Alain Moréac

Raman Modes of Index-Identified Freestanding Single-Walled Carbon Nanotubes

Using electron diffraction on freestanding single-walled carbon nanotubes, we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3 nm. On the same freestanding tubes, we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7 nm), these measurements confirm previously established radial breathing mode frequency versus diameter relations and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.

Ge–Sb–Te thin films deposited by pulsed laser: An ellipsometry and Raman scattering spectroscopy study

Pulsed laser (532 nm) deposited Ge2Sb2Te5 thin films were investigated by means of spectroscopic ellipsometry and Raman scattering spectroscopy. Tauc–Lorentz and Cody–Lorentz models were employed for the evaluation of optical functions of thin films in as-deposited (amorphous) and crystalline (cubic) phases. The models’ parameters (Lorentz oscillator amplitude, resonance energy, oscillator width, optical band gap, and Urbach energy) calculated for amorphous and crystalline states are discussed. The vibrational modes observed in Raman spectra of amorphous layers are attributed to GeTe4−nGen (n=1, 2, eventually 0) tetrahedra connected by corners (partly by edges) and SbTe3 units. The Raman spectra of crystalline thin films suggest that the local bonding arrangement around Ge atoms changes; GeTe component is thus mainly responsible for the phase transition in Ge2Sb2Te5 alloys.

Photo-stability of pulsed laser deposited Ge(x)As(y)Se(100-x-y) amorphous thin films.

Quest for photo-stable amorphous thin films in ternary Ge(x)As(y)Se(100-x-y) chalcogenide system is reported. Studied layers were fabricated using pulsed laser deposition technique. Scanning electron microscope with energy dispersive X-ray analyzer, Raman scattering spectroscopy, transmittance measurements, variable angle spectroscopic ellipsometry, and non-linear imaging technique with phase object inside the 4f imaging system were employed to characterize prepared thin films. Their photo-stability/photo-induced phenomena in as-deposited and relaxed states were also investigated, respectively. In linear regime, we found intrinsically photo-stable relaxed layers within Ge(20)As(20)Se(60) composition. This composition presents also the highest optical damage threshold under non-linear optical conditions.

Chalcogenide coatings of Ge15Sb20S65 and Te20As30Se50

Chalcogenide coatings are investigated to obtain either optical components for spectral applications or optochemical sensors in the mid-infrared. The deposition of Ge(15)Sb(20)S(65) and Te(20)As(30)Se(50) chalcogenide glasses is performed by two physical techniques: electron-beam and pulsed-laser deposition. The quality of the film is analyzed by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy to characterize the morphology, topography, and chemical composition. The optical properties and optical constants are also determined. A CF(4) dry etching is performed on these films to obtain a channeled optical waveguide. For a passband filter made by electron-beam deposition, cryolite as a low-refractive-index material and chalcogenide glasses as high-refractive-index materials are used to favor a large refractive-index contrast. A shift of a centered wavelength of a photosensitive passband filter is controlled by illumination time.

RF sputtered amorphous chalcogenide thin films for surface enhanced infrared absorption spectroscopy

The primary objective of this study is the development of transparent thin film materials in the IR enabling strong infrared absorption of organic compounds in the vicinity of metal nanoparticles by the surface plasmon effect. For developing these optical micro-sensors, hetero-structures combining gold nanoparticles and chalcogenide planar waveguides are fabricated and adequately characterized. Single As2S3 and Ge25Sb10Se65 amorphous chalcogenide thin films are prepared by radio-frequency magnetron sputtering. For the fabrication of gold nanoparticles on a chalcogenide planar waveguide, direct current sputtering is employed. Fabricated single layers or hetero-structures are characterized using various techniques to investigate the influence of deposition parameters. The nanoparticles of gold are functionalized by a self-assembled monolayer of 4-nitrothiophenol. Finally, the surface enhanced infrared absorption spectra of 4-nitrothiophenol self-assembled on fabricated Au/Ge-Sb-Se thin films hetero-structures are measured and analyzed. This optical component presents a ~24 enhancement factor for the detection of NO2 symmetric stretching vibration band of 4-nitrothiophenol at 1336 cm−1.

Molecular dynamics of a short-range ordered smectic phase nanoconfined in porous silicon

4-n-octyl-4-cyanobiphenyl has been recently shown to display an unusual sequence of phases when confined into porous silicon (PSi). The gradual increase of oriented short-range smectic (SRS) correlations in place of a phase transition has been interpreted as a consequence of the anisotropic quenched disorder induced by confinement in PSi. Combining two quasielastic neutron scattering experiments with complementary energy resolutions, the authors present the first investigation of the individual molecular dynamics of this system. A large reduction of the molecular dynamics is observed in the confined liquid phase, as a direct consequence of the boundary conditions imposed by the confinement. Temperature fixed window scans reveal a continuous glasslike reduction of the molecular dynamics of the confined liquid and SRS phases on cooling down to 250 K, where a solidlike behavior is finally reached by a two-step crystallization process.

