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This paper reports the effect of incorporation of Yb 3+ ions on the frequency downconversion luminescence and thermal properties of triply ionised Ho 3+ doped zinc tellurite (TZ) glasses. The photoluminescence spectra of both the Ho 3+ /Yb 3+ doped and codoped glasses have been recorded and observed a green emission band corresponding to the 5 F 4 , 5 S 2 → 5 I 8 (∼550 nm) transition upon various excitations. In the downconversion (DC) emission process, the back energy transfer (BET) mechanism from Ho 3+ ions to Yb 3+ ions has also been explored. The colour emitted in the downconversion process is found to be non-tunable at different excitations. Thus, the Ho 3+ :TZ glass can be utilised for non-colour tunable optical devices under various UV excitations. Also the glass transition (T g ) and crystallisation (T c ) temperatures have been measured for both the doped and codoped glasses and found to be increased in the codoped glass. The singly Ho 3+ ions doped TZ glass shows better optical downconversion and glass forming ability., (© 2024 IOP Publishing Ltd.)

A sequence of coactivated divalent-metal tungstate Er 3+ /Yb 3+ /Mn 4+ : MgWO 4 phosphors have been successfully developed to study the effect of synthesis temperature on the crystal structure, surface morphology, fluorescence, temperature sensing and the dynamics involved in the processes. Upconversion (UC) intensity of the Er 3+ /Yb 3+ : MgWO 4 phosphors increased by ∼109 and ∼778 times on increasing the synthesis temperature from 800 °C to 1000 °C and 1200 °C. UC intensity of the Er 3+ /Yb 3+ /Mn 4+ : MgWO 4 phosphors has been significantly improved up to ∼90 times via charge compensation. The incorporation of Mn 4+ in the Er 3+ /Yb 3+ codoped crystal system shifted the UC spectra from sharp green peaks to broadband emission along with amended sensing abilities. The ratiometric techniques of thermally coupled stark sublevels of the Er 3+ have been used to achieve a wide temperature range (300-623 K). The prepared nanophosphors show maximum absolute & relative sensitivities ∼25.86 × 10 -3 K -1 @453 K and ∼10.39 × 10 -3 K -1 @303 K respectively with an accuracy of ±0.42 K@303 K., (© 2022 IOP Publishing Ltd.)

The ability of rare-earth-doped ferroelectric oxides to achieve outstanding upconversion (UC) performances under NIR irradiation despite possessing intrinsic electric properties drives researchers all over the globe to work in this field. The structural and spectroscopic characteristics of the Bi 4 Ti 3 O 12 phosphor integrated with Er 3+ , Yb 3+ , and Al 3+ have been thoroughly investigated in this study. The considerable increase in UC emission ∼three times caused by the addition of Al 3+ ions has been observed and discussed. The processes connected with the UC emission related to the pump power variation have been realized using the rate law equation. Aside from having high sensitivity of 0.011 K -1 at room temperature, the prepared phosphor possesses excellent thermal stability, i.e., it retains ∼73% of its initial intensity with the addition of Al 3+ ions., (© 2022 IOP Publishing Ltd.)

Photoluminescence study of the Ho 3+ -Yb 3+ codoped ZrO, AlZrO and YZrO nanophosphors, synthesized by chemical co-precipitation method, upon excitation at 450 and 980 nm radiations have been performed. An improvement of about ∼4.5 times in the downconversion emission intensity of green band corresponding to the 5 F 4 , 5 S 2  →  5 I 8 transition for codoped YZrO nanophosphors compared to ZrO codoped nanophosphors has been observed. On varying the pump power density upon 980 nm excitation the colour tunability in the codoped YZrO nanophosphors has been observed. UC emission intensity of the green band arising from the Ho 3+ ion in the codoped YZrO nanophosphors is enhanced about ∼22 times compared to that of the codoped ZrO nanophosphors. The absorption and UC emission study for the codoped YZrO nanophosphors dispersed in different biologically compatible solvents viz. water, methanol, ethanol and dimethyl sulfoxide (DMSO) has been performed. The green UC emission intensity of about ∼1.3 and ∼1.7 times for the efficient codoped YZrO nanophosphors dispersed in methanol compared to that dispersed in water and DMSO respectively has been observed. The absorption spectra of an efficient upconverting YZrO nanophosphors dispersed in methanol exhibit no change with the passage of time.

