H. Bulou

One-step waveguide and optical circuit writing in photopolymerizable materials processed by two-photon absorption

Two-photon absorption process is known to be a convenient tool to create three-dimensional microstructures in photopolymerizable materials. In this context, we have fabricated stable optical waveguides. The features of these waveguides (in particular, transmission losses) have been compared to the results of numerical simulations. We have also demonstrated the possibility of connecting two optical fibers via a curved guide and to realize Y splitters. The technique allows one to fabricate operational integrated optical circuits in photopolymerizable resins.

Charge injection through single and double carbon bonds.

The contact conductance of oriented C(60) molecules is probed with a scanning tunneling microscope as a function of the lateral position of the tip in contact to the molecular cage. Together with first principles calculations, these measurements reveal variations of the efficiency of charge injection to the fullerene molecule with the order of the contacted carbon-carbon bond.

Oligothiophene nanorings as electron resonators for whispering gallery modes.

Structural and electronic properties of oligothiophene nanowires and rings synthesized on a Au(111) surface are investigated by scanning tunneling microscopy. The spectroscopic data of the linear and cyclic oligomers show remarkable differences which, to a first approximation, can be accounted by considering electronic state confinement to one-dimensional boxes having, respectively, fixed and periodic boundary conditions. A more detailed analysis shows that polythiophene must be treated as a ribbon (i.e., having an effective width) rather than a purely 1D structure. A fascinating consequence is that the molecular nanorings act as whispering gallery mode resonators for electrons, opening the way for new applications in quantum electronics.

Narrow-line single-molecule transducer between electronic circuits and surface plasmons

A molecular wire containing an emitting molecular center is controllably suspended between the plasmonic electrodes of a cryogenic scanning tunneling microscope. Passing current through this circuit generates an ultranarrow-line emission at an energy of ≈1.5  eV which is assigned to the fluorescence of the molecular center. Control over the linewidth is obtained by progressively detaching the emitting unit from the surface. The recorded spectra also reveal several vibronic peaks of low intensities that can be viewed as a fingerprint of the emitter. Surface plasmons localized at the tip-sample interface are shown to play a major role in both excitation and emission of the molecular excitons.

High Spin Polarization at Ferromagnetic Metal–Organic Interfaces: A Generic Property

A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metals electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.

Polarization state studies in second harmonic generation signals to trace atherosclerosis lesions

We have performed multi-photon image reconstructions as well as polarization state analyses inside an artery wall affected by atherosclerosis to investigate the changes in collagen structure. Mice, either healthy or affected by spontaneous atherosclerosis, have been used for this purpose. A two-photon imaging system has been used to investigate atherosclerotic lesions in the ascending aorta of mice. Second harmonic imaging has been performed alternatively on healthy samples and on affected region. The reconstructed images show that the spatial distribution of the collagen network seems disorganized by the disease. The polarization state studies reveal however that the apparent disorganization of the collagen is related to its spatially diffuse distribution and that the internal structure of the collagen fibers is not affected by the disease. In addition, a theoretical simulation of the second harmonic polarization states shows that they are consistent with the known 3D structure of the collagen network.

Time-resolved measurement of the refractive index for photopolymerization processes

A double-interferometer technique is employed to examine the dynamics of a photopolymerization process. The dye molecule is eosine Y. The refractive index and the thickness of the photopolymerizable film are measured as a function of time. During the photopolymerization process, the first quantity increases by 2%, while the second quantity decreases by more than 4%. Therefore, the refractive index cannot be measured by means of single-interferometer techniques. By fitting our experimental curves to a rate equation, the quantum yield and the absorption coefficient of the sample can be determined with good accuracy.

EXAFS study of the crystallographic structure of cobalt thin films on Pt(111)

Abstract The growth and structure of cobalt on a platinum (111) single crystal is studied using X-ray absorption spectroscopy at the cobalt K-edge. A polarisation dependent analysis of the EXAFS spectra shows first neighbours distances in Co Pt (111) films identical to those in bulk cobalt for thicknesses between 2 and 10 monolayers. The full shell spectra polarisation dependence indicates the hexagonal stacking of the thicker film. Influences of annealing and recovering by platinum on the structure of the cobalt films are also described.

Pulling and Stretching a Molecular Wire to Tune its Conductance.

A scanning tunnelling microscope is used to pull a polythiophene wire from a Au(111) surface while measuring the current traversing the junction. Abrupt current increases measured during the lifting procedure are associated with the detachment of molecular subunits, in apparent contradiction with the expected exponential decrease of the conductance with wire length. Ab initio simulations reproduce the experimental data and demonstrate that this unexpected behavior is due to release of mechanical stress in the wire, paving the way to mechanically gated single-molecule electronic devices.

Magnetism of CoPd self-organized alloy clusters on Au(111)

Magnetic properties of gold-encapsulated CoxPd1−x self-organized nano-clusters on Au(111) are analyzed by x-ray magnetic circular dichroism for x = 0.5, 0.7, and 1.0. The clusters are superparamagnetic with a blocking temperature decreasing with increasing Pd concentration, due to a reduction of the out-of-plane anisotropy strength. No magnetic moment is detected on Pd in these clusters, within the detection limit, contrary to thick CoPd films. Both reduction of anisotropy and vanishing Pd moment are attributed to strain.