Ensembles of plasmonic nanospheres at optical frequencies and a problem of negative index behavior

Arrays of metallic nanoparticles support individual and collective plasmonic excitations that contribute to unusual phenomena like surface-enhanced Raman scattering, anomalous transparency, negative index, and subwavelength resolution in various metamaterials. We examined the electromagnetic response of dual Kron’s lattice and films containing up to three monolayers of metallic nanospheres. It appears that open cubic Kron’s lattice exhibits ‘soft’ electromagnetic response but no negative index behavior. The close-packed arrays behave similarly: there are plasmon resonances and very high transmission at certain wavelengths that are much larger than the separation between the particles, and a ‘soft’ magnetic response, with small but positive effective index of refraction. It would be interesting to check these predictions experimentally. PACS 78.20.Ci; 42.30.Wb; 73.20.Mf; 42.25.Bs     

Near-field spectroscopy of surface plasmons in flat gold nanoparticles

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.     

Optical resonances and angular filtering functionality of subwavelength hyperbolic etalons

We show that a slab of subwavelength thickness can exhibit etalon resonances and can provide angular filtering functionality at optical wavelengths if it is filled by an anisotropic medium whose principal permittivities have different signs (hyperbolic medium) and have amplitudes smaller than one. This is possible since extraordinary plane waves hyperbolic dispersion allows the vacuum radiation to couple with medium plane waves whose longitudinal wavenumbers are sufficiently large to allow the settlement of standing waves within the nanometric slab thickness. We consider a mixture of metal nanoparticles dispersed within a liquid crystal matrix and we show that it can be designed to exhibit the considered unusual optical response.     

Electromagnetic interaction of fluorophores with designed two-dimensional silver nanoparticle arrays

We study the fluorescence enhancement of dye molecules adsorbed on regular two-dimensional arrays of designed silver nanoparticles. The silver particles show two orthogonal optical resonances at different wavelengths because of their elongated shape. The short-wavelength resonance was designed to fit to the absorption maximum of the fluorophore. When the excitation light drives the short-wavelength resonance, the measured fluorescence intensity is strongly enhanced compared to that for the orthogonal particle orientation. This shows directly a strong electromagnetic coupling between the nanoparticles and the fluorophore. Additionally enhanced photochemical bleaching is observed due to the interaction of fluorophores with the particles. Using a rate model describing the fluorescence enhancement and the bleaching enhancement, an average value for the particle-induced increase in the radiative fluorescence rate is obtained, together with a lower limit for the averaged particle-induced field intensity enhancement factor. Received: 3 July 2001 / Revised version: 3 September 2001 / Published online: 15 October 2001     

Size-controlled aerosol synthesis of silver nanoparticles for plasmonic materials

Aerosol techniques were used to synthesize spherical and monodisperse silver nanoparticles for plasmonic materials. The particles were generated with an evaporation-condensation technique followed by size selection and sintering with a differential mobility analyzer and a tube furnace, respectively. Finally, the nanoparticles were collected on a glass substrate with an electrostatic precipitator. The particle size distributions were measured with a scanning mobility particle sizer and verified with a transmission electron microscope. A spectrophotometer was used to measure the optical extinction spectra of the prepared samples, which contained particles with diameters of approximately 50, 90 and 130?nm. By controlling the particle size, the dipolar peak of the localized surface plasmon resonance was tuned between wavelengths of 398 and 448?nm. In addition, quadrupolar resonances were observed at shorter wavelengths as predicted by the simplified theoretical model used to characterize the measured spectra.     

Formation of Sodium and Potassium Nanoparticles upon Local Electron Bombardment of Alkali-Halide Crystals

Technical Physics - It has been shown experimentally that upon local bombardment of NaCl, KCl, and KBr crystals with 50-keV electrons, sodium and potassium nanoparticles with plasmon resonances in...     

Mode-matched Fano resonances for all-optical switching applications

We introduce the concept of mode-matched Fano resonances in one-dimensional, subwavelength, diffraction gratings. These resonances are characterized by an extremely high quality (Q)-factor and lend themselves well for applications to ultra-low power, all-optical switching devices at telecommunication wavelengths. We provide examples of diffraction gratings made of a chalcogenide glass (As₂S₃) where, thanks to the mode-matched Fano resonances, all-optical switching is obtained at local field intensities well below the photo-darkening threshold of the material.     

