Intense 974 nm emission from ErxYb2−xSi2O7 films through efficient energy transfer up-conversion from Er3+ to Yb3+ for Si solar cell

The optical properties of the Er_xYb_2?xSi₂O₇ thin films were investigated by photoluminescence measurements and the intense 974 nm light emission was observed. The 974 nm emission was mainly from the transition ²F_5/2 to ²F_7/2 level of Yb³⁺ upon exploring energy-transfer via up-conversion at Er^{3+ 4}I_13/2 level. Under 972 nm excitation, the lifetime at Er^{3+ 4}I_13/2 level reaches up to 4 ms for film containing 2 at% Er³⁺, while decreases to about 20 μs as the film is pumped by 488 nm. This confirmed that the energy transfer up-conversion process was the dominant transition at Er^{3+ 4}I_13/2 level. This may be of interest to improve the solar cells′ efficiency by placing this film at the rear of cell, converting the near-infrared photons between 1480 nm and 1580 nm to just above the Si bandgap.     

Comparative study of the spectroscopic properties of Yb3+/Er3+ codoped tellurite glasses modified with R2O (R=Li,Na and K)

Spectroscopic characterization of Yb³⁺/Er³⁺ codoped TeO₂–R₂O–ZnO–Ln₂O₃ glasses as a function of network modifiers (R=Li, Na and K) has been investigated. The Judd–Ofelt parameters (Ω_t), quantum efficiency in near infrared (1.55 μm) and visible up-conversion (546 and 660 nm) and quality factor spectroscopy (χ) were calculated. Three up-conversion emission bands centered at 525, 546 and 660 nm were observed as maxima for glasses containing potassium. The measured lifetime of ⁴I_13/2, ⁴F_9/2 and ⁴S_3/2 from Er³⁺ and ⁴F_5/2 from Yb³⁺ levels increased when potassium (K) replaced lithium (Li) and Na. The maximum emission cross-section (ECS) for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ was calculated to be 1.02×10^?20 cm² for TeO₂–Li₂O–ZnO–Ln₂O₃ glasses. The energy transfer efficiency (ET) from Yb³⁺ to Er³⁺, (⁴F_5/2)+(⁴I_15/2)→(⁴F_7/2)+(⁴I_13/2), was calculated using the measured lifetimes of Yb³⁺ with and without the presence of acceptor (Er³⁺). The maximum calculated ET was 58% for 0.25 mol% of Er³⁺ and 3 mol% of Yb³⁺ for TeO₂–K₂O–ZnO–Ln₂O₃ glass composition.     

Selective enhancement of green upconversion emissions of Er3+:Yb3Al5O12 nanocrystals by high excited state energy transfer with Yb3+–Mn2+ dimer sensitizing

An innovative upconversion (UC) emissions route of Er³⁺ by Yb³⁺–Mn²⁺ dimer sensitizing in Er³⁺–Mn²⁺:Yb₃Al₅O₁₂ (YbAG) nanocrystals is reported here, which resulted in the selective enhancement of green UC emission and suppression of red UC emission by a 976 nm laser diode excitation. By codoping of Mn²⁺, the green UC emission intensity increased about 260 times, while the red UC emission intensity decreased about 20 times than that of Er³⁺:YbAG nanocrystals. It indicates that the green enhancement and red suppression arise from the high excited state energy transfer with |²F_7/2, ⁴T_1g> (Yb³⁺–Mn²⁺ dimer) to the ⁴F_7/2 (Er³⁺), which partly decreases the nonradiative processes happened in the lower levels of Er³⁺. The proposed sensitizing route here may constitute a promising step to realize high-efficient UC emissions of rare-earth ions doped oxides and significantly extend their scope of applications.     

Enhanced spectroscopic properties and Judd–Ofelt parameters of Er-doped tellurite glass: Effect of gold nanoparticles

