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Influence of Nd3+ concentration on up-conversion fluorescence colour in YVO4 co-doped with Ho3+, Yb3+ and Nd3+ ions
Abstract
Influence of Nd3+ concentration on up-conversion fluorescence colour in YVO4 triple doped with Ho3+, Yb3+ and Nd3+ ions excited by 980 nm laser diode was investigated. A serials of NdxHo0.02Yb0.05Y0.93−xVO4 (where x=0, 0.25, 0.5, 0.75 and 1 at%) crystalline materials were synthesised by a solid state reaction. Excitation in the 2F5/2 bands of Yb3+ produced a strong red and a weaker green emissions at 300 K for Ho3+,Yb3+:YVO4 whereas for Ho3+,Yb3+,Nd3+:YVO4 a strong green emission was observed. Concentration dependence studies indicated that Nd3+ concentrations had significant influences on up-conversion mechanisms and emission colour.
... In addition, the energy gap between 5 F 5 and 5 I 6 of Ho 3+ is close to the energy interval between 4 I 15/2 and 4 I 9/2 of Nd 3+ . Cross-relaxation process may occur between these two sets of energy levels, resulting in the decrease of the photon numbers residing at the red emission level of Ho 3+ [38,39]. Fig. 5(b) describes the UC luminescence processes of Ho 3+ under 808 nm excitation. ...
... Because of the eminent thermal, mechanical and optical characteristics, yttrium orthovanadate (YVO 4 ) is found as an attractive luminescent host material, which has been widely used as the host material for many phosphors [9]. So far, there are kinds of Ln 3+ ions doped YVO 4 powders been reported as eminent phosphors with various color emissions [10][11][12]. ...
Yb3+,Er3+,Eu3+ triply-doped in YVO4 with varying molar ratio of Er/Eu were synthesized in a sol–gel method with a subsequent heat treatment for the first time. The optimal molar ratio of Er/Eu for the maximum energy transfer efficiency was 1/39. After molar ratio, the influences of different heat treatment temperatures were also researched and the maximal heat temperature was 1300 °C. Besides, the properties of YVO4:Yb3+,Er3+,Eu3+ upconversion phosphors were investigated by X-ray diffraction, scanning electron microscopy and photo luminescent spectra, respectively. In sum, XRD results indicate the crystal structure of as-prepared samples, which is the tetragonal phase of YVO4 with no other diffraction peaks. SEM images show the morphology of as-prepared samples, which are granular-like nanoparticles. PL spectra demonstrate the upconversion luminescence of as-prepared samples, which emit strong green lights (at 525 nm, 550 nm) and slight red lights (at 590 nm, 615 nm, 695 nm) under the NIR irradiation at 980 nm. Two strong green emissions are attributed to the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+ ions. Meanwhile, three slight red emissions are attributed to the 5D0→7F1, 5D0→7F2 and 5D0→7F4 transitions of Eu3+ ions. All in all, the colorful emissions endow YVO4:Yb3+,Er3+,Eu3+ phosphors great potential for some applications, such as display devices, bio-labeling and infrared detection.
... The energy diagram of Yb 3+ /Ho 3+ in La 2 O 2 S has rarely been reported and is not available to us for comparison. Nonetheless, UC luminescence involving three phonons was seen from the previous work on Yb 3+ /Ho 3+ -codoped other material systems [42,43]. Therefore, the energy diagram and UC process of (La 0.97 Ho 0.01 Yb 0.02 ) 2 O 2 S were constructed in Fig. 5 by referring to these previous studies and are detailed below: (1) excitation of Yb 3+ by laser photons [ESA; 2 F 7/2 (Yb 3+ ) + hν (978 nm) → 2 F 5/2 (Yb 3+ )]; (2) population of the 5 I 6 energy level of Ho 3+ after Yb 3+ absorbing the first laser photon and transferring energy to Ho 3+ [ET1; 2 F 5/2 (Yb 3+ ) + 5 I 8 (Ho 3+ ) → 2 F 7/2 (Yb 3+ ) + 5 I 6 (Ho 3+ )]; ...
