Advances in the prediction, design, fabrication and characterization of 2-dimensional crystal and metamaterial nanostructures for nanophotonics

Resume : Metasurfaces can show unique properties and can be designed e.g. as flat lenses by using well-shaped metallic nanoparticles. Nanosphere lithography (NSL) is one possibility to produce such tailored nanoparticles on large scale. Thereby, a self-assembled hexagonally closed packed mono- or bilayer of nanospheres covers a substrate and acts in a following metal evaporation process as a shadow mask. There, material is evaporated onto the substrate through mask openings formed by sphere triples. The three-dimensional (3D) morphology of the obtained triangular shaped nanoparticles determines their plasmon resonance and depends on the hole mask itself (arrangement, diameter of spheres, etc.) and the time-dependent modification of the holes during the evaporation process. With ongoing evaporation, material is deposited not only on the substrate, but also on top of the mask itself. Therefore, the mask openings become smaller and more round shaped. This change of the mask openings affects directly the shape of the obtained Au nanoparticles. Their morphology was analyzed by means of energy filtered transmission electron microscopy (EFTEM) thickness maps. By comparison of the found thickness distributions of the Au nanoparticles with those computed by a ray-trace type algorithm the clogging rate of the mask openings is deduced. This procedure allows to predict the morphology of NSL nanoparticles correctly, as long as preferential growth along low-index crystal directions is neglected.

Resume : Hyperbolic metamaterials, a class of artificially engineered materials with a highly anisotropic permittivity response originating from opposite signs of the principal components of the electric tensor, have attracted significant interest in recent years due to their ability to manipulate the propagation light in exotic ways. Such materials enable distinctive optical phenomena such as negative refraction, super-resolution imaging, and enhanced spontaneous emission. Here we exploit the hyperbolic iso-frequency characteristic of a planar type-II HMM (composed of a template-stripped stack of alternating, 25-nm-thick, sputtered films of Ag and SiO2) to achieve high-sensitivity proximity detection of nanoparticles in transmission. The iso-frequency surface of the HMM is unique in that propagation of light over the entire visible-range is allowed only for electromagnetic modes having tangential spatial frequencies kx exceeding the free-space wavevector k0 by over a factor of two [1]. Due to its high sensitivity to nanoparticles in deep-subwavelength proximity to a surface, achieved without the use of dark-field optics, this HMM-based device hints at promising applications in bio-chemical sensing, particle tracking and contamination analysis. Template-stripped refractive-index-change sensors consisting of ultra-high quality factor open-cavity resonators for surface-plasmon polaritons will also be discussed. 1. T. Xu & H.J. Lezec, Nat. Comm. 5, 4141 (2014).

Resume : The light management becomes the key-point for photoactive material like solar cells or OLED. The optimization of photonic and optoelectronic systems with engineered dielectric structures provide a promising versatile solution. In this contribution, we demonstrate highly efficient and tunable light trapping structures prepared using simple and scalable Sol-Gel chemistry, wet coating processing, and Nanoimprint techniques. Defect-free Distributed Bragg Reflectors are obtained through combination of the materials with high (dense TiO2 layers, n=2.08 in the visible range) and low refractive index (macroporous silica layers, n=1.24 with 50% of porosity) in the periodic dielectric stack. Such high index contrast allows for specular reflectivity of up to 96% at the normal incidence for as little as 9 layers in the stack. Apart from its simplicity, proposed technique has a high potential for industrial deployment, as the porous layers are prepared thanks to a porogen latex that is calcinated during a single annealing step of the full stack. We also demonstrate the flexibility of our process by making highly reflective DBRs with a tunable reflection range from UV to IR (from 400nm to 1300nm). A prove of concept has been obtained by increasing the efficiency of a-Si:H thin-film solar cell while containing our optimized 8-layer DBR. Moreover, we will show how the integration of diffraction gratings fabricated by Nanoimprint into our DBR structures improves their efficiency.

Resume : Plasmonic nanorod metamaterials display extremely high sensitivity to the surrounding refractive index, and have already been employed as ultrasensitive bio-, ultrasound and hydrogen sensors. They may exhibit hyperbolic dispersion and a spectrally tunable epsilon near zero (ENZ) frequency. Typically, metamaterials comprised of arrays of free standing metallic nanorods are synthesised from porous alumina template obtained using conventional electrolytes such as sulphuric acid and oxalic acid and have an ENZ wavelength which is limited to the visible spectral range 530-650 nm. In order to provide ENZ-related functionalities in the infrared, including at telecom wavelengths, a new approach is needed. Here we describe the design and fabrication of periodic arrays of free standing Au nanorods which allows the ENZ frequency of a nanorod metamaterial to be tuned from the visible to infra-red region using porous alumina templates obtained using Selenic acid. Perfectly ordered nanoporous alumina templates with ~15 nm pore diameter and ~125 nm interpore spacing, having large domain areas, are fabricated by employing two-step anodization in Selenic acid. The ENZ frequency is then tuned by controlling the nanorod diameter via post processing of the pores while keeping the inter-rod spacing and length constant. These metamaterials open up new possibilities for a variety of applications in the fields of bio- and chemical sensing and nonlinearity enhancement in the infrared.

Resume : Parallel fabrication of nano-sized sources is one of the major challenges facing nanophotonics today. During the past decade, plasmonics have demonstrated excellent potential in their ability to confine and process optical signals on nano-scale dimensions. In recent years, research in plasmonics-based nanophotonics has been extended to include the interaction of plasmonic structures with active materials, such as fluorescent media, with the aim of developing functional nano-scale optical components. The application of this research to the development of plasmonics-based sources has, however, been hindered by the limitations imposed by the properties of the fluorescent materials in high electric fields, such as those typically produced by dimer nanoantennas. Alternate geometries of plasmonic structures, such as Plasmonic Ring Cavities, offer several advantages despite the reduced electric field enhancements. In particular, the large hotspot sizes of these structures allow coupling of the collective emission of a large number of colloidal semiconductor quantum dots (QDs), selectively placed in their vicinity. In this coupled system, the radiative behaviour of the QDs is altered deterministically, i.e. it can be predicted theoretically given the dimensions of the plasmonic structure and the separation of the fluorescent and plasmonic components. These results have great implications for the scalable and parallel fabrication of plasmonic sources, with the promise of the manufacturing of devices to exact specifications.