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.     

Upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped fluorogermanate glass and glass ceramic

Energy-transfer excited upconversion luminescence in Ho³⁺/Yb³⁺- and Tb³⁺/Yb³⁺-codoped PbGeO₃–PbF₂–CdF₂ glass and glass–ceramic under infrared excitation is investigated. In Ho³⁺/Yb³⁺-codoped samples, green (545 nm), red (652 nm), and near-infrared (754 nm) upconversion emission corresponding to the ⁵S₂(⁵F₄) → ⁵I₈, ⁵F₅ → ⁵I₈, and ⁵S₂(⁵F₄) → ⁵I₇ transitions, respectively, was observed. Blue (490 nm) emission assigned to the ⁵F_2,3 → ⁵I₈ transition was also detected. In the Tb³⁺/Yb³⁺-codoped system, bright UV–visible emission around 384, 415, 438, 473–490, 545, 587, and 623 nm, identified as due to the ⁵D₃(⁵G₆) → ⁷F_J(J = 6, 5, 4) and ⁵D₄ → ⁷F_J(J = 6, 5, 4, 3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicated that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed.     

Strong green emission from Er3+ in a fluorine containing (Pb,La)–tellurite glass without using up-conversion route

Absorption, emission, excitation spectra and the lifetime of the ⁴S_3/2 excited luminescent state of Er³⁺ ions in a fluorine containing (lead, lanthanum)–tellurite glass have been studied. The glass exhibits a strong green luminescence upon excitation through 380 nm (⁴I_15/2 → ⁴G_11/2) absorption band of its Er³⁺ ions. The spectrum consists of a strong green component in the wavelength range 534–553 nm due to luminescence transitions ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 and a very weak red component in the range 650–710 nm due to ⁴F_9/2 → ⁴I_15/2 transition. The Stark split components of the ⁴S_3/2 state are not very clear in the spectrum, but the biexponential luminescence decay of the ⁴S_3/2 state confirms the presence of the Stark levels. A rapid conversion of the upper Stark level to the lower level is also evident from the decay kinetics which helps greater number of ions to populate in the lower stark level of the ⁴S_3/2 state. Thus, the present study indicates that the glass may be a suitable candidate for use as a laser medium in making a solid state green laser by pumping the later by normal route.     

Influence of Zn2+ ions concentration on the optical properties of Zn/Er:LiNbO3 crystals

Congruent Er³⁺(3 mol%):LiNbO₃ crystals codoped with ZnO (X mol %, X=0, 3, 6 and 7) were grown by the Czochralski technique. The Er contents in the crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP‐AES). Under 800 nm excitation, the upconversion emission spectra reveal an enhancement of the green emission with respect to the red emission when the Zn²⁺ ions are introduced into Er:LiNbO₃ crystal. The effect of Zn²⁺ ions concentration on the intensity ratio of the green to red emission has been investigated. Two cross‐relaxation processes (²H_11/2 + ⁴I_13/2 → ⁴I_11/2 + ⁴F_9/2 and ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2) are involved in populating the ⁴F_9/2 state, which bypass the green‐emitting states. The OH^‐ absorption spectra indicate that the Zn²⁺ codoping leads to a decreased concentration of Er³⁺ cluster sites contributing to the enhancement of the green emission. The studies on UV‐vis absorption spectra show that the heavily codoped with Zn²⁺ results in the reformation of the Er³⁺ cluster sites in Er:LiNbO₃. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and upconversion luminescence characterization of 12CaO·7Al2O3:Ho3+/Yb3+ polycrystals

The effect of Yb³⁺ concentration on the fluorescence of 12CaO·7 Al₂O₃:Ho³⁺/Yb³⁺ polycrystals is investigated. The Raman spectra of pure C12A7 under 633‐nm excitation show that the highest photon energy is 787.267 cm⁻¹, which is not much bigger than general fluorides, so it can realize high efficiency upconversion. The upconversion emission spectra suggest that the green upconversion emission centered at 548 nm and the red upconversion emission at 662 nm correspond to the ⁵F₄/⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transition of Ho³⁺ ions, respectively. The intensity of the upconversion luminescence and the ratio of red to green are changed with Yb³⁺ ion concentration. The pump dependence and luminescence decay dynamics spectra show the green and red upconversion emissions are populated by a two‐photon process, and the upconversion mechanisms are analyzed. The relative luminous efficiencies of green and red emissions are 2.035% and 0.7%, respectively. The normalized efficiency obtained for green emission of Ho³⁺ at RT when the sample is excited by 980‐nm light with an absorbed intensity of 7.5 W/cm² is 0.27 cm²/W. This result is comparable to the values obtained in YF₃ for the Yb³⁺, Er³⁺ green emission. The C12A7 with upconversion red and green light will be a promising luminous material.     

