Spectroscopic properties of 1.8 μm emission of thulium ions in germanate glass

A new type host of germanate glass (GeO₂− BaO−BaF₂−Ga₂O₃−La₂O₃) codoped with Tm₂O₃ has been investigated for application as laser material. It possesses a large emission cross section with the value of 9.3×10⁻²¹ cm² at 1.8 μm. Judd-Ofelt intensity parameters and radiative transition probability are calculated and analyzed by Judd-Ofelt theory and absorption spectra. The infrared emission spectra at 1.8 μm have been obtained by using a 794 nm laser diode as excitation resource. The emission intensity ratio of 1.8 (³F₄→³H₆) to 1.47 μm (³H₄→³F₄) increases, while the experimental lifetime of the Tm³⁺:³H₄ level decreases by increasing Tm₂O₃ concentration, which is attributed to the presence of a cross relaxation process. The most intensive emission at 1.8 μm is achieved from the germanate glass with the concentration of Tm₂O₃ reaches 1.0 wt%. The extended overlap integral method is used to calculate the microparameter of the energy transfer and the critical distance, which are derived to better understand the energy transfer process of thulium ions in the germanate glass responsible for emission at 1.8 μm.     

Self-pulsing dynamics of ytterbium-doped fiber laser with pump-bypassed cavity

In addition to efficient continuous wave laser action at 1.06 μm (⁴ F _3/2→⁴ I _11/2 transition), with a slope efficiency 30% and 30 mW pump power threshold, this work presents a throughout study of the spectroscopic, and thermo-optical properties of a 2.0 mol % Nd³⁺ doped fluoroindogallate glass. The characterization of the sample using the pump probe experimental technique indicated the presence of two broad excited state absorption bands from 0.94 to 1.05 μm (⁴ F _3/2→² D _3/2, ² G _9/2, ⁴ G _11/2, ² K _15/2) and from 1.16 to 1.42 μm (⁴ F _3/2→⁴ G _9/2, ⁴ G _7/2, ² K _13/2), found not to interfere with the stimulated emission at 1.06 μm, with a peak cross section value σ_SE = 2.55×10^-21 cm² and a full width at half maximum of 22 nm. Thermal lens measurements were performed to quantify the Auger upconversion parameter (γ=(1.26±0.09)×10^-16 cm³/s) and evaluate the thermal loading in the sample in comparison to other commercial glasses. PACS 181.05.Kf; 78.45.+h; 42.70.Hj     

Excited state absorption in thulium doped YVO4 crystals

Excited state absorption (ESA) of Tm³⁺ ions in YVO₄ crystal was measured using a pump and probe technique. The measurements have been performed in the wide spectral range in both the near infrared and visible region between 6000 cm^-1 (NIR) and 24000 cm^-1 (VIS). ESA absorption cross section spectra have been calculated for transitions from the ³ F ₄ and ³ H ₄ states of Tm³⁺ and compared to those evaluated experimentally. The excited state absorption in YVO₄:Tm³⁺ should not influence the laser operation related to the ³F₄→³H₆ transition around 1900 nm, but it will be a significant loss factor for a potential laser action associated with the ³H₄→³F₄ transition around 1.48 μm. PACS 42.55.Xi; 42.62.Fi     

Relaxation of excited states of Tm3+ and Tm3+-Eu3+ energy transfer in YVO4 crystal

Tm³⁺-Eu³⁺ energy transfer processes and relaxation dynamics of the ³ H ₄ and ³ F ₄ excited states of Tm³⁺ ions in 1 at. % Tm³⁺, 5 at. % Eu³⁺:YVO₄ single crystal were studied. Contribution of Tm³⁺-Eu³⁺ energy transfer reduces effectively the lifetime of terminal level in a potential ³ H ₄–³ F ₄ laser transition at around 1.48 μm. Adverse quenching of the ³ H ₄ emission by Eu³⁺ ions is found to be less efficient than that reported for Tm³⁺ + Tb³⁺ system in YVO₄. The classical Inokuti–Hirayama model accounts well for an experimental decay curve of the ³ H ₄ emission recorded for co-doped crystal. Stimulated emission cross section for ³ H ₄–³ F ₄ transition of Tm³⁺ at around 1.48 μm was analyzed taking into account the anisotropy of YVO₄ crystal. PACS 42.55.Xi; 42.62.Fi     

