Upconversion Excitation Research Articles

Resonant excited state absorption and relaxation mechanisms in Tb^3+-doped calcium aluminosilicate glasses: an investigation by thermal mirror spectroscopy

Resonant excited state absorption (ESA) and relaxation processes in Tb(3+)-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level (5)D(4) was found to be close to unity and concentration independent while, for the level (5)D(3), it decreases with Tb(3+) concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.

Citric acid-assisted phase controlled synthesis of NaYF4:Yb3+,Tm3+ crystals and their intense ultraviolet upconversion emissions

The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+, 0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.

Three- and four-photon excited upconversion luminescence in terbium doped lutetium silicate powders by femtosecond laser irradiation

Frequency upconversion (UC) luminescence was investigated in terbium (Tb3+) doped lutetium silicate powders when the samples were irradiated with femtosecond lasers operating either at 800 nm or 1500 nm. The samples with three different Tb3+ concentrations were prepared by the combustion synthesis method. Rietveld analysis of the X-ray powder diffraction data showed the predominance of monoclinic Lu2SiO5 phase. UC luminescence signals induced by three- and four-photon absorption were identified. The mechanisms that originate the anti-Stokes luminescence were discussed.

Study on the construction and performance of a Hg(II)-sensing system having an “off–on” probe derived from rhodamine and an up-conversion excitation core

In this work, a rhodamine derivative was synthesized and grafted onto the surface of an up-conversion host of β-NaYF4:Yb3+/Er3+, resulting in a Hg(II)-sensing system. The obtained nanocomposite was fully characterized and studied by electron microscopy images, wide-angle XRD diffraction patterns, IR spectra and thermogravimetry, which confirmed the successful construction of the nanocomposite. The analysis on absorption and emission spectra suggested that the up-conversion host could efficiently excite the probe, which was later proved by the emission decay characteristics. The sensing performance of the nanocomposite towards Hg(II) was studied by steady emission spectra. It was found that the fluorescence from the probe increased with the increasing Hg(II) concentration, showing an “off–on” effect. In addition, the fluorescence enhancement effect could be activated only by Hg(II), suggesting that the nanocomposite owned a high selectivity towards Hg(II).

An “off–on” fluorescence probe for Hg(II) detection using upconversion nanobars as the excitation source: Preparation, characterization and sensing performance

In this paper, we reported an “off–on” optical sensor for Hg(II) detection using upconversion β-NaYF4:Yb3+/Er3+ nanobars as the excitation source and a rhodamine derivative as the probe. The nanocomposite was characterized and studied by electron microscopy, XRD analysis, IR spectra, thermogravimetry and photophysical measurements. The energy transfer mechanism between the upconversion excitation source and the rhodamine based probe was analyzed by their emission/excitation spectra and excited state lifetime comparison. Experimental results suggested that the upconversion emission form the excitation source could effectively excite the probe. The sensing performance of the nanocomposite towards Hg(II) was also studied. It was found that the emission intensity of the nanocomposite increased with the increasing Hg(II) concentration, showing an “off–on” phenomenon. In addition, the emission signal of the nanocomposite was immune to other metal ions, providing a high selectivity towards Hg(II).

Tunable plasmon resonance modes on gold nanoparticles in Er3 +-doped germanium–tellurite glass

Relative to the Er3+:gold-nanoparticle (Er3+:Au–NP) axis, the polarization of the gold nanoparticle can be longitudinal (electric dipole parallel to the Er3+:Au–NP axis) or transverse (electric dipole perpendicular to the Er3+:Au–NP axis). For longitudinal polarization, the plasmon resonance modes of gold nanoparticles embedded in Er3+-doped germanium–tellurite glass are activated using laser lines at 808 and 488nm in resonance with radiative transitions of Er3+ ions. The gold nanoparticles were grown within the host glass by thermal annealing over various lengths of time, achieving diameters lower than 1.6nm. The resonance wavelengths, determined theoretically and experimentally, are 770 and 800nm. The absorption wavelength of nanoparticles was determined by using the Frohlich condition. Gold nanoparticles provide tunable emission resulting in a large enhancement for the 2H11/2→4I13/2 (emission at 805nm) and 4S3/2→4I13/2 (emission at 840nm) electronic transitions of Er3+ ions; this is associated with the quantum yield of the energy transfer process. The excitation pathways, up-conversion and luminescence spectra of Er3+ ions are described through simplified energy level diagrams. We observed that up-conversion is favored by the excited-state absorption due to the presence of the gold nanoparticles coupled with the Er3+ ions within the glass matrix.