Optical absorption and photoluminescence studies of Nd doped alkali boro germanate glasses

Optical absorption and photoluminescence studies have been carried out at room temperature in 25 R₂O-25 GeO₂-49.5 B₂O₃-0.5 Nd₂O₃ glass systems, (Composition in mol%, R = Li, Na, K and Rb). Judd Ofelt Intensity parameters and other parameters like Racah (E¹, E² and E³), Slater-Condon-Shortley (F₂, F₄ and F₆) Spin-Orbit Coupling (ξ_4f) and Configuration Interaction (α,β and γ) for Nd³⁺ ion in the glass system are calculated. The variation of the Ω₂ parameters are interpreted in terms of the covalency of the RE ion in the glass matrix. Further the hypersensitive transition ⁴I_9/2 → ⁴ G_5/2, ² G_7/2 is analyzed with respect to the intensity ratio I_L/I_S and is found to be dependent on the type of alkali in the glass matrix. The Photoluminescence studies do not show any appreciable shift in the peak emission wavelength of the ⁴ F_3/2 to ⁴I_11/2 transition with the change in alkali type.     

Compositional trends in low-temperature photoluminescence of heavily Er-doped GeS2–Ga2S3 glasses

The influence of temperature and glass composition on the photoluminescence (PL) efficiency of Er³⁺ ions embedded in (GeS₂)_100?x(Ga₂S₃)_x (x = 20, 25 and 33 mol%) glasses has been studied. The typical 4f–4f emission bands of Er³⁺ ions at around 830, 1000 and 1550 nm have been observed in the whole investigated temperature range from 300 K down to 10 K for all the compositions. New 4f–4f luminescence bands, in excess of the three basic ones, have been observed at 670, 870, 1120, 1260 and 1350 nm for (GeS₂)₇₅(Ga₂S₃)₂₅ glass composition, and are tentatively assigned to ²H_9/2 → ⁴I_11/2, ⁴G_11/2 → ⁴F_9/2, ²H_11/2 → ⁴I_11/2, ⁴F_7/2 → ⁴I_9/2 and ⁴F_3/2 → ⁴I_9/2 transitions, respectively. Thus a considerable influence of GeGaS host composition on the efficiency of 4f–4f transitions of embedded Er³⁺ ions is documented with the outcome that (GeS₂)₇₅Ga₂S₃)₂₅ composition appears near optimal for the emission efficiency of Er³⁺ ions. With decreasing temperature the PL efficiency is enhanced considerably with pronounced narrowing of all bands. In the case of the strongest PL band at ~ 1550 nm, corresponding to ⁴I_13/2 → ⁴I_15/2 transition, the narrowing at low temperature is further accompanied by the resolution of well pronounced fine structure due to “crystal field” splitting of corresponding electronic terms. The relationship between the photoluminescence and reflectance spectra as a function of Er content has been discussed.     

Crystal growth and spectroscopic properties of erbium doped Lu2SiO5

A high optical quality erbium doped Lu₂SiO₅ single crystal has been grown by the Czochralski method. The distribution coefficient of Er³⁺ was measured to be ∼0.926. The absorption and emission spectra as well as the fluorescence decay curve of the excited state ⁴I_13/2 were measured at room temperature. The spectroscopic parameters were calculated using the Judd–Ofelt theory, and the J–O parameters Ω₂, Ω₄ and Ω₆ were found to be 4.451×10^-20, 1.614×10^-20 and 1.158×10^-20 cm², respectively. The room-temperature fluorescence lifetime of the Er³⁺⁴I_13/2→⁴I_15/2 transition was measured to be 7.74 ms. The absorption and emission cross-section as well as the gain cross-section in the eye-safe regime of 1400–1700 nm were also determined and discussed.     

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.     

1540 nm fiber laser excited upconversion luminescence in erbium-doped lead-fluoride nanocrystals

Upconversion luminescence in erbium-doped PbGeO₃-PbF₂-CdF₂-based vitroceramic under 1540 nm infrared excitation is investigated. Luminescence signals around 410, 525, 550, 660 and 850 nm were generated and attributed to the ²H_9/2, ²H_11/2, ⁴S_3/2 and ⁴F_9/2 transitions to the ⁴I_15/2 ground-state, and ⁴S_3/2-⁴I_13/2, respectively. The erbium ions excited-state emitting levels were populated through a combination of stepwise ground-state absorption, phonon-assisted excited-state absorption and cross-relaxation processes. The results also disclosed that all emission signals obtained with vitroceramic samples presented intensities three times higher when compared to the precursor glass samples. In addition, the red emission signal at 660 nm for 1540 nm pumping exhibited an expressively high intensity when compared to the green signal.     

