Thin Films of Nonlinear Metallic Amorphous Composites.

We studied the nonlinear optical response of metallic amorphous composite layers in terms of a self-phase-modulated, third-order Kerr nonlinearity. A nonlinear effective medium theory was used to describe low densities of gold and iridium nanoparticles embedded in an equally nonlinear host material. The fill fraction strongly influences the effective nonlinear susceptibility of the materials, increasing it by orders of magnitude in the case of gold due to localized surface plasmonic resonances. The enhancement of the nonlinear strength in amorphous composites with respect to the bulk material has an upper limit in metallic composites as dominating absorption effects take over at higher fill factors. Both saturated and induced absorption in the thin films of amorphous composites were observed depending on the selected frequency and relative position to the resonant frequency of electron excitation in the metallic inclusions. We demonstrated the depths to which thin films are affected by nonlinear enhancement effects.

Second harmonic generation under doubly resonant lattice plasmon excitation.

Second harmonic generation is enhanced at the surface lattice resonance in plasmonic nanoparticle arrays. We carried out a parametric investigation on two-dimensional lattices composed of gold nanobars where the centrosymmetry is broken at oblique incidence. We study the influence of the periodicity, the incidence angle and the direction of the linear input polarization on the second harmonic generation. Excitation of the surface lattice resonance either at the fundamental or second harmonic wavelength, achieved by varying the incidence angle, enhance the conversion efficiency. As a special case, we demonstrate that both the wavelengths can be simultaneously in resonance for a specific period of the lattice. In this double resonant case, maximum second harmonic power is achieved.

Electromagnetic Field Enhancement of Nanostructured TiN Electrodes Probed with Surface-Enhanced Raman Spectroscopy.

We present a facile approach for the determination of the electromagnetic field enhancement of nanostructured TiN electrodes. As model system, TiN with partially collapsed nanotube structure obtained from nitridation of TiO2 nanotube arrays was used. Using surface-enhanced Raman scattering (SERS) spectroscopy, the electromagnetic field enhancement factors (EFs) of the substrate across the optical region were determined. The non-surface binding SERS reporter group azidobenzene was chosen, for which contributions from the chemical enhancement effect can be minimized. Derived EFs correlated with the electronic absorption profile and reached 3.9 at 786 nm excitation. Near-field enhancement and far-field absorption simulated with rigorous coupled wave analysis showed good agreement with the experimental observations. The major optical activity of TiN was concluded to originate from collective localized plasmonic modes at ca. 700 nm arising from the specific nanostructure.

Light Scattering from Rough Silver Surfaces: Modeling of Absorption Loss Measurements.

We consider two series of experimental setups of multilayered Ag/ZnO thin films with varying surface morphologies given by atomic force microscopy images. The absorption loss under diffuse scattering is studied theoretically by applying a combination of the scattering matrix approach with diffraction theory for randomly nanotextured interfaces. Our modeling is in excellent agreement with the respective measurements. The theoretical approach is applicable to a wide range of wavelengths, surface morphologies, and materials for both measured and computed rough surface morphologies.

Application of Core-Shell Metallic Nanoparticles in Hybridized Perovskite Solar Cell-Various Channels of Plasmon Photovoltaic Effect.

We analyze the microscopic mechanism of the improvement of solar cell efficiency by plasmons in metallic components embedded in active optical medium of a cell. We focus on the explanation of the observed new channel of plasmon photovoltaic effect related to the influence of plasmons onto the internal cell electricity beyond the previously known plasmon mediated absorption of photons. The model situation we analyze is the hybrid chemical perovskite solar cell CH 3 NH 3 PbI 3 - α Cl α with inclusion of core-shell Au/Si0 2 nanoparticles filling pores in the Al 2 O 3 or TiO 2 porous bases at the bottom of perovskite layer, application of which improved the cell efficiency from 10.7 to 11.4% and from 8.4 to 9.5%, respectively, as demonstrated experimentally, mostly due to the reduction by plasmons of the exciton binding energy.

Mode Splitting Induced by Mesoscopic Electron Dynamics in Strongly Coupled Metal Nanoparticles on Dielectric Substrates.