Ultraviolet and visible upconversion luminescence of Tm(0.1)Yb(5):FOV oxyfluoride nanophase vitroceramics

The ultraviolet upconversion luminescence of Tm³⁺ ions sensitized by Yb³⁺ ions in oxyfluoride nanophase vitroceramics when excited by a 975 nm diode laser was studied. An ultraviolet upconversion luminescence line positioned at 363.6 nm was found. It was attributed to the fluorescence transition of ¹D₂→³H₆ of Tm³⁺ ion. Several visible upconversion luminescence lines at 450.7 nm, (477.0 nm, 462.5 nm), 648.5 nm, (680.5 nm, 699.5 nm) and (777.2 nm, 800.7 nm) were also found, which result respectively from the fluorescence transitions of ¹D₂→³F₄, ¹G₄→³H₆, ¹G₄→³F₄, ³F₃→³H₆ and ³H₄→³H₆ of Tm³⁺ ion. The careful measurement and analysis of the variation of upconversion luminescence intensity F as a function of the 975 nm pumping laser power P prove that the upconversion luminescence of ¹D₂ state is partly a five-photon upconversion luminescence, and the upconversion luminescence of ¹G₄ state and ³H₄ state are respectively the three-photon and two-photon upconversion luminescence. The theoretical analysis suggested that the upconversion mechanism of the 363.6 nm ¹D₂→³H₆ upconversion luminescence is partly the cross energy transfer of {³H₄(Tm³⁺), ³F₄(Tm³⁺), ¹G₄(Tm³⁺)→¹D₂(Tm³⁺)} and {¹G₄(Tm³⁺)→³F₄(Tm³⁺), ³H₄(Tm³⁺)→¹D₂(Tm³⁺)} between Tm³⁺ ions. In addition, the upconversion luminescence of ¹G₄ and ³H₄ state results respectively from the sequential energy transfer {²F_5/2(Yb³⁺)→²F_7/2(Yb³⁺), ³H₄(Tm³⁺)→¹G₄(Tm³⁺)} and {²F_5/2(Yb³⁺) →²F_7/2(Yb³⁺), ³F₄(Tm³⁺)→³F₂(Tm³⁺)} from Yb³⁺ ions to Tm³⁺ ions. Supported by the National Natural Science Foundation of China (Grant No. 10674019)     

Relaxation dynamics of excited states of Tm3+ in SrGdGa3O7 crystals activated with Tm3+ and Tb3+

3 H₄ and ³F₄ states of Tm in SrGdGa₃O₇ single crystal was evaluated. Tm-Tb energy transfer reduces effectively the lifetime of terminal level in a potential ³H₄-³F₄ laser transition but gives rise to parasitic nonradiative relaxation of the initial ³H₄ level. SrGdGa₃O₇ crystal single doped with thulium possesses combination of properties advantageous for laser oscillations near 1.9 μm, namely high rate of cross relaxation process leading to population buildup on the initial ³F₄ level and relatively high crystal field splitting of the ground ³H₆ level. Received: 25 April 1996/Revised version: 10 October 1996     

Transition intensities and excited state relaxation dynamics of Tm3+ in Tm:PbF2 crystal

High-quality Tm doped β-PbF₂ crystal is grown by the Bridgman method in a nonvacuum atmosphere. Optical absorption spectra were acquired and analysed in the framework of the Judd-Ofelt theory. Values of radiative transition rates and luminescence branching ratios calculated using phenomenological intensity parameters Ω_λ were compared to corresponding experimental values derived from measurement of luminescence spectra and decay curves. The emission cross-sections of the ³ F ₄ → ³ H ₆ transitions of the Tm³⁺ ions at 1.86 μm wavelengths were estimated by the use of the Fuchtbauer-Ladengurg method.     

