Simulation of the optical properties of Tm:ZBLAN glass. II. Energy transfer between Tm ions under single- and dual-wavelength excitation

The energy transfer rate between Tm³⁺ ions in ZBLAN glass was estimated from the optical spectra and transition rates on the basis of a method proposed by Kushida. The optical spectra and radiative transition rates were obtained from the structural models prepared by molecular dynamic simulation and the crystal field theory. The lifetimes of ³H₄, ¹G₄ and ¹D₂ levels were estimated from the calculation and the results were experimentally confirmed. The numerical model was also used to predict emission intensities under dual-wavelength excitation and decay curves under CW excitation.     

Enhanced frequency upconversion in Er doped fluoroindate glass due to energy transfer from Tm

Energy transfer processes between Er³⁺ and Tm³⁺ were investigated examining the frequency upconversion emissions in a fluoroindate glass pumped at 790 nm. A 60-fold enhancement in the emission at ≈670 nm originating from Er³⁺ was observed when Tm³⁺ at concentration of 2% was introduced in a sample containing 2% of Er³⁺. The results are explained considering the influence of cross-relaxation processes between the active ions.     

Sol-gel preparation and optical properties of SiO2-Ta2O5 glass

Tantalum-doped silica glass was fabricated by the sol-gel process in order to obtain a glass with a high refractive index for optical use. A crack-free, clear glass rod was successfully prepared from a low-density gel and used as the core material for fabricating optical fibers. Transmission loss in the fabricated fibers was high, in the range of 10³-10⁴ dB/km, which may be caused by coloration due to the multivalency of tantalum; however, the loss was reduced by nearly one order of magnitude by heat treatment at 800 °C, that is, to 75 dB/km at a wavelength of 0.8 μm.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Two crystallographic sites in erbium-doped fluoride glass by frequency-resolved and site-selective spectroscopies

Erbium-doped PZG fluoride glass (lead-zinc-gallium) has been studied using site-selective spectroscopy associated to frequency-resolved spectroscopy. Possibilities of site-selective emission using frequency-resolved spectroscopy are discussed. The existence of two crystallographic sites is demonstrated and the spectroscopic properties of Er³⁺ in each site is analyzed. The minority site distribution peaks at high energy at 6575 cm⁻¹, its ⁴I_13/2 lifetime is non-exponential and lower than 5 ms and its ground state spreads over 302 cm⁻¹. The majority site distribution peaks at 6530 cm⁻¹, its ⁴I_13/2 lifetime is 11 ms and its ground state spreads over 333 cm⁻¹. The structural origin of these two sites is discussed.     

Concentration quenching of fluorescence from the (P0 + P1) manifold in heavily-doped Pr: ZBAN glasses

Fluorescence waveforms from the (³P₀ + ³P₁) manifold in Pr³⁺ doped ZBAN glass at wavelengths of 520, 635 and 695 nm were measured for Pr³⁺ concentrations from 4 to 12 mol%. The waveforms were found to be non-exponential with decay rates rapidly increasing with Pr³⁺ concentration and independent of whether the ³P₀ or the ³P₁ level was excited. The multipolar energy transfer model was used to analyse the waveforms and this showed that concentration quenching was due to cross-relaxation by dipole-dipole interaction. The critical concentration, at which the cross-relaxation rate equals the intrinsic decay rate, was found to be of 2.06 × 10²⁶ m⁻³ (1.20 mol%). There was no evidence of excitation diffusion for Pr³⁺ concentrations of up to 12 mol%.     

The spectrum and laser properties of ytterbium doped phosphate glass at low temperature

Low-temperature absorption and fluorescence spectra of the Yb³⁺ ions were measured in phosphate glass with compositions of (60-65)P₂O₅-(4-8)B₂O₃-(5-10)Al₂O₃-(10-15)K₂O-(5-10)BaO-(0-2)La₂O₃-(0-2)Nb₂O₅-(4-8)Yb₂O₃ (mol%). Temperature dependence of lifetime of Yb³⁺:²F_5/2 level was investigated. Laser performance of sample pumped by 940 nm laser diode at low temperature were presented. At 8 K, laser oscillation of diode pumped Yb³⁺: phosphate glass yielded a slope efficiency of 4% and a maximum power of 2 mW, the peak laser wavelength is 1001 nm.     

Infrared fibers based on Te-As-Se glass system with low optical losses

Unclad optical fibers based on high-purity Te-As-Se glasses prepared by chemical and physical methods of purification have been drawn. The optical, thermal and mechanical properties of glasses and fibers were investigated. The minimum optical losses were 0.07 dB/m at 7.3 μm for Te₂₅As₄₀Se₃₅ glass fiber and 0.04 dB/m at 6.7 μm for Te₂₀As₃₀Se₅₀ glass fiber. Sixty five percent of input power of a tunable CO₂ laser emitting at 9.3 μm was transmitted through a 1 m long fiber with diameter of 900 μm.     