Emissions of 1.20 and 1.38 μm from Ho3+-doped lithium–barium–bismuth–lead oxide glass for optical amplifications

Efficient infrared emissions at 1.20 μm [⁵I₆ → ⁵I₈ transition] and 1.38 μm [(⁵ F₄, ⁵ S₂) → ⁵I₅ transition] from Ho³⁺-doped lithium–barium–bismuth–lead (LBBP) glass were observed. The stimulated emission cross-sections were calculated to be 0.29 × 10^?20 and 0.25 × 10^?20 cm² for 1.20 and 1.38 μm emissions, respectively. Judd-Ofelt characteristic parameters Ω₂, Ω₄ and Ω₆ for Ho³⁺ in LBBP glass were calculated to be 6.72 × 10^?20, 2.35 × 10^?20 and 0.61 × 10^?20 cm², respectively, which indicates a strong asymmetry and a covalent environment between the Ho³⁺ ions and the ligands in this glass. The optical amplifications operating at these relatively unexplored wavelength regions were evaluated and discussed.     

Features in the photoluminescence line-shape of heavily Er-doped Ge–S–Ga glasses

Erbium doped chalcogenide glasses are of great interest in the integrated optoelectronic technology due to their Er³⁺ intra-4f emission at the standard telecommunication wavelength of 1.54 μm. In this paper, the photoluminescence (PL) of a series of (GeS₂)_x(Ga₂S₃)_100−x (x = 75 and 67) glasses doped with high amounts of Er₂S₃ (1.8, 2.1, 2.4 and 2.7 mol%) under excitation with 1064 nm light has been studied. A quenching PL effect at 1.22 аt.% Er-doped (GeS₂)₇₅(Ga₂S₃)₂₅ and 1.39 аt.% Er-doped (GeS₂)₆₇(Ga₂S₃)₃₃ glasses has been established. The relative changes in PL line-shape at around 1540 nm have been estimated by deconvoluting the spectra to Gaussian sub-bands centered at 1519 ± 1, 1537 ± 1, 1546 ± 1, 1555 ± 1 and 1566 ± 4 nm, which correspond to F₂₁, F₁₁, F₂₂, F₁₂ and F₁₃ transitions in the ⁴I_13/2 and ⁴I_15/2 energy levels and have intensity and manifestation that are strongly depend on the Er-doping level. The influence of gallium on the PL efficiency has been evaluated with a view to enhanced emission cross-section.     

Optical properties of Er3+-doped alkali fluorophosphate glasses

A systematic study of the optical absorption and luminescence spectra of Er³⁺-doped alkali fluorophosphate glasses (RTFP) 50(NaPO₃)–10TeO₂–20AlF₃–19RF–1Er₂O₃ (R = Li, Na and K) has been performed. The phenomenological Judd–Ofelt intensity parameters have been determined from the spectral intensities of the absorption bands in order to calculate the radiative transition probabilities, radiative lifetimes and branching ratios for various excited luminescent states. Using the visible and near infrared emission spectra, full width at half maximum (FWHM), emission cross-sections (σ_e) and figure of merit (FOM) were evaluated and compared with other hosts. Especially, the numerical values of these parameters indicate that the emission transition ⁴I_13/2 → ⁴I_15/2 transition at 1.534 μm in Er³⁺-doped fluorophosphate glasses may be highly useful in optical communication. The decay characteristic of ⁴S_3/2 excited level has also been recorded and analyzed. The calculated and experimental lifetimes were compared in terms of quantum efficiencies and multiphonon relaxation rates.     

Studies on the growth and optical characterization of Tm3+‐doped BaY2F8 single crystals

The BaY₂F₈ crystals doped with different concentrations of Tm³⁺ ions were prepared by the temperature gradient technique (TGT). X‐ray powder diffraction was applied to analyze the phase. The cracking phenomenon along (010) and (100) planes of the crystals grown by temperature gradient technique was studied on the basis of the structure of BaY₂F₈ crystals. The absorption spectra were measured and investigated in the ultraviolet‐visible and near‐infrared ranges at room temperature. Several characteristic absorption bands of Tm³⁺‐doped BaY₂F₈ crystal were observed. The emission and excitation spectra were obtained and investigated at room temperature and 12 K, showing the characteristic emission peaks of Tm³⁺ ions. The temperature dependence of Photoluminescence curve was also investigated in the range of 12–296 K. The luminescence intensity of emission bands decreased with increasing temperature, while the effective bandwidth increased. The up‐conversion spectrum excited at 650 nm was recorded and up‐conversion mechanism was analyzed in detail. The result showed the purple, green and yellow emissions corresponding to ³P₁→³F₃, ¹D₂→³H₅ and ³P₀→¹G₄ transitions, respectively.     