Spin-Flip and Elastic Processes in Slow Collisions of Mesic Atoms

The reduced adiabatic hyperspherical (RAHS) basis suggested previously is used for the calculation of elastic and spin-flip cross sections in the processes (aμ) _F + a → (aμ) _F′ + a, a = (p, d, t), for collision energies 10⁻³ ≤ ε ≤ 10² eV. The rapid convergence of the method is demonstrated. A comparison of the obtained results with previous ones is presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Growth and spectral properties of Tm3+-doped NaGd(WO4)2 crystal

Thulium doped sodium gadolinium tungstate crystals with sizes of about 20 mm×45 mm were grown successfully by the Czochralski technique along the (001) orientation. X-ray powder diffraction (XRD) of sodium gadolinium tungstate was described and the linear thermal expansion coefficients for the c- and a-axes were measured as 1.60×10^-5 and 7.89×10^-6 °C^-1. Polarized absorption spectra, and fluorescence spectra, as well as fluorescence decay curves of Tm³⁺-doped NaGd(WO₄)₂, have been recorded at room temperature and used to calculate the absorption and stimulated emission cross-sections. Based on the Judd–Ofelt theory, three intensity parameters were obtained. The luminescent quantum efficiency of the ³ H ₄ level was determined to be approximately 65% for this material. The emission cross-section of the ³ F ₄→³ H ₆ IR transition at about 1.8 μm was estimated by the reciprocity method. PACS 78.55.Hx; 65.40.De; 78.20.-e     

IR luminescence of thulium-activated crystals of double sodium yttrium fluoride Na0.4Y0.6F2.2:Tm3+: Self-quenching of luminescence from the 3H4 level

Na_0.4Y_0.6F_2.2:Tm³⁺ crystals with thulium contents of 0.5–20% have been grown by the Stockbarger-Bridgman method. The concentration dependences of the steady-state spectra and luminescence kinetics from the ³ H ₄ and ³ F ₄ thulium levels upon selective laser excitation of a series of Na_0.4Y_0.6F_2.2:Tm³⁺ crystals (0.5–20% Tm) are investigated. It is shown that the luminescence from the ³ H ₄ level becomes significantly quenched with an increase in the concentration due to the nonradiative energy transfer. No concentration quenching of luminescence from the ³ F ₄ level is observed. Possible schemes of self-quenching are considered for the ³ H ₄ thulium level, the self-quenching microparameters and macrorates are estimated using model quantum-mechanical calculation, and the most likely self-quenching mechanisms and schemes are found from comparison of the calculated and experimental self-quenching rates. Good agreement is obtained between the experimental and calculated kinetic curves. It is shown that, under pumping near ∼0.8 μm, self-quenching of the ³ H ₄ thulium level leads to effective occupation of the ³ F ₄ level and increases the luminescence intensity in the range of 1.7–1.9 μm. It is concluded that Na_0.4Y_0.6F_2.2:Tm³⁺ crystals are promising active media for tunable lasers in the range of 1.7–1.9 μm, with small thermal loss under laser-diode pumping near 0.8 μm. Original Russian Text ? S.é. Ivanova, A.M. Tkachuk, A.A. Mirzaeva, F. Pellé, 2009, published in Optika i Spektroskopiya, 2009, Vol. 106, No. 6, pp. 922–930.     

Spectroscopic characterisation of LaGaO3:Er3+ crystals

LaGaO₃ crystals doped with Er³⁺ ions were grown by the Czochralski method and their optical properties were examined. The Er³⁺ energy levels have been determined from the low-temperature absorption and emission spectra. The results of Judd–Ofelt analysis are presented and compared with experimental data. The emission cross sections are determined for the ⁴ I _13/2→⁴ I _15/2 (1.55 μm) and ⁴ I _11/2→⁴ I _13/2 (2.85 μm) transitions of erbium. Received: 6 December 1999 / Revised version: 10 February 2000 / Published online: 27 April 2000     

Visible up-conversion and NIR luminescence studies of LiAl<Subscript>5</Subscript>O<Subscript>8</Subscript>:Er phosphor co-doped with Yb<Superscript>3+</Superscript> and Zn<Superscript>2+</Superscript>

The frequency up-conversion, an efficient laser emission and amplification in Er³⁺:LiAl₅O₈ phosphors co-doped with Yb³⁺ and Zn²⁺ phosphor powders in the 520–560, 640–680 nm regions and at ∼1.5 μm, respectively, have been reported. The emission corresponds to the ²H_11/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions upon direct excitation into the intermediated ⁴I_11/2 level using ∼980 nm radiation from a CW laser. Possible mechanisms involved for the up-conversion processes based on the energy level matching scheme, the pump-power dependence and the dynamical behaviour have been discussed. The effect of the addition of Yb³⁺ and Zn²⁺ for the amplification in the 1.5 μm eye-safe telecommunication window has been elaborated and characterized in detail.     