Phase-pure (La0.97RE0.01Yb0.02)2O2S upconversion (UC) nanophosphors (average crystallite size ~ 45 nm; RE=Ho, Er) were annealed from their hydrothermally crystallized layered hydroxyl sulfate precursors in flowing hydrogen at 1200 °C for 1 h, with water vapor as the only exhaust. Under 978-nm laser excitation (up to 2.0 W), the Ho³⁺-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~ 546 (⁵F4 → ⁵I8), 658 (⁵F7 → ⁵I8), and 763 nm (⁵F4 → ⁵I7), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400–700 nm). The Er³⁺-doped UC phosphor, on the other hand, showed weak green (~ 527/549 nm, ²H11/2,⁴S3/2 → ⁴I15/2), weak red (~668/672 nm, ⁴F9/2 → ⁴I15/2), and strong near-infrared (~ 807/58 nm, ⁴I9/2 → ⁴I15/2) luminescence, whose emission color in the visible region drifted from yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] with increasing excitation power. Analysis of the power-dependent UC luminescence found three- and two-photon processes for RE=Ho and Er, respectively, and the possible UC mechanisms were proposed.
Electronic supplementary material
The online version of this article (doi:10.1186/s11671-017-2277-4) contains supplementary material, which is available to authorized users.
... Lanthanide ions are desirable, because of the unique characteristics of their intraconfigurational 4f-4f electronic transitions, sharp emission lines, longlifetimes and high quantum yields [4,5]. The inorganic solids doped with trivalent lanthanide ions (Er 3+ , Tm 3+ , Ho 3+ , Pr 3+ , Tb 3+ , Eu 3+ ) gives the mechanism of anti-Stokes luminescence and have been studied extensively by the researchers [6][7][8][9]. In general, four different mechanisms are involved in the UC process: (a) excited-state absorption (ESA), (b) energy transfer upconversion (ETU), (c) co-operative energy transfer (CET) and (d) photon avalanche (PA). ...
... Nd 3+ ions are recognized as one of the best RE for developing solid state lasers in amorphous and crystalline state [18]. Few studies report the correlation of Nd 3+ ions with up-conversion properties [18][19][20]. ...
... Since the absorption bands at around 800 nm, which correspond to the 4 I 9/2 ? 4 F 5/2 transition of the Nd 3+ ion are broad, an efficient pumping is possible. Studies of concentration dependence of luminescence indicated that Nd 3+ concentration had significant influences on up-conversion mechanisms and emission colour in YVO 4 triply doped with Ho 3+ , Yb 3+ and Nd 3+ ions [10]. For Nd 3+ : KZnLa(VO 4 ) 2 crystal a strong near infrared emission was observed under 808 or 354.5 nm excitation to the 4 F 5/2 level or CT bands, respectively [11]. ...
Single crystals of α-Nd3+: Na3Y(VO4)2 have been successfully grown by the flux growth method and their optical properties were investigated. The α-Na3Y(VO4)2 host crystal belongs to the monoclinic system with the space group P21/n (No. 14). Infrared and Raman spectra were recorded and the assignment of the stretching and bending vibrations of VO43- tetrahedra were performed. IR and R spectra were applied in order to assign the vibronic components of the electronic transitions of the Nd3+ ion. The absorption (4.2 and 300 K) and fluorescence spectra (77 and 300 K) as well as the fluorescence dynamics of the Nd3+-doped title crystals are presented and analyzed in detail. The 300 K absorption spectra have been analyzed using the Judd–Ofelt approach. The Ωλ (λ = 2, 4, 6) intensity parameters have been evaluated and subsequently used to calculate the spontaneous fluorescence probabilities, the branching ratios and the radiative lifetime for the 4F3/2 emitting level of Nd3+. The fluorescence branching ratio for the 4F3/2 → 4I11/2 laser transition is 47.3%. The maximum emission cross-section has been calculated to be 17.81 × 10−20 cm2 at 1064.3 nm using the Füchtbauer–Landenburg equation. The spectroscopic properties show that the α-Nd3+: Na3Y(VO4)2 crystal is a good candidate for a solid-state laser.
