Hernando Garcia

Phonon-assisted two-photon absorption in the presence of a dc-field: the nonlinear Franz–Keldysh effect in indirect gap semiconductors

The two-photon absorption coefficient of an indirect gap semiconductor (phonon-assisted two-photon absorption) in the presence of a strong dc-electric field applied perpendicular to the direction of propagation of the optical field is calculated using the formalism developed elsewhere (Aspnes 1996 Phys. Rev. B. 147 554). We show that depending on the type of transition (i.e., allowed–allowed, allowed–forbidden or forbidden–forbidden), the absorption coefficient followed different dispersion relations. In the limit of a weak electric field, we recovered results previously calculated using perturbation theory. In the strong dc-field regime, we found that below the rescaled energy gap given by Eg/N, where N is the number of photons, the tunnelling effect is present, but to our surprise, above the rescaled gap, the Franz–Keldysh oscillations are present only for the allowed–allowed transition. This absence of the oscillations in the allowed–forbidden and forbidden–forbidden transitions is possible due to the weak coupling of the tails of the electron and hole wavefunctions.

Self-organized bimetallic Ag-Co nanoparticles with tunable localized surface plasmons showing high environmental stability and sensitivity.

We demonstrate a promising synthesis route based on pulsed laser dewetting of bilayer films (Ag and Co) to make bimetallic nanoparticle arrays. By combining experiment and theory we establish a parameter space for the independent control of composition and diameter for the bimetallic nanoparticles. As a result, physical properties, such as the localized surface plasmon resonance (LSPR), that depend on particle size and composition can be readily tuned over a wavelength range one order of magnitude greater than for pure Ag nanoparticles. The LSPR detection sensitivity of the bimetallic nanoparticles with narrow size distribution was found to be high-comparable with pure Ag (∼60 nm/RIU). Moreover, they showed significantly higher long-term environmental stability over pure Ag.

Oxidation-resistant silver nanostructures for ultrastable plasmonic applications.

Reduced degradation (oxidation) of silver nanoparticles (NPs) is achieved by contacting Ag with immiscible Co NPs. The relative decay of the plasmon peak (plot) shows that pure Ag NPs (blue dashed curve) decay by 25% in ca 20 days, whereas AgCo NPs last about 10 times longer, requiring nearly five months for a similar decay (red solid curve). The TEM images for both Ag and AgCo were taken after 50 days of storage under ambient conditions.

Nanostructure and microstructure of laser-interference-induced dynamic patterning of Co on Si

We have investigated the nanostructure and microstructure resulting from simultaneous ns laser irradiation with deposition of Co films on Si(001) substrates. The spatial order and length scales of the resulting nanopatterns and their crystalline microstructure were investigated as a function of film thickness h and laser energy density E using a combination of atomic force, scanning electron and transmission electron microscopies. The results could be classified into two distinct categories based on the laser energy density used. It was observed that the thickness-dependent E required to melt the Co film (ECo) was lower than for Si (ESi) primarily because of the higher reflectivity of Si. Consequently, for energy densities ECo < E1 < ESi that preferentially melted the Co film, spatially ordered nanoparticles were formed and were attributed to capillary-driven transport in the liquid phase. The ordering length scale corresponded to the interference fringe spacing Λ and the microstructure was primarily Co metal and a metal-rich silicide phase. The native oxide layer played an important role in minimizing the Co–Si reaction. For laser energy densities E2 ≥ ESi, spatially ordered patterns with periodic length scales L < Λ were observed and resulted from interference of an incident laser beam with a beam transmitted into the Si substrate. These nanopatterns showed one- as well as two-dimensional spatial ordering of the nanostructures. The microstructure in this laser energy regime was dominated by silicide formation. These results suggest that various nanostructures and microstructures, ranging from nearly pure metal to a Si-rich silicide phase, can be formed by an appropriate choice of the laser energy simultaneously with film growth.

