Measurements of refractive indices and thermo-optical coefficients using a white-light Michelson interferometer.

A dispersive white-light Michelson interferometer was used to determine the wavelength dependence of the refractive index (n) in the visible range from 425 to 775 nm and the thermo-optical coefficient (dn/dT) of fused silica (FS) and borosilicate glass (BK7). For FS, the values obtained for n and dn/dT at 546 nm were 1.46079 and 11.3×10-6 K-1, respectively, while the values for BK7 glass were 1.51825 and 2.2×10-6 K-1, respectively, which is in good agreement with the literature. The accuracy of the methodology used for n was almost 10-6, enabling precise spectroscopic characterization of materials across a wide spectral range.

Enhanced optical properties and (Zn, Mg) interdiffusion in vapour transport grown ZnO/MgO core/shell nanowires.

ZnO/MgO (core/shell) nanowires (NWs) grown by a two-step vapour transport method under different MgO shell growth conditions are examined by x-ray diffraction, photoluminescence (PL) excitation and temperature (10-300 K) dependent PL. The excitonic-to-defect PL ratio is increased by more than two orders of magnitude in the core/shell as compared to bare ZnO NWs. Concomitantly, a strong depression of the PL thermal quenching, most particularly for the visible part of the PL spectrum, occurs. Using a semi-quantitative model, results are interpreted as a strong radiative to non-radiative lifetime ratio reduction due to defect passivation at the ZnO NW walls and photocarrier confinement within the ZnO core by the MgO shell. These beneficial effects are, however, significantly weakened when metal interdiffusion across the core/shell interface is favoured during the shell growth. Non-radiative recombination lifetime in the sample with sharp core/shell interface is described by a single activation energy of 15 meV (bound exciton release). For interdiffused cases and bare ZnO an additional activation energy of 60 meV (free exciton breakup) is observed.

Resonant excited state absorption and relaxation mechanisms in Tb3+-doped calcium aluminosilicate glasses: an investigation by thermal mirror spectroscopy.

Resonant excited state absorption (ESA) and relaxation processes in Tb(3+)-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level (5)D(4) was found to be close to unity and concentration independent while, for the level (5)D(3), it decreases with Tb(3+) concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.

Near-infrared quantum cutting through a three-step energy transfer process in Nd3+-Yb3+ co-doped fluoroindogallate glasses.

The Nd(3+)-Yb(3+) couple was investigated in fluoroindogallate glasses using optical spectroscopy to elucidate the energy transfer mechanisms involved in the downconversion (DC) process. Upon excitation of a Nd(3+) ion by an ultraviolet photon, DC through a three-step energy transfer process occurs, in which the energy of the ultraviolet photon absorbed by the Nd(3+) ion is converted into three infrared photons emitted by Yb(3+) ions, i.e. quantum cutting (QC). In addition, with excitation in the visible, our results confirm that the DC process occurs through a one-step energy transfer process, in which the energy of a visible photon absorbed by the Nd(3+) ion is converted into only one infrared photon emitted by an Yb(3+) ion. Time-resolved measurements enabled the estimation of the efficiencies of the cross-relaxation processes between Nd(3+) and Yb(3+) ions.

Correlations between conjugation length, macromolecular dynamics, and photophysics of phenylene-vinylene/aliphatic multiblock copolymers.

This work reports a detailed spectroscopy study of a series of multiblock conjugated nonconjugated copolymers built by p-phenylene vinylene type units (PV) and octamethylene spacers, namely, poly(1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene) (LaPPS18). The relative proportions of the PV and aliphatic segments were estimated on the basis of solid-state NMR and Raman spectroscopy. The overall structure was characterized by wide angle X-ray diffraction; (1)H wide-line dipolar chemical shift correlation (DIPSHIFT), and centerband-only detection of exchange (CODEX) NMR data, that together with glass transition temperatures allowed us to identify the groups involved in the molecular dynamics. These different structural properties were used to explain the photoluminescence properties in terms of peak position and spectral profile.

From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties.

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36-0.42 in Si(y)O(1-y)), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T(a) = 900-1100 °C) under either Ar or (Ar + 5%H(2)) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate T(a) can be qualitatively understood in terms of a 'crystalline core-amorphous shell' Si-ncl model. The amorphous shell, which is invisible in most diffraction and electron microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing T(a), the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.

