Codoped Fluoroindate Glass Research Articles

Effect of Yb3+/Er3+ co-doping on the emission properties of fluoroindate glass and glass optical fiber

In this study, a comprehensive analysis of emission in erbium-doped and erbium and ytterbium co-doped fluoroindate glasses has been presented. The luminescence of the erbium transitions and the effect of ytterbium co-doping under 488, 808 and 976 nm laser excitation have been discussed. Optimization of rare-earth (RE) ions doping concentration and the energy transfer processes related to the sensibilization effect in Er3+/Yb3+ co-doped fluoroindate glasses focusing on the reciprocal distribution of radiative transitions, energy transfer parameters, and lifetimes have been analyzed. The luminescence consists of several bands in the visible (525, 546 and 660 nm) and the infrared (1550 and 2700 nm) spectral regions that correspond to characteristic radiative energy transitions within the erbium ions. In glasses co-doped with the Er3+/Yb3+ system, an augmentation in the emission intensity was observed in the visible and at 1550 nm, while a decrease was observed in the 2700 nm infrared band. The increase of the Yb3+: 2F5/2 lifetime under 488, 808 and 976 nm excitation showed significant Er3+: 4I11/2 → Yb3+: 2F5/2 back energy transfer. Based on the analysis of the available literature, it can be concluded that energy transfer upconversion and cooperative energy upconversion play a significant role in modulating luminescence intensity values within the examined emission bands. This effect is likely enhanced due to the incorporation in a low phonon energy glass matrix. As a result of the above optimization, a glass optical fiber co-doped with 0.8 YbF3/1.4 ErF3 (in mol%) was drawn and characterized, showing a strong emission at 2.77 μm. The optimized Er3+/Yb3+ co-doped fluoroindate glass and glass optical fiber confirmed their ability to be used in the visible and near-infrared spectral regions as efficient optical fiber sources.

Deactivation effects of Tb3+ on Ho3+ emission in fluoroindate glasses for 3.9 μm laser applications

A series of Ho3+/Tb3+ co-doped fluoroindate glasses with good thermal stability have been synthesized to study the deactivation effects of Tb3+ on the Ho3+: 3.9 μm emission. Efficient 3.9 μm emission enhancement is obtained under excitation by an 888 nm laser diode (LD). The Judd-Ofelt (J-O) intensity parameters and radiative properties are calculated to evaluate the spectroscopic properties. Possible energy transfer processes resulting in emission reinforcement are discussed. A higher spontaneous transition probability and larger peak emission cross section are achieved with the inclusion of Tb3+. This analysis supports the conclusion that Ho3+/Tb3+ co-doped fluoroindate glass is a potentially useful laser material for highly efficient 3.9 μm fiber lasers.

Efficient ∼4 µm emission from Pr3+/Yb3+ co-doped fluoroindate glass.

Pr3+/Yb3+ co-doped fluoroindate (InF3) glasses were prepared by using a traditional melt-quenching method in dry N2 atmosphere. Pumped by a 976 or 1570 nm laser diode, efficient emissions at ∼4µm were obtained from the Pr3+/Yb3+ co-doped glasses, which could be ascribed to the transition 3F4→3H6 of Pr3+ ions. The relative stimulated emission cross section was calculated to be ∼1.44×10-24m2 at 4 µm, which was ∼4.2 times larger than that of transition Ho3+:5I5→5I6 (3.4×10-25m2). In addition, combined with transitions 1G4→3F3 and 1G4→3F4 of Pr3+ ions, ultra-broadband emission ranging from 2.7 to 4.2 µm was also obtained. Our results indicate that Pr3+/Yb3+:InF3 glasses could be used to develop efficient ∼4µm lasers and widely tunable mid-infrared lasers.

Enhanced 3.9 µm emission from diode pumped Ho3+/Eu3+ codoped fluoroindate glasses.

The use of Eu3+ codoping for enhancing the Ho3+:5I5→5I6 emission in fluoroindate glasses shows that Eu3+ could depopulate the lower laser state Ho3+:5I6 while having little effect on the upper state Ho3+:5I5, resulting in greater population inversion. The Ho3+/Eu3+ codoped glass has high spontaneous transition probability (6.31s-1) together with large emission cross section (7.68×10-21cm2). This study indicates that codoping of Ho3+ with Eu3+ is a feasible alternative to quench the lower energy level of the 3.9 µm emission and the Ho3+/Eu3+ codoped fluoroindate glass is a promising material for efficient 3.9 µm fiber lasers.

