The observation of very-low-frequency bands by Raman scattering in a nucleated cordierite glass is described. The frequency of the maximum of scattering is proportional to the inverse diameter of the particles, which are spherical spinel microcrystallites. It is shown that vibrational surface modes of particles are responsible for this Raman scattering.PACS numbers: 68.35.Ja, 81.20.Qf After the specific-heat measurements of small metal particles, carried out by Novotny, Meincke, and Watson, 1 and Novotny and Meincke, 2 many theoretical researches have been made on the lattice dynamics 3 " 7 of small particles. The vibrations of spherical particles have been studied a century ago by Lamb 8 who discussed the problem of the homogeneous elastic body in a spherical shape. Two types of modes, spheroidal and torsional, are derived from the stress-free boundary condition of a spherical surface. The recent theoretical works of Tamura and co-workers 6,7 are an extension of Lamb's theory and consider various effects surrounding a small particle: matrix effect, surface relaxation effect, local clamping effect, and shape effect.The aim of the latest theoretical studies was principally to interpret the specific-heat measurements. However, it should be possible to observe by infrared absorption or Raman scattering the modes which obey the transition selection rules. Some years ago Hayashi and Kanamori 9 observed the Raman scattering from the surface phonon mode in GaP microcrystals. The energy of the observed modes was between the bulk TO and LO phonon peaks. In the work which is presented in this Letter we were rather interested in the Raman scattering from particle modes, the energies of which fall in the acoustic of the bulk, and which contribute to specific heat.Actually, the first observation of particle modes by Raman scattering at very low frequency in a nucleated glass is presented in this Letter. This Raman spectroscopy follows a detailed study of nucleation and precise characterization of nuclei or microcrystals, carried out partially in our research group. 10,11 The base glass, nucleation of which has been studied, has a composition close to that of the mineral cordierite: 52 Si0 2 , 34.7 A1 2 0 3 , 12.5 MgO, with 0.8 Cr 2 0 3 (wt.%). Nucleation was induced at a temperature higher than 800 °C by Cr 3 + ions which are well known as a good nucleating agent. The characterization of nuclei or microcrystallites was carried out by several techniques. The size of the particles was determined principally by small-angle neutron scattering, 10 and also by small-angle x-ray scattering and electron microscopy. 12 The nature and structure of the microcrystallites were precisely determined by electron paramagnetic resonance, 10 " 13 laser spectroscopy, 11 and electron diffraction. 12 The process of nucleation and crystallization was described as follows. At first there is a diffusion of Cr 3+ ions in the glassy matrix giving rise to a clustering of these ions and the formation of a mixed MgCr 2 0 4 -MgAl 2 04 spinel. Microcr...

International audienceIn this review paper, we present radiation effects on silica-based optical fibers. We first describe the mechanisms inducing microscopic and macroscopic changes under irradiation: radiation-induced attenuation, radiation-induced emission and compaction. We then discuss the influence of various parameters related to the optical fiber, to the harsh environments and to the fiber-based applications on the amplitudes and kinetics of these changes. Then, we focus on advances obtained over the last years. We summarize the main results regarding the fiber vulnerability and hardening to radiative constraints associated with several facilities such as Megajoule class lasers, ITER, LHC, nuclear power plants or with space applications. Based on the experience gained during these projects, we suggest some of the challenges that will have to be overcome in the near future to allow a deeper integration of fibers and fiber-based sensors in radiative environments

A model assuming a non-continuous structure of glasses is established to interpret the inelastic neutron scattering and Raman scattering. The 'boson' peak in Raman scattering is related to the vibrational density of states 'excess'. These two related features are a result of vibrations localized in the blobs that compose the glass. Size distributions of the blobs are deduced from neutron and Raman scatterings.

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In this topical review, the recent progress on radiation-hardened fiber-based technologies is detailed, focusing on examples for space applications. In the first part of the review, we introduce the operational principles of the various fiber-based technologies considered for use in radiation environments: passive optical fibers for data links, diagnostics, active optical fibers for amplifiers and laser sources as well as the different classes of point and distributed fiber sensors: gyroscopes, Bragg gratings, Rayleigh, Raman or Brillouin-based distributed sensors. Second, we describe the state of the art regarding our knowledge of radiation effects on the performance of these devices, from the microscopic effects observed in the amorphous silica glass used to design fiber cores and cladding, to the macroscopic response of fiber-based devices and systems. Third, we present the recent advances regarding the hardening (improvement of the radiation tolerance) of these technologies acting on the material, device or system levels. From the review, the potential of fiber-based technologies for operation in radiation environments is demonstrated and the future challenges to be overcome in the coming years are presented.

International audienceUltrashort laser pulses can modify the inner structure of fused silica, generating refractive index changes varying from soft positive (type I) light guiding forms to negative (type II) values with void presence and anisotropic sub-wavelength modulation. We investigate electronic and structural material changes in the type I to type II transition via coherent and incoherent secondary light emission reflecting free carrier behavior and post-irradiation material relaxation in the index change patterns. Using phase contrast microscopy, photoluminescence, and Raman spectroscopy, we determine in a space-resolved manner defect formation, redistribution and spatial segregation, and glass network reorganization paths in conditions marking the changeover between type I and type II photoinscription regimes. We first show characteristic patterns of second harmonic generation in type I and type II traces, indicating the collective involvement of free carriers and polarization memory. Second, incoherent photoemission from resonantly and non-resonantly excited defect states reveals accumulation of non-bridging oxygen hole centers (NBOHCs) in positive index domains and oxygen deficiency centers (ODCs) with O 2 ions segregation in voidlike regions and in the nanostructured domains, reflecting the interaction strength. Complementary Raman investigations put into evidence signatures of the different environments where photochemical densification (bond rearrangements) and mechanical effects can be indicated. NBOHCs setting in before visible index changes serve as precursors for subsequent compaction build-up, indicating a scenario of cold, defect-assisted densification for the soft type I irradiation regime. Additionally, we observe hydrodynamic effects and severe bond-breaking in type II zones with indications of phase transition. These observations illuminate densification paths in fused silica in low power irradiation regimes, and equally in energetic ranges, characterized by the onset of thermo-mechanical effects

We investigated the efficiencies of two different approaches to increase the radiation hardness of optical amplifiers through development of improved rare-earth (RE) doped optical fibers. We demonstrated the efficiency of codoping with Cerium the core of Erbium/Ytterbium doped optical fibers to improve their radiation tolerance. We compared the γ-rays induced degradation of two amplifiers with comparable pre-irradiation characteristics (~19 dB gain for an input power of ~10 dBm): first one is made with the standard core composition whereas the second one is Ce codoped. The radiation tolerance of the Ce-codoped fiber based amplifier is strongly enhanced. Its output gain decrease is limited to ~1.5 dB after a dose of ~900 Gy, independently of the pump power used, which authorizes the use of such fiber-based systems for challenging space missions associated with high total doses. We also showed that the responses of the two amplifiers with or without Ce-codoping can be further improved by another technique: the pre-loading of these fibers with hydrogen. In this case, the gain degradation is limited to 0.4 dB for the amplifier designed with the standard composition fiber whereas 0.2 dB are reported for the one made with Ce-codoped fiber after a cumulated dose of ~900 Gy. The mechanisms explaining the positive influences of these two treatments are discussed.

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