Upconversion-pumped population kinetics for4I13/2 and4I11/2 laser states of Er3+ ion in several host crystals

Population kinetics of the upper⁴I_11/2 and lower⁴I_13/2 laser states of the Er³⁺ ion were studied experimentally and theoretically in (Er)BaY₂F₈, (Er)YLF, (Er)YSGG, (Er)CaF₂ and (Er)YALO. Fluorescence from these states to the⁴I_15/2 ground state was excited through upconversion simultaneously with the⁴I_11/2 ⁴I_13/2 lasing using 1.53 µm radiation from an erbium : glass laser for optical pumping. Lifetimes of both states are altered during lasing by co-operative energy transfer processes: the lifetime of the lower state ₁ is shortened and that of the upper state ₂ increased with the resultant ratio ₂/₁>1. After lasing the lifetime ratio returns to the normal value ₂/₁ <1; that=" is,=" one=" obtained=" under=" weak=" ultraviolet=" excitation.=" kinetic=" rate=" equations=" for=" the=" population=" density=" functions=" for=" both=" laser=" states=" were=" set=" up=" and=" solved=" by=" approximation=" in=" three=" time=" domains.=" it=" was=" assumed=" that=" only=" one=" co-operative=" energy=" transfer=" process=" operates=" in=" the=" laser=" crystals=" and=" determines=" the=" population=" inversion=" kinetics.=" consistency=" relationships=" for=" comparison=" of=" the=" theoretical=" results=" with=" the=" experiment=" were=" developed.=" only=">₂F₈ spectral features showed close agreement with theory, resulting in a high score of 94% for the overall correlation in the consistency test, whereas all other crystals scored <50%. as=" a=" result=" of=" this=" high=" correlation,=" a=" close=" match=" between=" theoretical=" and=" experimental=" population=" decay=" curves=" was=" shown=" for=">₂F₈. Most probably, more than one energy transfer process shapes the decay curves and determines the population inversion kinetics for the other laser crystals. (Er)YALO showed little lifetime change for the laser states, apparently due to inefficient co-operative energy transfer processes. As a result it probably lased in a self-terminating short-pulse mode.     

Doubly doped BaY2F8:Er,Nd VUV scintillator

Doubly doped BaY₂F₈:Er,Nd scintillation crystals were grown by modified micro-pulling-down method. The Er co-doping was chosen to enhance the energy transfer from the host lattice to the Nd³⁺ luminescence center via the 5d-levels of Er³⁺, which can be enabled by the overlap of Er³⁺ 5d-4f emission spectrum with the Nd³⁺ 4f-5d absorption. The energy transfer was clearly evidenced in the BaY₂F₈:Er,Nd. The processes are complicated by energy migration to killer centres and/or cross-relaxation processes. The luminescence and energy transfer mechanism are discussed.     

Efficiency of the generation and self-addition of laser line frequencies upon the diode pumping of Er:BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals

Features of the generation of laser lines upon the diode pumping of Er:BaY₂F₈ crystals are studied. Blue, violet, and UV laser lines are most efficiently generated via the nonlinear frequency self-addition of 541 and 553 nm lines of Er³⁺ radiation with the corresponding IR line of Er³⁺ for Er:BaY₂F₈ crystals pumped by a laser diode with a wavelength of 972 nm.     

VUV luminescence of Er<Superscript>3+</Superscript> ions in LiYF<Subscript>4</Subscript> and BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals

Vacuum ultraviolet luminescence of Er³⁺ ions in LiYF4 and BaY₂F₈ crystals has been investigated. It is revealed that under excitation by 193 nm radiation from an ArF excimer laser the interconfigurational 5d–4f radiative transitions in Er³⁺ ions are observed. It is shown that from the LiYF₄:Er crystal only the spin-forbidden luminescence (λ = 165 nm) is detected, whereas both the spin-forbidden (λ = 169 nm) and spin-allowed (λ = 160.5 nm) components are observed from the BaY₂F₈:Er crystal.     