... Trivalent neodymium has been recognized as one of the most efficient lanthanides for solid state lasers in different crystalline and amorphous materials, due to its intense emissions at about 1064 and 910 nm [1][2][3][4][5][6]. The studies of the concentration dependence of luminescence indicated that Nd 3 þ ions concentration possess significant influences on the up-conversion mechanisms and fluorescence color in triply doped with Ho 3 þ , Yb 3 þ and Nd 3 þ ions oxides [7]. ...
Telluride glasses of the composition xNd2O3–(7−x)La2O3–3Na2O–25ZnO–65TeO2, where (0≤x≤7) were prepared by the melt quench technique. Some physical and optical properties of the glasses were evaluated. The thermal behavior i.e. glass transition and crystallization temperatures were studied by using TGA–DTA technique. Optical properties of Nd3+-doped telluride glasses were investigated between 298 and 700 K. Basing on the obtained values of J–O parameter values (×10−20 cm2: Ω2=4.49±0.84, Ω4=5.03±0.61, Ω6=4.31±0.73), the radiative transition probabilities (AT), radiative lifetimes (τR), fluorescence branching ratios (β) and emission cross-sections (σem) were calculated for the 4F3/2→4IJ/2 (where J=9, 11 and 13) transitions of Nd3+ ions. The τR value of the 4F3/2 level amount to 164 μs and is slightly higher than the measured decay time of 162 μs. With the increasing of Nd2O3 concentration from 0.5 to 7.0 mol% the experimental lifetime of the fluorescent level decreases from 162 to 5.6 μs. The estimated quantum efficiency amount to 100%, based on a comparison of τR and the experimental decay time of a slightly doped Nd3+ telluride glass. An analysis of the non-radiative decay was based on the cross-relaxation mechanisms. The 4F3/2→4I9/2 and 4F5/2→4I9/2 transitions were analyzed with respect to the fluorescence intensity ratio (FIR) and were found to be temperature dependent. Infrared-to-visible up-conversion emissions with a maximum at 603.0 and 635.3 nm were observed at high temperatures using the 804 nm excitation and are due to the 4G5/2→4I9/2 and 4G5/2→4I11/2 transitions of Nd3+ ions, respectively. The near quadratic dependence of fluorescence on excitation laser power confirms that two photons contribute to up-conversion of the orange emissions. The temperature-stimulated up-conversion excitation processes have been analyzed in detail. The optical results indicate that the investigated glasses are potentially applicable as a 1063 nm laser host as well as an optical sensor for temperature measurements.
... Europium doped vanadates have been used as phosphors for X-ray intensifying screen and X-ray computed tomography [4]. Nd 3+ -doped YVO 4 single crystals are very efficient commercial laser materials operating at 1064 nm and pumped by semiconductor laser at 808 nm due to a broad and strong Nd 3+ absorption bands [5][6][7]. Neodymium and erbium ions-doped vanadates(V) shows a strong emission under UV excitation due to the efficient energy transfer from the CT state to the 4f 3 (Nd 3+ ) or 4f 11 (Er 3+ ) electronic states [8,9]. Relatively strong fluorescence at 1064 nm from Nd 3+ ions was observed also in Nd 3+ :KZnLa(VO 4 ) 2 under 808 or 354.5 nm excitation to the 4 F 5/2 or CT bands, respectively [9]. ...