3.9 μm emission and energy transfer in ultra-low OH−, Ho3+ /Nd3+ co-doped fluoroindate glasses

Ho3+/Nd3+ co-doped fluoroindate glass samples were prepared by melt-quenching. The absorption and emission spectra, and the differential scanning calorimetry (DSC) curve were measured and used to evaluate the spectroscopic parameters and thermal properties. An intense ~3.9 μm emission, ascribed to the transition Ho3+:5I5 →5I6, was observed under the excitation of an 808 nm laser diode and was ascribed to the efficient energy transfer process from Nd3+: 4F3/2 to Ho3+: 5I5, showing the Nd3+ role as a sensitizer. The optimal concentration ratio of Ho3+ and Nd3+ for ~3.9 μm emission was estimated to be 1:1. The spectroscopic performance suggests that the Ho3+/Nd3+ co-doped fluoroindate glass is a potential gain material for ~3.9 μm laser applications.

Enhancement mechanisms of Tm3+-codoping on 2 μm emission in Ho3+ doped fluoroindate glasses under 888 nm laser excitation

Ho3+ doped and Ho3+/Tm3+ co-doped fluoroindate glass samples were prepared and their emission properties were compared. Under 888 nm laser excitation, the emission at 2 μm of Ho3+ ions with co-doping 2 mol% Tm3+ ions had a 2.9-fold improvement compared with that of Ho3+ doped. The absorption and emission spectra, and energy level lifetime of Ho3+/Tm3+ co-doped glass samples were measured to analyze the energy transfer processes and enhancement mechanisms. The luminescence intensity at 2 μm can be greatly increased due to the bidirectional energy transfer between Tm3+ and Ho3+ ions.

Visible to infrared energy conversion in Pr3+–Yb3+ co-doped fluoroindate glasses

Processes involving visible to infrared energy conversion are presented for Pr3+–Yb3+ co-doped fluoroindate glasses. The emission in the visible and infrared regions, the luminescence decay time of the Pr3+:3P0→3H4 (482nm), Pr3+:1D2→3H6 (800nm), Yb3+:2F5/2→2F7/2 (1044nm) transitions and the photoluminescence excitation spectra were measured in Pr3+ samples and in Pr3+–Yb3+ samples as a function of the Yb3+ concentration. In addition, energy transfer efficiencies were estimated from Pr3+:3P0 and Pr3+:1D2 levels to Yb3+:2F7/2 level. Down-Conversion (DC) emission is observed due to a combination of two different processes: 1-a one-step cross relaxation (Pr3+:3P0→1G4; Yb3+:2F7/2→2F5/2) resulting in one photon emitted by Pr3+ (1G4→3H5) and one photon emitted by Yb3+ (2F7/2→2F5/2); 2-a resonant two-step first order energy transfer, where the first part of energy is transferred to Yb3+ neighbor through cross relaxation (Pr3+:3P0→1G4; Yb3+:2F7/2→2F5/2) followed by a second energy transfer step (Pr3+:1G4→3H4; Yb3+:2F7/2→2F5/2). A third process leading to one IR photon emission to each visible photon absorbed involves cross relaxation energy transfer (Pr3+:1D2→3F4; Yb3+:2F7/2→2F5/2).

Enhanced energy upconversion and super-resolved focused spot generation in Tm^3+-Yb^3+ codoped glass using silica microspheres

We report the enhancement of the blue upconversion emission in a Tm3+-Yb3+ codoped fluoroindate glass covered with silica microspheres. Each microsphere produces a photonic nanojet that concentrates the illuminating beam in a tiny region of the glass, increasing the intensity per unit area and so the upconversion emission by a factor of 3. Moreover, the mean size of the emission area has been reduced by a factor of 3 due to the three-photon process involved in the blue emission band. The experimental values of the full width at half-maximum of the emission spots have been found to be in agreement with the theoretical values obtained from finite-difference time-domain simulations.

