel features. The envelope (profile) of the beating SP waves could be stationary, which means its shape will not change in time. Our simulation results clearly showed such phenomena, which is a strong evidence of the SPs dispersion relations at the thin metal film. It is a proof of the existence of negative group velocity of SPs. Beats can help us determine the difference in k and the amplitudes ratio of the two beating waves. We also studied beating between anti-symmetric mode and symmetric mode SPs with the same frequency. The study of the energy density distribution showed that the output from such system can be well controlled through beats formation. Example by using NSOM (Near-field Scanning Optical Microscopy) has been simulated. The beating phenomena have a potential application in the integrated optical circuits.

By tailoring the dispersion curve of surface plasmons (SPs) of a thin metallic film surrounded by dielectric half-spaces, it is shown that the group velocity of the symmetric mode is always positive, while the group velocity of the anti-symmetric mode can be negative. Consequently, the forward and backward propagation of SPs, in which the energy flow is respectively parallel or antiparallel to the wave vector, can be realized. The physical origin of the intriguing backward SPs is given. Furthermore, schemes for the negative refraction and imaging of SPs are proposed by incorporating two plasmon modes with group velocities of opposite signs.

This study utilizes a surface-enhanced Raman spectroscopy (SERS) based on the attenuated-total-reflection method to investigate the secondary structures of long oligonucleotides and their influence on the DNA hybridization. It is found that the ring-breathing modes of adenine, thymine, guanine, and cytosine in Raman fingerprint associated with three 60mer oligonucleotides with prominent secondary structures are lower than those observed for the two oligonucleotides with no obvious secondary structures. It is also determined that increasing the DNA hybridization temperature from 35°C to 45°C reduces secondary structure effects. The kinetics of biomolecular interaction analysis can be performed by using surface plasmons resonance biosensor, but the structural information of the oligonucleotides can not observed directly. The SERS spectrum provides the structural information of the oligonucleotides with the help of a silver colloidal nanoparticle monolayer by control of the size and distribution of the nanoparticles adapted as a Raman active substrate. Also, the detection limit of the DNA Raman signal has been successfully improved to reach sub-micro molarity of DNA concentration.

Three kinds of colloids containing gold nanoparticles (AuNP) were obtained by three different methods of synthesis, using castor oil as dispersant agent and tetrachloroauric (III) acid as gold source. The colloidal systems were characterized by Uv-vis spectroscopy and transmission electron microscopy (TEM). Each method gave rise to quasispherical shape and different size distribution of AuNP. The TEM images of the nanostrutured systems show that from each method of synthesis, nanoparticles of different average sizes, equal to 7, 15, and 55 nm, were produced. These characteristics are reflected by the presence of different maximum wavelength absorption, indicating that each colloid presents distinct surface Plasmon resonance bands.

We report on the observation of a large thermal nonlinearity of an organic material enhanced by the presence of gold nanoparticles. The studied system consisted of a colloid of castor oil and gold particles with average diameter of 10 nm, with filling factor of 4.0x10^-5. Z-scan measurements were performed for an excitation wavelength tuned at 810 nm in the CW regime. It was observed that this colloidal system presents a large thermal nonlinear refractive index, which was equal to -7.4x10^-8 cm²/W. This value is about 41 times larger than the n₂ of the host material. The thermo-optic coefficient of the colloid was also evaluated, and a large enhancement was observed in its value owing to the presence of the gold nanoparticles in the organic material.

This paper is intended to demonstrate the effect of coupled long-range surface plasmon polaritons (LRSPPs) on the optical response of a lamellar grating nanostructure with organic material on the surface. This phenomenon gives rise to a selective spectral response and a local field enhancement which can be used in the context of nano-optics. This novel structure of nanofabricated device, consisting of coupled organic/metal nanostructure with specific width and symmetric dielectric structure. By designing the size and shape of the grating nanostructure, and the location of the organic Alq3 relative to the surfaces, Alq3 can be quenched, display increases in emission quantum yield, and decreasing the lifetimes. The combinations of organic/metal interface LRSPP mode can emit specific direction rather than isotropic emission. We discuss recent experimental results and potential applications in biosensor, organic light emitting diodes (OLEDs), polymer laser and organic solar cells of organic/metal grating enhanced emission resonance energy interactions.

The near and far-field transmission characteristics of nanoscale annular array metamaterials fabricated using ion beam lithography are investigated both computationally and experimentally. Experimental results in the far-field regime demonstrate high transmission efficiencies in the near infra-red region of the electromagnetic spectrum for these devices, in excellent qualitative agreement with a previously developed numerical model. The diffractive near-field behavior of such structures is discussed, with a particular emphasis on the implications associated with verifying such predictions experimentally.

We obtain the surface mode solution for the Helmholtz equation using an analog formalism for diffracted free beams. By means of a linear superposition of the surface modes we obtain an expression similar to the angular spectrum model which allows us the generation of surface diffraction fields. For the understanding of the physical features implicit in this representation, we describe the interaction between two counter-propagating surface modes, generating a standing plasmonic wave, whose nodes then generate a stationary local charge distribution. The study is reinforced by associating extremal features to the surface modes and an eikonal equation for plasmonic fields is obtained, where a plasmonic refractive index appears naturally. This representation allows us to interpret the surface optical field as a geodesic flow which in principle enables us to associate coherence features to plasmon modes and to analyze the stability of the surface fields under small perturbations of the refractive index.

Enhanced transmission through structures consisting of linear gratings surrounding a single subwavelength aperture in an opaque gold film is modeled using a commercial finite element model (FEM). The stability of the FEM and boundary conditions are discussed, and different field visualizations are explored to gain insight into field behavior. The results from the FEM were compared with experimental results, yielding excellent agreement. This lends confidence that the FEM is giving an accurate representation of the field behavior around the structure. The FEM was then used to examine how transmission enhancement depends on geometric properties of the structure and to gain insight into the mechanisms of transmission enhancement.

The design principle of split-ring resonators (SRRs) for realizing the negative magnetic metamaterial is proposed through the theoretical investigation of magnetic properties of the SRRs from THz to the visible light region. To describe the frequency dispersion of metal throughout the frequency range, we consider the exact expression of the internal impedance formula. This formula can describe not only the conduction characteristics but also the dielectric behavior of metal in the optical frequency region. Based on this investigation, we successfully determine the magnetic responses of the SRRs, which are characterized by the metal's dispersive properties, from THz to the visible light region. Our results indicate that the design principle should be changed considerably at the transition frequency of 100 THz. Below 100THz, since the resistance of the SRRs determines the magnetic responses, the low resistance structures are essential. On the other hand, above 100THz region, because the decrease of the geometrical inductance dominantly reduces the magnetic responses, we should design the SRRs' structures maintaining large geometrical inductance. The theoretical limitation of the SRR's operating frequency with a negative permeability is also discussed from the viewpoint of the saturation of the magnetic response.

We study surface-enhanced infrared absorption, including multiphoton processes, due to the excitation of surface plasmons on metal nanoparticles. The time-dependent Schroedinger equation and finite-difference time-domain method are self-consistently coupled to treat the problem.