Materials that generate pure, single-color emission are desirable in the development and manufacturing of modern optoelectronic devices. This work shows the possibility of generating pure, green up-conversion luminescence upon the excitation of Er3+-doped nanomaterials with a 785 nm NIR laser. The up-converting inorganic nanoluminophores YVO4: Er3+ and YVO4: Yb3+ and Er3+ were obtained using a hydrothermal method and subsequent calcination. The synthesized vanadate nanomaterials had a tetragonal structure and crystallized in the form of nearly spherical nanoparticles. Up-conversion emission spectra of the nanomaterials were measured using laser light sources with λex = 785 and 975 nm. Importantly, under the influence of the mentioned laser irradiation, the as-prepared samples exhibited bright green up-conversion luminescence that was visible to the naked eye. Depending on the dopant ions used and the selected excitation wavelengths, two (green) or three (green and red) bands originating from erbium ions appeared in the emission spectra. In this way, by changing the UC mechanisms, pure green luminescence of the material can be obtained. The proposed strategy, in combination with various single-doped UC nanomaterials activated with Er3+, might be beneficial for modern optoelectronics, such as light-emitting diodes with a rich color gamut for back-light display applications.
LaNbO4: Nd³⁺/Yb³⁺/Ho³⁺ phosphors with various concentrations of Nd³⁺, Yb³⁺ and Ho³⁺ were prepared by a traditional high temperature solid-state reaction method. The crystal structure of the as-prepared samples was characterized by X-ray diffraction (XRD) and the results showed that monoclinic LaNbO4 samples were obtained. Green and red up-conversion (UC) emissions of Ho³⁺ were observed under the excitations of 808 nm and 980 nm. Concentration dependent studies indicated that Nd³⁺ and Yb³⁺ played different roles in the UC emissions of Ho³⁺ under different wavelength excitations. The same optimal concentrations of Yb³⁺ and Ho³⁺ ions were obtained when excited by 808 nm and 980 nm lasers. The UC luminescence mechanisms under different excitation sources were discussed by detecting the UC luminescence intensity related to the laser pumped power density. The energy transfer efficiencies from Nd³⁺ to Yb³⁺ and Yb³⁺ to Ho³⁺ were confirmed via lifetime measurement. By comparing with the commercial green NaYF4: Er³⁺/Yb³⁺ UC luminescence phosphor, the UC luminescence performance of Ho³⁺ doped sample was evaluated. The results showed that LaNbO4: Nd³⁺/Yb³⁺/Ho³⁺ phosphors had excellent green UC luminescence properties under both 808 nm and 980 nm excitations.
YVO4:Yb³⁺,Er³⁺; YVO4:Yb³⁺,Tm³⁺; and YVO4:Yb³⁺,Er³⁺,Tm³⁺ were all synthesized via sol-gel method with a subsequent thermal treatment. Specifically, YVO4:Yb³⁺,Er³⁺,Tm³⁺ phosphors were prepared with different annealing temperatures to study the influence of temperature. The transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffractometer (XRD), and photoluminescent (PL) spectrofluorometer were used to investigate the morphology, crystal structure, and up-conversion luminescent properties of all samples. In summary, all samples were granular-like nanoparticles and well crystallized with the same tetragonal phase as YVO4. Under the irradiation at 980 nm, YVO4:Yb³⁺,Er³⁺ phosphors can generate green emission at 525 and 553 nm and red emission at 657 nm, while YVO4:Yb³⁺,Tm³⁺ phosphors can generate blue emission at 476 nm, red emission at 648 nm, and near-infrared emission at 800 nm. Notably, YVO4:Yb³⁺,Er³⁺,Tm³⁺ samples can exhibit green emission, blue emission, red emission, and near-infrared emission at the same time, which might endow the as-prepared samples with potential applications in many fields, such as luminous paint, infrared detection, and biological label.
A detailed analysis of the luminescence behaviour of Yb³⁺-doped and Yb³⁺-Tb³⁺ co-doped strontium aluminates powders: Sr1-xYbxAl4O7 (x=0.001–0.07) and Sr1-x-yYbxTbyAl4O7 (x=0.03; y=0.02–0.2) were performed. The studies of singly doped samples show that direct excitation of Yb³⁺ by means of ²F7/2-²F5/2 absorption at 900–980 nm leads to Stokes Yb³⁺ emission in the range of 970–1130 nm as well as bluish-green Yb³⁺ cooperative luminescence (CL) whose energy doubles that of the NIR one. The effect of activator concentration and charge compensation through Na⁺ co-doping on both Yb³⁺ emissions were also studied. It was found that Na⁺ addition enhanced Stokes Yb³⁺ photoluminescence brightness, while the cooperative emission intensity appeared to be lower. In doubly Yb³⁺-Tb³⁺ doped materials excitation at 980 nm led to cooperative sensitization of the Tb3+ 5D4 level giving rise to its green ⁵D4→⁷FJ (J=⁷F6-⁷F3) up-conversion luminescence with the dominant component around 542 nm. The cooperative energy transfer (CET) mechanism was proposed basing on the results obtained from emission and absorption spectra, decay kinetics as well as the dependence of UC luminescence intensity on NIR excitation power.
