Ref HAL: hal-01265608_v1
DOI: 10.1177/0022034516629400
WoS: 000374580000013
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7 Citations
Résumé:

Multiphoton confocal microscopy and nonlinear spectroscopy are used to investigate the caries process in dentin. Although dentin is a major calcified tissue of the teeth, its organic phase comprises type I collagen fibers. Caries drive dentin demineralization and collagen denaturation. Multiphoton microscopy is a powerful imaging technique: the biological materials are transparent to infrared frequencies and can be excited to penetration depths inaccessible to 1-photon confocal microscopy. The laser excitation greatly reduces photodamage to the sole focal region, and the signal-to-noise ratio is improved significantly. The method has been used to follow pathologic processes involving collagen fibrosis or collagen destruction based on their 2-photon excited fluorescence (2PEF) emission and second harmonic generation (SHG). Combining multiphoton imaging with nonlinear spectroscopy, we demonstrate that both 2PEF and SHG intensity of human dentin are strongly modified during the tooth caries process, and we show that the ratio between SHG and 2PEF signals is a reliable parameter to follow dental caries. The ratio of the SHG/2PEF signals measured by nonlinear optical spectroscopy provides valuable information on the caries process, specifically on the degradation of the organic matrix of dentin. The goal is to bring these nonlinear optical signals to clinical application for caries diagnosis.

PMID 26678677
DOI: 10.1021/acs.nanolett.5b02901
WoS: 000368322700013
95 Citations
Résumé:

Topological insulators (TIs) represent a novel quantum state of matter, characterized by edge or surface-states, showing up on the topological character of the bulk wave functions. Allowing electrons to move along their surface, but not through their inside, they emerged as an intriguing material platform for the exploration of exotic physical phenomena, somehow resembling the graphene Dirac-cone physics, as well as for exciting applications in optoelectronics, spintronics, nanoscience, low-power electronics, and quantum computing. Investigation of topological surface states (TSS) is conventionally hindered by the fact that in most of experimental conditions the TSS properties are mixed up with those of bulk-states. Here, we activate, probe, and exploit the collective electronic excitation of TSS in the Dirac cone. By engineering Bi2Te(3-x)Sex stoichiometry, and by gating the surface of nanoscale field-effect-transistors, exploiting thin flakes of Bi2Te2.2Se0.8 or Bi2Se3, we provide the first demonstration of room-temperature terahertz (THz) detection mediated by overdamped plasma-wave oscillations on the "activated" TSS of a Bi2Te2.2Se0.8 flake. The reported detection performances allow a realistic exploitation of TSS for large-area, fast imaging, promising superb impacts on THz photonics.

Ref HAL: hal-01261847_v1
DOI: 10.1038/nphoton.2015.277
WoS: 000372978900017
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476 Citations
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Hexagonal boron nitride is a wide bandgap semiconductor with a very high thermal and chemical stability used in devices operating under extreme conditions. The growth of high-purity crystals has recently revealed the potential of this material for deep ultraviolet emission, with an intense emission around 215 nm. In the last few years, hexagonal boron nitride has been raising even more attention with the emergence of two-dimensional atomic crystals and Van der Waals heterostructures, initiated with the discovery of graphene. Despite this growing interest and a seemingly simple structure, the basic questions of the bandgap nature and value are still controversial. Here, we resolve this long-debated issue by bringing the evidence for an indirect bandgap at 5.955 eV by means of optical spectroscopy. We demonstrate the existence of phonon-assisted optical transitions, and we measure an exciton binding energy of about 130 meV by two-photon spectroscopy.

