Biological samples studied by optical nanospectroscopy

The different parts of the electromagnetic spectrum result in diverse effects upon interaction with matter: according to the wavelength, the radiation has energy appropriate for the excitation of a specific physical process. X-rays can be used as a tool to analyze the structure of matter since their wavelength is comparable with the interatomic distances. Infrared light is in the spectral region that excites molecular vibrations and is employed to investigate the chemical composition of a material. Visible radiation can study the optical properties of a sample, such as the fluorescence and the absorbance, and provide a chemical fingerprint when the inelastically scattered light is detected. In this thesis work these light sources are used in diverse experimental approaches to study structured biological specimens, resulting in a detailed chemical and physical characterization at the atomic and molecular scale. Conventional spectroscopy is often not enough sensitive and spatially resolved to detect specific elements or domains in a sample. The need of imaging objects on increasingly finer scales and spatially localize specific molecules, brought to combine infrared, visible and Raman spectroscopy with scanning near-field microscopy giving rise to a powerful nanospectroscopic tool used to perform simultaneous topographical measurements and optical/chemical characterizations with subwavelength resolution, overcoming the diffraction limit of light. Our study combines X-ray diffraction and reflectivity with optical nanospectroscopy to investigate the order and clustering of lipid bilayers, the interaction between solid-supported membranes and embedded alamethicin peptides, the optical and chemical properties of hippocampal neuron cells and the trafficking mechanism of specific neuron receptors.

Electronic and optical properties of supported C60 molecules studied by scanning tunneling microscopy

Elizabeta Cavar

This thesis is primarily concerned with optical properties of adsorbed C60 molecules on metal and insulating substrates. These properties were studied on a single-molecule scale, using Scanning Tunneling Microscope-induced light emission spectroscopy (STM-LES). In the first part of the thesis (Chapter 2), we present the investigation of the structural and electronic properties of C60 islands on Au(111) and on ultrathin NaCl films. We further present a study of the structure of ultrathin sodium chloride films grown on Au(111), and the influence of the herringbone reconstruction on the interface state of a NaCl-covered Au(111) surface. In the second part of the thesis (Chapter 3 and 4), we show that applying a local technique such as STM induced optical spectroscopy enables the exploration of the optical characteristics of individual objects on the nanoscale. Contrary to conventional non-local techniques, the local character of this technique offers the unique possibility to select and probe individual atoms, molecules or clusters on surfaces. Here, we demonstrate the capability of STM-induced light emission spectroscopy (STM-LES) for the chemical recognition on a single-molecule scale. In Chapter 3, we present STM-induced light emission spectroscopy (STM-LES) of a clean Au(111) surface, Au(111) surface covered with islands of C60 molecules and of Au(111) covered with ultrathin NaCl films. We have performed an extensive investigation of the dependence of the gold plasmon spectra on tunneling current and voltage. We show the influence of nano-objects on the shape and intensity of the spectra. Furthermore, we investigated the influence of this experimental technique on morphological changes of surfaces and adsorbed layers. In Chapter 4, we present the first observation of energy resolved luminescence from selected C60 molecules excited by electrons tunnelling through a double barrier STM junction. A comparison with the luminescence spectra obtained by non-local spectroscopy from dispersed C60 molecules in rare gas matrices enables us to demonstrate the molecular origin of the detected light and to identify the observed spectral features. The present novel observation of both, fluorescence and phosphorescence constitutes a solid basis for the chemical identification of an individual C60 molecule.

EPFL2005

Scanning near-field infrared microscopy and spectromicroscopy applied to nano-systems and cells

