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

A serials of Ho³⁺/Yb³⁺ 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 Ho³⁺/Yb³⁺ co-doped tellurite glasses centered at 549 and 664 nm corresponding to Ho³⁺: ⁵S₂(⁵F₄)→⁵I₈ and ⁵F₅→⁵I₈ transitions, respectively. The upconversion intensities of red and green emissions in Ho³⁺/Yb³⁺ co-doped glasses were enhanced largely when increasing Yb₂O₃ 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_D2, C_D3 and C_D4 were C_D2=5.0×10⁻¹⁸ cm³/s, C_D3=1.5×10⁻¹⁷ cm³/s, C_D4=9.0×10⁻¹⁷ cm³/s.     

Energy transfer and infrared-to-visible upconversion luminescence of Er/Yb-codoped halide modified tellurite glasses

We report on the energy transfer and frequency upconversion spectroscopic properties of Er³⁺-doped and Er³⁺/Yb³⁺-codoped TeO₂-ZnO-Na₂O-PbCl₂ halide modified tellurite glasses upon excitation with 808 and 978 nm laser diode. Three intense emissions centered at around 529, 546 and 657 nm, alongwith a very weak blue emission at 410 nm have clearly been observed for the Er³⁺/Yb³⁺-codoped halide modified tellurite glasses upon excitation at 978 nm and the involved mechanisms are explained. The quadratic dependence of fluorescence on excitation laser power confirms the fact that the two-photon contribute to the infrared to green-red upconversion emissions. And the blue upconversion at 410 nm involved a sequential three-photon absorption process.     

Conversion of infrared radiation into visible emission in NaY(WO4)2:Yb, Ho crystal

The spectroscopic characteristics and fluorescence dynamics for Yb³⁺/Ho³⁺:NaY(WO₄)₂ crystal were investigated. The parameters of oscillator strengths, the spontaneous transition probabilities, the fluorescence branching ratios, the radiative lifetimes and the stimulated emission cross sections have been calculated based on Judd-Ofelt theory and Füchtbauer-Ladenburg method. The energy transfer efficiency from Yb³⁺ to Ho³⁺ was 65.85%. The green emission (530-570 nm) corresponding to (⁵F₄, ⁵S₂)→⁵I₈ transition, red emission (640-670 nm) due to ⁵F₅→⁵I₈ transition and NIR emission (740-770 nm) attributed to (⁵F₄, ⁵S₂)→⁵I₇ transition were observed on 974 nm excitation at room temperature. Under low pump power, the intensity of green light emission is weaker than that of the red light, while under high pump power, the case is on the contrary. The upconversion is based on the two-photon process either the energy transfer from Yb³⁺ ions or by the excited state absorption. The proposed mechanisms of upconversion emissions were provided.     

Upconversion luminescence and mechanisms in Yb-sensitized Tm-doped oxyhalide tellurite glasses

Effect of Yb₂O₃ content on upconversion luminescence and mechanisms in Yb³⁺-sensitized Tm³⁺-doped oxyhalide tellurite glasses were investigated under 980 nm excitation. Intense blue and relatively weak red upconversion emission centered at 476 and 649 nm corresponding to the transitions ¹G₄→³H₆ and ¹G₄→³H₄ of Tm³⁺, respectively, are simultaneously observed at room temperature. The results show that upconversion blue and red emission intensities of Tm³⁺ first increase, reach its maximum at Yb₂O₃%=3 mol%, and then decrease with increasing Yb₂O₃ content. The effect of Yb₂O₃ content on upconversion intensity is discussed, and possible effect mechanisms are evaluated. The investigated results were conducing to increase upconversion luminescence efficiency of Tm³⁺.     

Optical properties of Ho-doped novel oxyfluoride glasses

Novel oxyfluoride glasses SiO₂-Al₂O₃-Na₂O-ZnF₂ doped with Ho³⁺ and Ho³⁺/Yb³⁺ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Ho³⁺ were determined by measurement of absorption spectrum of Ho³⁺-singly doped glass. According to the Judd-Ofelt theory, the Judd-Ofelt parameters were calculated, by which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. Visible upconversion luminescence was observed under 980 nm diode laser excitation and the influence of Yb³⁺ concentration on the emission bands were also investigated. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms were discussed.     