Optical amplification by upconversion in Tm–Yb fluoroindate glass

Evidence of a positive optical gain at 650 nm is observed in Tm 3+–Yb 3+ codoped fluoroindate glass in an upconversion pump and probe experiment. The 1G 4 level of the Tm 3+ ions is populated by an upconversion mechanism under excitation of the Yb 3+ ions at 975 nm with high power pulsed laser and give rise to an intense emission from the 1G 4 to the 3F 4 levels. The 1G 4 → 3F 4 electronic transition is stimulated with a low signal at 650 nm as a probe. Under this condition, we reach the population inversion necessary between the Tm 3+ levels of the transition 1G 4 → 3F 4 and we observed an increase of the emission intensity at the signal wavelength due to the stimulated emission. Transient positive optical gain around 3 cm −1 (∼12 dB/cm) has been measured in Tm–Yb codoped fluoroindate glass during the first 400 μs.

Infrared-laser induced photon avalanche upconversion in Ho3+–Yb3+ codoped fluoroindate glasses

An intense green upconversion emission due to a photon avalanche mechanism has been observed in Ho3+–Yb3+ codoped fluroindate glasses, measured at 200 K, under excitation at around 750 nm. The influence of the Yb3+ concentration on the photon avalanche process has been studied. The pump power threshold of the avalanche mechanism decreases as the Yb3+ concentration is increased. A reduction by about a factor of 3 is observed from the 2.25 mol % Ho3+ single doped glass to the 2.25 mol % Ho3+–2.25 mol % Yb3+ codoped glass. This effect has been related to an extra feeding of the I47 intermediate level of Ho3+ ions via Ho3+→Yb3+ and Yb3+→Ho3+ energy transfer and back transfer processes, respectively.

Phonon-assisted cooperative energy transfer and frequency upconversion in a Yb3+/Tb3+ codoped fluoroindate glass

We report large thermal enhancement of the frequency upconversion (UC) process due to cooperative energy transfer (CET) in a Yb3+/Tb3+ codoped fluoroindate glass obtained by heating the sample from 308 to 530 K. To study the influence of multiphonon transitions in the UC process we chose anti-Stokes quasiresonant excitation of Yb3+ ions which were used as sensitizers. UC of radiation at 1064 nm into blue and green light was obtained. Various emission lines of Tb3+ were observed between 400 and 700 nm due to CET from Yb3+ to Tb3+ ions. A rate equation model was used to describe the temperature dependence of the UC emission intensities and the theoretical results are in good agreement with the experimental data.

Increase of the 800 nm excited Tm3+ blue upconversion emission in fluoroindate glasses by codoping with Yb3+ ions

Intense Tm3+ blue upconversion emission has been observed in Tm3+–Yb3+ codoped fluoroindate glasses under direct excitation into Tm3+ ions with a diode laser at 796 nm. The dependence of the intensity of this upconversion emission on the Tm3+ and Yb3+ concentration has been studied in order to determine the optimum ion concentrations and the involved mechanisms. The blue upconversion emission is highly increased with the Yb3+ concentration. For fixed Yb3+ concentrations, the maximum upconversion efficiency was obtained with Tm3+ concentration of about 0.5 mol%. For this optimum Tm3+ concentration, the upconversion emission intensity is increased by a factor about 100 by codoping with 2.25 mol% of Yb3+. The results are well explained by a proposed rate equation model.

Frequency upconversion involving triads and quartets of ions in a Pr3+/Nd3+ codoped fluoroindate glass

Frequency upconversion (UC) processes involving energy transfer (ET) among Nd3+ and Pr3+ ions in a fluoroindate glass are reported. In a first experiment, the excitation of Pr3+ [transition H34→D32] and of Nd3+ [transition I49/2→(G27/2+G45/2)] was achieved with a dye laser operating in the 575–590 nm range. In a second experiment, the Nd3+ ions were excited with the second harmonic of a Nd: YAG laser at 532 nm. The ET processes leading to UC in both experiments were studied by monitoring the blue fluorescence decay at 480 nm due to the transition P30→H34 in Pr3+. In the more relevant UC process, quartets of ions (Nd–Nd–Pr–Pr) are excited due to absorption of three laser photons by two Nd3+ ions which transfer their energy to two Pr3+ ions. Each Pr3+ ion promoted to the P30 level decays to the ground state emitting one photon in the blue region. This conclusion was achieved investigating the dependence of the UC fluorescence intensity as a function of laser intensity, samples concentrations, and temporal behavior of the UC signal. Other UC processes involving nonisoionic groups of three ions are also reported.