Glasses with composition 70TeO₂–20ZnO–10Na₂O–0.5Er₂O₃–(x)Au are synthesized by melt-quenching technique. Their spectroscopic and optical characterizations are made. The presence of gold nanoparticles (Au NPs) with average size ～9 nm is confirmed from TEM micrograph. The value of E_dir is found to lie between 3.082 and 3.073 eV, while E_indir lies within 2.765 to 2.724 eV. The observed visible up-conversion (UC) emission under 779 nm excitation wavelength exhibits two bands centered at 546 nm (green) and 637 nm (red) in all samples. The glass with 0.4 mol% Au NPs shows dominant enhancement in the UC emission intensity of the order of 3.5 times for the green band (⁴S_3/2 → ⁴I_15/2) and a weaker enhancement for the red (⁴F_9/2 → ⁴I_15/2) band. The optical properties of the system are affirmed to depend strongly on the Au concentration in the dielectric medium. The absorption spectra consist of six bands attributed to absorption from ground state (⁴I_15/2) to ⁴I_13/2, ⁴I_11/2, ⁴I_9/2, ⁴F_9/2, ²H_11/2, and ⁴F_7/2 excited states. The structural reveals that the types of bonding and difference in electro-negativity can be manipulated by the presence of metallic NPs in glass matrix. Judd–Ofelt analysis asserts the increase in Ω₂ and Ω₆ parameters with the addition of Au NPs and the enhancement of green and red emissions. The enhancement is mainly attributed to large plasmonic effect of Au NPs. The proposed glasses can be nominated as potential materials for solid state laser developments.     

Near-infrared broadband luminescence and energy transfer in Bi–Tm–Er co-doped lanthanum aluminosilicate glasses

The Bi–Tm–Er co-doped SiO₂–Al₂O₃–La₂O₃ (SAL) glasses, which exhibited a broadband near-infrared (NIR) emission, were investigated by the optical absorption and photoluminescence spectra. A super broadband NIR emission extending from 0.95 to 1.6 μm with a full-width at half-maximum (FWHM) of 430 nm which covered the whole O, E, S, C and L bands, was observed in Bi–Tm–Er co-doped samples under 808 nm excitation, as a result of the overlap of the Bi-related emission band (centered at 1270 nm) and the emission from Tm^{3+ 3}H₄→³F₄ transition (1450 nm) as well as Er^{3+ 4}I_13/2→⁴I_15/2 transition (1545 nm). In addition, a super broadband emission with amplitude relatively flat from 0.95 to 2.1 μm has been observed. The possible energy transfer between Bi-related centers, Tm³⁺ ions and Er³⁺ ions was proposed.     

Microwave sol–gel derived NaCaGd(MoO4)3:Er3+/Yb3+ phosphors and their upconversion photoluminescence properties

Ternary molybdate NaCaGd_1−x(MoO₄)₃:Er³⁺/Yb³⁺ phosphors with the proper doping concentrations of Er³⁺ and Yb³⁺ (x = Er³⁺ + Yb³⁺, Er³⁺ = 0, 0.05, 0.1, 0.2 and Yb³⁺ = 0, 0.2, 0.45) were successfully synthesized by microwave sol–gel method for the first time. Well-crystallized particles formed after heat-treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 3–5 μm. The optical properties were examined comparatively using photoluminescence emission and Raman spectroscopy. Under excitation at 980 nm, the doped particles exhibited a strong 525-nm emission band, a weak 550-nm emission band in the green region, which correspond to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, and a very weak 655-nm emission band in the red region, which corresponds to the ⁴F_9/2 → ⁴I_15/2 transition. The optimal Yb³⁺:Er³⁺ ratio was obtained to be 9:1, as indicated by the composition-dependent quenching effect of Er³⁺ ions. The pump power dependence of upconversion emission intensity and Commission Internationale de L’Eclairage chromaticity coordinates of the phosphors were evaluated in detail.     

Visible luminescence of nanocrystalline AlN:Er thin film by co-deposition of AlN,Er, and SiO2

We report a visible luminescence of Er³⁺ ions in an amorphous-nanocrystalline AlN:Er thin film prepared by co-deposition using AlN, Er, and SiO₂ targets. A PL emission spectrum of Er³⁺ in the AlN:Er film annealed at 750 °C showed a strong bluish green emission of Er³⁺ in the amorphous-nanocrystalline AlN:Er thin film, which is attributed to the intra-4fEr³⁺ transitions of ²H_11/2 → ⁴I_15/2 and ⁴F_7/2 → ⁴I_15/2. It was found that crystallite diameters were between 3 and 5 nm by high-resolution transmission electron microscopy. The occurrence of the strong Er³⁺ emission in the annealed AlN:Er thin film with a mixture of amorphous and nanocrystalline phases may be contributed to an increase in the number of excitation Er³⁺ centers and a presence of oxygen related to Er³⁺ excitation and recombination process in the AlN:Er thin film.     

