Characterization of Eu3+-doped fluorophosphate glasses for red emission

Fluorescence spectra and decay curves of the ⁵D₀ level for different concentrations of Eu³⁺ (4f⁶) ions in K–Ba–Al fluorophosphate glasses have been measured at room temperature and are analyzed. The Judd–Ofelt intensity parameters Ω₂ and Ω₄ have been determined from the intensity ratios of emission peaks corresponding to ⁵D₀ → ⁷F_J (J = 2 and 4) to ⁵D₀ → ⁷F₁ transitions for 1.0 mol% glass. The intensity parameters thus obtained are in turn used to calculate the radiative properties of the fluorescent levels of Eu³⁺ ions. Second and fourth rank crystal-field parameters have been evaluated by assuming a C_2V site symmetry for the local environment of Eu³⁺ ions to estimate the crystal-field strength experienced by Eu³⁺ ions in the present host. The decay profiles of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions in the present glasses are found to be single exponential for all the studied Eu³⁺ ion concentrations. A marginal increase in lifetime of the ⁵D₀ level has been noticed with Eu³⁺ ion concentration up to 2.0 mol% and then the lifetime marginally decreases for higher Eu³⁺ ion concentrations.     

Laser transition characteristics of Nd3+-doped fluorophosphate laser glasses

Fluorophosphate glasses of composition P₂O₅–K₂O–MgO–Al₂O₃–AlF₃ and P₂O₅–K₂O–MgO–Al₂O₃–BaF₂ were prepared with different Nd³⁺ ion concentrations. The absorption and emission spectra in the UV–VIS–NIR region were measured for these glasses. Judd–Ofelt analysis has been carried out using the absorption spectra of 1.0 mol% Nd³⁺-doped glasses to evaluate the radiative properties for some luminescent levels of the Nd³⁺ ion. The stimulated emission cross-sections of the ⁴F_3/2 → ⁴I_11/2 laser transition for the present glasses are found to be higher than for other Nd³⁺-doped glasses. Branching ratio calculations also revealed the potentiality of the ⁴F_3/2 → ⁴I_11/2 transition for laser action in these glasses. The observed concentration quenching of the lifetime of the ⁴F_3/2 level is explained as a result of cross-relaxation process between the Nd³⁺ ions.     

Thermo-optical properties and nonradiative quantum efficiency of Er3+-doped and Er3+/Tm3+-co-doped tellurite glasses

Thermal lens (TL) measurements were performed in tellurite glasses, 70TeO₂–19WO₃–7Na₂O–4Nb₂O₅ (mol%), undoped, doped with Er³⁺ (1.19 × 10²⁰ ions/cm³) and co-doped with Er³⁺ (1.19 × 10²⁰ ions/cm³)/Tm³⁺ (1.56 × 10²⁰ ions/cm³). The absolute nonradiative quantum efficiency (ϕ) was determined by the TL method. The ϕ values for Er³⁺/Tm³⁺-co-doped and Er³⁺-doped tellurite glasses were 0.98 and 0.74, respectively. Fluorescence spectra were performed at λ_e = 488 nm and used to estimate the fluorescence quantum efficiency (η) using the TL results. These values were compared with results obtained by Judd–Ofelt calculations.     

Optical properties of Er3+-singly doped and Er3+/Yb3+-codoped novel oxyfluoride glasses

Novel oxyfluoride glasses SiO₂–Al₂O₃–Na₂O–ZnF₂ doped with Er³⁺ and Er³⁺/Yb³⁺ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Er³⁺ were determined by measurement of the absorption spectrum of Er³⁺-singly doped glass. According to the Judd–Ofelt theory, the experimental intensity parameters were calculated, from which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. The fluorescence lifetime and quantum efficiency for the near-infrared emission of Er³⁺-singly doped glass were determined to be 3.0 ms, and 42%, respectively. Visible upconversion luminescence was observed under 980 nm diode laser excitation. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms are discussed.     

Sodium-Iron fluorophosphate glasses part 1: Optical and magnetic properties

Sodium-iron fluorophosphate glasses have been prepared by the action of NaPO₃ on FeF₃ in a fluorinated reducing atmosphere. Optical absorption spectra show the presence of 6-coordinated Fe²⁺ and Fe³⁺ ions. Magnetic ordering occurs for iron-rich samples at low temperature.     