Nd3+-Sensitized Upconversion Nanophosphors: EfficientIn VivoBioimaging Probes with Minimized Heating Effect

Upconversion (UC) process in lanthanide-doped nanomaterials has attracted great research interest for its extensive biological applications in vitro and in vivo, benefiting from the high tissue penetration depth of near-infrared excitation light and low autofluorescence background. However, the 980 nm laser, typically used to trigger the Yb(3+)-sensitized UC process, is strongly absorbed by water in biological structures and could cause severe overheating effect. In this article, we report the extension of the UC excitation spectrum to shorter wavelengths, where water has lower absorption. This is realized by further introducing Nd(3+) as the sensitizer and by building a core/shell structure to ensure successive Nd(3+) → Yb(3+) → activator energy transfer. The efficacy of this Nd(3+)-sensitized UC process is demonstrated in in vivo imaging, and the results confirmed that the laser-induced local overheating effect is greatly minimized.

Triplet–Triplet Annihilation Upconversion Based Nanocapsules for Bioimaging Under Excitation by Red and Deep‐Red Light

Non-toxic and biocompatible triplet-triplet annihilation upconversion based nanocapsules (size less than 225 nm) were successfully fabricated by the combination of miniemulsion and solvent evaporation techniques. A first type of nanocapsules displays an upconversion spectrum characterized by the maximum of emission at λmax = 550 nm under illumination by red light, λexc = 633 nm. The second type of nanocapsules fluoresces at λmax = 555 nm when excited with deep-red light, λexc = 708 nm. Conventional confocal laser scanning microscopy (CLSM) and flow cytometry were applied to determine uptake and toxicity of the nanocapsules for various (mesenchymal stem and HeLa) cells. Red light (λexc = 633 nm) with extremely low optical power (less than 0.3 μW) or deep-red light (λexc = 708 nm) was used in CLSM experiments to generate green upconversion fluorescence. The cell images obtained with upconversion excitation demonstrate order of magnitude better signal to background ratio than the cell images obtained with direct excitation of the same fluorescence marker.

Engineering the Upconversion Nanoparticle Excitation Wavelength: Cascade Sensitization of Tri‐doped Upconversion Colloidal Nanoparticles at 800 nm

Cascade-sensitized 800 nm excited tri-doped upconversion nanoparticles (UCNPs) are developed for the first time. This novel class of UCNPs employ Nd3+ as an 800 nm photon sensitizer and Yb3+ as a bridging ion, and show strong upconversion emission without photobleaching. They outperform classical 980 nm excited UCNPs in regard to significantly decreased NIR photon absorption in water and decreased laser-induced water heating effects.

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.

Near-infrared emission and upconversion in Er3+-doped TeO2–ZnO–ZnF2 glasses

We have investigated the near infrared emission and upconversion of Er3+ ions in two different compositions of glasses based on TeO2, ZnO, and ZnF2 for different ErF3 concentrations (0.5, 1, 2, and 3wt%). Judd–Ofelt intensity parameters have been determined and used to calculate the radiative transition probabilities and radiative lifetimes. The infrared emission at around 1532nm corresponding to the 4I13/2→4I15/2 transition is broader by nearly 30nm if compared to silica based glasses. The stimulated emission cross section is higher for the glass with the lowest content of ZnF2 which also shows higher values of the figure of merit for bandwidth. On the other hand, the lifetimes of the excited states are longer for the glass with the highest content of ZnF2. Green and red emissions corresponding to transitions (2H11/2,4S3/2)→4I15/2 and 4F9/2→4I15/2 have been observed under excitation at 801nm and attributed to a two photon process. The temporal evolution of the green emission suggests the presence of excited state absorption and energy transfer upconversion processes to populate the 4S3/2 level. In the case of the red emission, its increase as ErF3 concentration increases together with its temporal behavior indicate that for ErF3 concentrations higher than 0.5wt%, level 4F9/2 is populated by multiphonon relaxation from level 4S3/2 and energy transfer processes.