Optical properties of Tm,Mg:LiTaO3 laser crystal

A Tm, Mg co-doped LiTaO₃ crystal has been grown by Czochralski method. Room temperature polarized absorption spectra and fluorescence spectrum of the Tm, Mg:LiTaO₃ crystal were measured and analyzed. The maximum absorption cross-section is 6.0791 × 10⁻²⁰ cm² at around 790 nm with full width at half maximum of 5 nm. The emission cross-section of ³ F ₄ manifold was 2.2 × 10⁻²⁰ cm². The spectroscopic parameters of Tm³⁺ ion were calculated by applying the Judd-Ofelt approach, and the intensity parameters Ω₂, Ω₄, and Ω₆ were obtained to be 7.71 × 10⁻²⁰, 1.09 × 10⁻²⁰, and 1.16 × 10⁻²⁰ cm², respectively. The branching ratios and radiative lifetimes were also presented and the radiative lifetime of Tm^{3+ 3} F ₄ → ³ H ₆ transition is 968.3 μs. The results were also analyzed and compared with other Tm³⁺ doped hosts.     

The special features of the luminescence and light resistance of europium complexes and their mixtures with lanthanum complexes in polymethyl methacrylate

The kinetics of luminescence and transformation of short-lived products of the photolysis of europium and lanthanum complexes with thenoyltrifluoroacetone and 1,10-phenanthroline and their mixtures in polymethyl methacrylate films was studied by the nanosecond laser photolysis method with recording both light emission and absorption. Fast (535 and 585 nm, ⁵ D ₁ → ⁷ F ₀, ⁷ F ₃, decay time 0.7 μs) and slow (613 nm, ⁵ D ₀ → ⁷ F ₂, luminescence rise and decay times 0.7 μs and 0.5 ms, respectively) luminescence was studied. Induced absorption with a maximum at 600 nm and decay time ∼3 ms was observed; this absorption was assigned to triplet states of the deprotonated form of thenoyltrifluoroacetone. The dependences of luminescence intensity on the concentration of the components in a mixture of complexes were analyzed, and synergistic effects of luminescence strengthening were estimated. The kinetics of a decrease in luminescence intensity during photolysis was studied. Possible mechanisms of a decrease in the relative initial process rate and an increase in the quasi-stationary value of relative luminescence intensity as the concentration of complexes in the polymer increased were discussed.     

Thermal and optical properties of Tm<Superscript>3+</Superscript>:NaLa(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

A Tm³⁺-doped NaLa(WO₄)₂ single crystal with a dimension of Φ20 mm×40 mm was grown by the Czochralski method. Anisotropic thermal expansion coefficients of this crystal were investigated. Polarized absorption spectra, emission spectra and decay curve were recorded at room temperature. The absorption and emission cross-section were presented. Based on the Judd–Ofelt analysis, we obtained the three intensity parameters: Ω₂=10.21×10^-20, Ω₄=2.66×10^-20, and Ω₆=1.46×10^-20 cm². The radiative probabilities, radiative lifetimes, and branch ratios of Tm³⁺:NaLa(WO₄)₂ were calculated, too. Luminescence lifetime of the ³ H ₄ level was measured to be 220 μs. The stimulated emission cross-section for the ³ F ₄→³ H ₆ transition were determined using the reciprocity method, potential laser gain for this transition were also investigated, the gain curves implied that the tunable range is up to 200 nm. PACS 42.70.Hj; 78.20.-e     

Broadband 2 μm emission and energy-transfer properties of thulium-doped oxyfluoride germanate glass fiber

High near-infrared transparency Tm³⁺-doped germanate bulk glass and fiber have been fabricated and 2 μm emission properties demonstrated. Energy-transfer processes in the germanate glass and fiber are discussed. The emission spectra are obtained from both bulk glass and fiber with the excitation of a 794 nm laser diode. The results indicate that the line width of the Tm³⁺:³F₄→³H₆ emission spectra measured in fibers is narrower than that of the bulk glass sample and shifts to longer wavelengths with increment of fiber length. The extended overlap integral method is used to calculate the microparameters of energy transfer and critical distance. A model is derived to better understand of the energy-transfer process of thulium ions in the germanate glasses responsible for emission at 2 μm. The study indicates that Tm³⁺-doped germanate fibers with a large core diameter has proved to be promising infrared optical and high-power level laser materials.     