Broadband near-infrared emission from Bi–Er–Tm Co-doped germanate glasses

Bi–Er–Tm co-doped germanate glasses and Bi, Er, Tm singly doped glasses were prepared and characterized through absorption spectra, NIR emission spectra and decay lifetime. A super broadband near-infrared emission from 1000 nm to 1600 nm, covering the whole O, E, S, C, and L bands, was observed in the Bi–Er–Tm co-doped samples due to the result of the overlapping of the Bi related emission band (centered at 1300 nm), the emission from Er^{3+ 4}I_13/2 → ⁴I_15/2 transition (centered at 1534 nm) as well as the emission from Tm^{3+ 3}H₄ → ³F₄ transition (centered at 1440 nm), which is essential for broadly tunable laser sources and broadband optical amplifiers. The energy transfer process was also discussed at the end of the paper.     

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).     

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.     

Photoluminescence lifetime spectrum in erbium doped Ge–Ga–S glasses

Ge₃₀Ga₄S_65.5:Er_0.5 glasses with stoichiometric composition have been prepared using a conventional melt-quenching technique. The PL lifetime distribution corresponding to ⁴I_13/2 → ⁴I_15/2 transition in Er³⁺ ion has been measured experimentally using quadrature frequency-resolved PL spectroscopy (QFRS) from 2 ns to 160 s at room temperature and at 3.7 K. The distribution seems to be principally single-peaked at around τ_QFRS ≈ 3–4 ms with a half width <3 ms irrespective of PL energy and temperature. The Judd–Ofelt analysis based on optical transitions from ⁴I_15/2 level to ⁴I_13/2, ⁴I_11/2, ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²H_11/2 and ⁴F_7/2 levels in Er³⁺ ions gives the radiative lifetime of ⁴I_13/2 → ⁴I_15/2 transition τ_JO ≈ 2.6 ms. The closeness of τ_JO and τ_QFRS implies the dominant role of the radiative relaxation in ⁴I_13/2 → ⁴I_15/2 transitions. The shape of steady-state PL spectra can be predicted by using a modified McCumber theory taking into account the possibility of absorption and re-emission of PL quanta in heavily doped materials under weak excitation. The latter calculations used Monte Carlo simulations taking into account the re-absorption and re-emission of the PL radiation. The non-radiative lifetime for the ⁴I_13/2 → ⁴I_15/2 transition has been estimated to be more than 100 times larger than the radiative lifetime.     

Effect on the fluorescence branching ratio of different synergistic ligands in neodymium complex doped PMMA

A series of neodymium complexes Nd(TTA)₃L_x (where TTA = α-thenoyltrifluoroacetonato, L_x (x = 1–5) = H₂O, triophenylphosphine oxide (Tppo), 2,2-bipyridine (Bipy), 1,10-phenanthroline (Phen) and 2- (N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (Dpbt) were synthesized and incorporated in poly(methyl methacrylate). Their absorption spectra were measured and analyzed using Judd–Ofelt theory. Near-infrared luminescent spectra were studied and the radiative properties have been stimulated. Laser parameters such as effective bandwidths (Δλ_eff), stimulated emission cross-sections (σ_e), and gain bandwidth (σ_g) had also been calculated and compared with other systems. The effect on the fluorescence branching ratio (β) in the ⁴F_3/2→⁴I_11/2 transition of different synergistic ligands had been investigated and the relation between β and Judd–Ofelt parameter Ω₂, Ω₄, Ω₆ was discussed. In conclusion, among the five neodymium complexes, Nd(TTA)₃Dpbt has the largest Ω₂ parameter (33.72 × 10^? 20 cm²), stimulated emission cross-sections, which is found promising to be a candidate for laser materials in further application.     

Energy transfer between Tm and Er ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in Tm³⁺/Er³⁺ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er³⁺ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm³⁺ ions. This is evidence of the relevant contribution of the route Er₁(⁴I_11/2) + Er₂(⁴I_13/2) → Er₁(⁴I_15/2) + Er₂(⁴F_9/2) to the process. Energy migration among pumped ⁴I_9/2 level reducing the efficiency of the upconversion emission rate (³H_11/2, ⁴S_3/2, and ⁴F_9/2) is observed for Er³⁺ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.