Phosphor powders of CaZr4O9:Er³⁺ and CaZr4O9:Er³⁺,Yb³⁺ were prepared by the urea combustion route. The crystal structure was characterized by powder XRD measurement. The optical properties were studied using absorption and photoluminescence (PL) spectroscopy. Upon excitation at 978 nm, the upconversion (UC) luminescence properties of Er³⁺ and Er³⁺,Yb³⁺ co-doped CaZr4O9 phosphors were analyzed. Energy transfer mechanism was discussed to understand the UC luminescence process for the Er³⁺ and Er³⁺,Yb³⁺ co-doped CaZr4O9 phosphors. From the pump power dependent green and red luminescence intensity analysis, required number of photons to populate the excited emitting levels are estimated and observed that the green emission intensity exhibits faster increase rates than red emission as a function of pump power. Upon excitation at 978 nm, the UC mechanism was discussed in terms of the experimental results.
A neodymium-doped silica microsphere (NDSM) laser is described in this work, which was a pure silica sphere coated with Nd3+ doped sol-gel film and showed a rather simple production procedure, good characteristics, low cost and flexibility. To create the NDSM lasers, the neodymium-doped sol-gel film was applied to the surface of a silica microsphere, an optical tapered fiber (OTF) which was used to couple an 808 nm continuous-wave diode laser into the inner surface of the NDSM, served as a pump light source. Multi-mode and single-mode laser oscillations around 1100 nm were observed due to the 4F3/2-4I11/2 transitions of Nd3+ ions. Highly confined whispering gallery modes of pump laser made an ultralow threshold microlaser, we could observe the laser oscillations when the pump power was as low as 15.7 μW. The multi-mode laser oscillation spectrum depends on the microsphere morphology characteristics. The free spectral range (FSR) was in good agreement with the calculated value which was based on the Mie scattering theory.
YVO4: Yb3+, Ln3+ (Ln = Er, Tm, Ho) microrods were synthesized via a simple two-step hydrothermal method followed by a subsequent heat treatment process for the first time. The structures, morphologies and up-conversion luminescent properties of the samples were investigated. All samples are well crystallized to a pure tetragonal phase of YVO4. The as-synthesized microrods are composed of self-assembled granular-like nanoparticles. Under 973 nm laser diode excitation, intense and nearly monochromatic green, blue, and red up-conversion emissions were observed for the Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ co-doped YVO4 microrods, respectively, which might find potential applications in fields such as phosphors, infrared detection and display devices.
The synthesis and upconversion properties of trigonal glaserite-type K3Y(VO4)2 co-doped with Er3+/Yb3+, Ho3+/Yb3+, or Tm3+/Yb3+ were studied. Powder samples were synthesized by solid state reactions at 1000 °C for 48 h, while well-formed hexagonal single crystals of the same were grown hydrothermally using 10 M K2CO3 at 560-650 °C. Infrared-to-visible upconversion by Er3+/Yb3+, Ho3+/Yb3+, or Tm3+/Yb3+ codoped-K3Y(VO4)2 glaserite powder and single crystals was observed, and the upconversion spectral properties were studied as a function of different Er3+, Tm3+, Ho3+, and Yb3+ ion concentrations. The process is observed under 980 nm laser diode excitation and leads to strong green (552 nm) and red (659 nm) emission for Er3+/Yb3+, green (549 nm) and red (664 nm) emission for Ho3+/Yb3+, and blue (475 nm) and red (647 nm) emission for Tm3+/Yb3+. The main mechanism that allows for up-conversion is attributed the energy transfer among Yb3+ and the various Er3+/Ho3+/Tm3+ ions in excited states. These results illustrate the large potential of co-doped alkali double vanadates for photonic applications involving optoelectronics devices.