The spectroscopic investigations on the Er 3+ /Yb 3+ ions doped/codoped TeO 2 -ZnO (TZ), TeO 2 -ZnO-WO 3 (TZW) and TeO 2 -ZnO-WO 3 -TiO 2 (TZWTi) heavy metal oxide (HMO) glasses have been made. The absorption, photoluminescence, decay curve and Judd-Ofelt analysis have been performed to optimise the optical properties of the Er 3+ /Yb 3+ ions. The effect of incorporation of HMOs like WO 3 and TiO 2 in the Er 3+ /Yb 3+ doped/codoped TZ glass on its optical properties have been investigated. The enhancement in upconversion emission intensity has been explained on the basis of efficient energy transfer and inhomogeneous local field generation around the rare earth ions. The spectroscopic quality factor, absorption and stimulated emission cross-sections, optical gain, quantum efficiency (∼17.53%), energy transfer efficiency (∼61.64%), colour purity (∼94.7%) and ionic nature of the bonding have been determined. The Er 3+ -Yb 3+ -TZWTi glass can be used in visible lasers, yellowish green optical devices and home appliances.

Tm 3+ /Yb 3+ /Zn 2+ :yttrium metavanadate (YVO 4 ) phosphors prepared through chemical coprecipitation and the solid state reaction method have been structurally characterized by an x-ray diffraction (XRD) study. Photoluminescence study of the developed phosphors under ultraviolet (UV) and near infrared (NIR) excitation has been performed. The excitation spectrum of the tetragonal zircon type YVO 4 phosphors corresponding to the emission at ∼476 nm exhibits a broad excitation peak in the 250-350 nm region, which is due to charge distribution in the [Formula: see text] group. Under 980 nm CW diode laser excitation, enhancements of about ∼3000 times and ∼40 times have been observed for the blue band in the tridoped Tm 3+ Yb 3+ Zn 2+ :YVO 4 phosphors compared to those of the Tm 3+ :YVO 4 singly and Tm 3+ /Yb 3+ :YVO 4 codoped phosphors, respectively. A downconversion (DC) emission study shows an enhancement of about ∼50 times for the blue band in the tridoped phosphors compared to that of the singly doped phosphors. Optical bistability (OB) behavior of the developed phosphors has been also investigated upon 980 nm excitation. The calculated Commission Internationale de l'Éclairage (CIE) color coordinates lie in the blue region with 96.5% color purity under 980 nm excitation, having a color temperature of ∼3400 K. Our observations show that the developed phosphors may be suitably used in dual mode luminescence spectroscopy, display devices, and UV LED chips.

Ho 3+ /Yb 3+ /Tm 3+ /Li + :Gd 2 (MoO 4 ) 3 nanophosphors successfully synthesised via solid state reaction method have been structurally and optically characterised. Under 980 nm diode laser excitation the nanophosphors emit intense blue, green, red and NIR emissions peaking at ∼476 nm, ∼543 nm, ∼646 nm and ∼798 nm corresponding to the 1 G 4  →  3 H 6 (Tm 3+ ), 5 F 4 , 5 S 2  →  5 I 8 (Ho 3+ ), 5 F 5  →  5 I 8 (Ho 3+ ) and 3 H 4  →  3 H 6 (Tm 3+ ) transitions respectively. The upconversion emission intensity enhancement in the Ho 3+ -Yb 3+ -Tm 3+ -Li + :Gd 2 (MoO 4 ) 3 nanophosphors for the green band is found to be ∼367, ∼50 and ∼9 times compared to the singly Ho 3+ doped, Ho 3+ -Yb 3+ co-doped and Ho 3+ -Yb 3+ -Tm 3+ tri-doped Gd 2 (MoO 4 ) 3 nanophosphors. The enhancement observed has been explained on the basis of energy transfer process and local field modifications around the rare earth ions. The energy transfer efficiency ∼5% is determined in the tridoped nanophosphors. The interaction involved between rare earth ions for energy transfer process is found to be dipole-dipole type. On changing the Tm 3+ ions concentration the colour emitted from the tridoped nanophosphors is tuned from near white to blue region. In the tridoped nanophosphors, on varying the pump power the colour tunability has been observed.