Quasi-one dimensional Er3+–Yb3+ codoped single-crystal MoO3 ribbons: Synthesis,characterization and up-conversion luminescence

The quasi-one dimensional (Q1D) Er³⁺–Yb³⁺ codoped single-crystal MoO₃ ribbons with width range from 1 to 5 μm, and maximum length about 30 μm have been synthesized by the vapor transport method. The samples were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscope, and luminescence spectra. By a 975 nm laser diode (LD) as excitation source, the blue, green and red emission bands centered at about 408, 532, 553 and 657 nm were detected, which attributed to the ²H_9/2 → ⁴I_15/2, ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The three-, and two-photon process was responsible for the blue, green and red up-conversion emissions mechanism for the Q1D Er³⁺–Yb³⁺ codoped single-crystal MoO₃ ribbons, respectively. The results suggested that the Q1D Er³⁺–Yb³⁺ codoped single-crystal MoO₃ ribbons will have potential applications in remote bio-imaging and surface enhanced Raman scattering.     

Synthesis and luminescence properties of Gd6MoO12:Yb3+, Er3+ phosphor with enhanced photoluminescence by Li+ doping

Yb³⁺/Er³⁺ co-doped Gd₆MoO₁₂ and Yb³⁺/Er³⁺/Li⁺ tri-doped Gd₆MoO₁₂ phosphors were prepared by adjusting the annealing temperature via the high temperature solid-state method. Under the excitation of 980 nm semiconductor, the upconversion luminescence properties were investigated and discussed. In the experimental process, we get the optimum Yb³⁺ concentration and the concentration quench effect will happen while the concentration extends the given region. According to the Yb³⁺ concentration quenching effects, the critical distance between Yb³⁺ ions had been calculated. The measured UC luminescence exhibited a strong red emission near 660 nm and green emission at 530 nm and 550 nm, which are due to the transitions of Er³⁺(⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺(⁴I_15/2). Then the effect of excitation power density in different regions on the upconversion mechanisms was investigated and the calculated results demonstrate that the green and red upconversion is a two-photon process. A possible mechanism was discussed. After Li⁺ ions mixing, the upconversion emission enhanced largely, and the optimum Li⁺ concentration was obtained while fixed the Yb³⁺ and Er³⁺ on the above optimum concentration. This enhancement owns to the decrease of the local symmetry around Er³⁺ after Li⁺ ions doping into the system. This result indicates that Li⁺ is a promising candidate for improving luminescence in some case.     

Positive influence of Tm3+ on effective Er3+: 3 μm emission in fluoride glass under 980 nm excitation

Er³⁺ and Tm³⁺ singly doped and codoped new fluoride glasses were prepared by traditional melt-quenching method. Efficient 3 μm emission was obtained under 980 nm laser excitation. It is worthy to notice that one of the two ions can be the sensitizer to the other one by depressing the Er³⁺: 1.5 μm emission through the energy transfer process from Er³⁺:⁴I_13/2 level to Tm³⁺:³F₄ level. On the basis of measured absorption spectra, the Judd-Ofelt intensity parameters and radiation emission probability were calculated to evaluate the spectroscopic properties. Additionally, the micro-parameters together with the phonon assistance of Er³⁺:⁴I_13/2 → Tm³⁺:³F₄ and Er³⁺:⁴I_11/2 → Tm³⁺:³H₅ processes were quantitatively analyzed by using Dexter model. The theoretical micro-parameters results meet well with the experiments which indicates that Er³⁺/Tm³⁺ codoped fluoride glass is a potential kind laser glass for 3 μm laser.     

Up conversion luminescence of Yb3+–Er3+ codoped CeO2 nanocrystals with imaging applications

The effects of Yb³⁺ doping on up conversion in Yb³⁺–Er³⁺ co-doped cerium oxide nanocrystals are reported. Green emission around 545 and 560 nm attributed to the ²H_11/2, ⁴S_3/2→⁴I_15/2 transitions and red emission around 660 and 680 nm due to ⁴F_9/2→ ⁴I_15/2 transitions under 975 nm excitation were studied at room temperature. Both green and red emission intensities increase as the Yb³⁺ concentration increases from 0%. Emission strength starts to decrease after the Yb³⁺ concentration exceeds a critical amount. The green emission strength peaks around 1% Yb³⁺ concentration while the red emission strength peaks around 4%. An explanation of competition between different decay mechanisms is presented to account for the luminescence dependence on Yb³⁺ concentration. Also, the application of up converting nanoparticles in biomedical imaging is demonstrated.     

Thermal stability and spectroscopic properties of erbium-doped niobic-tungsten–tellurite glasses for laser and amplifier devices

Er³⁺ doped niobic-tungsten–tellurite glasses doped with concentration of Er³⁺ ion up to 3 wt% were fabricated. The effect of Er³⁺ doping concentration on thermal stability and optical properties was investigated in order to obtain the most suitable rare earth content for developing 1.5 μm compact fiber amplifier pumped with a commercial telecom 980 nm laser diode. The maximum doping concentration allowed was found to be around 1.77×10²⁰ ions/cm³, for which a broad 1.5 μm emission spectra of 65 nm FWHM and a lifetime of 3.4 ms for the ⁴I_13/2 level was measured.     