Tb3+ and Yb3+ co-doped calcium aluminate powders: Ca1−x−yYbxTbyAl4O7 (x = 0.02–0.06; y = 0.02), Ca1−x−yYbxTbyAl4O7 (x = 0.03, y = 0.01–0.07), Ca1−x−yYbxTbyAl4O7 (x = 0.01–0.05, y = 0.01) were synthesized by a modified Pechini citrate process and their optical properties were investigated. Upon excitation at 245 nm all studied samples yielded relatively strong Tb3+ luminescence corresponding to the 5D4 → 7FJ and 5D3 → 7FJ transitions with the dominant component around 542 nm. The direct excitation of two Yb3+ ions at 980 nm led to cooperative sensitization of the Tb3+ 5D4 level giving rise to its green up-conversion luminescence. The up-conversion emission was also observed from higher 5D3 level of Tb3+, which is rather rare in high phonon frequency host materials. The effect of activator concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied. It was found that Na+ addition lowers disturbance/distribution of activator ions local symmetries and enhanced Stokes Tb3+ photoluminescence brightness, while the up-conversion emission intensity does not appear to change noticeably then.
Calcium aluminate powders co-doped with Yb3+ and Eu3+: Ca1−x−yYbxEuyAl4O7 (x = 0.02–0.06; y = 0.02), Ca1−x−yYbxEuyAl4O7 (x = 0.05, y = 0.01–0.07) were synthesized by a modified Pechini citrate process and their optical properties at 298 K and 77 K were investigated. Direct excitation of Yb3+ by means of 2F7/2 → 2F5/2 absorption at 980 nm leads to bluish-green luminescence centered around 488 nm due to the cooperative emission of Yb3+ and red emission of Eu3+ corresponding to 5D0 → 7FJ (J = 0–4) transitions. The near-infrared to red up-conversion luminescence of Eu3+ was assigned to a cooperative sensitization mechanism. The effect of activators concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied in detail. Na+ addition led to narrowing of luminescence bands, suggesting smaller disturbance/distribution of activator ions local symmetries. This was associated with significantly enhanced luminescence brightness of Eu3+ under UV excitation and, surprisingly, with noticeably reduced efficiency of Eu UC emission upon near-infrared excitation. It was also concluded that Yb3+ pairs transfer preferentially their excitation energy to Eu3+ ions with the local symmetry perturbed by certain defects present in the crystal lattice.
The processes of excitation, nonradiative energy transfer, and relaxation of excited states relevant to the laser performance of YVO4:Yb, Ho and YVO4:Yb, Tm crystals are investigated. The main shortcoming of holmium-doped laser materials is a lack of pump bands corresponding to the emission wavelengths of powerful diode lasers. The incorporation of ytterbium ions into the YVO4:Ho crystal provides an efficient means of population buildup on the /I7 level via a Yb 3+-Ho3+ energy transfer. This transfer process has to be phonon-assisted; nevertheless, its efficiency is quite high owing to the extended spectrum of lattice vibrations of the YVO4 matrix. The YVO4:Tm system is better suited for laser diode pumping thanks to a relatively intense pump band associated with the 3H6- 3H4 transition around 800 nm. In the limit of a low Tin3+ concentration, the quantum efficiency of the 3H 4 level is sufficiently high for the consideration of laser operation associated with the 3H4-3F4 transition. At a higher thulium concentration, the 3F4 excitation is transferred to the 3F4 level by a cross relaxation process and the laser operation associated with the 3F4-3H6 transition near 1800 nm is feasible, but the desired efficiency of this process sets a lower limit on the thulium concentration. In the YVO4:Yb, Tm system, an alternative means of optical pumping may be used, consisting of excitation of ytterbium ions followed by Vb3+-Tm3+ energy transfer. In the assessment of the efficiency of energy transfer from ytterbium ions to the upper laser level 5I7 (Ho3+) or 3F4 (Tm3+), additional active losses associated with up-conversion phenomena should be accounted for, however.