Optical transitions and frequency upconversions of Ho and Ho/Yb ions in Al(NO3)3-SiO2 sol-gel glasses

Infrared-to-visible upconversion processes and Judd Ofelt intensity parameters were analyzed for Ho³⁺ singly doped and Ho³⁺/Yb³⁺ co-doped Al(NO₃)₃-SiO₂ glasses with a fixed Ho³⁺ and Yb³⁺ concentrations prepared by sol-gel method. Blue and intense green upconversion emissions centered at 467 and 538 nm, corresponding to the and transitions, respectively, were observed under 800 nm excitation. The analysis of the dynamics of upconversion emissions suggest excited state absorption, energy transfer and back transfer as the possible causes for the observed transitions. Significant enhancement of upconversion intensities in Ho³⁺/Yb³⁺ co-doped glass compared to the Ho³⁺ singly doped one confirms efficient energy transfer between Yb³⁺ and Ho³⁺ ions. Intense upconversion emissions shown by the glasses in the present study indicate their potential in upconversion device applications.     

Investigation of 2.0 μm emission in Tm and Ho co-doped oxyfluoride tellurite glass

This paper reports 2.0 μm emission properties of Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass exited by 808 nm laser diode (LD). Mid-infrared transmittance property of glass was investigated by Fourier transform infrared (FTIR) spectrometer. The real chemical composition of investigated glass was identified by X-ray photoelectric spectroscopy (XPS). Thermal stability of the glass was determined by differential thermal analysis (DTA) measurement. The Judd-Ofelt parameters, spontaneous radiative transition probabilities, branching ratios and radiative lifetime of Ho³⁺ were calculated based on the absorption spectra by using Judd-Ofelt theory. Results indicate that the maximum 2.0 μm emission intensity attributed to the ⁵I₇→⁵I₈ transition of Ho³⁺ was achieved at 1.5 mol% Tm₂O₃ and 1 mol% Ho₂O₃ concentrations in oxyfluoride tellurite glass. OH⁻ absorption at 3000 cm⁻¹ was greatly depressed by introduction of 10 mol% F⁻. The maximum absorption and stimulated emission cross-section of Ho³⁺ near 2.0 μm are 7.0×10⁻²¹ cm² at 1950 nm and 8.8×10⁻²¹ cm² at 2048 nm, respectively. The calculated radiative lifetime of 4.4 ms for ⁵I₇→⁵I₈ transition and large stimulated emission cross-section of the Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass indicate that the glass has a potential application in efficient 2.0 μm laser.     

Up and down conversion fluorescence studies on combustion synthesized Yb/Yb: MO-Al2O3 (M=Ca, Sr and Ba) phosphors

The ytterbium ions doped MO-Al₂O₃ (M=Ca, Sr and Ba) phosphors have been synthesized through combustion technique and their up and down conversion fluorescence properties have been studied and compared. The samples were calcinated at different temperatures and their FTIR and XRD spectra have shown a close relationship. With 976 nm excitation all these phosphors show cooperative upconversion emission at 488 nm from the pairs of two Yb³⁺ ions along with an unexpected broad upconversion band in the blue green region and has been assigned to arise from the defect centers. Contrary to this upconversion emission, calcium aluminate phosphor exhibits bright and very broad down-conversion fluorescence (FWHM≈160 nm) upon UV (266 nm) excitation due to Yb²⁺ ions. The inter-conversion between the 3+ and 2+ valence states of Yb ion has been observed on calcinations of samples in open atmosphere and has been correlated to the emission properties. The Yb²⁺ ions containing calcium aluminate phosphor has been found suitable for producing broad band light in the visible region (white light). Lifetime of the emitting states of Yb³⁺ and Yb²⁺ ions have also been measured and discussed.     

Yb3+敏化的Er3+/Ho3+共掺碲酸盐玻璃的上转换发光研究

用高温熔融法制备了系列Er³⁺/Yb³⁺共掺，Ho³⁺/Yb³⁺共掺，和Er³⁺/Yb³⁺/Ho³⁺三掺碲酸盐玻璃，在975nm激光抽运下三种掺杂玻璃中都出现了较强的绿光和红光上转换.研究了Yb³⁺离子对Er³⁺和Ho³⁺离子上转换发光强度的影响以及Yb³⁺→Er关键词： 3+/Yb³⁺/Ho³⁺共掺')" href="#">Er³⁺/Yb³⁺/Ho³⁺共掺 碲酸盐玻璃 光谱性质 上转换     

Energy transfer and upconversion luminescence in Tm/Yb co-doped lanthanum-zinc-lead-tellurite glasses