TeO 2 -Pb 3 O 4 (TPO) glasses codoped with Nd 3+ and Yb 3+ ions have been fabricated by conventional melting technique. The absorption, emission and excitation spectra of the samples have been recorded. The optical band gap in both the doped/codoped glasses is found to be ∼3.31 eV. Judd-Ofelt analysis has been carried out by using the absorption spectrum of 0.8 mol% Nd 3+ doped glass to determine the radiative properties viz radiative transition probabilities, branching ratios, radiative lifetimes, quality factor and emission cross sections of some emitting levels for Nd 3+ ions. The radiative transition probability for the 4 G 7/2  →  4 I 9/2 transition (∼1926 Hz) is found to be maximum compared to other 4 G 5/2  →  4 I 9/2 (∼1622 Hz) and 4 F 5/2  →  4 I 9/2 (∼865 Hz) transitions. Upconversion (UC) luminescence of the samples has been examined by the 980 nm CW diode laser excitation. Effect of addition of Yb 3+ ions in the Nd 3+ doped glasses on UC emission intensity has been discussed. The UC emission intensity corresponding to the green, red and NIR bands in the codoped glass has been enhanced by ∼17, ∼12 and ∼42 times as compared to that of the Nd 3+ singly doped glass. The quantum efficiency for the 4 G 7/2 level is found to be ∼32%. The nephelauxetic ratio, bonding parameter and covalency of Nd 3+ ions have been found positive which represents the covalent bonding between Nd 3+ ion and oxygen atom. The colour tunability from yellowish-green to dominant green region has been obtained in the optimized codoped TPO glass.

Near infrared (NIR) to visible frequency upconversion emission studies in Er 3+ -Eu 3+ /Er 3+ -Eu 3+ -Yb 3+ co-doped/tri-doped Gd 2 O 3 phosphors prepared by the co-precipitation technique have been explored under 980 nm laser diode radiation. The developed phosphors were characterized with the help of XRD, FE-SEM and FTIR analysis. No upconversion (UC) emission was found in the Eu 3+ -doped Gd 2 O 3 phosphor. UC emission from Eu 3+ ions along with Er 3+ ions was observed in Er 3+ -Eu 3+ and Er 3+ -Eu 3+ -Yb 3+ co-doped/tri-doped phosphors. The UC emission arising from the Er 3+ and Eu 3+ ions was enhanced several times due to the incorporation of Yb 3+ ions. The processes involved in the UC emission were obtained on the basis of the effect of energy transfer/sensitization through the Yb 3+ → Er 3+ → Eu 3+ process. The red/green intensity ratio was improved from 0.16 to 1.50 and 1.01 to 1.50 for Er 3+ -Eu 3+ -Yb 3+ tri-doped phosphors as compared to the Er 3+ -doped and Er 3+ -Yb 3+ co-doped phosphors, respectively, at a fixed pump power density. A UC fluorescence intensity ratio (FIR)-based temperature sensing study was performed in the prepared Er 3+ -Eu 3+ -Yb 3+ tri-doped Gd 2 O 3 phosphors for green upconversion emission bands in the 300 K-443 K temperature range. A maximum sensor sensitivity of about ∼0.0043 K -1 at 300 K was achieved for the synthesized tri-doped phosphors upon excitation with a 980 nm laser diode. The colour coordinates lying in the green-yellow region are invariant, with variation in pump power density and temperature. The observed results support the utility of the prepared tri-doped phosphors in optical temperature sensing, display devices and NIR to visible upconverters.