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Photon avalanche (PA) is a highly nonlinear mode of upconversion that is characterized by 100–1000‐fold increase in luminescence intensity upon minute increments of pumping power. The practical realization of numerous possible nano‐bio‐technology applications utilizing the PA phenomenon will require information on its susceptibility to the material volume and surface. Here, these parameters are investigated via experimental and theoretical PA. The two‐color, highly nonlinear PA emission at 475 and 800 nm is clearly observed in bulk single crystal, individual microcrystals, and ensembles of colloidal core and core–shell nanoparticles of LiYF4 host doped with either 3 or 8% of thulium ions. The properties of PA emission, such as PA nonlinearity, PA gain, PA intensity, and luminescence kinetics in these materials show dependence on crystal volume and surface quenching. Theoretical simulations provide understanding of key physical processes that influence PA performance. Moreover, photon avalanche single beam super‐resolution imaging is realized for the first time in 3% Tm3+ doped LiYF4 core–shell nanoparticles. The obtained insights and predictions form a solid background for further development and applications of new optimized PA materials. [ABSTRACT FROM AUTHOR]

Gd 3 (Al,Ga) 5 O 12 :RE3+, Yb3+ single crystals (RE3+ = Tm3+, Er3+, Ho3+, Pr3+) were fabricated by the Czochralski technique. Effect of Yb3+ concentration on spectral distribution of luminescence emitted by interacting RE3+-Yb3+ ion pairs and on the dynamics of excitation energy flow in systems under study was investigated and interpreted referring to previously reported spectroscopic features of Gd 3 (Al,Ga) 5 O 12 :RE3+ systems. It was concluded that observed quenching of the Yb3+ emission by co-doped rare earth ions results from an interplay between phenomena of quantum cutting, down shifting and energy dissipation by multiphonon emission. [ABSTRACT FROM AUTHOR]

Abstract Spectroscopic properties of Gd 3 Ga 3 Al 2 O 12 (GGAG) single crystals co-doped with various configurations of (Eu + Tb); (Eu + Ce); (Tb + Ce) and (Eu+ Tb + Ce) ions have been investigated. The effect of distinct Ln3+ co-doping variations on luminescence of garnet crystals was studied using the different excitation wavelengths. The Ln3+-Ln3+ and host-Ln3+ interplays have been considered in doubly and triply doped GGAG:Ln3+. The crystals containing cerium ions are characterized by mainly broadband emission in the 450–750 nm spectral region that is adequately modified in green and red spectral range in the samples co-doped with terbium and europium, respectively. The Eu3+ luminescence in materials under study is selectively excited employing solely f-f europium transitions. Contrary to that, emission of terbium and cerium occur as result of interaction with Gd3+ and adjacent co-dopants. Graphical abstract Image 1 Highlights • Novel doubly and triply - doped GGAG garnet crystals for lighting system applications. • Luminescent and excitation properties of GGAG:Ce,Tb,Eu crystals. • Optimization of phosphor color by various combination of Ce, Tb, Eu dopants. • Evaluation of Ln3+-Ln3+ and host-ion interactions. [ABSTRACT FROM AUTHOR]

The CsCaI 3 single crystals doped with Tm 2+ were grown by Bridgman technique. Divalent thulium luminescence related to the 4f 13 -4f 13 intra-configurational and 4f 12 5d 1 -4f 13 inter-configurational transitions were analyzed as a function of temperature. Excited state excitation spectroscopy has been applied within a wide spectral range to examine peculiarities of transitions between different 4f 12 5d 1 excited states of Tm 2+ . This two-color excitation experiment was useful to assess d-d photon-absorption processes that are relevant to observed up-conversion phenomena attributed to effective anti-Stokes green luminescence around 550 nm. Furthermore, activation energy of 4f 12 5d 1 → 4f 13 luminescence quenching was estimated and high value of 2 F 5/2 → 2 F 7/2 emission cross-section was determined in CsCaI 3 :Tm 2+ . Our spectroscopic study was performed to complement and verify the previous investigation in this field. [ABSTRACT FROM AUTHOR]

Optical absorption and emission spectra of Dy 3+ ions in Gd 3 Ga 3 Al 2 O 12 single crystal were investigated at room temperature and T = 5 K in the visible (VIS) and near-infrared (NIR) region. Low-temperature spectra were utilized to determine the energy level structure of Dy 3+ in the GGAG host. The detailed analysis implied that optically active ions reside mainly into well-defined sites in the crystal studied. Judd–Ofelt (JO) theory was used to calculate oscillator strengths, transitions intensity parameters Ω 2,4,6 , radiative transition probabilities, luminescence branching ratios and radiative lifetime of dysprosium luminescent level. Relaxation dynamic of 4 F 9/2 excited state was examined as a function of temperature. High quantum efficiency of dysprosium luminescence level, value of the 4 F 9/2 → 6 H 13/2 yellow emission cross-section and the quality of remaining spectroscopic parameters indicate that the Gd 3 Ga 3 Al 2 O 12 :Dy 3+ optical system can be considered as promising phosphor and visible laser material. [ABSTRACT FROM AUTHOR]