Twentyfold blue upconversion emission enhancement through thermal effects in Pr3+/Yb3+-codoped fluoroindate glasses excited at 1.064 μm

Infrared-to-visible upconversion emission enhancement through thermal effects in Yb3+-sensitized Pr3+-doped fluoroindate glasses excited at 1.064 μm is investigated. A twentyfold increase in the 485 nm blue emission intensity as the sample temperature was varied from 20 to 260 °C was observed. The visible upconversion fluorescence enhancement is ascribed to the temperature dependent multiphonon-assisted anti-Stokes excitation of the ytterbium sensitizer and excited-state absorption of the praseodymium acceptor. A model based upon conventional rate equations considering a temperature dependent effective absorption cross section for the 2F7/2→2F5/2 transition of the Yb3+ and 1G4→3P0 excited-state absorption of the Pr3+, agrees very well with the experimental results.

Transfer and back transfer processes in Yb3+–Er3+ codoped fluoroindate glasses

The kinetics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb3+ and Er3+ has been studied. It was possible to excite selectively Yb3+ ions and detect emission due only to these ions or combined with the emission coming from the Er3+ ions. At low concentration of Yb3+ (0.1 mol %) the emission decay of these ions is nonexponential when there is back transfer from Er3+ ions; the dynamics is well described using the fluorescence “transfer function” model. Moreover, the evolution of the emission from the I11/24 (Er3+) level is in good agreement with the behavior predicted by the model. The critical radii obtained from the fitting for forward and backward energy transfer are compared with those calculated with the Dexter formula. When the Yb3+ concentration is increased, the migration among these ions is very important and the emission decay of the Yb3+ ions is nearly exponential with the lifetime of these ions. This behavior is explained considering that, at high Yb3+ concentration and due to the migration, the transfer from Yb3+ to Er3+ ions is restricted to very closed couples of ions with high back transfer probability. The limitations of the fluorescence “transfer function” model associated with the correlation effect in the excitation of Er3+ ions by energy transfer are discussed.

Cooperative frequency upconversion in Yb 3+–Tb 3+ codoped fluoroindate glass

We report the observation of cooperative frequency upconversion in a fluoroindate glass. The experiments were performed by exciting Yb 3+–Tb 3+ codoped samples with an infrared diode laser. The process is monitored through the green fluorescence emitted by Tb 3+ ions due to a cooperative energy transfer from a pair of excited Yb 3+ ions.

Upconversion of infrared-to-visible light in Pr 3+–Yb 3+ codoped fluoroindate glass

Bright fluorescence in the visible range has been observed in Pr 3+–Yb 3+ doped fluoroindate glass under infrared diode laser irradiation. The mechanism which contributes for the upconversion emission is identified and the energy transfer rate between Pr 3+–Yb 3+ is obtained for different concentrations.

Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses

Optical absorption, Stokes, and anti-Stokes photoluminescence were performed on Er3+–Yb3+ co-doped fluoroindate glasses. For compounds prepared with a fixed 2 mol % ErF3 concentration and YbF3 contents ranging from 0 to 8 mol %, important upconversion processes were observed as a function of temperature and photon excitation energy. Based on the experimental data, two mechanisms for the upconversion (or anti-Stokes photoluminescence) processes were identified and analyzed in detail. At high Yb contents, the upconversion mechanisms are mostly determined by the population of the F5/22 levels of Yb3+ ions (or I11/24 levels of Er3+ ions, by energy transfer) regardless of the photon excitation energy and temperature of measurement. Moreover, green and red light emission have similar intensities when a large Yb3+ content is present.

Kinetics of transfer and backtransfer in Yb 3+-Er 3+ codoped fluoroindate glasses

The dynamics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb 3+ and Er 3+ have been studied using the fluorescence ‘transfer function’. From the fitting to the decay curves of the luminescence the critical radii for these processes have been obtained. These values are also calculated with the Dexter formula.