A novel synthesis method and up-conversion properties of hexagonal-phase NaYF4:Er nano-crystals

A novel synthesis method for hexagonal (β)-phase NaYF4:Er nano-crystals (NCs) which showed up-conversion (UC) from infrared to visible spectral region was developed. The NaYF4:Er NCs were synthesized in oleic acid (OA) and 1-octadecene (ODE) with Y2(CO3)3·xH2O, Er2(CO3)3·xH2O, Na2CO3 and NH4F as precursors. This proposed method was simple and less toxic compared with generally used method so far. The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control of NaYF4:Er NCs. The UC emission spectra were obtained with the emission wavelength at about 980 nm (4I11/2→4I15/2), 800 nm (4I9/2→4I15/2), 660 nm (4F9/2→4I15/2) and 540 nm (4S3/2→4I15/2) from Er3+ ions, by excitation wavelength of 1550 nm. The slope values, n, in the pump-power dependence, showed that the emission at 980 and 800 nm were generated by 2-step UC and at 660 nm and 540 nm were 3-step UC. The optical process for the UC excitation was discussed.

Bright white up-conversion emission from Er3+/Ho3+/Tm3+/Yb3+ co-doped YVO4 phosphors

Er3+/Ho3+/Yb3+/Tm3+ co-doped YVO4 phosphors were synthesized by the conventional solid-state reaction. Under 980nm laser diode excitation, intense red, green and blue up-conversion emissions were observed for the Yb3+/Er3+, Yb3+/Ho3+ and Yb3+/Tm3+ couples in the YVO4 host, respectively, which were due to the successive energy transfer from Yb3+ to Er3+, Ho3+ or Tm3+. Bright white luminescence upon 980nm near-infrared excitation can be observed for the sample at the optimum chemical composition of YVO4:10%Yb3+/1%Tm3+/0.3%Ho3+/0.2%Er3+. Under different excitation pump powers, the obtained optimal color coordinate (0.323, 0.325) is very close to the standard white light (0.333, 0.333) coordinate.

Giant enhancement of phonon-assisted one-photon excited frequency upconversion in a Nd3+-doped tellurite glass

Changing the sample's temperature from 200 K to 535 K, we observed 670-fold enhancement of a phonon-assisted upconversion emission at ≈754 nm obtained from a Nd3+-doped tellurite glass excited by 5 ns laser pulses at 805 nm. A rate-equation model, including the relevant energy levels and temperature dependent transition rates, is proposed to describe the process. The results fit well with the data when one considers the nonradiative transitions contributing for the 754 nm luminescence are promoted by an effective phonon mode with energy of 700 cm−1.

Tailorable Multicolor Up‐conversion Emissions in Tm3+/Ho3+/Yb3+ Co‐Doped LiLa(MoO4)2

Using a modified sol–gel method, LiLa(MoO4)2: Tm3+/Ho3+/Yb3+ phosphors with tailorable up‐conversion (UC) emission colors were prepared. Under the excitation of a 980 nm laser diode, up‐conversion red and green emissions in Ho3+/Yb3+ co‐doped and blue emission in Tm3+/Yb3+ co‐doped LiLa(MoO4)2 were observed, respectively. The intensities of the RGB (red, green, and blue) emissions could be controlled by varying concentrations of Tm3+ or Ho3+, and the optimal composition was also determined. In Tm3+/Ho3+/Yb3+ co‐doped LiLa(MoO4)2, the UC emission colors could be tuned from blue through white to yellow by adjusting the concentrations of Tm3+ or Ho3+. The UC excitation mechanisms were also investigated based on the power dependence of UC luminescence intensity.

Temperature-dependent luminescence and temperature-stimulated NIR-to-VIS up-conversion in Nd3+-doped La2O3–Na2O–ZnO–TeO2 glasses

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 700K. Basing on the obtained values of J–O parameter values (×10−20cm2: 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 164μs and is slightly higher than the measured decay time of 162μs. With the increasing of Nd2O3 concentration from 0.5 to 7.0mol% the experimental lifetime of the fluorescent level decreases from 162 to 5.6μs. 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 635.3nm were observed at high temperatures using the 804nm 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 1063nm laser host as well as an optical sensor for temperature measurements.