Spectroscopic investigation and Judd–Ofelt analysis of silver nanoparticles embedded Er3+-doped tellurite glass

A series of silver nanoparticles (NPs) embedded zinc-tellurite glass is prepared by melt-quenching technique. The transmission electron microscopic images reveal spherical as well as anisotropic silver NPs having average diameter in the range of 14–48 nm. The Er³⁺-free glass sample containing AgCl exhibits surface plasmon resonance (SPR) band of Ag NPs centered at ∼ 501 nm. From Judd–Ofelt analysis, it is found that by increasing the concentration of NPs, the value of Ω₂ is enhanced suggesting increased covalency and decreased symmetry around the Er³⁺ ions. Integrated emission cross-section (IEC) is enhanced as the concentration of silver NPs is increased up to 0.5 mol% AgCl. Fourier infrared spectra show that the intensity of the vibrational band of the water molecule and fundamental stretching band of OH group are suppressed. Furthermore, under an excitation wavelength of 786 nm, three prominent upconversion emissions are observed at 520 nm, 550 nm and 650 nm which are attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. The upconversion emissions are enhanced significantly by introduction of silver NPs. The enhancement is mainly attributed to the local field effect of silver NPs. Studied nanocomposites are potential candidates for the development of solid state lasers.     

Structural and luminescence properties of Nd3+-doped PbO–B2O3–TiO2–AlF3 glass for 1.07 μm laser applications

Trivalent neodymium doped multi-component lead borate titanate aluminumfluoride (LBTAFNd) glasses were prepared and characterized as a function of Nd³⁺ ions concentration through optical absorption, NIR luminescence and decay measurements. The intensity (Ω_2,4,6) and other radiative parameters were determined within the frame work of Judd–Ofelt theory. The intensities of absorption bands were expressed in terms of experimental oscillator strengths. Reasonably small root mean square deviation of ±0.384×10^?6 obtained between the experimental and calculated oscillator strengths indicates the validity of intensity parameters. Upon 805 nm laser excitation, the NIR emissions at 0.92 μm (⁴F_3/2→⁴I_9/2), 1.07 μm (⁴F_3/2→⁴I_11/2) and 1.35 μm (⁴F_3/2→⁴I_13/2) were observed. The spectroscopic quality factor has been determined from the Ω₄ and Ω₆ intensity parameters as well as the intensities of emission bands centered at 1.07 and 1.35 μm. The decay curves of the ⁴F_3/2 excited state were recorded by monitoring the emission and excitation wavelengths at 1.07 μm and 805 nm, respectively. The decay curves exhibit single exponential behavior for all the glasses. The laser characteristic parameters of ⁴F_3/2→⁴I_11/2 (1.07 μm) transition were determined and compared with other reported glasses.     

Infrared to visible upconversion luminescence in Er3+/Yb3+ doped titanate glass prepared by containerless processing

Er³⁺ doped TiO₂–La₂O₃ glasses modified by ZrO₂ have been successfully fabricated by the containerless method with incorporated Yb³⁺ ions as sensitizers. Under the excitation of 980 and 808 nm diode lasers, visible emissions centered at 534, 554 and 674 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2, respectively. The emission signals are so strong that they can be observed by naked eyes even at pumping power as low as 20 mW. Measurements of pump-power dependent intensity and time-resolved decay behavior of upconversion luminescence show that two-photon excited state absorption (ESA) and energy transfer (ET) between rare earth ions are the predominant mechanisms for upconversion emissions. Besides, the intensity of upconversion luminescence has been enhanced by increasing the concentration of ZrO₂ in these rare earth doped bulk titanate glasses.     

Synthesis of TiO2 nanoparticles and spectroscopic upconversion luminescence of Nd3+-doped TiO2–SiO2 composite glass

Nd³⁺-doped TiO₂–SiO₂ composites were prepared by sol–gel method. Optical properties such as radiative life-time (τ), stimulated emission cross-section (σ_p) and branching ratio (β) were calculated using Judd–Ofelt theory. Violet to blue upconversion emissions at 380 nm (⁴D_3/2→⁴I_11/2), 399 nm (²P_3/2→⁴I_11/2), 420 nm (²D_5/2→⁴I_9/2) and 452 nm (²P_3/2→⁴I_13/2) were obtained under 578 nm xenon-lamp excitation. The choice of 578 nm is justified by the absorption spectra of the same samples, which shows a strong absorption peak at 578 nm. This 578 nm excitation pump produces upconversion in Nd³⁺ by a sequential two-photon absorption process.     