Spectroscopic and lasing properties of Er3+-doped glass microspheres

We report experimental results on the characterization of microspherical cavities fabricated in Er³⁺-doped modified-silica and phosphate glasses. The spectroscopic properties of the bulk precursor glasses as compared to the obtained microspheres were investigated by photoluminescence spectroscopy and lifetime measurement for the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions. In both types of glasses we demonstrate whispering gallery mode laser action at various wavelengths around 1550 nm by using a 1480 nm pump laser coupled through a tapered fiber.     

Multiple sites for Er3+ in alkali silicate glasses (II). Evidence of four sites for Er3+

The low-temperature absorption spectrum of a series of binary alkali silicate glasses doped with Er³⁺ shows the presence of four different sites for the Er³⁺. The near-octahedral A site reported in Part I of this series is the principal site and is present in all these glasses. The B site appears in the binary lithium and sodium silicate glasses. The binary potassium, rubidium and cesium glasses show the presence of the C and D sites. The C site absorption spectrum is similar to that found by Gruber et al. for Er³⁺ in Er₂O₃. The D site is probably a variation of the B site. Tentative sixfold-coordinated models are suggested for the B and D sites.     

Optical properties and infrared-to-visible upconversion in Er3+-doped GeO2–Bi2O3 and GeO2–PbO–Bi2O3 glasses

Luminescence properties and upconversion studies of germanate glasses in ternary GeO₂–PbO–Bi₂O₃ and binary GeO₂–Bi₂O₃ systems containing Er₂O₃ (0.1–1.0 wt%) are presented for the first time. The Judd-Ofelt parameters found for these glasses are: Ω₂ = 4.50 × 10⁻²⁰ cm², Ω₄ = 1.55 × 10⁻²⁰ cm² and Ω₆ = 0.69 × 10⁻²⁰ cm² for binary glasses and Ω₂ = 4.44 × 10⁻²⁰ cm², Ω₄ = 1.82 × 10⁻²⁰ cm² and Ω₆ = 0.39 × 10⁻²⁰ cm² for ternary glasses. The refractive index of these glasses is found to be ∼2. The transition ⁴I_13/2 → ⁴I_15/2 is peaked at ∼1.53 μm and shows a radiative lifetime around 5 ms. Both systems exhibit similar emission cross-section at 1.53 μm around 0.8 × 10⁻²⁰ cm². Upconverted green emission at ∼530 nm (²H_11/2 → ⁴I_15/2) and ∼550 nm (⁴S_3/2 → ⁴I_15/2) and red emission at ∼668 nm (⁴F_9/2 → ⁴I_15/2) are observed under 980 nm cw excitation. Our results suggest that these glasses are promising candidates for applications in photonics.     

Blue,green and red upconversions in Ho2O3-doped fluorophosphate glasses

Near infrared (NIR) to visible upconversions of a fluorophosphate glass of composition (mol%) 7Ba(PO₃)₂–32AlF₃–30CaF₂–18SrF₂–13MgF₂ doped with various concentrations (0.1, 0.3 and 1.0 mol%) of Ho₂O₃ have been investigated by exciting at 892 nm at room temperature. Three upconverted bands originated from the ⁵F₃ → ⁵I₈, (⁵S₂, ⁵F₄) → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions have been found to center at 491 nm (blue), 543 nm (green) and 658 nm (red), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been interpreted by the excited state absorption (ESA), energy transfer (ET) and cross relaxation (CR) mechanisms involving population of the metastable (storage) energy levels by multiphonon deexcitation effect. It is evident from the infrared reflection spectral (IRRS) analysis that the upconversion phenomena are expedited by the low multiphonon relaxation rate in fluorophophate glasses owing to their high intense low phonon energy of ∼600 cm⁻¹ which is very close to that of fluoride glasses (500–600 cm⁻¹).     

Optical properties of Nd3+ in lead phosphate glasses

Measurements of optical absorption and emission spectra of Nd³⁺ in lead phosphate glasses at 295 K are compared with results reported earlier for various metaphosphate glasses. The Judd-Ofelt intensity parameters for high-lead-content phosphate glasses are the smallest observed thus far for metaphosphate glasses with divalent network modifier cations.     