A series of Tm³⁺/Yb³⁺ co-doped lanthanum-zinc-lead-tellurite (TPZL) glasses pumped by a 980 nm laser diode (LD) were demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Effects of PbO content on the thermal stability, structure and upconversion properties of Tm³⁺/Yb³⁺ co-doped TPZL glasses had been investigated. The efficient visible upconversion fluorescences corresponding to the ¹G₄→³H₆, ¹G₄→³F₄ and ³H₄→³H₆ transitions of Tm³⁺ were observed under 980 nm excitation. The upconversion intensities of blue, red and near infrared emissions in Tm³⁺/Yb³⁺ co-doped TPZL glasses were obviously enhanced with increasing PbO content. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model. Population density in different levels and coefficients of the energy transfer rate C_Di (i=2, 4, 6) between Tm³⁺ and Yb³⁺ were estimated by fitting the simulated curves to the measured ones. The obtained three energy transfer coefficients C_D2, C_D4, and C_D6 were determined to be 5.7×10⁻¹⁷, 1.3×10⁻¹⁶ and 8.6×10⁻¹⁷ cm³/s, respectively.     

Enhanced blue-green-red up-conversion and 1.3-μm emission of Pr/Yb-codoped oxyhalide tellurite glasses with PbCl2 doping

We report on the blue-green-red up-conversion spectroscopic properties of Pr³⁺/Yb³⁺-codoped oxyhalide tellurite glasses upon excitation of a conventional 980 nm laser diode (LD). Significant enhancement of the blue-green-red up-conversion emission intensity has been observed with increasing PbCl₂ doping. The up-conversion intensity has a quadratic dependence on incident pump laser power, indicating a two-photon process. The population of the Pr³⁺ upper ³P₀ emitting level was accomplished through a combination of a ground state absorption, energy transfer and excitated state absorption. 1.3-μm emission in the second telecom window originated from Pr³⁺:¹G₄→³H₅ transition has also been investigated upon excitation at 980 nm LD. The measured peak wavelength and full width at half-maximum of the fluorescent are 1335 nm and ∼100 nm, respectively. An enhanced 1.3μm emission with increasing PbCl₂ doping has also been observed. Codoping of Yb³⁺ significantly enhance both the blue-green-red up-conversion emission and 1.3-μm emission intensity by way of a nonradiative Yb³⁺:²F₅→Pr³⁺:¹G₄ energy transfer.     

Upconversion in Ho<Superscript>3+</Superscript>-doped YbF<Subscript>3</Subscript> particle prepared by coprecipitation method

YbF₃ particles doped with Ho³⁺ were synthesized by coprecipitation method, from which the ultraviolet and visible emission bands of the Ho³⁺ and the 480 nm cooperative upconversion emission of Yb³⁺–Yb³⁺ are observed under 980 nm excitation. Under the same excitation power, the emission intensity of Ho³⁺ in coprecipitation method is enhanced by about two times comparing to that in solid-state reaction method. The novel ultraviolet and violet emissions of the Ho³⁺ are firstly obtained which are centered at 360 (⁵G₂→⁵I₈),391 (³K₇→⁵I₈),412 (⁵G₄→⁵I₈), and 446 nm (⁵G₅→⁵I₈). The luminescence decay profiles of 545 and 652 nm visible emissions were obtained with a 980 nm pulsed laser. The excitation power dependence of the emission intensity was also measured and intensity saturation was observed. Based on the level structures of Ho³⁺, two- and three-photon processes are suggested to perform populations of ⁵S₂ and ⁵G₃ (Ho³⁺) levels, respectively. The dominant upconversion mechanism may be attributed to a cooperative sensitization process of two excited states of Yb³⁺ and energy transfers from Yb³⁺ to Ho³⁺.     

Towards efficient upconversion and downconversion of NaYF4:Ho3+,Yb3+ phosphors

NaYF₄ microcrystals co-doped with Ho³⁺ and Yb³⁺ were prepared by a facile hydrothermal synthesis. The products were characterized by X-ray diffractometer, scanning electron microscopy, and photoluminescence spectroscopy. Upon excitation with a 980 nm laser diode, the sample shows an intense green upconversion emission centered at 540 nm corresponding to the ⁵S₂→⁵I₈ transition of Ho³⁺. The quadratic dependence of the green emission intensity on the excitation power reveals a two-phonon upconversion process. On the contrary, upon excitation with 448 nm, both visible and near-infrared emissions peaked at 483, 540, 644, 749, and 978 nm are simultaneously observed, which could be assigned to the electronic transitions of Ho³⁺: ⁵F₃→⁵I₈, ⁵S₂→⁵I₈, ⁵F₅→⁵I₈, ⁵S₂→⁵I₇, and Yb³⁺: ²F_5/2→²F_7/2, respectively. The energy transfer processes between Ho³⁺ and Yb³⁺ ions and the involved mechanisms have been investigated and discussed.     