The laser-diode-excited 5d-4f luminescence of Ce3+ and Pr3+ ions embedded into a BaR2F8 matrix

We show the possibility of obtaining UV luminescence from 5d-4f transitions of rare-earth ions in the BaY₂F₈: (Yb³⁺, Pr³⁺, Ce³⁺) crystal under upconversion excitation by standard laser diodes with lasing wavelengths of 960, 808, and 840 nm. Various upconversion mechanisms of pumping for populating the higher-lying energy levels of the active ions, as well as methods of adaptation of the active medium BaY₂F₈: (Yb³⁺, Pr³⁺, Ce³⁺) to these mechanisms, are considered.     

Energy transfer and fluorescence characteristics of erbium-doped ytterbium aluminum garnet

We have examined the fluorescence characteristics of the garnet-type crystal Yb₃Al₅O₁₂ : Er³⁺ (YbAlG : Er³⁺) and studied the energy transfer process between the two rare earth ions over a temperature range 78–297 K. Certain data were compared with those of YAlG : Er³⁺. In YbAlG : Er³⁺, Yb fluorescence is observed at ?1.03 μm (corresponding to the ²F_5/2 → ²F_7/2 transition); Er fluorescence occurs at ?8500 Å (⁴S_3/2 → ⁴I_13/2 transition) and ?1.6 μm (⁴I_13/2 → ⁴I_15/2 transition). In YAlG : Er³⁺, the same Er lines are observed with the addition of a band at ?1 μ (⁴I_11/2→⁴I_15/2 transition). In YbAlG : Er³⁺, the decay pattern of the Yb emission is purely exponential at all the temperatures examined; the fluorescence lifetime ranges from 36 μ s (at 78 K) to 74 μs(at 269 K). The lifetime of the Er⁴I_13/2 level in the same sample increases from 5.4 ms (at 78 K) to 6.85 ms (at 294 K). The lifetime of this Er level in YAlG : Er³⁺ is weakly temperature dependent over the same range with a value of ?12 ms. Excitation spectra were obtained for the Er 1.53 μm fluorescence in YbAlG : Er³⁺ in order to verify the presence of Yb → Er energy transfer in this sample. The presence of the Yb absorption band (?1 μm) in these spectra provides direct evidence of this energy transfer. The relative enhancement of this Yb band with respect to the Er bands in going from 78 K to 175 K is an indication of a more efficient transfer at the higher temperature. Excitation spectra obtained for the Yb 1.03 μm fluorescence in YbAlG : Er³⁺ revealed the presence of Er → Yb energy transfer as well in this sample. The existence of both Yb → Er and Er → Yb transfer is expected, due to the resonance between the ⁴I_11/2 → ⁴I_15/2 transition of Er and the ²F_5/2 → ²F_7/2 transition of Yb. The above results are explained in terms of a rate equation model in which transfer in both directions is treated in the following manner: Yb → Er transfer is considered to be much more probable than decay processes originating at the Yb ²F_5/2 level; Er → Yb transfer is treated as much more probable than decay processes originating at the Er ⁴I_11/2 level.     

Upconversion from the I13/2 and I11/2 levels in Er:YAG

The efficiency of erbium three-micron laser (laser transition ⁴I_11/2→⁴I_13/2) depends essentially on the ratio of the parameters of active energy transfer upconversion (ETU) from the laser levels. The parameters of both ETU processes can be obtained from the analysis of the shape of the kinetics of the ⁴I_11/2 level in concentrated Er:YAG crystals, under short pulse pumping. Mathematical modeling is used to evaluate the sensitivity of the method and to estimate the errors which can be introduced by the inhomogeneous pumping and accidental impurities. It was found that the ratio of the parameters corresponding to the ETU from the laser levels is less sensitive to the pumping inhomogeneities than that corresponding to the lower laser level. A reduction of this ratio with increasing erbium concentration is observed.     

