Energy Transfer Upconversion Research Articles

Excited state dynamics of the Ho3+ ions in holmium singly doped and holmium, praseodymium-codoped fluoride glasses

The deactivation of the two lowest excited states of Ho3+ was investigated in Ho3+ singly doped and Ho3+, Pr3+-codoped fluoride (ZBLAN) glasses. We establish that 0.1–0.3mol% Pr3+ can efficiently deactivate the first excited (I75) state of Ho3+ while causing a small reduction of ∼40% of the initial population of the second excited (I65) state. The net effect introduced by the Pr3+ ion deactivation of the Ho3+ ion is the fast recovery of the ground state of Ho3+. The Burshstein model parameters relevant to the Ho3+→Pr3+ energy transfer processes were determined using a least squares fit to the measured luminescence decay. The energy transfer upconversion and cross relaxation parameters for 1948, 1151, and 532nm excitations of singly Ho3+-doped ZBLAN were determined. Using the energy transfer rate parameters we determine from the measured luminescence, a rate equation model for 650nm excitation of Ho3+-doped and Ho3+, Pr3+-doped ZBLAN glasses was developed. The rate equations were solved numerically and the population inversion between the I65 and the I75 excited states of Ho3+ was calculated to examine the beneficial effects on the gain associated with Pr3+ codoping.

Stokes luminescence and frequency upconversion in Pr3+ doped TeO2–PbO glass

Optical properties of Pr3+ doped TeO2–PbO glass were investigated by resonant excitation of H43→D21 transition at ∼586nm. The decay time of D21 level, the energy transfer (ET) rates from the D21 level to other Pr3+ states, and the frequency upconversion process which leads to emission at 484nm (P03→H43 transition) were studied. The ET rates between Pr3+ ions were determined and their interaction was identified as due to dipole-dipole potential. A model used to describe the emission P03→H43 provided information on the dynamics of Pr3+ pair states. The upconversion ET rate was also determined.

Upconversion and excited-state absorption in the lower levels of Er:KPb2Cl5

Polarized excited-state absorption spectra for the 4I13/2→4I9/2 transition in Er:KPb2Cl5 were determined from the measured 4I9/2→4I13/2 emission spectra using the reciprocity relation of [D.E. McCumber, Phys. Rev. 136 (1964) A954]. From these spectra, the microparameters for energy transfer were found to be CDA=2.6×10−42cm6/s for (4I13/2,4I13/2)→(4I15/2,4I9/2) and CDD=6.9×10−39cm6/s for (4I13/2,4I15/2)→(4I15/2,4I13/2). The macroscopic upconversion coefficient was measured by comparing fluorescence intensities from the 4I13/2 and 4I9/2 levels, and found to be Cup=(2.6±0.4)×10−20cm3/s, in good agreement with the value calculated from the microparameters CDA and CDD. Energy transfer upconversion between two ions in the 4I13/2 level is orders of magnitude weaker than in other typical hosts, and this is predicted to lead to dramatically reduced heat loading in a resonantly pumped 1.5μm Er laser.

Investigation of energy transfer and frequency upconversion in Ho 3+/Yb 3+ co-doped tellurite glasses

A serials of Ho 3+/Yb 3+ co-doped tellurite glasses by pumping 970 nm laser diode (LD) were demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Two intense emission bands were observed in Ho 3+/Yb 3+ co-doped tellurite glasses centered at 549 and 664 nm corresponding to Ho 3+: 5S 2( 5F 4)→ 5I 8 and 5F 5→ 5I 8 transitions, respectively. The upconversion intensities of red and green emissions in Ho 3+/Yb 3+ co-doped glasses were enhanced largely when increasing Yb 2O 3 content. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model. The energy transfer coefficients were estimated by fitting the simulated curves to the measured ones. The obtained three energy transfer coefficients C D 2, C D 3 and C D 4 were C D 2=5.0×10 −18 cm 3/s, C D 3=1.5×10 −17 cm 3/s, C D 4=9.0×10 −17 cm 3/s.

Energy transfer and frequency upconversion in Ho 3+/Yb 3+ co-doped bismuth-germanate glasses

We report an intense room-temperature green and red upconversion in Ho 3+/Yb 3+ co-doped xBi 2O 3–(90 − x)GeO 2–10Na 2O glasses upon excitation with a convenient 980 nm laser diode. Effect of Bi 2O 3 on upconversion properties of Ho 3+/Yb 3+ co-doped bismuth-germanate glasses have been investigated based on IR spectra. We find that the increase of Bi 2O 3 decreasing the phonon energy of host glasses. The upconversion intensity of green and red emissions in Ho 3+/Yb 3+ co-doped glasses is enhanced largely with increasing Bi 2O 3 content. Three intense emission bands were observed in Ho 3+/Yb 3+ co-doped BGN glasses centered at 549, 644 and 750 nm correspond to Ho 3+: 5S 2( 5F 4) → 5I 8, 5F 5 → 5I 8 and 5S 2( 5F 4) → 5I 7 transitions, respectively. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model.