We study strong optical coupling of metal nanoparticle arrays with dielectric substrates. Based on the Fermi Golden Rule, the particle-substrate coupling is derived in terms of the photon absorption probability assuming a local dipole field. An increase in photocurrent gain is achieved through the optical coupling. In addition, we describe light-induced, mesoscopic electron dynamics via the nonlocal hydrodynamic theory of charges. At small nanoparticle size (<20 nm), the impact of this type of spatial dispersion becomes sizable. Both absorption and scattering cross sections of the nanoparticle are significantly increased through the contribution of additional nonlocal modes. We observe a splitting of local optical modes spanning several tenths of nanometers. This is a signature of semi-classical, strong optical coupling via the dynamic Stark effect, known as Autler-Townes splitting. The photocurrent generated in this description is increased by up to 2%, which agrees better with recent experiments than compared to identical classical setups with up to 6%. Both, the expressions derived for the particle-substrate coupling and the additional hydrodynamic equation for electrons are integrated into COMSOL for our simulations.

TiO2 Self-Assembled, Thin-Walled Nanotube Arrays for Photonic Applications.

Two-dimensional arrays of hollow nanotubes made of TiO 2 are a promising platform for sensing, spectroscopy and light harvesting applications. Their straightforward fabrication via electrochemical anodization, growing nanotube pillars of finite length from a Ti foil, allows precise tailoring of geometry and, thus, material properties. We theoretically investigate these photonic crystal structures with respect to reduction of front surface reflection, achievable field enhancement, and photonic bands. Employing the Rigorous Coupled Wave Analysis (RCWA), we study the optical response of photonic crystals made of thin-walled nanotubes relative to their bare Ti foil substrate, including under additional charge carrier doping which might occur during the growth process.

Microscopic Electron Dynamics in Metal Nanoparticles for Photovoltaic Systems.

Nanoparticles—regularly patterned or randomly dispersed—are a key ingredient for emerging technologies in photonics. Of particular interest are scattering and field enhancement effects of metal nanoparticles for energy harvesting and converting systems. An often neglected aspect in the modeling of nanoparticles are light interaction effects at the ultimate nanoscale beyond classical electrodynamics. Those arise from microscopic electron dynamics in confined systems, the accelerated motion in the plasmon oscillation and the quantum nature of the free electron gas in metals, such as Coulomb repulsion and electron diffusion. We give a detailed account on free electron phenomena in metal nanoparticles and discuss analytic expressions stemming from microscopic (Random Phase Approximation—RPA) and semi-classical (hydrodynamic) theories. These can be incorporated into standard computational schemes to produce more reliable results on the optical properties of metal nanoparticles. We combine these solutions into a single framework and study systematically their joint impact on isolated Au, Ag, and Al nanoparticles as well as dimer structures. The spectral position of the plasmon resonance and its broadening as well as local field enhancement show an intriguing dependence on the particle size due to the relevance of additional damping channels.

Two-fluid, hydrodynamic model for spherical electrolyte systems.

Spatial interaction effects between charge carriers in ionic systems play a sizable role beyond a classical Maxwellian description. We develop a nonlocal, two-fluid, hydrodynamic theory of charges and study ionic plasmon effects, i.e. collective charge oscillations in electrolytes. Ionic spatial dispersion arises from both positive and negative charge dynamics with an impact in the (far-)infrared. Despite highly classical parameters, nonlocal quenching of up to 90% is observed for particle sizes spanning orders of magnitude. Notably, the ionic system is widely tunable via ion concentration, mass and charge, in contrast to solid metal nanoparticles. A nonlocal soft plasmonic theory for ions is relevant for biological and chemical systems bridging hard and soft matter theory and allowing the investigation of non-classical effects in electrolytes in full analogy to solid metal particles. The presented semi-classical approach allows studying plasmonic photo-catalysis introducing nonlocal aspects into electrolyte-metal interactions.

High Electromagnetic Field Enhancement of TiO2 Nanotube Electrodes.

We present the fabrication of TiO2 nanotube electrodes with high biocompatibility and extraordinary spectroscopic properties. Intense surface-enhanced resonance Raman signals of the heme unit of the redox enzyme Cytochrome b5 were observed upon covalent immobilization of the protein matrix on the TiO2 surface, revealing overall preserved structural integrity and redox behavior. The enhancement factor could be rationally controlled by varying the electrode annealing temperature, reaching a record maximum value of over 70 at 475 °C. For the first time, such high values are reported for non-directly surface-interacting probes, for which the involvement of charge-transfer processes in signal amplification can be excluded. The origin of the surface enhancement is exclusively attributed to enhanced localized electric fields resulting from the specific optical properties of the nanotubular geometry of the electrode.