Low-temperature investigation of electronic excitations in wide-gap materials doped with RE3+ or Cr3+ ions

The processes of photon multiplication in insulators have been considered. The luminescence of Tb³⁺ ions (⁵ D ₃ → ⁷ F _J , ⁵ D ₄ → ⁷ F _J transitions) upon intracenter excitation, the optical excitation of oxyanions, or the formation of separated electrons and holes has been studied for CaSO₄ doped with Tb³⁺ and Na⁺ ions at 6–9 K. An increase in Tb³⁺ concentration from 0.2 to 4 at % and transition from single Tb³⁺-Na⁺ states to centers that contain two or three terbium ions leads to the redistribution of the luminescence intensities in favor of the ⁵ D ₄ → ⁷ F _J transitions and increase in their efficiency due to the possibility of the cooperative ⁵ D ₃ → ⁵ D ₄ and ⁷ F ₆ → ⁷ F _J transitions and the 4f ⁷5d ¹ → ⁵ D ₃ and ⁷ F ₆ → ⁵ D ₄ transitions in the two- and three-terbium centers. Based on the example of MgO single crystals with highly mobile excitons, holes, and electrons, the migration of free excitons and holes toward Cr³⁺ ions in the crystal bulk and their exit from the bulk to the surface have been revealed at 9 K. Surface losses limit the luminescence quantum yield of MgO:Cr³⁺, CaSO₄:Tb³⁺, and many other materials.     

Er3+-doped sol–gel SnO2 for optical laser and amplifier applications

Erbium-doped tin dioxide (SnO₂:Er³⁺) was obtained by the sol–gel method. Spectroscopic properties of the SnO₂:Er³⁺ are analyzed from the Judd–Ofelt (JO) theory. The JO model has been applied to absorption intensities of Er³⁺ (4f¹¹) transitions to establish the so-called Judd–Ofelt intensity parameters: Ω₂, Ω₄, and Ω₆. With the weak spectroscopic quality factors Ω₄/Ω₆, we expect a relatively prominent infrared laser emission. The intensity parameters are used to determine the spontaneous emission probabilities of some relevant transitions, the branching ratios, and the radiative lifetimes of several excited states of Er³⁺. The emission cross section (1.31×10^-20 cm²) is evaluated at 1.54 μm and was found to be relatively high compared to that of erbium in other systems. Efficient green and red up-conversion luminescence were observed, at room temperature, using a 798-nm excitation wavelength. The green up-conversion emission is mainly due to the excited state absorption from ⁴ I _11/2, which populates the ⁴ F _3/2,5/2 states. The red up-conversion emission is due to the energy transfer process described by Er³⁺ (⁴I_13/2)+Er³⁺(⁴I_11/2)→Er³⁺(⁴F_9/2)+Er³⁺ (⁴ I _15/2) and the cross-relaxation process. The efficient visible up-conversion and infrared luminescence indicate that Er³⁺-doped sol–gel SnO₂ is a promising laser and amplifier material. PACS 71.20.Eh; 74.25.Gz; 78.55.-m     

Growth, optical parameters, and spectroscopic properties of crystals of disordered scheelite-like molybdates NaLaxGd1 ? x(MoO4)2 (x = 0–1) activated by Tm3+ ions

Polarized optical absorption spectra in the range of 450–2000 nm have been recorded for the Czochralski-grown concentration series of single crystals of mixed scheelite-like molybdates NaLa _x Gd_{1 − x} (MoO₄)₂(x = 0–1) activated by Tm³⁺ ions. The intensity parameters Ω _t (t = 2, 4, 6) of the crystals grown were determined by the Judd-Ofelt method, and the values found were used to estimate the probabilities and emission branching ratios for a number of radiative transitions of Tm³⁺ ions. The main optical constants of an Na_0.909La_0.490Gd_0.498Tm_0.035Mo_2.020O_8.048 crystal were found and the temperature dependences of the refractive index for the ordinary and extraordinary waves were investigated in the temperature range of 293–373 K. The spectral dependences of the emission cross section for the expected ³ F ₄ → ³ H ₆ laser transition of Tm³⁺ ions in an Na_0.908La_0.301Gd_0.676Tm_0.048Mo_2.019O_8.043 crystal were measured in the spectral range of 1500–2000 nm. The gain cross section of the active medium on the ³ F ₄ → ³ H ₆ laser transition of Tm³⁺ ions in this crystal was calculated using the spectral dependences of the absorption and emission cross sections.