Results of a systematic spectroscopic study of YVO4 crystals singly doped with Pr, Ho, Er, Tm and the YVO4: Yb + Ho, YVO 4: Yb + Er, YVO4: Yb + Tm systems are summarized. Anomalous quenching of the 3P0 and 1D 2 emissions of Pr, the 5S2 emission of Ho, and the 4S3/2 emission of Er has been revealed. Upon continuous wave excitation at around 980 nm, all doubly doped systems mentioned above exhibited intense up-converted emission. Up-converted green emission in YVO4:Yb + Er and blue emission in YVO4:Yb + Tm are qualitatively similar to those observed in other matrices containing ytterbium and erbium or thulium. The up-conversion process in YVO4: Yb + Ho is unusual in that it brings about an intense red emission centered at 650 nm instead of the green emission encountered in other matrices containing ytterbium and holmium. Based on data obtained using short pulse excitation, it has been concluded that the mechanism of excitation of the red emission involves Yb3+-Ho3+ energy transfer followed by excited-state absorption from the long lived 5I7 state of Ho3+. Singlecrystals of undoped YVO4 exhibited a broadband emission centered at 450 nm when excited at any wave-length between 620 and 440 nm. Both the emission wavelength and kinetics are consistent with transitions of the vanadate group, but the mechanism of up conversion is not clear. In doped YVO4, the excitation of the vanadate group is transferred efficiently to rare earth ions.
Upconversion by Yb3+ sensitization in Er3+-doped
YVO4 crystals has been studied under excitation with a 976 nm
laser diode. In addition to the previously observed upconverted 526 and
554 nm Er3+ emission bands, three weak upconverted
Er3+ emission bands are observed at 490, 660, and 670 nm at
room temperature. From the quadratic pump-power dependence, it is
confirmed that the upconversion is induced by a two-photon excitation
process involving excited-state absorption from the
4I11/2 state of Er3+. In addition to
these emission bands, downconverted Yb3+ and Er3+
emission bands are simultaneously observed at 1010 and 1470-1630 nm,
respectively. It is observed that the infrared Er3+ emission
intensity increases linearly with pump power, while the Yb3+
emission intensity saturates at higher pump powers.
In an attempt to find a neodymium–vanadate system with long lifetime of 4F3/2 level and relatively strong 4F3/2→4I11/2 emission for laser applications, the optical properties of Nd3+ in a new KZnLa(VO4)2 host is reported. The crystalline samples were obtained at 900 °C in air. The samples were crystallized in monoclinic system and were isostructural with KZnLa(PO4)2. KZnLa0.99Nd0.01(VO4)2 strongly emits in the near infrared range with the maxima at 871.6 and 1057 nm upon excitation through the 4F5/2 level (808 nm) or by the charge transfer bands of VO43−. The lifetime of 4F3/2 level of Nd3+ ion is larger than that observed in other neodymium–vanadates systems.
Er3+-doped KCaY(VO4)2 microcrystalline samples were synthesized using a high temperature solid-state reaction technique. Spectroscopic properties of Er3+: KCaY(VO4)2 are studied and the nature of emissions is discussed. A strong green and infrared luminescence were observed under excitation at 314 nm in the O2−→V5+ charge-transfer transitions and direct excitation of Er3+ ions at 435 nm. A strong emission lines in the blue region are due to the transitions of VO43− ions have been observed at 77 K. The Judd–Ofelt parametrization scheme has been applied to the analysis of the room temperature absorption spectra in order to evaluate the intensity parameters, the branching ratios and the radiative lifetimes of the 4I13/2, 4I11/2, 4F9/2 and 4S3/2 emitting levels. The effective cross-section has been calculated for the 4I13/2→4I15/2 transition, indicating that the title compounds is a promising active medium for application in the three-level laser system. The up-conversion emission in Er3+: KCaY(VO4)2 was investigated at 300 K. The decay profiles of the Stokes and anti-Stokes emissions were measured and the mechanism of up-conversion luminescence is discussed.
