José Dintinger

A bottom-up approach to fabricate optical metamaterials by self-assembled metallic nanoparticles

We introduce a novel bottom-up approach to fabricate by self assembly a metamaterial from metallic nanoparticles in a two-step process. In the first step, a metamaterial made of densely packed silver nanoparticles is required. The material dispersion with increasing nanoparticle densities, from dispersed to randomly packed nanoparticles, was measured by spectroscopic ellipsometry, demonstrating high permittivity values in the visible. In the second step, this material was used to prepare spherical clusters by a method based on oil-in-water emulsion. The optical properties of these clusters were equally investigated by spectroscopic means. Comparisons with rigorous numerical simulations clearly indicate that, depending on the cluster size, their spectral response can be unambiguously associated with the excitation of a magnetic dipole resonance. As a consequence, such spherical clusters are promising building blocks for future metamaterials possessing a magnetic response in the visible range.

A Self-Organized Anisotropic Liquid-Crystal Plasmonic Metamaterial

A composite material that leads to self organization of mesogen-coated gold nanospheres is synthesized and shows enhanced anisotropic optical properties due to synergistic effects of the mesogens intrinsic birefringence and its ability to drive the self-assembly process into highly anisotropic architectures with densely packed nanospheres. Such nanoengineered matter sustains a response beyond that achievable by its individual constituents, i.e., a metamaterial.

Channel and wedge plasmon modes of metallic V-grooves with finite metal thickness.

We investigate numerically the effect of a finite metal film thickness on the propagation characteristics of the channel Plasmon polariton (CPP) and wedge plasmon polariton (WPP) modes, both in a symmetric and asymmetric environment. We observe that decreasing the metal thickness results in an improvement of the field localization near the groove tip and an increase of the losses for both types of mode. This behavior stems from the typical symmetric charge distribution of both modes across the metal film. When considering an asymmetric dielectric environment, the CPP mode is found to evolve into short range Plasmon modes propagating along the groove walls, in contrast to the WPP mode which remains essentially confined at the tip apex. These results can be useful to tailor the properties of such plasmon modes, using the metal thickness as the variable parameter.

Self-assembled plasmonic metamaterials

Abstract Nowadays for the sake of convenience most plasmonic nanostructures are fabricated by top-down nanofabrication technologies. This offers great degrees of freedom to tailor the geometry with unprecedented precision. However, it often causes disadvantages as well. The structures available are usually planar and periodically arranged. Therefore, bulk plasmonic structures are difficult to fabricate and the periodic arrangement causes undesired effects, e.g., strong spatial dispersion is observed in metamaterials. These limitations can be mitigated by relying on bottom-up nanofabrication technologies. There, self-assembly methods and techniques from the field of colloidal nanochemistry are used to build complex functional unit cells in solution from an ensemble of simple building blocks, i.e., in most cases plasmonic nanoparticles. Achievable structures are characterized by a high degree of nominal order only on a short-range scale. The precise spatial arrangement across larger dimensions is not possible in most cases; leading essentially to amorphous structures. Such self-assembled nanostructures require novel analytical means to describe their properties, innovative designs of functional elements that possess a desired near- and far-field response, and entail genuine nanofabrication and characterization techniques. Eventually, novel applications have to be perceived that are adapted to the specifics of the self-assembled nanostructures. This review shall document recent progress in this field of research. Emphasis is put on bottom-up amorphous metamaterials. We document the state-of-the-art but also critically assess the problems that have to be overcome.

Optical properties of mesogen-coated gold nanoparticles

In this work, the physical and optical properties of gold nanoparticles functionalized with laterally grafted nematic ligands were studied. In particular, the influence of the nanoparticle size on the mesomorphic behavior and optical properties of the composite was investigated. To obtain an in-plane alignment of the mesogens, thin oriented films were prepared by shearing and characterized by polarized absorption spectroscopy. While the sub-2nm nanoparticle thin film only showed birefringence due to a strong damping of the plasmon resonance, larger NPs exhibit a strong dichroism with a shift of the NP plasmon resonance by about 50 nm. These results demonstrate the possibility to obtain a bulk NP metamaterial with tunable plasmonic properties by chemical engineering of the NP ligands.

Liquid crystal plasmonic metamaterials

The electromagnetic response of metamaterial can be managed by combining resonances and interferences of different materials and on different lengths scales. In our contribution we study composite metamaterials containing resonant plasmonic metallic nanoparticles that show organization. The material bases its non-conventional properties on short distance self-organization by mesogens that form a liquid crystal material. We analyze the properties of such materials with a structural model containing organized nanoparticles. Theoretically insight of the electromagnetic properties is provided and we give details on their optical properties.

Plasmonic Nanoparticle-Based Metamaterials: From Electric to Magnetic Response

The self-assembly of nanoparticles into hierarchical architectures is currently attracting a lot of interest due to their potential applications in a wide range of fields like nanophotonics, nanoelectronics or catalysis. In the present chapter, we discuss the potential of metal nanospheres for the bottom-up fabrication of optical metamaterials. Controlling the spatial arrangement of the nanoparticles in these composites offers a promising route to engineer unique optical responses originating from their collective plasmonic resonance. Here we explore experimentally how different types of NP arrangements can give rise to distinct macroscopic effective properties, including both electric and magnetic optical responses. For each of the structures investigated, we propose a brief overview of the current state-of-the-art of the appropriate bottom-up fabrication methods and analyze their optical properties in details. First, the optical constants of “bulk” amorphous nanoparticle metamaterials are investigated by ellipsometry, demonstrating that controlling the nanoparticle filling fraction provides an efficient route to tune the metamaterial permittivity. As an example of a potential application, the realization of a hybrid plasmonic Bragg mirror is discussed. Finally, we focused on the fabrication and characterization of dense spherical nanoclusters that can sustain a magnetic response at optical frequencies. In doing so, we demonstrate the possibility to engineer the permeability of nanocluster-based metamaterials, thereby opening interesting perspectives for the realization of isotropic negative index materials operating in the visible.

Bottom-up metamaterials with an isotropic magnetic response in the visible

A theoretical framework to analyze the optical properties of amorphous metamaterials made from meta-atoms which are amenable for a fabrication with bottom-up technologies is introduced. The achievement of an isotropic magnetic resonance in the visible is investigated by suggesting suitable designs for the meta-atoms. Furthermore, two meta-atoms are discussed in detail that were fabricated by self-assembling plasmonic nanoparticles using techniques from the field of colloidal nanochemistry. The metamaterials are experimentally characterized by spectroscopic means and the excitation of the magnetic dipole moment is clearly revealed. Advantages and disadvantages of metamaterials made from such meta-atoms are discussed.