Biomimetic soft lithography on curved nanostructured surfaces

In this paper a nano-molding process using a nature-created master is demonstrated. The eye of night moth Agotis exclamationis having 100 nm-scale structures on a curved surface is used as biomimetic master mold from which nanostructures are replicated onto a flat substrate. Suitable conditions of this simple and cost-efficient process allows for minimal texture damage. The fabrication consists of two steps: first, a negative PDMS mold of the curved eye surface is made, and second, the flexible mold is replicated into a hybrid UV sensitive polymer, on a flat substrate. An accurate copy of the master surface with dense arrays of 200 nm high and 100–120 nm wide posts are generated, thus preserving the integrity of the nanostructures. The known anti-reflecting optical properties of the moth eye were reproduced with a reflectivity reduced by a factor of 2

Planar Optical Nanoantennas Resolve Cholesterol-Dependent Nanoscale Heterogeneities in the Plasma Membrane of Living Cells

Optical nanoantennas can efficiently confine light into nanoscopic hotspots, enabling single-molecule detection sensitivity at biological relevant conditions. This innovative approach to breach the diffraction limit offers a versatile platform to investigate the dynamics of individual biomolecules in living cell membranes and their partitioning into cholesterol-dependent lipid nanodomains. Here, we present optical nanoantenna arrays with accessible surface hotspots to study the characteristic diffusion dynamics of phosphoethanolamine (PE) and sphingomyelin (SM) in the plasma membrane of living cells at the nanoscale. Fluorescence burst analysis and fluorescence correlation spectroscopy performed on nanoantennas of different gap sizes show that, unlike PE, SM is transiently trapped in cholesterol-enriched nanodomains of 10 nm diameter with short characteristic times around 100 μs. The removal of cholesterol led to the free diffusion of SM, consistent with the dispersion of nanodomains. Our results are consistent with the existence of highly transient and fluctuating nanoscale assemblies enriched by cholesterol and sphingolipids in living cell membranes, also known as lipid rafts. Quantitative data on sphingolipids partitioning into lipid rafts is crucial to understand the spatiotemporal heterogeneous organization of transient molecular complexes on the membrane of living cells at the nanoscale. The proposed technique is fully biocompatible and thus provides various opportunities for biophysics and live cell research to reveal details that remain hidden in confocal diffraction-limited measurements.Peer ReviewedPostprint (author's final draft

Mode Coupling in Plasmonic Heterodimers Probed with Electron Energy Loss Spectroscopy

While plasmonic antennas composed of building blocks made of the same material have been thoroughly studied, recent investigations have highlighted the unique opportunities enabled by making compositionally asymmetric plasmonic systems. So far, mainly heterostructures composed of nanospheres and nanodiscs have been investigated, revealing opportunities for the design of Fano resonant nanostructures, directional scattering, sensing and catalytic applications. In this article, an improved fabrication method is reported that enables precise tuning of the heterodimer geometry, with interparticle distances made down to a few nanometers between Au–Ag and Au–Al nanoparticles. A wide range of mode energy detuning and coupling conditions are observed by near field hyperspectral imaging performed with electron energy loss spectroscopy, supported by full wave analysis numerical simulations. These results provide direct insights into the mode hybridization of plasmonic heterodimers, pointing out the influence of each dimer constituent in the overall electromagnetic response. By relating the coupling of nondipolar modes and plasmon–interband interaction with the dimer geometry, this work facilitates the development of plasmonic heterostructures with tailored responses, beyond the possibilities offered by homodimers

Novel manufacturing methods for optical antennas:controlling light down to the single nanometer scale

The resonant excitation of free electrons in metallic nanostructures enables extreme near field intensities along with a deep sub-wavelength localization of the electromagnetic energy. This has been exploited to enhance light-matter interaction down to the single molecule level, to localize heat, and tailor radiation into the far field, all applications of optical antennas. Importantly, the rise and advances in the field of plasmonics over the last ten years have been tightly bound to the development of nanofabrication techniques. The present thesis shows how different nanofabrication approaches can be combined and exploited to produce optical antennas with single nanometer interparticle distances, full geometric and material control and increased fabrication throughput and reliability. Different nanofabrication techniques are first developed for the manufacturing of aperture-type antennas that target applications in fluorescence enhancement for the study of biological systems. The fabrication of high-resolution (~20 nm) bowtie nanoapertures via reactive ion etching through stencils is demonstrated to be an efficient approach to upscale the lithographic fabrication of a master, the stencil, to produce a number of functional samples in an aluminum thin film. Fluorescence measurements are performed on living cells, a proof of concept of the system for the sub-wavelength illumination of dynamic lipid membranes. This initial work preludes to the investigation of the dimer in a box geometry that provides further near field enhancement and confinement in order to reach measurement volumes down to the few tens of zeptolitres and over three orders of magnitude fluorescent enhancement. A second part of the work is dedicated to the investigation and exploitation of colloidal nanoparticle assembly. Although these elements possess unique intrinsic properties, their deterministic positioning is crucial to harness their full potential. An initial study enables understanding and exploitation of the various mechanisms underlying the capillary assembly of gold nanorods onto topographic templates. Novel topographic trap geometries are realized, including three dimensional barriers and funnels to reach unity yield, nanometric positioning and 1° orientation accuracy. Further on, complex nanorod trimers of dolmen geometry are assembled and characterized, revealing the unique potential of nanoparticle assembly in creating plasmonic structures with few-nm gaps. ¿ Finally, two projects exploit the developed nanofabrication capabilities to investigate material-based opportunities for nanoantennas. First, heterostructures composed of two coupled elements made of different material, respectively gold-silver and gold-aluminum, are investigated. High-resolution electron beam lithography and sub-10 nm layer-to-layer alignment is used to produce dimers with fully controlled geometry and interparticle gap in order to reveal the underlying mechanisms of detuned plasmonic pairs. Finally, the very last section offers an outlook beyond metallic nanostructures relying on high index dielectric, silicon nanodiscs. These silicon elements rely on Mie resonances rather than their plasmonic counterpart in metals. The gradual contribution of both strong electric and magnetic dipoles in silicon structures is compared to the properties of metal discs and used to produce vivid color palettes visible under bright field microscopy and naked eye

High-yield and high-precision nanoparticle assembly: towards complex plasmonic antennas

We present a high-confinement approach to capillarity-assisted particle assembly (CAPA) enabling both high-precision and high-yield assembly of single nanoparticles and of multi-particle clusters into silicon and glass substrates.info:eu-repo/semantics/publishe

3D nanostructures fabricated by advanced stencil lithography

This letter reports on a novel fabrication method for 3D metal nanostructures using high-throughput nanostencil lithography. Aperture clogging, which occurs on the stencil membranes during physical vapor deposition, is leveraged to create complex topographies on the nanoscale. The precision of the 3D nanofabrication method is studied in terms of geometric parameters and material types. The versatility of the technique is demonstrated by various symmetric and chiral patterns made of Al and Au