Localized surface plasmon resonance interaction with Er3+-doped tellurite glass.

We show the annealing effect on silver and Erbium-doped tellurite glasses in the formation of nanoparticles (NPs) of silver, produced by the reduction of silver (Ag+ → Ag0), aiming to an fluorescence enhancement. The absorption spectra show typical Localized Surface Plasmon Resonance (LSPR) band of Ag0 NP in addition to the distinctive absorption peaks of Er3+ ions. Both observations demonstrate that the photoluminescence enhancement is due to the coupling of dipoles formed by NPs with the Er3+ 4I(13/2) → 4I(15/2) transition. This plasmon energy transfer to the Er3+ ions was observed in the fluorescence spectrum with a blue-shift of the peaks.

Study on the observation of Eu2+ and Eu3+ valence states in low silica calcium aluminosilicate glasses.

The optical, magnetic and structural properties of Eu doped low silica calcium aluminosilicate glasses were investigated. The optical absorption coefficient presented two bands at 39,246 and 29,416 cm(-1), which were assigned respectively to the [Formula in text], and [Formula in text] transitions of Eu(2+). The fluorescence measured at 300 K on a sample doped with 0.5 wt% of Eu(2)O(3) exhibited a broad band centered at 17,350 cm(-1), which is attributed to the [Formula in text] transition of Eu(2+), whereas the additional peaks are due to the [Formula in text] transitions of Eu(3+). From magnetization and XANES data it was possible to evaluate the fractions of Eu(2+) and Eu(3+) for the sample doped with 0.5 and 5.0 wt% of Eu(2)O(3), the values of which were approximately 30 and 70%, respectively.

Short-range structure and cation bonding in calcium-aluminum metaphosphate glasses.

Comprehension of short- and medium-range order of phosphate glasses is a topic of interest, due to the close relation between network structure and mechanical, thermal, and optical properties. In this work, the short-range structure of glasses (1 - x)Ca(PO(3))(2).xAl(PO(3))(3) with 0 < or = x < or = 0.47 was studied using solid-state nuclear magnetic resonance spectroscopy, Raman spectroscopy, density measurements, and differential scanning calorimetry. The bonding between a network modifier species, Al, and the network forming phosphate groups was probed using high-resolution nuclear magnetic resonance spectroscopy of (27)Al and (31)P. Changes in the compositional behavior of the density, glass transition temperature, PO(2) symmetric vibrations, and Al coordination number were verified at around x = 0.30. (31)P NMR spectra show the presence of phosphorus in Q(2) sites with nonbridging oxygens (NBOs) coordinated by Ca ions and also Q(2) sites with one NBO coordinated by Al (namely, Q(2)(1Al)). The changes in the properties as a function of x can be understood by considering the mean coordination number measured for Al and the formation of only Q(2) and Q(2)(1Al) species. It is possible to calculate that a network formed only by Q(2)(1Al) phosphates can just exist up to the upper limit of x = 0.48. Above this value, Q(2)(2Al) species should appear, imposing a major reorganization of the network. Above x = 0.30 the network undergoes a progressive reorganization to incorporate Al ions, maintaining the condition that only Q(2)(1Al) species are formed. These observations support the idea that bonding principles for cationic species inferred originally in binary phosphate glasses can also be extended to ternary systems.

Nonlinear refraction spectroscopy in resonance with laser lines in solids.

We report a simple extension of the Z-scan technique that permits a spectral line-shape measurement of the real and the imaginary parts of n(2) . In this technique the sample is placed at the peak position of the usual Z-scan curve while the laser frequency is scanned. We employed this method to investigate the nonlinear susceptibility of the R lines of ruby and alexandrite, using a cw dye laser. This susceptibility can be explained by the resonant interaction and by a nonresonant contribution that is due to the difference in polarizability between Cr(3+) excited and ground states. For ruby, the nonresonant contribution to the technique is 1 order of magnitude larger than the resonant contribution. However, for alexandrite both contributions are comparable, and their interference leads to a shift between n(2)(?) and n(2)(??) spectra that is not observed in ruby.