Spectroscopic and laser properties of Er3+-doped monoclinic BaY2F8 single crystals

The luminescence and absorption properties of Ba(Y_1−x Er _x )₂F₈ (x=0.001, 0.01, 0.05, 0.1, 0.2 and 0.3) and the Er³⁺-ion decay kinetics of luminescent transitions from three initial laser states, the⁴S_3/2,⁴F._9/2 and⁴I_11/2 manifolds, were measured. The crystal-field splitting schemes for allJ-manifolds which are involved in (J →J′)-luminescence transitions and stimulated emission parameters of Er³⁺ -ions in BaY₂F₈ were determined. A comparison of laser powers and efficiencies of BaY₂F₈ and Y₃Al₅O₁₂, Lu₃Al₅O₁₂ and LiYF₄ single crystals doped with Er³⁺-ions shows the similar performance of these materials.

     

Spectroscopic and laser properties of Er3+-doped monoclinic BaY2F8 single crystals

The luminescence and absorption properties of Ba(Y_1–x Er _x )₂F₈ (x=0.001, 0.01, 0.05, 0.1, 0.2 and 0.3) and the Er³⁺-ion decay kinetics of luminescent transitions from three initial laser states, the⁴S_3/2,⁴F._9/2 and⁴I_11/2 manifolds, were measured. The crystal-field splitting schemes for allJ-manifolds which are involved in (J J)-luminescence transitions and stimulated emission parameters of Er³⁺ -ions in BaY₂F₈ were determined. A comparison of laser powers and efficiencies of BaY₂F₈ and Y₃Al₅O₁₂, Lu₃Al₅O₁₂ and LiYF₄ single crystals doped with Er³⁺-ions shows the similar performance of these materials.     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

Optical losses in YVO4: RE (RE = Nd3+, Er3+, Tm3+) laser crystals

The relevance of processes contributing to depletion of pump and upper laser levels has been assessed based on experimental data obtained during measurement of excited state absorption, steady state emission and dynamics of excited states as a function of excitation power and activator concentration. It has been concluded that the excited state absorption in YVO₄: Nd and YVO₄: Er is not significant except for that from the ⁴ I _11/2 level of Er³⁺. In these systems, the interionic processes are dominant. In particular, the reported decrease of the YVO₄: Er laser slope efficiency when the Er³⁺ concentration increased from 0.5 to 1 at % is due mainly to the up-conversion by energy transfer from the pump level and upper laser level. Excited state absorption cannot contribute to depletion of excited states involved in the ³ F ₄-³ H ₆ laser operation near 1800 nm in the YVO₄: Tm crystal. However, the heavy doping required to enhance the cross-relaxation process which feeds the upper laser level brings about the migration-accelerated energy transfer to energy sinks.     

Excited-state absorption in Er: BaY2F8 and Cs3Er2Br9 and comparison with Er: LiYF4

The influence of Excited-State Absorption (ESA) on the green laser transition and the overlap of Ground-State Absorption (GSA) and ESA for 970 nm upconversion pumping in erbium is investigated in Er³⁺ : BaY₂F₈ and Cs₃Er₂Br₉. Results are compared to Er³⁺ : LiYF₄. In Er³⁺: BaY₂F₈, a good overlap between GSA and ESA is found at 969 nm in one polarization direction. The emission cross section at 550 nm is a factor of two smaller than in LiYF₄. In Cs₃Er₂Br₉, the smaller Stark splitting of the levels shifts the wavelengths of the green emission and ESA from⁴ I _{1 3/2} off resonance. It enhances, however, ground-state reabsorption. The emission cross section at 550 nm is comparable to LiYF₄. Upconversion leads to significant green fluorescence from² H _9/2. A significant population of the⁴ I _11/2 level and ESA at 970 nm are not present under 800 nm pumping.     