Absorption and emission characteristics of Er 3NbO 7 phosphor: A comparison with ErNbO 4 phosphor and Er:LiNbO 3 single crystal

Er 3NbO 7 phosphor was synthesized by sintering a mixture of Er 2O 3 and Nb 2O 5 powder in a molar ratio of 3:1 at 1600 °C over 55 h. Optical absorption and emission characteristics of Er 3+ ions in the calcined Er 3NbO 7 powder were investigated and discussed compared with ErNbO 4 phosphor and a Z-cut congruent Er (2 mol%):LiNbO 3 single crystal. The absorption and emission studies show that, due to different crystal structures, the spectroscopic properties of these niobates have some differences in spectral shape, peak position, and relative intensity, especially at 1.5 μm. The most obvious spectral feature of the Er 3NbO 7 is that the spectral structure of band instead of peak is observed in its absorption or emission spectrum due to the existence of local structural disorder and multiple Er 3+ sites. The Er 3NbO 4 shows stronger upconversion emission than the single crystal but weaker than the ErNbO 4. Experimental results show that energy transfer upconversion and/or excited state absorption play a dominant role in the upconversion emissions, and, at higher pump level (>200 mW), the thermal effect becomes significant and results in drop of the upconversion intensity. The 1.5 μm lifetimes of Er 3+ ion in the Er 3NbO 7, ErNbO 4 phosphor, and in the Er:LiNbO 3 crystal are measured to be ∼5.3, 2.0, and 2.4 ms, respectively. In combination with the measured Raman spectra, the quantum efficiency, multiphonon nonradiative decay rate, and theoretical radiative lifetime of the 1.5 μm emission of the two powder materials are expected. The differences in upconversion intensity and measured 1.5 μm lifetime between the three materials are explained qualitatively.

Thermo-optic effects affecting the high pump power end pumped solid state lasers: Modeling and analysis

A complete solution of the thermal effect for continuous wave laser diode array fiber coupled high pump power end pumped actively solid state laser is presented. In this paper, the thermal loading resulted from energy transfer up-conversion effect for laser operated under different pulse repetition frequency has been taken into account in the overall thermal effects. The thermal loading transfer the absorbed pump power into non-uniform temperature distribution in the laser rod, and thus results in thermal strain, thermal stress, thermal dispersion of index, and thermally induced diffraction losses. For a practical laser cavity configuration containing the internal optical elements, we use spherical aberration theory to give a better explanation of experimental results via calculating diffraction losses quantitatively. According to analysis of thermal focal length experimental data, the results show that our calculated value of thermal loading under fixed Nd 3+ doped concentration laser crystal is not a constant but a function of pump power instead. The theoretical predicted curve of thermal focal length based on our model is in good agreement with experimental measurements. At low pump power, the thermal loading is a linear function varying with pump power, but it begins to saturate at pump powers exceeding 6 W. This phenomenon is consistent with our physical intuitions.

Diode-cladding-pumped singly Ho3+-doped silica fibre laser

An unsaturated continuous-wave output power of 109 mW was produced from a diode-pumped Ho3+-doped double-clad silica fibre laser that did not require the use of a sensitiser. The laser operated with a slope efficiency of 7% and an emission wavelength of 2.14 µm. Energy transfer upconversion may have adversely affected the performance of the laser.

Fractional thermal loading and thermal lensing in end-pumped Tm,Ho:YLF lasers

Based the energy transfer between ions and the transition between energy levels of diode-end-pumped Tm,Ho:YLF laser, and considering the energy transfer up-conversion and ground state re-absorption, the rate equations are set up. The analytical formula of fractional thermal loading is deduced from the rate-equations, and the results show that the fractional thermal loading critically depends on the pump-to-mode size ratio. The focal length of the thermal lens as a function of pump power is obtained in experiment. The experimental results are compared with theoretical results, which shows that the theoretical results are reasonable.

Near-Infrared (NIR) to Red and Green Up-Conversion Emission from Silica Sol−Gel Thin Films Made with La0.45Yb0.50Er0.05F3 Nanoparticles, Hetero-Looping-Enhanced Energy Transfer (Hetero-LEET): A New Up-Conversion Process

Bright green and red luminescence has been generated with a 980 nm diode laser from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles through a newly described hetero-looping-enhanced energy-transfer (hetero-LEET) up-conversion process, which exhibits a power dependence similar to that of a photon avalanche (PA). The hetero-LEET mechanism is potentially more efficient than PA, ground-state absorption/excited-state absorption (GSA/ESA), and energy-transfer (ETU) mechanisms because it combines resonant ground-state absorption with a looping or feedback process.