The neutral-to-ionic phase transition of TTF-CA: a Raman and infrared study versus temperature at atmospheric pressure

The very first low-frequency infrared absorption and Raman scattering spectra of TTF-CA are presented. Several studies have been performed as a function of temperature at atmospheric pressure. In the neutral phase by infrared absorption, we have emphasized the critical behaviour of an antisymmetric lattice mode (located around at 150 K) which shows the displacive nature of the neutral-to-ionic phase transition of TTF-CA at atmospheric pressure. In the ionic phase, the Raman scattering study has permitted confirmation of this result, the soft mode being located around at 16 K. Besides, the Raman study of several totally symmetric internal modes has confirmed the clearly first-order character of this transition. Finally, this study permits an estimation of the e - mv (electron-molecular vibrations) coupling constants in the neutral and ionic phases.

Design of fiber coupled Er3+:chalcogenide microsphere amplifier via particle swarm optimization algorithm

Abstract. A mid-IR amplifier consisting of a tapered chalcogenide fiber coupled to an Er3+-doped chalcogenide microsphere has been optimized via a particle swarm optimization (PSO) approach. More precisely, a dedicated three-dimensional numerical model, based on the coupled mode theory and solving the rate equations, has been integrated with the PSO procedure. The rate equations have included the main transitions among the erbium energy levels, the amplified spontaneous emission, and the most important secondary transitions pertaining to the ion-ion interactions. The PSO has allowed the optimal choice of the microsphere and fiber radius, taper angle, and fiber-microsphere gap in order to maximize the amplifier gain. The taper angle and the fiber-microsphere gap have been optimized to efficiently inject into the microsphere both the pump and the signal beams and to improve their spatial overlapping with the rare-earth-doped region. The employment of the PSO approach shows different attractive features, especially when many parameters have to be optimized. The numerical results demonstrate the effectiveness of the proposed approach for the design of amplifying systems. The PSO-based optimization approach has allowed the design of a microsphere-based amplifying system more efficient than a similar device designed by using a deterministic optimization method. In fact, the amplifier designed via the PSO exhibits a simulated gain G=33.7  dB, which is higher than the gain G=6.9  dB of the amplifier designed via the deterministic method.

Laser desorption ionization time-of-flight mass spectrometry of erbium-doped Ga-Ge-Sb-S glasses

RATIONALE Rare earth-doped sulphide glasses in the Ga-Ge-Sb-S system present radiative emissions from the visible to the middle infrared range (mid-IR) range, which are of interest for a variety of applications including (bio)-chemical optical sensing, light detection, and military counter-measures. The aim of this work was to reveal structural motifs present during the fabrication of thin films by plasma deposition techniques as such knowledge is important for the optimization of thin film growth. METHODS The formation of clusters in plasma plume from different concentrations of erbium-doped Ga5Ge20Sb10S65 glasses (0.05, 0.1, and 0.5 wt. % of erbium) using laser (337 nm) desorption ionization (LDI) was studied by time-of-flight mass spectrometry (TOF MS) in both positive and negative ion mode. The stoichiometry of the Ga(m)Ge(n)Sb(o)S(p)(+/-) clusters was determined via isotopic envelope analysis and computer modelling. RESULTS Several Ga(m)Ge(n)Sb(o)S(p)(+/-) singly charged clusters were found but, surprisingly, only four species (Sb3S4(+/-), GaSb2S(p)(+/-) (p = 4, 5), Ga3Sb2S7(+/-) ) were common to both ion modes. For the first time, species containing rare earths (GaSb2SEr(+) and GaS6 Er2(+)) were identified in the plasma formed from rare earth-doped chalcogenide glasses, directly confirming the importance of gallium presence for rare earth bonding within the glassy matrix. CONCLUSIONS The local structure of Ga-Ge-Sb-S glasses is at least partly different from the structure of species identified in plasma by mass spectrometry, as deduced from Raman scattering spectroscopy analysis; these glasses are mainly formed by [GeS4/2]/[GaS4/2] tetrahedra and [SbS3/2] pyramids. Extended X-ray absorption fine structure measurements show that Er(3+) ions in Ga-Ge-Sb-S glasses are surrounded by 7 sulphur atoms.

Femtosecond spin-state photo-switching dynamics in an FeIII spin crossover solid accompanied by coherent structural vibrations

We investigate light-induced excited spin-state trapping (LIESST) dynamics of an FeIII spin-crossover material from low (S = 1/2) to high (S = 5/2) spin states. Our results show that this process occurs only at the molecular level as evidenced by the linear dependence of the fraction of photo-switched molecules with the excitation density as well as with the initial fraction of low spin molecules. The inter-system crossing from photoexcited LS (S = 1/2) to HS (S = 5/2) occurs within ≈200 fs and is accompanied by coherent non-equilibrium vibrational relaxation in the photo-induced HS state. These results reveal similar dynamical features to those already reported for LIESST in FeII systems. The activation of coherent molecular vibrations is essential for rapidly reaching the HS potential on the timescale of molecular motions, whereas their fast damping allows an efficient trapping in the HS potential. The observed coherent oscillations are attributed to photoinduced molecules in the HS states, as supported by Raman spectroscopy at thermal equilibrium, and DFT analyses of molecular vibrations and TD-DFT calculations of optical absorption.