Photoluminescence properties of Sm3+-doped SFB glasses for efficient visible lasers

The photoluminescence properties of different concentrations of Sm³⁺ ions doped sodium fluoroborate (SFB) glasses of composition Na₂O–LaF₃–CaF₂–AlF₃–B₂O₃–SmF₃ have been investigated. The energy level analysis is carried out using free-ion Hamiltonian model. The Judd–Ofelt intensity parameters are used to evaluate the laser characteristic parameters such as spontaneous transition probability, radiative lifetime and fluorescence branching ratio for ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2 and 11/2) emission transitions of Sm³⁺ ion. The principal photoluminescence transitions of interest are identified by recording the emission spectra for different Sm³⁺ ion concentrations and measuring their emission cross-sections, integrated absorption cross-sections and optical gain parameters. The decay profiles from the ⁴G_7/2 excited manifold state to its lower lying energy levels have been recorded by monitoring the excitation and emission wavelengths at 402 and 600 nm, respectively. The dependence of effective fluorescence lifetime on the Sm³⁺ concentration is also discussed.     

1.5 μm Emission and infrared-to-visible frequency upconversion in Er+3/Yb+3-doped phosphoniobate glasses

Sodium phosphoniobate glasses with the composition (mol%) 75NaPO₃–25Nb₂O₅ and containing 2 mol% Yb³⁺ and x mol% Er³⁺ (0.01 ⩽ x ⩽ 2) were prepared using the conventional melting/casting process. Er³⁺ emission at 1.5 μm and infrared-to-visible upconversion emission, upon excitation at 976 nm, are evaluated as a function of the Er³⁺ concentration. For the lowest Er³⁺ content, 1.5 μm emission quantum efficiency was 90%. Increasing the Er³⁺ concentration up to 2 mol%, the emission quantum efficiency was observed to decrease to 37% due to concentration quenching. The green and red upconversion emission intensity ratio was studied as a function of Yb³⁺ co-doping and the Er³⁺–Er³⁺ energy transfer processes.     

Upconversion and concentration quenching in Er3+-doped TeO2–Na2O binary glasses

The spectroscopic properties of Er³⁺-doped alkali tellurite TeO₂–Na₂O glasses are investigated. Infrared-to-visible upconversion emission bands are observed at 410, 525, 550 and 658 nm using 797 nm excitation wavelength. These bands are assigned to the ²H_9/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transition, respectively. The power dependence study reveals that the ²H_9/2 → ⁴I_15/2 transition involves a three-step process while the other upconversion transitions involve only two steps. An excitation with 532 nm wavelength, two upconversion bands are observed in the UV region at 380 and 404 nm in addition to bands in the visible region at 410, 475, 525, 550, 658 and 843 nm. These bands are ascribed to ⁴G_11/2 → ⁴I_15/2, ²P_3/2 → ⁴I_13/2, ²H_9/2 → ⁴I_15/2, ²P_3/2 → ⁴I_11/2, ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_13/2 transition, respectively. Increasing Er³⁺ concentration leads to a rapid growth in the intensity of red emission relative to that for the green emission. An explanation for this observation has been suggested. The temperature dependence profile for the two thermally coupled levels (²H_11/2, ⁴S_3/2) shows that they can be used for measuring the temperature.     

Crystallization behavior of Dy3+-doped selenide glasses

Rare earth (RE)-doped chalcogenide glasses are an important promising material for active photonic devices, including mid-infrared (mid-IR) fiber lasers and amplifiers. Here we report on dysprosium ion (Dy³⁺)-doped GeAsGaSe chalcogenide glasses based on 10 atomic (at.) % Ga. A series of Dy³⁺-doped GeAsGaSe glasses, with increasing levels of Dy³⁺ dopant from 0 ppm to 2000 ppm added to the Ge_16.5As₉Ga₁₀Se_64.5 (at. %) base glass, is synthesized and characterized using: Fourier transform infrared spectrometry; X-ray diffraction (XRD); imaging and analysis using a high resolution transmission electron microscope, with selected area electron diffraction (HRTEM-SAED), and energy dispersive X-ray spectroscopy (HRTEM-EDX) and an environmental scanning electron microscope with energy dispersive X-ray spectroscopy (ESEM-EDX) and with secondary electron mapping. At the higher levels of Dy³⁺ doping, the glasses exhibit bulk crystallization; XRD, HRTEM-EDX and ESEM-EDX indicate the crystals are predominantly a modified, face centered cubic α-Ga₂Se₃, with some substitution of Ge. In addition, features on the bulk glass surface are shown to comprise Dy³⁺, sometimes accompanied by Si and [O] which, it is suggested, are due to contamination from the silica glass melting ampoule.     