The fluorescence lifetime of the 4F3/2 state of Nd3+ in YVO4 crystals has been determined as a function of temperature and concentration. Over the temperature range 90-300 °K the lifetime is approximately 100 musec and decreases to 87 musec at 453 °K. The lifetime at 300 °K is found to depend on the Nd3+ concentration decreasing from 100 musec at 0.4 at.% to 60 musec at 1.6 at.%.
The upconversion optical characteristic and brightness of Y2O2S:Yb,Ho phosphors are investigated. It is shown that Y2O2S:Yb,Ho exhibits NIR, red, green, blue and even ultraviolet-blue emission bands under 980 nm pumping. Moreover, Y2O2S:Yb,Ho shows excellent upconversion luminescence and its upconversion brightness is 2.2 times to that of the commercial Y2O2S:Yb,Er phosphors. The upconversion mechanism is discussed: the green and NIR emissions are due to two consecutive energy transfer from Yb3+ to Ho3+, and the blue and red emissions can be explained as a loop-like mechanism.
Investigation of excitation and decay of Er3+in YVO4 single crystals has been carried out at temperature range from 5 to 500 K. In the limit of low Er3+ content, the temperature dependence of the 4S3/2 decay is not consistent with predictions of multiphonon relaxation model. In heavily doped crystals the 4S3/2 decay is governed by cross relaxation process, whose rate grows steadily with increasing temperature. An efficient upconverted luminescence near 550 nm following a cw and short pulse (5 ns) excitation of the 4I9/2 level in the system under study has been recorded and analysed. Contribution of the excited state absorption to these phenomena has been considered.
Conversion of the infrared (IR) radiation at 975 nm into the visible emission in the Czochralski grown YVO4 crystals containing several different concentrations of Yb3+ and Ho3+ has been investigated. Unlike other matrices doped with ytterbium and holmium, the YVO4:Yb,Ho system exhibits intense red upconverted emission originating from the 5F5 level, whereas, green upconverted emission originating from the 5S2 level of Ho3+ is found to be unusually weak at room temperature but dominating at 4.2 K. Power dependencies of the emissions are consistent with two-step excitation process but time dependencies differ and imply that the longlived 5I7 level of Ho3+ is involved in the second-step excitation of red emission. Intensity distribution of σ-polarized emission band associated with the 5F5-5I8 transition with peak value of effective stimulated emission cross section amounting to 10−20 cm2 combined with the possibility of tailoring of optical pumping efficiency by appropriate choice of dopant concentrations offer a potential of IR-pumped visible laser emission.
The following is a brief summary of the results of this investigation: (1) The orthovanadates of cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutecium, yttrium and scandium possess the zircon structure. (2) The oxygen parameters for fourteen of these orthovanadates are x = 0.19 ± 0.01 and z = 0.35 ± 0.01. For scandium orthovanadate the parameters are x = 0.20 ± 0.01 and z = 0.32 ± 0.01. (3) The oxygen tetrahedra in scandium orthovanadate are elongated in the direction of the c-axis. In the other orthovanadates of this series the oxygen tetrahedra are almost regular.
Upon continuous wave excitation around 1 μm, a YVO 4 crystal codoped with ytterbium and holmium exhibits intense red emission originating in the 5F5 level and considerably weaker green emission originating in the 5S2 level of Ho 3+ . The ratio of the red to green emission intensities is 17:1 at 300 K. The dependence of the intensity of both emissions on the pump power is nearly the same but the mechanisms determined on the basis of short pulse excitation are found to be different. It is concluded that the green emission is excited by two consecutive energy transfers from Yb 3+ to Ho 3+ , whereas excited state absorption is involved in the excitation of red emission. © 2001 American Institute of Physics.
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