Conversion of infrared radiation into visible emission in NaGd(WO4)2:Yb3+, Ho3+ crystals

NaGd(WO₄)₂:Yb³⁺, Ho³⁺ single crystals have been grown by the Czochralski technique along the (0 0 1) orientation. Conversion of the infrared (IR) radiation at 980 nm into the visible emission in NaGd(WO₄)₂ crystals containing several different concentrations of Yb³⁺ and Ho³⁺ has been investigated. The NaGd(WO₄)₂: 8 at. % Yb³⁺, 4 at. % Ho³⁺ system exhibits intense red upconverted emission originating from the ⁵F₅ level. The upconversion mechanism in a Ho³⁺-Yb³⁺ system under near infrared excitation is discussed. It is concluded that the green emission is excited by energy transfers from Yb³⁺ to Ho³⁺, whereas excited state absorption is involved in the excitation of red emission. The emission cross-section of the ⁵F₅→⁵I₈ transition at about 660 nm was estimated by using the Füchtbauer–Ladengurg formula. PACS 78.55.Hx; 78.20.-e     

Infrared to visible upconversion fluorescence in Yb,Tm:YAG single crystal

Absorption spectrum from 400 to 2000 nm and upconversion fluorescence spectra under 940 nm pumping of YAG single crystal codoped with 5 at.% Yb³⁺ and 4 at.% Tm³⁺ were studied at room temperature. The blue upconversion emission centered at 483 nm corresponds to the transition ¹G₄ → ³H₆, the emission band around 646 nm corresponds to the transition ¹G₄ → ³F₄ of Tm³⁺. Energy transfer from Yb³⁺ to Tm³⁺ is mainly nonradiative and the transfer efficiency was experimentally assessed. The line strengths, transition probabilities and radiative lifetimes of ¹G₄ level were calculated by using Judd-Ofelt theory. Gain coefficient calculated from spectra shows that the upconversion corresponding with transitions ¹G₄ → ³H₆ in YAG doped with Yb³⁺ and Tm³⁺ is potentially useful for blue light output.     

Downconversion for the Er, Yb couple in KPb2Cl5—A low-phonon frequency host

Downconversion of a single blue/green photon to two near-infrared photons offers a promising route to increase the efficiency of photovoltaic cells. Here we report on downconversion for the well-known upconversion couple (Er³⁺, Yb³⁺) doped into a host with low (∼200 cm⁻¹) maximum phonon energy (KPb₂Cl₅). The intermediate energy level in both the upconversion and downconversion processes is the ⁴F_7/2 level around 490 nm. While fast multi-phonon relaxation to the lower energy ²H_11/2/⁴S_3/2 levels is beneficial for upconversion, it prevents efficient downconversion. To reduce multi-phonon relaxation, a low-phonon energy host (KPb₂Cl₅) was doped with Er³⁺ and varying amounts of Yb³⁺ co-dopant. The results show that downconversion from the ⁴F_7/2 level occurs, exciting two neighboring Yb³⁺ ions to the ²F_5/2 level. The efficiency is however low due to multi-phonon relaxation from the ⁴F_7/2 to the ⁴S_3/2 level via the intermediate ²H_11/2 level. Based on the results it is clear that efficient downconversion for the (Er³⁺, Yb³⁺) couple requires even lower phonon energy hosts (e.g. bromide host lattices). A Cl⁻-Yb³⁺ charge transfer absorption band is observed between 300 and 400 nm. Excitation in this band results in two broad emission bands centered around 430 and 700 nm at temperatures below 30 K, which are assigned to Cl⁻-Yb³⁺ charge transfer emission.     