Ferroplasmons: Intense Localized Surface Plasmons in Metal-Ferromagnetic Nanoparticles

Interaction of photons with matter at length scales far below their wavelengths has given rise to many novel phenomena, including localized surface plasmon resonance (LSPR). However, LSPR with narrow bandwidth (BW) is observed only in a select few noble metals, and ferromagnets are not among them. Here, we report the discovery of LSPR in ferromagnetic Co and CoFe alloy (8% Fe) in contact with Ag in the form of bimetallic nanoparticles prepared by pulsed laser dewetting. These plasmons in metal-ferromagnetic nanostructures, or ferroplasmons (FP) for short, are in the visible spectrum with comparable intensity and BW to those of the LSPRs from the Ag regions. This finding was enabled by electron energy-loss mapping across individual nanoparticles in a monochromated scanning transmission electron microscope. The appearance of the FP is likely due to plasmonic interaction between the contacting Ag and Co nanoparticles. Since there is no previous evidence for materials that simultaneously show ferromagnetism and such intense LSPRs, this discovery may lead to the design of improved plasmonic materials and applications. It also demonstrates that materials with interesting plasmonic properties can be synthesized using bimetallic nanostructures in contact with each other.

Self-consistent determination of plasmonic resonances in ternary nanocomposites

We have developed a self consistent technique to predict the behavior of plasmon resonances in multi-component systems as a function of wavelength. This approach, based on the tight lower bounds of the Bergman-Milton formulation, is able to predict experimental optical data, including the positions, shifts and shapes of plasmonic peaks in ternary nanocomposites without using any ftting parameters. Our approach is based on viewing the mixing of 3 components as the mixing of 2 binary mixtures, each in the same host. We obtained excellent predictions of the experimental optical behavior for mixtures of Ag:Cu:SiO2 and alloys of Au-Cu:SiO2 and Ag-Au:H2 O, suggesting that the essential physics of plasmonic behavior is captured by this approach.

Pump-induced nonlinear refractive-index change in erbium- and ytterbium-doped fibers: theory and experiment

A 980-nm pump-induced nonlinear refractive-index (n2) change in erbium-doped (20-m) and ytterbium-doped (20-m) fibers has been measured at 1064 nm by time-delayed photorefractive beam coupling in a co-propagating and counterpropagating geometry. It was found that n2 decreases at the wavelength of the probe beam when the pump beam is present. We present a semiclassical theory based on a four-state system that accounts for the pump-induced change of n2. Both theoretical and experimental results show that a significant index change, of the order of 10%, can be obtained for cw pump powers as small as 10 mW.

Nonlinear optical properties of multi-metal nanocomposites in a glass matrix

The lower and upper bounds for the nonlinear susceptibility of a multi-component nanocomposite composed of a glass host and nanoparticles of multiple metals are calculated within the context of the Hashin–Shtrikman formulation. We have applied this effective medium theory to the following ternary composites: AgCu:SiO2 and AgAu:SiO2 in the form of mixtures. These bounds were used to estimate the limits of the nonlinear effective third-order susceptibility of the composite as a function of wavelength, volume fraction and particle size. It is shown that the modulation of the third-order susceptibility of the metal by local-field enhancement due to plasmon resonance results in unexpected nonlinear responses which could be of interest to surface-enhanced Raman scattering, plasmonics and nanophotonics.

From Mie to Fresnel through effective medium approximation with multipole contributions

The Mie theory gives the exact solution to scattering from spherical particles while the Fresnel theory provides the solution to optical behavior of multilayer thin film structures. Often, the bridge between the two theories to explain the behavior of materials such as nanoparticles in a host dielectric matrix, is done by effective medium approximation (EMA) models which exclusively rely on the dipolar response of the scattering objects. Here, we present a way to capture multipole effects using EMA. The effective complex dielectric function of the composite is derived using the Clausius–Mossotti relation and the multipole coefficients of the approximate Mie theory. The optical density (OD) of the dielectric slab is then calculated using the Fresnel approach. We have applied the resulting equation to predict the particle size dependent dipole and quadrupole behavior for spherical Ag nanoparticles embedded in glass matrix. This dielectric function contains the relevant properties of EMA and at the same time predicts the multipole contributions present in the single particle Mie model.

Optical Plasmon Properties of Co-Ag Nanocomposites Within the Mean-Field Approximation

The optical properties of multi-metal nanocomposites made from Cobalt (Co), and Silver (Ag) are analyzed theoretically within the mean field approximation, and experimentally verified using absorption spectroscopy. The experimental system was modeled as a thin layer composed of hemispherical nanoparticles formed by grains of Co and Ag in contact with air and the SiO