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RAS spectra of 8 ML Pb deposited on Si (5 5 7)-5 × 1-Au. Data points and model spectra (solid line) (a) uncapped; the inset shows an SEM image with a 200 nm scale bar; (b) capped with 4.5 nm of a-Si.
Source publication
Anisotropic nanoparticle (NP) arrays with useful optical properties, such as localized plasmon resonances (LPRs), can be grown by self-assembly on substrates. However, these systems often have significant dispersion in NP dimensions and distribution, which makes a numerical approach to modeling the LPRs very difficult. An improved analytic approach...
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The aggregation state of nanoparticles (NPs) must be precisely known in order to study the structure-property relationship and to evaluate the exploitability of NPs dispersion for a given appli-cation. Here we report a general technique for sample preparation to investigate with transmission electron microscopy (TEM) the state of aggregation of NPs...
Citations
... In such structures, the optical response can be made strongly dependent on the direction of incident light and/or its polarization [16]; however, even more freedom in designing a desired functionality can be obtained by using anisotropic materials. At the very basic level an illumination/polarizationdependent optical response can be achieved not by tuning the shape of the particle [17,18], but rather the birefringence of the constituent material [19,20]. ...
Hyperbolic materials offer much wider freedom in designing optical properties of nanostructures than ones with isotropic and elliptical dispersion, both metallic or dielectric. Here, we present a detailed theoretical and numerical study on the unique optical properties of spherical nanoantennas composed of such materials. Hyperbolic nanospheres exhibit a rich modal structure that, depending on the polarization and direction of incident light, can exhibit either a full plasmonic-like response with multiple electric resonances, a single, dominant electric dipole or one with mixed magnetic and electric modes with an atypical reversed modal order. We derive conditions for observing these resonances in the dipolar approximation and offer insight into how the modal response evolves with the size, material composition, and illumination. Specifically, the origin of the magnetic dipole mode lies in the hyperbolic dispersion and its existence is determined by two diagonal permittivity components of different sign. Our analysis shows that the origin of this unusual behavior stems from complex coupling between electric and magnetic multipoles, which leads to very strong scattering or absorbing modes. These observations assert that hyperbolic nanoantennas offer a promising route towards novel light–matter interaction regimes.
... At the very basic level an illumination/polarization-dependent optical response can be achieved not by tuning the shape of the particle [17,18], but rather the birefringence of the constituent material [19,20]. ...
Hyperbolic materials offer a much wider freedom in designing optical properties of nanostructures than ones with isotropic and elliptical dispersion, both metallic or dielectric. Here, we present a detailed theoretical and numerical study of the unique optical properties of spherical nanoantennas composed of such materials. Hyperbolic nanospheres exhibit a rich modal structure that, depending on the polarization and direction of incident light, can exhibit either a full plasmonic-like response with multiple electric resonances, a single, dominant electric dipole or one with mixed magnetic and electric modes with an atypical reversed modal order. We derive resonance conditions for observing these resonances in the dipolar approximation and offer insight into how the modal response evolves with the size, material composition, and illumination. Specifically, the origin of the magnetic dipole mode lies in the hyperbolic dispersion and its existence is determined by two diagonal permittivity components of different sign. Our analysis shows that the origin of this unusual behavior stems from complex coupling between electric and magnetic multipoles, which leads to very strongly scattering or absorbing modes. These observations assert that hyperbolic nanoantennas offer a promising route towards novel light-matter interaction regimes.
... This multivariate dependence makes unclear the interpretation of the SRF. Persechini et al. (2014) have introduced the nanoparticle shape in effective medium theory to exploit reflectance anisotropy spectroscopic measurements. However, the authors consider monodispersed NPs. ...
This paper reports the complete ellipsometric characterization of gold nanoparticles (NPs) embedded in a photoresist films. The effective dielectric function of nanocomposite films as well as the shape distribution and the volume fraction of NPs are extracted from ellipsometric measurements by introducing an effective medium theory which takes into account the NP shape distribution and the intrinsic confinement effect. This theory remains valid as long as the nanoparticle interaction is negligible. We show that the magnitude of the confinement depends on the nanoparticle shape and the environment through chemical damping. This suggests that the NP shape distribution can be directly estimated by ellipsometry, while the determination of absolute radius distribution requires transmission electron microscopy measurements. The imaginary part of the effective dielectric function exhibits a strong asymmetric surface plasmon band, while a large variation of the real part occurs close to the resonance. The redshift and the broadening of the plasmon band as the gold volume fraction increases are correlated to the evolution of NP shape distribution. This evolution is attributed to a competition between the nucleation and the coalescence of NPs. This unambiguously demonstrates that ellipsometry combined with a shape-distributed effective medium theory is a powerful alternative tool to transmission electron microscopy for the NP shape analysis.
... This part of the spectrum is different in the single-crystal sample and in the N-O array target on which the surface plasmon is excited. The problem addressed here is quite remote from the one that underlies the description of an optical absorption spectrum [51,52]. Therefore, we focus our attention onto the fact that it is the roughness of the surface of the N-O array target, that triggers the excitation of the SP wave. ...