Multi-phonon-assisted relaxation and Yb3+ sensitized bright red-dominant upconversion luminescence of Ho3+ in YF3-BaF2-Ba(PO3)2 glass

Unusual bright red-dominant upconversion light was observed in Ho³⁺/Yb³⁺ co-doped YF₃-BaF₂-Ba(PO₃)₂ glasses excited by the 980-nm laser diode at room temperature. The integral intensity ratios of the red upconversion emission to the green one reached about 10:1 in optimized 0.125Ho³⁺-15Yb³⁺ co-doped sample. In order to find out its behind-the-scene mechanism, the optical properties and the phonon-assisted relaxations on the excited levels of Ho³⁺ in our samples were investigated. Additionally, the effects of the concentrations of the doping ions, excitation pump power, and temperature on the upconversion emissions were also systematically studied. These results revealed that the proper phonon frequency of fluorophosphate glasses, the efficient phonon-assisted relaxations from ⁵I₆ to ⁵I₇ levels (4,960 s^?1), and the long lifetime of the ⁵I₇ (about 2.8 ms) levels should be responsible for bright red upconversion emission at a much greater concentration ratio of C _Yb ³⁺ /C _Ho ³⁺ .     

Role of Yb and Tm ions in upconversion emission of Tb under 798 and 980 nm laser excitations in Tb-Tm-Yb doped tellurite glass

Upconversion emission and energy transfer processes in singly, doubly and triply doped tellurite glasses have been studied under 798 and 980 nm laser excitations. Emissions have been observed at 482, 544, 584, 655 nm and at 477, 655, 698, 800 nm corresponding to Tb³⁺: ⁵D₄ → ⁷F₆, ⁷F₅, ⁷F₄, ⁷F_J (J = 0, 1, 2, 3) and Tm³⁺: ¹G₄ → ³H₆, ¹G₄ → ³F₄, ³F₃ → ³H₆, ³H₄ → ³H₆ transitions, respectively. Among Tm³⁺, Yb³⁺and Tb³⁺ ions only Tm³⁺ has a ground state absorption at 798 nm excitation due to ³H₄ ← ³H₆ transition. For 980 nm excitation only Yb³⁺ can absorb the incident radiation. However, for both types of excitations, emission from all the three ions Tb, Yb and Tm has been observed. Possible mechanisms are proposed as follows: under 798 nm excitation Tm³⁺ ions are excited which excite Yb³⁺ ions through energy transfer. Finally “cooperative energy transfer” from a pair of Yb³⁺ ions to Tm³⁺ and Tb³⁺ ions takes place. Under 980 nm excitation Yb³⁺ ions absorb the incident energy and excite Tm³⁺ and Tb³⁺ ions via cooperative energy transfer. Variation of emission intensity with the ion concentrations of Yb³⁺, Tm³⁺ and Tb³⁺ has been studied. The lifetime of the ¹G₄ level has also been measured.     

Synergistic upconversion effect in NaYF4:Yb,Tm nanorods under dual excitation of 980 nm and 808 nm

NaYF₄:Yb³⁺,Tm³⁺ nanorods are prepared with hydrothermal method. The upconversion luminescent properties are investigated under dual excitation of 980 nm and 808 nm. The blue emission is observed at about 475 nm under dual excitation. The intensity is 2.6 times higher than the total intensity of the two corresponding single wavelength excitations, showing a synergistic upconversion effect occurring there. The dual wavelength excitation not only effectively decreases non-radiative relaxation pumped by 980 nm but also reduces the rate of the back energy transfer from Tm³⁺ to Yb³⁺ pumped by 808 nm. The result provides a possible new way to further improve the upconversion efficiency of rare earth doped phosphor.     

Experimental investigation on the upconversion mechanism of 754 nm NIR luminescence of Ho/Yb:Y2O3, Gd2O3 under 976 nm diode laserexcitation

Upconversion (UC) spectra of Ho³⁺/Yb³⁺ codoped Y₂O₃, Gd₂O₃ bulk ceramics were obtained under the excitation of a 976 nm diode laser. Systematic experimental studies, including power dependence, luminescence lifetime, and the intensity ratio σ for the green to NIR emissions, were carried out in order to confirm the UC mechanism of Ho³⁺ ions. Our results demonstrated that the NIR emission was associated with the ⁵F₄/⁵S₂→⁵I₇ transition of Ho³⁺ ions without the contribution of the ⁵I₄→⁵I₈ transition for Ho³⁺/Yb³⁺ codoped Y₂O₃ and Gd₂O₃ bulk ceramics. Additionally, population saturation in the ⁵I₇ energy level had been observed in Ho³⁺/Yb³⁺ codoped Y₂O₃, Gd₂O₃ bulk ceramics. All experimental observations can be well explained by the steady-state rate equations.