Photoluminescence of Er-doped Si-SiO2 and Al–Si-SiO2 sputtered thin films

Er-doped Si-SiO₂ and Al–Si-SiO₂ films have been deposited by rf-sputtering being annealed afterwards. Annealing behavior of the Er³⁺: ⁴I_13/2→⁴I_15/2 emission of Er-doped Si-SiO₂ yields a maximum intensity for annealing at 700–800 °C. ⁴I_13/2→⁴I_15/2 peak emission for Er-doped Al–Si-SiO₂ at 1525 nm is shifted from that for Er-doped Si-SiO₂ at 1530 nm and the bandwidth increases from 29 to 42 nm. ⁴I_13/2→⁴I_15/2 emission decays present a fast decaying component related to Er ions coupled to Si nanoparticles, defects, or other ions, and a slow decaying component related to isolated Er ions. Excitation wavelength dependence and excitation power dependence for the ⁴I_13/2→⁴I_15/2 emission correspond with energy transfer from Si nanoparticles. Populating of the ⁴I_11/2 level in Er-doped Si-SiO₂ involves branching and energy transfer upconversion involving two or more Er ions. Addition of Al reduces the populating of this level to an energy transfer upconversion involving two ions.     

Upconversion photoluminescence properties of SrY2(MoO4)4:Er3+/Yb3+ phosphors synthesized by a cyclic microwave-modified sol–gel method

SrY_2−x(MoO₄)₄:Er³⁺/Yb³ phosphors with doping concentrations of Er³⁺ and Yb³⁺ (x = Er³⁺ + Yb³⁺, Er³⁺ = 0.05, 0.1, 0.2 and Yb³⁺ = 0.2, 0.45) have been successfully synthesized by a cyclic microwave-modified sol–gel method, and the upconversion photoluminescence properties have been investigated. Well-crystallized particles showed a fine and homogeneous morphology with particle sizes of 1–3 μm. Under excitation at 980 nm, SrY₂(MoO₄)₄:Er³⁺/Yb³⁺ particles exhibited a strong 525-nm, weak 550-nm emission bands in the green region, and a very weak 655-nm emission band in the red region. The possible mechanism of the green and red emissions was discussed in detail under consideration of a two-photon process. The Raman spectra of the particles indicated the presence of strong peaks at both higher and lower frequencies.     

Up-conversion luminescence and optical temperature-sensing properties of Er3+-doped perovskite Na0.5Bi0.5TiO3 nanocrystals

In this work we demonstrate the preparation of Er³⁺ doped perovskite ferroelectric Na_0.5Bi_0.5TiO₃ nanocrystals and their application in temperature sensing. The samples were synthesized via a facile hydrothermal method. Upconversion emission at 528 nm and 547 nm from two thermodynamically coupled excited states of Er³⁺ were recorded in the temperature from 80 K to 480 K under the excitation of a 980 nm diode laser. The emission intensity ratio (I₅₂₈/I₅₄₇) as a function of the temperature was investigated. A sensitivity of 0.0053 K⁻¹ is observed at 400 K, suggesting they are promising candidate for nanothermometers.     

Synthesis and near-infrared luminescence properties of LaOCl:Nd3+/Yb3+

Near-infrared emitting phosphors LaOCl:Nd³⁺/Yb³⁺ were prepared by the solid-state method, and their structures and luminescent properties were investigated by using X-ray diffraction and photoluminescence analysis, respectively. The studies shows that tetragonal LaOCl:Nd³⁺/Yb³⁺ can be synthesized by the solid-state reaction at 600 °C for 3 h. Upon 353 nm UV excitation, LaOCl:Nd³⁺/Yb³⁺ sample shows strong near-infrared emission lines in the region of 1060–1150 nm (corresponding to ⁴F_3/2 → ⁴I_J′ transition of Nd³⁺, J′ = 9/2, 11/2, 13/2, 15/2) and 980–1050 nm (corresponding to ²F_5/2 → ²F_7/2 transition of Yb³⁺). The decreasing emission intensity of Nd³⁺ with increasing doping concentration of Yb³⁺ proved the energy transfer in LaOCl:Nd³⁺/Yb³⁺. The possible near-infrared emission and energy transfer mechanism between Nd³⁺ and Yb³⁺, as well as the energy transfer efficiency of LaOCl:Nd³⁺/Yb³⁺ were discussed.