Up-conversion luminescence of Er 3+/Yb 3+/Nd 3+-codoped tellurite glasses

Up-conversion luminescence and energy transfer (ET) processes in Nd 3+–Yb 3+–Er 3+ triply doped TeO 2–ZnO–Na 2O glasses have been studied under 800 nm excitation. Intense green up-conversion emissions around 549 nm, which can be attributed to the Er 3+: 4S 3/2→ 4I 15/2 transition, are observed in triply doped samples. In contrast, the green emissions are hardly observed in Er 3+ singly doped and Er 3+–Yb 3+ codoped samples under the same condition. Up-conversion luminescence intensity exhibits dependence of Yb 2O 3-concentration and Nd 2O 3-concentration. Up-conversion mechanism in the triply doped glasses under 800 nm pump is discussed by analyzing the ET among Nd 3+, Yb 3+ and Er 3+. And a possible up-conversion mechanism based on sequential ET from Nd 3+ to Er 3+ through Yb 3+ is proposed for green and red up-conversion emission processes.

Investigation of energy transfer and frequency upconversion in Er 3+/Ho 3+ co-doped tellurite glasses

The energy-transfer mechanisms and frequency upconversion emissions in 0.5Er 3+/ xHo 3+ co-doped tellurite glasses by exciting at 980 nm have been investigated. Three intense upconversion luminescence emissions are observed at around 525, 548, and 660 nm, which correspond to Er 3+: 2H 11/2 → 4I 15/2, Er 3+: 4S 3/2 → 4I 15/2 + Ho 3+: 5S 2( 5F 4) → 5I 8, and Er 3+: 4F 9/2 → 4I 15/2 + Ho 3+: 5F 5 → 5I 8 transitions, respectively. The upconversion emissions reach the maximum values when Ho 2O 3 is 0.5 mol%, and the intensities of the green and red light emissions were about 4.5 and 6 times stronger than those un-doped Ho 2O 3, respectively. The possible upconversion mechanisms and energy transfer between Er 3+ and Ho 3+ were also estimated and evaluated. All the three emissions are based on two photon absorption processes.

Population dynamics of the 3F4 and 3H4 levels in highly-doped Tm3+:ZB(L)AN glasses

Population dynamics of the 3F4 and 3H4 levels in Tm3+ doped ZB(L)AN glasses was studied for Tm3+ concentrations from 0.5 to 12mol%. Fluorescence waveforms from these levels were measured at 1.8μm (3F4) and 800nm (3H4) with both direct and indirect pumping. Decay from the 3F4 level was found to be exponential with non-radiative decay rates proportional to the square of the Tm concentration. This indicated a process of energy migration by diffusion within the excited Tm3+ ions followed by quenching at sites to which the ions could migrate. The decay of the directly pumped 3H4 level exhibited both exponential and non-exponential behavior depending on the concentration. For the lowest concentration (0.5mol%) the decay was exponential, but at concentrations of 1, 2, 4 and 6mol% the decay waveforms were distinctly non-exponential. The non-exponential waveforms could be fitted by the Yokota–Tanimoto model for diffusion of excited donors and dipole–dipole interactions with acceptors. This model produced values for CDD and CDA, the donor–donor and donor–acceptor energy transfer parameters, respectively. At the higher concentrations (8, 10, 12mol%) the waveforms were exponential with decay rates from which the cross-relaxation parameter for the process 3H4, 3H6→3F4, 3F4 was obtained. When the 3F4 level is pumped at 1660nm, the decay of the 3H4 level confirmed the influence of the up-conversion energy transfer process 3F4, 3F4→3H4, 3H6.

Energy transfer and frequency upconversion in Pr3+-doped ZBLAN glass

The lifetimes of 3P0 and 1D2 levels of Pr3+ were measured in ZBLAN glasses of composition in mol% 53ZrF4–20BaF2–(4−x)LaF3–3AlF3–20NaF–xPrF3 (x=0.1, 0.5, 1, and 3) by resonant excitation. Energy transfer observed between Pr3+ ions was analyzed by means of the Inokuti–Hirayama theory. The non-exponential dependence of the 1D2 decay on Pr3+ concentration indicates that dipole–dipole interaction between Pr3+ is the main process. Orange-to-blue upconversion luminescence was observed and described by two-ion energy transfer mechanism. The temporal evolution of upconversion luminescence was characterized by rise and decay times related to the dopant concentration.