Luminescence Enhancement from Silica-Coated Gold Nanoparticle Agglomerates Following Multi-photon Excitation

Multi-photon absorption induced luminescence (MAIL) from bare gold nanoparticles, silica-coated particles, as well as silica-coated agglomerated gold nanoparticles suspended in aqueous solution was studied by using time-resolved and steady-state luminescence spectroscopy. The nanoparticles were excited by femtosecond pulses of wavelengths ranging from 630 nm to 900 nm. The luminescence from the particles exhibits a broad spectrum in the UV and VIS region. The time-resolved measurements indicate a luminescence lifetime of a few ps, limited by the response of the experimental system. The studied dependence of the MAIL efficiency on the excitation wavelength showed that the luminescence from silica-coated agglomerates was enhanced over the whole range of excitation wavelengths, when compared to the luminescence from individual gold nanoparticles. The agglomerates show an almost excitation wavelength independent efficiency of the MAIL, while for individual nanoparticles a rapid decrease of the MAIL efficiency was observed with increasing excitation wavelength. The observed enhancement of the MAIL from the agglomerated nanostructures can be attributed to the presence of localized surface plasmon resonances in the spectral region corresponding to the excitation wavelengths. The high MAIL efficiency from the agglomerated nanoparticle structures in the near-infrared could be an advantage in the expanding field of luminescence-based-imaging, as well as in biosensor technology.     

Control of light scattering by nanoparticles with optically-induced magnetic responses

Conventional approaches to control and shape the scattering pattems of light generated by different nanostructures are mostly based on engineering of their electric response due to the fact that most metallic nanostructures support only electric resonances in the optical frequency range. Recently, fuelled by the fast development in the fields of metamaterials and plasmonics, artificial optically-induced magnetic responses have been demonstrated for various nanostructures. This kind of response can be employed to provide an extra degree of freedom for the efficient control and shaping of the scattering patterns of nanoparticles and nanoantennas. Here we review the recent progress in this research direction of nanoparticle scattering shaping and control through the interference of both electric and optically-induced magnetic responses. We discuss the magnetic resonances supported by various structures in different spectral regimes, and then summarize the original results on the scattering shaping involving both electric and magnetic responses, based on the interference of both spectrally separated （with different resonant wavelengths） and overlapped dipoles （with the same resonant wavelength）, and also other higher-order modes. Finally, we discuss the scattering control utilizing Fano resonances associated with the magnetic responses.     

Observation of antiphase dynamics and harmonic resonance in a modulated dual-wavelength laser

We observe a nonlinear response of a dual-wavelength Nd:YAG laser when subjected to low-frequency periodic modulations of cavity losses. The modulation frequency is far from the relaxation oscillation frequency. The harmonic resonances of the two laser wavelengths associated with antiphase intensity oscillations are demonstrated and resonances up to the fourth order were observed. For relatively weak modulation, the intensity oscillation frequency of the laser is equal to the modulation frequency. Harmonic resonances occur under a stronger modulation. We find that more harmonic components appear when the modulation frequency is increased. Furthermore, with enhancing the modulation, the dominant frequency of the intensity oscillations of both wavelengths is shifted toward the higher-order harmonic frequency.     

Optical Resonances of the Systems of Nanoparticles with a Metallic Shell

Characteristic features of the formation of the plasma resonance absorption spectra of double-layer nanoparticles with a dielectric core and metal shell were investigated theoretically and experimentally. Two peaks of the surface plasma resonances were observed with the example of an AgI–Ag system. The model of the conductivity electron free path limitation suggested by Kreibig for describing the dimensional dependence of the optical constants of homogeneous spherical metal nanoparticles was extended to the case where metal is concentrated in the shell of the particle. It is established that allowance for the dimensional effect leads to a decrease in plasma resonance absorption and expansion, with the two-peak band structure being preserved. The influence of the metal shell granularity and the degree of the polydispersity of particles on the spectral position, halfwidth, and absolute value of absorption resonances was investigated.     

Efficient generation of far-infrared radiation in the vicinity of polariton resonance of lithium niobate

We efficiently generated far-infrared radiation at the wavelengths centered at 20.8?μm in the vicinity of one of the polariton resonances of lithium niobate. Such an efficient nonlinear conversion is made possible by exploiting phase matching for difference-frequency generation in lithium niobate. The highest peak power reached 233?W.     

Substrate-Modulated Electromagnetic Resonances in Colloidal Cu2O Nanospheres

Dielectric nanoparticles are expected to complement or even replace plasmonic nanoparticles in many optical and optoelectronic applications, because they exhibit small absorption losses and support strong electric and magnetic resonances simultaneously. Dielectric nanoparticles need to be deposited on various substrates in many applications. Understanding the substrate effect on the electromagnetic resonances of dielectric nanoparticles is of great importance for engineering their resonance properties and designing optical devices. In this study, moderate-refractive-index cuprous oxide nanospheres with uniform sizes and shapes are synthesized. The scattering spectra and images of the nanospheres deposited on three types of substrates are analyzed experimentally and theoretically. When supported on indium tin oxide–coated glass slides and Si wafers, the color of the nanospheres varies from blue, cyan, green, yellow, orange and red, covering almost the entire visible region. When deposited on gold films, the electromagnetic resonances of the nanospheres redshift intensively and a new effective magnetic resonance mode appears. The enhanced Raman scattering reveals that large electromagnetic field enhancements are produced in the gap region between the nanosphere and the substrate. The results shed light on the manipulation of the electromagnetic responses of dielectric nanoparticles and the design of dielectric metamaterials in the presence of various substrates.     