Luminescence of rare earth-doped Si–ZrO2 co-sputtered films

Er-doped Si-yttria-stabilized zirconia (YSZ) thin film samples were prepared by rf co-sputtering. Chemical composition of the samples was determined using energy-dispersive spectroscopy (EDS) and the structure of the films by X-ray diffraction (XRD). The samples were annealed to 700 °C. Photoluminescence (PL) measurements were performed for the visible and infrared. By exciting with the 488-nm-laser line the Er³⁺ emissions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and a narrow ⁴I_13/2→⁴I_15/2 emission were observed. The ⁴I_11/2→⁴I_15/2 emissions for the same excitation wavelength were weak. Excitation wavelength dependence of the ⁴I_13/2→⁴I_15/2 emissions indicated that the emissions were due to a combination of energy transfer from Si nanoparticles (np) to Er ions and energy transfer from defects in the matrix to the Er ions for excitations resonant with the energy levels of such defects. ⁴I_13/2→⁴I_15/2 emission decay measurements show two decaying populations of Er ions according to their locations with respect to other ions or any non-radiative defects. ⁴I_11/2→⁴I_15/2 emission dependence on ⁴I_13/2→⁴I_15/2 emission showed that the former was possibly due to a combination of downconversion from higher levels of the Er ions, energy transfer from Si nanoparticles and upconversion transfer processes. We concluded that Er-doped Si-YSZ is a promising material for photonic applications being easily broadband excited using low-pumping powers.     

Upconversion excitations in Pr<Superscript>3+</Superscript>-doped BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystal

We report the orange-to-blue and infrared-(IR)-to-blue wavelengths upconversion luminescence in Pr³⁺:BaY₂F₈ crystals. Mechanism of the orange light upconversion into blue ³P₀ state emission was confirmed to be energy transfer between two Pr³⁺ ions in the ¹D₂ state. IR-to-blue upconversion has only been observed under two different color IR pumping. The first resonant step was the ³H₄→¹G₄ ground state absorption transition, and the second resonant transition was the excited state absorption from the ¹G₄ to ¹I₆ and ³P_J levels. A comparison of the efficiency of the IR-to-blue upconversion in several praseodymium activated host is presented and discussed. A model of the IR pumped upconversion praseodymium blue laser is presented and the population inversion conditions are calculated.     

Afterglow phenomenon in Erbium and Titanium codoped GD2O2S phosphors

A series of new long-lasting phosphor Gd₂O₂S:xEr,Ti are prepared by the conventional high-temperature solid-state method and their luminescent properties are systematically investigated in this paper. The characteristic afterglow emission of Er, which is due to the transition of ⁴F_9/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2, is reported for the first time. XRD, photoluminescence, long-lasting phosphorescence and decay curves are used to characterize the synthesized phosphors. The possible energy transfer mechanism of Gd₂O₂S:xEr,Ti is also investigated.     

Up-conversion multiwave (White) luminescence in the visible spectral range under excitation by IR laser diodes in the active BaY2F8:Yb3+,Pr3+ medium

The possibilities of occupying high-lying 4f states of Pr³⁺ ions in the active BaY₂F₈:Yb³⁺,Pr³⁺ medium according to the photon avalanche and step-by-step sensitization mechanisms are compared. It is shown that the photon avalanche is unlikely to occur in the BaY₂F₈:Yb³⁺,Pr³⁺ crystal. The multiband luminescence spectra in the visible spectral range (white emission) under single- and multiwave pumping of BaY₂F₈:Yb³⁺,Pr³⁺ crystal by IR laser diodes are reported.     

Pulsed blue-light generation by the frequency doubling of the 4F3/2 to 4I9/2 transition in Nd:YAG and Nd:YAlO3

We report on the frequency doubling of Q-switchedNd:YAG and Nd:YAlO₃ lasers emitting at 946 and 930 nm, respectively (⁴F_3/2 to ⁴I_9/2 transition). The neodymium-doped laser host crystals were excited with a flashlamp-pumped Cr:LiSAF laser operating in a free-running mode. Blue-light pulses were obtained at both 473 nm (9 mJ, 25 ns FWHM) and 465 nm (4.4 mJ, 35 ns FWHM) by using a potassium niobate crystal as an extra-cavity frequency doubler. The second-harmonic generation conversion efficiencies reached 53% and 31%, respectively. Received: 23 June 1999 / Revised version: 8 August 1999 / Published online: 3 November 1999     