Vanadia-silica xerogels: optical and structural properties

Vanadium doped silica gels were prepared from tetraethyl orthosilicate and three different inorganic vanadium precursors with formal oxidation states of V³⁺, V⁴⁺ and V⁵⁺ respectively. Optical and EPR studies were carried out on the dried gels to understand the changes in the oxidation state and coordination of vanadium in the doped silica gel matrix. The observed optical and EPR results provide very strong evidence to establish that irrespective of the starting material, vanadium is stabilized as vanadyl ion in the gel monoliths. EPR studies on the powdered samples corroborate the optical data on the gel samples and confirmed that the stabilized vanadyl ion is situated in a distorted octahedral geometry in these silica gels.     

Analysis of spectral components in the 1.5 μm emission band of Er doped borosilicate glass

We measured the 1.5 μm emission spectra corresponding to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ in borosilicate glass within the temperature range from 11 to 300 K. The spectral components emitting from the lowest and upper Stark levels of ⁴I_13/2 state were distinguished by analyzing the spectra with normalized area. The effect of optical properties of the spectral components on the 1.5 μm emission bandwidth is investigated. The results indicate that to search for a host with higher spontaneous emission probability of the upper Stark levels of ⁴I_13/2 state for Er³⁺ ions is very important to broadening of the 1.5 μm emission band. An equivalent model of four-level system is presented and applied to explain the spectral shape and temperature characteristics of the 1.5 μm emission band.     

Nanosized gold clusters formation in selected areas of soda-lime silicate glass

Nanosized gold clusters were developed in a gold-doped soda-lime silicate glass after X-ray irradiation followed by annealing, which provided an alternative way of forming metallic nanoclusters in glass to ion implantation. The formation of gold nanoclusters can be controlled by irradiation time or annealing time.     

Efficient frequency upconversion emission in Er-doped novel strontium lead bismuth glass

Er³⁺-doped strontium lead bismuth glass for developing upconversion lasers has been fabricated and characterized. The Judd-Ofelt intensity parameters Ω_t (t = 2, 4, 6), calculated based on the experimental absorption spectrum and Judd-Ofelt theory, were found to be Ω₂ = 2.95 × 10⁻²⁰, Ω₄ = 0.91 × 10⁻²⁰, and Ω₆ = 0.36 × 10⁻²⁰ cm². Under 975 nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2, respectively, were observed. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for the green and red emissions. The long-lived ⁴I_11/2 level is supposed to serve as the intermediate state responsible for the upconversion processes.     

X-ray absorption and Raman characterizations of TeO2-Ga2O3 glasses

The thermodynamic properties of transparent glasses prepared in the TeO₂-Ga₂O₃ system were investigated by differential scanning calorimetry. The change of the thermal parameters as a function of the chemical composition is discussed. Raman and both Te L_III and Ga K edge X-ray absorption spectroscopies at room temperature were used to examine the short range order. Analyses of the spectra suggest that the addition of Ga₂O₃ content to the TeO₂ glass matrix induces the transformation of trigonal bipyramids (TeO₄E, E=lone electronic pair 5s² of Te) to trigonal pyramids (TeO₃E) with formation of Te-O-Ga bridging bonds. Furthermore, Ga K edge XANES and EXAFS studies show that Ga atoms exhibit both tetrahedral (GaO₄) and octahedral (GaO₆) environments.     

Effects of composition and catalyst on the optical properties of ZrO2-based Ormosil films

FTIR spectroscopy measurements were performed to investigate zirconia-based Ormosil thin films. Samples synthesized with a sol-gel technique were prepared starting from 3-glycidoxypropil-trimethoxysilane (GLYMO) and zirconium(IV)-propoxide (Zr-OPr). Acetic or nitric acid were used as a catalyst. The effects induced by the Zr content and the catalyst type on the mid-infrared spectra of the sol-gel hybrid films were studied and a detailed assignment of the vibrational absorption bands is presented. The thermal evolution of the hybrid systems was also followed by monitoring the release of the organic groups, the thickness, and refractive index changes by means of FTIR, diffuse reflectance spectroscopy, and ellipsometry.     

Mechanical reliability of silica optical fibers

Dynamic and static mechanical tests were implemented using a tensile test bench and a static fatigue test under uniform curve. The incidence of aging treatments at 65 and 85 °C was investigated on two standard silica optical fibers (with polyacrylate and fluorinated coatings). Microscopic observations helped the understanding of the failure mechanism. It appears that the cyclic variations of the failure stress phenomenon, with respect to the aging time, are the result of the silicate gel which migrates towards the polymer coating.     

The role of a three-ion energy transfer process in the violet fluorescence in highly doped Er:ZB(L)AN glasses

The violet (∼400-410-nm) fluorescence in Er³⁺:ZB(L)AN glasses has been previously attributed to transitions originating from the ²H_9/2 and ²P_3/2 levels. The study reported here found that, in high Er³⁺ concentrations and with 800-nm excitation, a significant source of violet fluorescence around 407-nm was due to the previously unreported ⁴G_9/2 → ⁴I_9/2 transition. The study also established that, under these conditions, a three-ion energy transfer process originating in the ⁴I_9/2 level is responsible for populating of the fluorescing ⁴G_9/2 level.