Dusan Vobornik

This thesis further explores the possibilities of scanning near-field optical microscopy (SNOM) in both materials and life sciences. Two experimental SNOM setups were developed: one designed for infrared spectroscopy applications and the other for the imaging of fluorescently labeled samples. The results of the experiments that were conducted with both setups are presented and analyzed in this thesis. Diffraction limits the resolution of lens-based microscopes to a value close to that of the wavelength of the light that is used to illuminate the samples. SNOM instruments overcome this limit by probing the near-field light — the light that remains within close vicinity of the sample and decays exponentially away from it. The SNOM concept and instrumental design are described. A comparison of SNOM with other novel microscopies (electron, atomic force, and scanning tunneling microscopies) outlines the main advantages of SNOM — the sole optical microscopy technique with a nanometer resolution. Aperture-based infrared SNOM (IR-SNOM), performed in the spectroscopic mode using the Vanderbilt University free electron laser, recently started delivering spatially resolved information on the distribution of chemical species and on other laterally fluctuating properties. The IR-SNOM combines the IR spectroscopy's chemical specificity and the SNOM's high optical and topographical resolutions. The IR-SNOM experimental setup is described in detail and results from the study of cells, boron nitride and lithium fluoride thin films are presented and analyzed. They demonstrate the great potential of this new technique. An overview of the development of IR-SNOM, as well as a consideration of its future possibilities are presented. The newly built SNOM instrument at EPFL was used for experiments that are complementary to the aforementioned IR-SNOM experiments. Specifically, the new SNOM enabled the study of fluorescently marked samples. Images of fluorescently labeled cells and carbon nanotubes are presented and analyzed. The results further demonstrate the exciting possibilities of SNOM.

EPFL2005

Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

Amazing and beautiful optical effects are present in Nature. Some examples are the iridescent colors produced by peacocks, butterflies and beetles. While the simulations of some of these structures have already been realized, only a few elements have been fabricated. We are interested to understand and to reproduce some of these amazing optical properties. Applications such as security or decorative elements in different fields like jewelery or horology can be imagined. SEM pictures of the structure of these animals show a frequent combination of periodic or pseudo-periodic elements such as interference filters, multilayer reflectors, microlenses and gratings. The different elements combine angular and spectral effects. This combination enables strong optical effects for a large range of viewing angles and spectra. Different technologies such as standard photolithography, recording of interference patterns, transfer of photoresist structures in glass substrates, replications, hot embossing and spincoating were used to fabricate a variety of optical elements. The fabricated elements were produced with polymer materials because of their ease of use, and the possibility to build multilevel structures with replication methods. A drawback is the small refractive index difference possible between polymers and compared to air. In the first part, different combinations of polymer micro- and nano-optical elements such as corrugated gratings, multilayer Bragg reflectors, microlens arrays, micro-prisms arrays and diffusers were successfully fabricated. Poly (vinyl alcohol) and poly (N-vinycarbazole) with refractive indices of 1.56 and 1.72, respectively, at a wavelength of 500nm were used to fabricate the polymer multilayer reflectors. The micro-optical elements used were: microlenses with diameter between 32 μm and 250 μm and around 20 μm height; prisms with 50 μm period and 25 μm height. The angularly dependent reflectivity of the different fabricated elements was studied. Thanks to the close combination of diffractive, refractive and reflective micro- and nano-optical elements, non-standard artificial visual color effects were produced. The fabricated elements were modeled with different tools according to the dimensions of the different optical elements. Ray tracing analysis was used in the case with micro-optical elements while the Fourier modal method permitted simulation of the interaction of the light with the periodic nano-structures. The specific effects of the variation of the different parameters were highlighted, the basic principles and the limitation of the polymer technology were identified. In the second part, the optical modeling tools and the fabrication technologies developed were used to model and fabricate polymer light emitting diodes in a distributed feedback regime. The optical properties of the different layers were modeled and the physical dimensions of elements with active conductive polymers me-LPPP and F8 were calculated, fabricated and tested. The sensitive dimensions and parameters were underlined. Polymer materials permitted rapid fabrication of complex and innovative optical elements with very simple technologies like spin-coating and replication methods. These technologies allow the combination of nano-optical elements with dimensions of the order of a hundred nanometers with micro-optical elements with dimension of the order of about ten to one hundred microns.

Institut de Microtechnique, Université de Neuchâtel2008

Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

Institut de Microtechnique, Université de Neuchâtel2008

Scanning near-field infrared microscopy and spectromicroscopy applied to nano-systems and cells

Dusan Vobornik

EPFL2005

Electronic and optical properties of supported C60 molecules studied by scanning tunneling microscopy

Elizabeta Cavar

EPFL2005