Luminescence study of the 4f5d configuration of Nd in LiYF4, LiLuF4 and BaY2F8 crystals

Ultraviolet fluorescence of Nd³⁺ ions induced by triphotonic excitation process was studied in Nd-doped LiYF₄, LiLuF₄ and BaY₂F₈ crystals using a technique of time-resolved spectroscopy. The observed ultraviolet luminescence was due to transitions between the bottom of 4f²5d configuration and 4f³ states of Nd³⁺ ions. Narrow emission lines superposed to the broadband emissions were observed. A detailed analysis of luminescence spectrum revealed that the narrow emissions are due to parity and spin allowed radiative transitions from the Stark levels of ⁴K_11/2(5d) state created by the electrostatic interaction between the 5d electron and the two electrons of the 4f² configuration. The narrow emissions are related to the high spin state (S=3/2) which gives f-f characteristics to the f-d broadband emissions. The narrow emissions superposed to the wide emission correspond to 18%, 34% and 43% of the integrated broadband emission at 262 nm observed in LiYF₄, LiLuF₄ and BaY₂F₈ crystals, respectively. Although the 5d-4f² interaction is observed to be weaker than 5d-crystal field interaction, it is stronger enough to select only the radiative transitions from 4f²5d configuration to 4f³ states that preserves the total spin S=3/2.     

SPECTROSCOPIC INVESTIGATION ON THE PROPERTIES OF VISIBLE AND NEAR INFRARED EMISSIONS IN Tm3+, Ho3+ Co-DOPED YVO4 CRYSTALS

With the help of absorption and fluorescence spectra, the spectroscopic properties of Tm³⁺, Ho³⁺:YVO₄ crystals have been investigated under different dopant concentrations. The absorption results show that the maximum absorption in the spectral range of currently available powerful laser diodes (LD) is at 796 nm, corresponding to the transition ³H₆→³H₄ of Tm³⁺ in the crystals. Upon the excitation with a LD at 808 nm, we have detected ten fluorescence bands in the spectral range spanning from visible to infrared. In visible region 4 upconversion bands have been observed centered at 475, 547, 660, and 701 nm. Discussions of the energy transfer processes suggest that cross relaxations and excited absorptions with the involvement of Tm^{3+ 3}F₄, ³H₅ and Ho^{3+ 7}I₅ states are responsible for the four upconversion emissions. Comprising the relative integrated intersities of 2 μm (Ho³⁺: ⁵I₇→⁵I₈) and 1.8 μm (Tm³⁺: ³F_4→³H₆) bands we have observed an efficient energy transfer from Tm³⁺ (³F₄) to Ho³⁺ (⁵I₇). Examination of the ratios of the intensity of 2 μm band to that of 1.8 μm band as a function of the ratios of Tm³⁺ concentration (c_Tm) to Ho³⁺ concentration (c_Ho) indicates that small concentration ratios (c_Tm/c_Ho) lead to high efficient energy transfers of Tm³⁺(³F₄)→Ho³⁺(⁵I₇).     

Spectroscopic analysis of Cr,Er:Gd3Ga5O12 crystals as candidates of potential xenon lamp pumped ~ 2.7 μm laser

A series of Cr,Er:Gd₃Ga₅O₁₂ crystals with high concentrations of Er^3 + were grown by Czochralski method. The absorption spectra, the up-conversion, near infrared (NIR) and mid-infrared (Mid-IR) luminescence spectra as well as the luminescence decay curves of Er: ⁴I_13/2 and ⁴I_11/2 levels were measured at room temperature. The spectroscopic properties of Cr,Er:Gd₃Ga₅O₁₂ crystals and Cr–Er energy transfer processes were investigated. The spectroscopy of the Er^3 +:⁴I_11/2 → ⁴I_13/2 transition was centralized to discuss, and the important optical parameters including luminescence lifetimes and the Cr–Er energy transfer efficiency are presented. Based on the comprehensive spectral analyses, 0.6 at.%Cr/50 at.%Er:GGG crystal is preferred as candidate of potential xenon lamp pumped ~ 2.7 μm laser in this work.