Archive: Seminars 2014
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|March 12, 2014
"Supra-molecular Architectures at Surfaces for Probing Structure, Electron and Spin States," by Thomas A. Jung, Paul Scherrer Institute, hosted by Saw Wai Hla
Abstract: Well-defined electronic and spintronic interfaces can be architected by combining self-assembly and surface science. The atomically clean metal surface in ultrahigh vacuum provides a very specific environment affecting the behavior of the ad-molecules as well as the adsorbent-adsorbate interaction. Depending on the bonding at the interface, complex electronic and magnetic interaction can occur that can be explored by using spectromicroscopy correlation, in this case, photoemission and photoabsorption spectroscopy and scanning tunnelling microscopy.
One example is provided by the emergence of quantum dot states from the interaction of a porous network with the two-dimensional (Shockley) surface state of Cu(111), which exhibit sufficient residual coupling to show the emergence of a band-like structure in angle-resolved photoemission experiments. In another example, specifically chosen surface supported molecules have been shown to exhibit ferromagnetic or antiferromagnetic exchange interaction, and their spin system has shown change induced by physical parameters and/or chemical stimuli. By combining supramolecular chemistry with on-surface coordination chemistry, the reversible spin switching of self-assembled bimolecular arrays has recently been demonstrated.
These examples all have in common that the molecular interfaces are well defined by their production from atomically clean substrates and molecular building blocks. The physics and chemistry of these unprecedented systems, which are addressable by scanning probes, provide insight into novel materials in their assembly and their electronic and spintronic properties, which emerge from the interaction of their components down to the scale of single atoms, molecules, and bonds.
|February 19, 2014
"When Structured Light Meets Structured Matter at the Nanoscale," by Xiaobo Yin, University of Colorado, Boulder, hosted by Jun Rho
Abstract: The rapid development of nanoscale science and technology not only permits exploration of advanced scientific ideas and observation of unprecedented phenomena, but also offers practical solutions to the world's most serious issues, such as energy and pollution crises, health and food safety concerns, and military and homeland security needs. Exploiting and enhancing originally weak light-matter interactions, we will be able to devise better imaging and manufacturing tools, catalyze more efficient photochemical reactions, and sense and diagnose contaminants at the single-molecule level.
This talk will focus on how judiciously designed nanostructures and materials can tailor and eventually control light-matter interactions at the deep-subwavelength scale. I will illustrate these design principles by using specific examples. In particular, I will elaborate strategies to harvest substantial amounts of energy-efficient emissions from a sub-wavelength laser cavity and to achieve close-to-unit utilization of light, providing coherent sources at the nanoscale. These nanolasers can perform much brighter and faster when quantum engineering is employed and show great potential in ultra-trace chemical sensing. More interestingly, introducing uniquely structured quantum materials, such as monoatomic layer transition metal dichalcogenides, the light-matter interaction at the nanoscale senses the atomic structural and topological symmetries that are embedded in the system, revealing the fascinating physics and redefining applications based on these unique physical processes. I will discuss some of the preliminary assessments of the observed valley physics and illustrate the structure and function relationship in these impactful materials that have been widely utilized in both mechanical systems and energy sciences.
|February 10, 2014
"Optical and electronic microscopic characterization of plasmonic modes on a self-assembled metallic grating," by Clotilde Lethiec, Universite Pierre et Marie Curie, hosted by Gary Wiederrecht
Abstract: Efficient coupling of single-photon nanoemitters to photonic or plasmonic structures requires spatial and spectral matching of the emitting dipoles to the nanostructure. It is especially crucial to match the orientation between the electrical field of the photonic or plasmonic mode and the fluorescent dipole. Therefore, it is necessary to determine the distribution of the electrical field associated to the excited mode as well as the dipole orientation.
This seminar will be divided into two parts. First, I will present the polarimetric method I have developed and applied to high-quality CdSe/CdS dot-in-rods and spherical nanocrystals in order to retrieve the three-dimensional orientation of the associated dipole. I could correlate optical dipolar properties to the shape of the emitter.
In the main part of my talk, I will focus on the coupling between light and surface plasmons polaritons (SPPs). SPPs are known to enhance light matter interaction with applications in fields such as bio-imaging, light-emitting diodes, photovoltaics, and single-photon sources. Metallic surface gratings offer the opportunity to absorb light with almost 100% of efficiency and to enhance the fluorescence of nanoemitters close to their surface. In order to take advantage of SPP modes, which are not coupled to far-field radiative modes in the case of a planar metallic surface, a periodically patterned metallic surface can be used. We used self-assembly to produce centimeter-sized plasmonic crystals with 400-nm periodic structure, by evaporating a thick layer of gold on artificial silica opals used as a periodic template. We performed optical specular reflection spectra and evidenced a dip of almost complete absorption. This dip is explained by theoretical calculations and can be attributed to a coupling to SPPs. We demonstrated, at a given incidence angle, a broad continuum of coupled wavelengths over the visible spectrum.
Complementary photo-emission electron microscopy (PEEM) measurements give a high-resolution (25-nm) map of the electric field of the photo-excited plasmonic modes. This technique enables us to distinguish between the coupling of incident light modes to SPP and localized plasmons by the observation of interference fringes and hot spots. The arrangement of the hot spots is discussed as a function of the crystallographic quality of the opal. These results stress the important role of disorder at different scales and open new possibilities for the study of optical disordered media.