The process of photoelectron emission from gold surfaces covered with nano-objects that are organized in the form of a periodic array is addressed in the short laser pulse regime ([Formula: see text] fs) at moderate intensities [Formula: see text] W cm(-2) and for various laser wavelengths. The emission spectrum from a gold single crystal measured under the same conditions is used for reference. The comparison of the photo-emission yield and the energy of the ejected electrons with their counterparts from the (more simple) reference system shows that the periodic conditions imposed on the target surface drastically enhance both quantities. In addition to the standard mechanism of Coulomb explosion, a second mechanism comes into play, driven by surface plasmon excitation. This can be clearly demonstrated by varying the laser wavelength. This interpretation of the experimental data is supported by predictions from model calculations that account both for the primary quantum electron emission and for the subsequent surface-plasmon-driven acceleration in the vacuum. Despite the fact that the incident laser intensity is as low as [Formula: see text] W cm(-2), such a structured target permits generating electrons with energies as high as 300 eV. Experiments with two incident laser beams of different wavelengths with an adjustable delay, have also been carried out. The results show that there exist various channels for the decay of the photo-emission signal, depending on the target type. These observations are shedding light on the various relaxation mechanisms that take place on different timescales.
... Specifically, all pressure ranges of gas-solid interfaces are accessible, the material damage and contamination associated with charged particle probes is eliminated, and insulating substrates can be studied without the problem of charging effects [15]. For example, it has been shown recently that reflection anisotropy spectroscopy (RAS), a surface sensitive form of polarized reflectance, is a particularly useful in situ probe of Ag NP growth on faceted Al 2 O 3 [7,16], while spectroscopic ellipsometry combined with analytic modelling of the optical response also appears promising [17,18]. ...
... Selected samples were analysed using linear optics to assess reflectivities and resonance energies: some of these results have been published elsewhere [18]. SHG was then performed: α-p and α-s measurements were made on Ag on Al 2 O 3 and on rippled Si samples, along both xand y-azimuths. ...
... The maximum of the α-p response is roughly 3 times larger along the y-azimuth than the x-azimuth, which is consistent with the higher NP density in the yz-plane (see Fig. 1). In addition, Table 1 shows an average spacing between NPs in the y direction of 6 nm, a separation where dipole-dipole interactions will have a significant effect on the local fields [18]. Table 1 also shows a NP aspect ratio of 1.2 and thus aligned NPs with an average 20% departure from isotropy in the y direction, combined with the dipole-dipole interaction between the NPs in arrays aligned in the same direction, produce the dramatic change in the SH response observed. ...
Off-normal, polarization dependent second–harmonic generation (SHG) measurements were performed ex situ on plasmonic nanostructures grown by self–assembly on nanopatterned templates. These exploratory studies of Ag nanoparticle (NP) arrays show that the sensitivity of SHG to the local fields, which are modified by the NP size, shape and distribution, makes it a promising fixed wavelength characterization technique that avoids the complexity of spectroscopic SHG. The off–normal geometry provides access to the out–of–plane SH response, which is typically an order-of-magnitude larger than the in–surface–plane response measured using normal incidence, for example in SHG microscopy. By choosing the plane of incidence orthogonal to the NP array direction, it was shown that the p–polarized SH response, as a function of input polarization, is very sensitive to NP morphology, with a change of 20% in the aspect ratio of the NPs producing a variation of a factor of 30 in the easily measureable ratio of the p–polarized SH field strength for s– and p–polarized input. The results show that such a fixed geometry could be used for the in situ characterization of anisotropic nanostructure morphology during growth by self–assembly, which could be particularly useful in situations where rotating the sample may be neither desirable nor easily accomplished.
Herein, the formation of Au nanoclusters on nitridized GaAs(001) surface is described, as well as the structure diagnostics and spectroscopic studies which reveal a strong anisotropy of the plasmons localized on the clusters. Principal aspects of the work are the following. Technologically, structures of Au/N/GaAs are fabricated with a monolayer of nitrogen atoms chemisorbed preliminary onto GaAs substrate to prevent its reaction with subsequently deposited Au film. Annealing of the structures Au/N/GaAs results in the appearance of anisotropic nanoclusters of chemically clean gold on GaAs surface. Experimentally, the existence of in‐surface anisotropy of Au clusters is verified with the atomic force microscopy and it is investigated with the resonant optical spectroscopies of anisotropy reflectance and polarized reflection. All the methods are applied jointly for the detailed study of anisotropic plasmons revealed in gold nanocluster arrays. Theoretically, the plasmon‐conditioned features observed in optical polarized spectra are interpreted using an optical model of in‐surface anisotropic plasmons in Au nanospheroids. As a result, the macroscopic anisotropy and orientation of gold nanoclusters and their plasmons relative to the crystallographic axes of GaAs substrate are unambiguously established and reliably specified. Chemically clean gold nanoclusters are fabricated on GaAs(001) surface covered by a monolayer of chemisorbed nitrogen atoms. The on‐surface Au clusters possess a pronounced anisotropy of both shape and orientation in crystallographic [11¯0] direction on GaAs surface. Localized plasmons of the clusters also reveal rather strong in‐surface anisotropy detected by polarized reflection spectroscopy.