Structure and luminescence properties of the novel multifunctional K2Y(WO4)(PO4):Ln3+ (Ln = Tb, Eu, Yb, Er, Tm and Ho) phosphors

Lanthanide ion (Tb, Eu, Yb, Tm, Er, and Ho) activated multifunctional phosphors K2Y(WO4)(PO4), (KYWP) were prepared via a conventional solid-state reaction. The crystal structure of KYWP as a new host matrix for luminescence was firstly defined to be the orthorhombic system with space group Ibca (73) via Rietveld refinement of the powder X-ray diffraction and the GSAS software. The vacuum ultraviolet (VUV) excited photoluminescence and cathodoluminescence (CL) analysis of individual Tb3+/Eu3+ activated KYWP phosphors exhibit excellent emission properties in their respective regions. Both KYWP:Tb3+ and KYWP:Eu3+ had high quenching concentration under 147 nm excitation attributed to a long Y–Y distance in KYWP structure, whereas they showed very low quenching concentration under low-voltage electron-beam excitation. This phenomenon was ascribed to the different mechanisms between CL and VUV excited luminescence. Under 980 nm laser excitation, yellow, purple, and red up-conversion (UC) emissions had been achieved in Yb3+–Er3+, Yb3+–Tm3+, and Yb3+–Ho3+ co-doped KYWP, respectively. In Yb3+–Tm3+–Ho3+ tridoped KYWP, red emissions emerging from transitions of Tm3+ changed to the transitions of Ho3+. And the possible reason had been elucidated by a cross-relaxation process between Tm3+ and Ho3+. Laser power dependence of the UC emissions and the energy level diagrams were studied to understand the UC and cross-relaxation mechanisms. The obtained results indicated that the multifunctional lanthanide ion doped KYWP exhibited excellent VUV photoluminescence, CL and multicolor UC luminescence properties.

Up-conversion luminescence in LaF3:Ho3+via two-wavelength excitation for use in solar cells

An efficient broadband excited near-infrared to visible up-conversion is observed in LaF3:Ho3+ as the result of a two-wavelength excitation. The visible up-conversion emission intensity is greatly enhanced upon simultaneous excitation at 970 nm and 1150 nm, due to an energy transfer up-conversion mechanism. Multi-wavelength excitation based on the ground-state absorption, excited-state absorption, and phonon-coupled absorption of rare-earth ions results in an efficient broadband excited up-conversion emission, which may provide a new approach to fully harvest NIR solar energy and has potential application in solar cells.

Effect of Excitation Migration and Upconversion in Highly Erbium-doped Glass Micro-sphere Lasers

Excitation migration and upconversion process in the Erbium-doped silica-alumina glass microsphere lasers with concentrations of 2500-4000~ppm were investigated in detail. The experiment shows that under 976~nm excitation, the intense of up-conversion emission at 523, 546 and 657~nm, corresponding to the transitions $^{2}$H$_{11 / 2 } \to ^{4}$I$_{15 / }$, $^{4}$S$_{3 / 2} \to ^{4}$I$_{15 / 2}$, and $^{4}$F$_{9 / 2} \to ^{4}$I$_{15 / 2}$, respectively, $^{ }$depends on the erbium content, migration of excitation and pump power. The excitation migration has strongly influenced on the threshold and Red shift of lasing wavelength and migration-assisted up-conversion process leads to degraded amplification performance of microsphere lasers made by silica-alumina glasses with different contents of Er-ions.

Flux growth of honeycomb-like β-NaYF4:Yb3+, Er3+/Tm3+ crystals with multicolor upconversion luminescence

Highly crystalline NaYF4:Ln3+ (Ln=Yb, Er, Tm) crystals with honeycomb structure were successfully synthesized for the first time by using a mixed NaCl–KCl flux cooling method at a holding temperature of 800°C for 2h in air. The results indicated that the crystals of NaYF4:Ln3+ belonged to hexagonal system and three-dimensional open pores had an average diameter of 1μm interconnected with each other. Upon 980nm laser diode excitation, yellow green and sky blue upconversion fluorescences of β-NaYF4:20%Yb3+, 2%Er3+ crystals and β-NaYF4:20%Yb3+, 0.5%Tm3+ crystals, which was caused by two- or three-photon upconversion processes, was observed clearly.