Yellow-to-violet, blue, and green frequency upconversions in Nd3+-doped PbWO4 single crystal

Upconversion emission properties of Nd3+ ions doped in PbWO4 crystal are investigated by pulsed laser excitation into the G5∕24+G2(1)7∕2 levels at temperatures from 10to295K. The main upconversion emission consists of strong blue emission bands in the wavelength region of 435–447nm corresponding to the P1∕22→I9∕24 transitions. Some UV bands located in the wavelength region of 356–397nm corresponding to the D3∕24→I9∕24, I11∕24, and I13∕24 transitions and weak green emission at 525.2nm corresponding to the G7∕24→I9∕24 transition are observed. Several upconversion blue emissions from the P1∕22 level and the absoprtion lines due to the I9∕24→P1∕22 transition indicate multisite structure of the Nd3+ ions in the PbWO4 lattices. We investigate the excitation spectra of the upconversion emissions and their pump power dependence and lifetimes from which the probable upconversion mechanisms are proposed and analyzed. The dominant mechanisms for the upconversion emission originating from the D3∕24 level (UV emission) and the P1∕22 level (blue emission) are proven to be the sequential two-photon excited-state absorption, while the energy transfer upconversion process is responsible for the green upconversion emission from the G7∕24 level of Nd3+.

Thermal lens and Auger upconversion losses' effect on the efficiency of Nd^3+-doped lead lanthanum zirconate titanate transparent ceramics

A thorough investigation of optical losses for the 1064nm emission in Nd3+-doped lead lanthanum zirconate titanate (PLZT) transparent ceramics is presented. Thermal lens experiments were carried out to evaluate thermo-optical properties and the fluorescence quantum efficiency of the emitting level 4F3/2. Excited-state absorption losses were measured in the emitting wavelength region, and the Auger upconversion energy transfer parameter γ was calculated. By using γ, the pump-intensity dependence of the optical gain at 1064nm, the fluorescence quantum efficiency, and the generation of heat in the ceramic were simulated for a high 803nm pump-power regime. Since the radiative and nonradiative losses in Nd:PLZT were verified to be considerably lower than in various commercial laser crystals and glasses, it is suggested that this material might become an interesting alternative for high-power laser emission.

The effects of energy transfer upconversion on the performance of Tm/sup 3+/, Ho/sup 3+/-doped silica fiber lasers

We report the performance characteristics of a diode-pumped Tm3+, Ho3+-doped silica fiber laser as a function of the dopant concentrations. We show, albeit indirectly, that the rates of energy transfer upconversion (ETU) are significantly increased when the Tm3+ concentration and hence rates of cross relaxation (CR) are increased. The net result of ETU is a reduction in the 3 F4 population of Tm3+ and heating of the fiber as a result of the large rates of multiphonon emission from the higher lying energy levels of Ho3+. Fibers with low Tm3+ concentrations display the highest slope efficiencies and lowest thresholds, with 95% of the Stokes efficiency limit being achieved. CR, therefore, cannot be completely harnessed as a mechanism to enhance the rate of upper laser level excitation in Tm3+, Ho3+-doped silica fiber lasers

Determination of the macroscopic upconversion parameter in Er^3+-doped transparent glass ceramics

A simple rate equation model is used to determine the macroscopic upconversion parameter of a heavily erbium-doped oxyfluoride transparent glass ceramic. The fraction of initially excited ions in the metastable state among erbium ions partitioned in the nanocrystallite phase is found and the decay of the 4S3/2 level is used to determine the upconversion parameter. The macroscopic energy-transfer upconversion parameter is found to be approximately 4×10−18cm3s−1, in good agreement with other erbium-doped fluoride materials.

Ultraviolet and visible upconversion dynamics in Er3+:YAlO3under2H11/2excitation

The luminescence of Er3+:YAlO3 in ultraviolet, visible and infrared ranges under the 518 nm excitation of the multiples 2H11/2 have been investigated. Ultraviolet (275 nm and 318 nm), violet (405 nm and 413 nm) and blue (474 nm) upconversion and infrared downconversion luminescence has been observed. By means of measuring the fluorescence decay curves and using the theory of rate equations, the luminescence kinetics was studied in detail and the processes of energy transfer upconversion (ETU) and excitation state absorption (ESA) were proposed to explain the upconversion phenomena.

Upconversion rate in Nd-doped Ta 2O 5 waveguides and influence on the cw laser performance

The non-linear upconversion processes in Nd-doped Ta 2O 5 waveguides have been investigated to determine the depopulation rate of the 4F 3/2 laser level due to energy transfer upconversion (ETU). A rate-equation analysis has been used to obtain analytical expressions of the population and average lifetime of the 4F 3/2 level under modulated cw excitation. The analysis of lifetime measurements as a function of excitation power yielded to an ETU rate of about 22 800 s −1. Equations for the four level cw laser performance have been obtained using a rate-equation approximation that includes ETU processes. The effect of upconversion on the laser performance has been estimated using the parameters obtained for the Nd-doped Ta 2O 5 waveguide lasers.