Ref HAL: hal-01261246_v2
Ref Arxiv: 1510.06320
DOI: 10.1063/1.4939640
WoS: 000368618400033
Ref. & Cit.: NASA ADS
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17 Citations
Résumé:

Cavity point-to-set correlations are real-space tools to detect the roughening of the free-energy landscape that accompanies the dynamical slowdown of glass-forming liquids. Measuring these correlations in model glass formers remains, however, a major computational challenge. Here, we develop a general parallel-tempering method that provides orders-of-magnitude improvement for sampling and equilibrating configurations within cavities. We apply this improved scheme to the canonical Kob-Andersen binary Lennard-Jones model for temperatures down to the mode-coupling theory crossover. Most significant improvements are noted for small cavities, which have thus far been the most difficult to study. This methodological advance also enables us to study a broader range of physical observables associated with thermodynamic fluctuations. We measure the probability distribution of overlap fluctuations in cavities, which displays a non-trivial temperature evolution. The corresponding overlap susceptibility is found to provide a robust quantitative estimate of the point-to-set length scale requiring no fitting. By resolving spatial fluctuations of the overlap in the cavity, we also obtain quantitative information about the geometry of overlap fluctuations. We can thus examine in detail how the penetration length as well as its fluctuations evolve with temperature and cavity size.

Commentaires: 12 pages, 7 figures; v2: minor revisions made, figure 1b added, Sec.V extensively revised for clarification, references added. Réf Journal: J. Chem. Phys. 144, 024501 (2016)

Ref HAL: hal-01259346_v1
PMID 26592422
DOI: 10.1039/c5sm02265g
WoS: 000369747900028
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We develop a theory which predicts curvature-related structural peculiarities of soft spherical nanostructures with a dodecagonal local arrangement of subunits. Spherical templates coated with a thin film of a soft quasicrystal (QC)-forming material constitute the most promising direction to realize these nanostructures. Disordered and perfect spherical nanostructures are simulated using two approaches. The first of them models a random QC-like spherical nanostructure with extended curvature-induced topological defects similar to scars in colloidal spherical crystals. The second approach is inspired by the physics of viral capsids. It deals with the most regular spherical nanostructures with a local QC-like order derived from three well-known planar dodecagonal tilings. We explain how the additional QC-like degrees of freedom assist the nanostructure stabilization and determine the point defect number and location without extended scar formation. Unusual for nanoassemblies snub cube geometry is shown to be the most energetically favorable global organization of these spherical QC nanostructures.

Résumé:

The competition between order and disorder in dense states of matter (liquids, crystals, glasses) is one of the most fascinating aspects of statistical physics. The microscopic mechanisms that allow us to transform some liquids into glass with ease, while others crystallize quickly when cooled or compressed, are subtle and still remain poorly understood. To try to understand how the collective behavior of these systems emerges from the interactions between the atoms, physicists often build on simple theoretical models, whose prototype is a system of purely repulsive hard spheres. These models, which were introduced during the second half of the twentieth century, have recently found important applications in glass transition studies, soft matter modeling, as well as in packing problems. The generalization to different spatial dimensions - from the Euclidean plane to the infinite dimensional limit - brings additional insight into which microscopic features stabilize disordered phases of matter and into the interplay between structure and dynamics. In reviewing these recent advances, I will emphasize the crucial role of numerical simulations, which help interpreting the experiments and provide stringent tests of theoretical models. I will discuss how to exploit powerful yet cheap architectures, such as graphics cards, and efficient algorithms to dramatically accelerate the sampling of configurational space.

Ref HAL: hal-01254327_v1
Ref Arxiv: 1602.00551
DOI: 10.1021/acs.macromol.5b01481
WoS: 000368322000045
Ref. & Cit.: NASA ADS
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4 Citations
Résumé:

By using the blob theory and computer simulations, we investigate the properties of a linear polymer performing a stationary rotational motion around a long impenetrable rod. In particular, in the simulations the rotation is induced by a torque applied to the end of the polymer that is tethered to the rod. Three different regimes are found, in close analogy with the case of polymers pulled by a constant force at one end. For low torques the polymer rotates maintaining its equilibrium conformation. At intermediate torques the polymer assumes a trumpet shape, being composed by blobs of increasing size. At even larger torques the polymer is partially wrapped around the rod. We derive several scaling relations between various quantities as angular velocity, elongation and torque. The analytical predictions match the simulation data well. Interestingly, we find a "thinning" regime where the torque has a very weak (logarithmic) dependence on the angular velocity. We discuss the origin of this behavior, which has no counterpart in polymers pulled by an applied force.

Commentaires: 30 pages, 8 figures, 1 TOC figure; video abstract at https://youtu.be/LwicoSkh3mI. Réf Journal: Macromolecules, 2016, 49 (1), 405-414