Optical properties of Tm3+ ions in alkali germanate glass

Tm-doped alkali germanate glass is investigated for use as a laser material. Spectroscopic investigations of bulk Tm-doped germanate glass are reported for the absorption, emission and luminescence decay. Tm:germanate shows promise as a fiber laser when pumped with 0.792 μm diodes because of low phonon energies. Spectroscopic analysis indicates low non-radiative quenching and pulsed laser performance studies confirm this prediction by showing a quantum efficiency of 1.69.     

The study of Sm3+-doped low-phonon-energy chalcohalide glasses

The Sm³⁺-doped low-phonon-energy (LPE) Ge–Ga–Se–CsI glasses were studied. Upon excitation at 980 nm diode laser, intense 1.25 and 1.49 μm near-infrared fluorescence bands with broad full width at half maximum (FWHM) of 49 and 53 nm were observed, respectively. About 180–300 μs fluorescence lifetimes were obtained for the 1.49 μm emission. The thermal properties and structure of glasses were investigated by differential thermal analysis (DTA) and Raman spectra, respectively. Spectroscopic characteristics of the optical transitions have been calculated by using the Judd–Ofelt theory and evaluated for excited levels.     

Optical properties of Pr3+ in selenide glasses modified with CsI

Effects of CsI content on the optical properties of Ge₃₀Ga₅Sb₅Se₆₀ glasses were evaluated. Linear and non-linear absorption properties of the glasses without Pr³⁺ were examined in addition to 1.6 μm emission properties of the Pr-doped glasses. Blueshift of the UV-side absorption edge was accompanied with increasing CsI concentration, while non-linear absorption coefficients measured at 1.06 μm by the Z-scan method remained unaffected. Measured lifetimes of the 1.6 μm emission from modified glasses were comparable to those of the unmodified glass. These experimental observations are discussed in connection with a pronounced weak absorption tail appeared in selenide glasses with the addition of CsI.     

Optical and lasing properties of fluorophosphate glass

Neodymium-doped fluorophosphate glass is a laser material newly-developed for use in high power laser fusion systems. The low refractive index (n_d ～ 1.45) and low dispersion (Abbe number ～90) of fluorophosphate glasses give them the properties of low nonlinear refractive indices and long Nd³⁺ fluorescence lifetimes, which are desirable for the high power laser applications. We have measured the intensity gain of 1.052 and 1.064 nm laser light produced by flashlamp-pumped fluorophosphate glass amplifiers, varying in size from 4–34 clear aperture. The measured gains are compared with those measured in other laser glass types and with those predicted from the spectroscopic properties of Nd³⁺. We estimate that the peak cross section for the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>12</mtext>}}$ transition in commercial fluorophosphate laser glasses is ～2.2 × 10^?20 cm².     

Design of Er3+-doped chalcogenide glass laser for MID-IR application

The feasibility of a photonic crystal fiber laser (PCF laser), made of a novel Er³⁺-doped chalcogenide glass and operating at the wavelength λ_s = 4.5 μm is investigated. The design is performed on the basis of spectroscopic and optical parameters measured on a fabricated Er³⁺-doped Ga₅Ge₂₀Sb₁₀S₆₅ chalcogenide bulk sample. The simulations have been performed by employing a home made numerical code that solves the multilevel rate equations and the power propagation equations via a Runge-Kutta iterative method. The numerical results indicate that a laser exhibiting slope efficiency close to the maximum theoretical one and a wide tunability in the wavelengths range where the atmosphere is transparent can be obtained.