Magnetron and pulsed laser deposition of silver and gold nanoparticles and discontinuous films and their optical properties

Ag and Au nanoparticles are obtained by magnetron sputtering and pulsed laser deposition under different conditions, and the features of their absorption spectra associated with plasmon resonances are investigated. Optimal deposition conditions for obtaining small (5?C10 nm) silver nanoparticles with a high density of surface distribution include an increased argon pressure (2.5 × 10^?2 Torr) and a low discharge voltage (100 V). Gold nanoparticle arrays obtained by pulsed laser deposition at a temperature of 200°C in vacuum are more uniformly distributed on the substrates than those deposited at room temperature in argon. It is shown that the maximum of the plasmon absorption shifts toward shorter wavelengths with a decrease in the equivalent thickness of metal films and depends not only on this thickness but also on the type of substrate, which is responsible for the morphology of nanoparticle arrays.     

Demonstration of polarization-independent resonant subwavelength grating filter arrays

We demonstrate a two-dimensional (2D) polarization-independent resonant subwavelength grating (RSG) in a filter array. RSGs, also called guided mode resonant filters, are traditionally one-dimensional gratings; however, this leads to TE and TM resonances at different wavelengths and with different spectral shape. A 2D grating can remove the polarization dependence at normal incidence, while maintaining the desirable RSG properties of high reflectivity, narrow passband, and low sidebands without ripple. We designed and fabricated 2D gratings with near-identical responses for both polarizations at normal incidence in the telecommunication band. Ninety percent reflectivity is achieved at the resonant wavelengths.     

New aspects of the electronic structure of the GaSb-InAs (001) superlattice

Relativistic pseudopotentials have been used to calculate the electronic structure of the GaSb-InAs (001) superlattice of period 36A.We present energy levels and charge densities of the states in the superlattice. In particular, new resonances and confined states with large amplitudes at interfaces are identified. We also show that scattering processes associated with wavelengths which are comparable with the width of the confining layers are not important in the formation of the confined states but are essential in the formation of the resonances.     

Transmission resonances of metallic compound gratings with subwavelength slits

Transmission metallic gratings with subwavelength slits are known to produce enhanced transmitted intensity for certain resonant wavelengths. One of the mechanisms that produce these resonances is the excitation of waveguide modes inside the slits. We show that by adding slits to the period, the transmission maxima are widened and, simultaneously, this generates phase resonances that appear as sharp dips in the transmission response. These resonances are characterized by a significant enhancement of the interior field.     

Temperature-dependent Sellmeier equation for the refractive index of stoichiometric lithium tantalate

We present a temperature-dependent Sellmeier equation for the refractive index of stoichiometric LiTaO3. The extraordinary refractive index, for the range 0.39-4.1 microm and for temperatures of 30-200 degrees C, are based on previously published data [Jpn. J. Appl. Phys. 41, 465 (2002)] and on measured data derived from quasi-phase-matched (QPM) resonances. We used the new Sellmeier coefficients that we obtained to calculate the QPM wavelengths for an optical parametric oscillator (OPO) based on periodically poled stoichiometric LiTaO3 pumped at 1064 nm. The measured wavelengths of the OPO were in good agreement with our predictions.     

Electrostatic and optical resonances of arrays of cylinders

The solutions of the electrostatic potential problem for the square and hexagonal arrays of circular cylinders with zero applied field (homogeneous or resonant solutions) are studied. We show that for non-touching cylinders, the set of resonances is discrete except in the neighbourhood of one point, at which the dielectric constant of the array has an essential singularity. For arrays of touching cylinders, the set is well represented by a continuous distribution. This representation enables the derivation of the asymptotic form of the expansion for the dielectric constant of the array when the dielectric constant of the cylinders is large. The known value of the first term in the expansion enables us to derive the second term. The physical characteristics of the resonant solutions are studied. Metals achieve values of dielectric constant which are close to the resonant values (real and negative) for certain wavelengths. Curves are given which enable the prediction of those wavelengths at which the optical resonances of both arrays occur, for any area fraction and composition of a columnar cerment film.     

Spiral morphology-dependent resonances in an optical fiber: effects of fiber tilt and focused Gaussian beam illumination

Spiral morphology-dependent resonances have been observed in a tilted optical fiber. The polarization-preserving and the cross-polarized elastic-scattering spectra for plane-wave illumination show that the wavelengths of the resonances are blueshifted quadratically as the fiber tilt angle increases. When a focused Gaussian beam illuminates the fiber at its edge, the resonances are blueshifted and broadened as the detector is offset from the scattering plane with the maximum scattering intensity. The blueshift with focused beam illumination is also a consequence of the spiral resonances.