Multi-type particle layer improved light trapping for photovoltaic applications.

This work discusses regular particle arrays as nanostructured front layers for possible application in photovoltaic devices yielding strongly increased forward scattering. I used a rigorous plane-wave method to investigate multi-type particle layers combining different radii and configurations. The absorbance was enhanced compared to the bare Si wafer and I demonstrated on mixing particles a broadband boost in the absorbance within the homogeneous wafer region, excluding parasitic absorption in the particle layer. I studied the efficiency enhancement for varying geometries. Multi-type layers made of Si disks with two different radii achieved up to 33% (24%) and with four different radii up to 40% (30%) improvement in the short circuit current and integrated absorbance, respectively, without yet standard anti-reflection coatings. Broadband efficiency enhancement for metal multi-type layers was not observed because they show strong parasitic absorption and boost the absorbance only in narrow wavelength regions.

Surface plasmon dependence on the electron density profile at metal surfaces.

We use an extension of the hydrodynamic model to study nonlocal effects in the collective plasmon excitations at metal surfaces and narrow gaps between metals, including the surface spill-out of conduction band electrons. In particular, we simulate metal surfaces consisting of a smooth conduction-electron density profile and an abrupt jellium edge. We focus on aluminum and gold as prototypical examples of simple and noble metals, respectively. Our calculations agree with the dispersion relations measured from planar surfaces for these materials. Systems involving small gaps display a regime of tunnelling electrons, which is partially captured by the overlap of electron densities. This extension of the hydrodynamic model to cope with inhomogeneous density profiles provides a relatively fast and accurate way of describing the optical response of metal surfaces at subnanometer distances.

Near-field nanoimprinting using colloidal monolayers.

We experimentally and theoretically explore near-field nanopatterning obtained by irradiation of hexagonal monolayers of micron-sized polystyrene spheres on photosensitive Ge(2)Sb(5)Te(5) (GST) films. The imprinted patterns are strongly sensitive to the illumination conditions, as well as the size of the spheres and the orientation of the monolayer, which we change to demonstrate control over the resulting structures. We show that the presence of multiple scattering effects cannot be neglected to describe the resulting pattern. The experimental patterns imprinted are shown to be robust to small displacements and structural defects of the monolayer. Our method enables the design and experimental verification of patterns with multiple focii per particle and complex shapes, which can be directly implemented for large scale fabrication on different substrates.

Image dipoles approach to the local field enhancement in nanostructured Ag-Au hybrid devices.

We have investigated the plasmonic enhancement in the radiation field at various nanostructured multilayer devices that may be applied in surface enhanced Raman spectroscopy. We apply an image dipole method to describe the effect of surface morphology on the field enhancement in a quasistatic limit. In particular, we compare the performance of a nanostructured silver surface and a layered silver-gold hybrid device. It is found that localized surface plasmon states provide a high field enhancement in silver-gold hybrid devices, where symmetry breaking due to surface defects is a supporting factor. These results are compared to those obtained for multishell nanoparticles of spherical symmetry. Calculated enhancement factors are discussed on the background of recent experimental data.

Novel Au-Ag hybrid device for electrochemical SE(R)R spectroscopy in a wide potential and spectral range.

A nanostructured gold-silver-hybrid electrode for SER spectroelectrochemistry was developed which advantageously combines the electrochemical properties and chemical stability of Au and the strong surface enhancement of (resonance) Raman scattering by Ag. The layered device consists of a massive nanoscopically rough Ag electrode, a thin (2 nm) organic layer, and a ca. 20 nm thick Au film that may be coated by self-assembled monolayers for protein adsorption. The SERR-spectroscopic and electrochemical performance of this device is demonstrated using the heme protein cytochrome c as a benchmark model system, thereby extending, for the first time, SE(R)R studies of molecules on Au surfaces to excitation in the violet spectral range. The enhancement factor is only slightly lower than for Ag electrodes which can be rationalized in terms of an efficient transfer of plasmon resonance excitation from the Ag to the Au coating. This mechanism, which requires a thin dielectric layer between the two metals, is supported by theoretical calculations.