Single crystals of YAl 3 (BO 3 ) 4 doped with trivalent samarium were grown by the top-seeded high temperature solution method and their absorption and emission spectra were investigated. Optical pumping into prominent absorption band around 405 nm feeds the 4 G 5/2 metastable level giving rise to intense visible luminescence distributed in several spectral lines with the most intense line around 600 nm characterized by a branching ratio of 0.42 and peak emission cross section of 0.25×10 −20 cm 2 . Optical amplification at 600 nm with a gain coefficient of 2.9 cm −1 was achieved during a pump-and-probe experiment. [ABSTRACT FROM AUTHOR]

Highlights: [•] Samarium doped langatate crystal was grown by Czochralski method. [•] Thermal dependences of Sm3+ emission and absorption have been measured. [•] Judd–Ofelt analysis was carried out and radiative transition rates were calculated. [•] Inhomogeneous broadening of spectral lines has been observed. [ABSTRACT FROM AUTHOR]

Single crystal of LaGaTaO (LGT) containing intentionally 0.5 % of Ho and 1 % of Yb was grown by the Czochralski method. Examination of chemical composition of the grown crystal revealed that luminescent holmium and ytterbium ions are preferably retained in the melt and their actual concentrations are 0.12 and 0.24 %, respectively. Spectroscopic investigation performed encompassed IR host absorption spectra and Raman spectra at room temperature, optical absorption and luminescence spectra of Ho and Yb at room temperature and at 5 K, and luminescence decay curves at room temperature. It was found that all spectral bands recorded show important inhomogeneous line broadening. This feature was attributed to structural disorder inherent to the crystal lattice in which pentavalent Ta ions occupy octahedral Ga(1) sites together with trivalent Ga ions. Despite small concentrations of luminescent ions, the occurrence of nonradiative interaction that feeds the I and I levels of Ho ions by transfer of an excitation from the F level of Yb ions was evidenced. Based on examination of spectroscopic parameters evaluated, it was concluded that LGT:Ho, Yb may be considered as a potential intermediate-gain laser active material able to emit infrared radiation from the I → I transition of Ho around 2 micrometres upon laser-diode pumping into Yb absorption band. [ABSTRACT FROM AUTHOR]

Highlights: [•] Optical and structural investigation of (Lu0.4Gd0.55Sm0.05)2SiO5 single crystal. [•] Absorption and emission spectra highly influenced by optical anisotropy of material. [•] Cross-relaxation between samarium ions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Influence of temperature on optical properties of the Gd3Ga5O12:Sm single crystals. [•] Analysis of absorption and emission spectra as well as luminescence lifetimes. [•] Estimation of energies of crystal field components of Sm3+ in the Gd3Ga5O12 crystal. [•] Study on energy transfer phenomena using the classical Inokuti–Hirayama model. [ABSTRACT FROM AUTHOR]

Spectra and decay curves of luminescence induced in a single crystal of LiTaO 3 :Pr3+ by femtosecond light pulses at different wavelengths from the UV-Blue and NIR regions were recorded and analyzed. It was found that excited levels involved in the broad UV absorption band of LiTaO 3 : Pr3+ are not able to relax via radiative transitions but they feed nonradiatively excited states of the ground Pr3+ configuration. Femtosecond pulse excitation in the NIR region results in an up-converted visible luminescence weakly affected by the pulse wavelength within the 1200 nm–1800 nm region. Spectra acquired imply that the supercontinuum (SC) generated does not promote single - or double-photon processes that may feed the 3P 0 or 1D 2 levels via transitions within the ground 4f2 configuration. It was concluded that the excitation mechanism is consistent with the multiphoton excitation (MPE) that feeds the 5d levels of Pr3+. • Conversion of femtosecond pulses into optically uniaxial system. • Down and up-conversion phenomena in LiTaO 3 :Pr3+ crystal. • Effect of supercontinuum generation on Pr3+ excited states excitation. • Ultrashort pulses induced change of electrical conductivity. [ABSTRACT FROM AUTHOR]

Single crystal of La 3 Ga 5.5 Ta 0.5 O 14 (LGT) doped with Dy3+ was fabricated by the Czochralski technique and investigated employing methods of optical spectroscopy. Polarized optical absorption spectra and luminescence spectra were recorded at room temperature. Unpolarized absorption and luminescence spectra were recorded at 10 K. Spectra of blue luminescence related to transitions 4I 15/2 , 4F 9/2 → 6H 15/2 were recorded at several temperatures in the 300–773 K region. Dy3+ ions have the same symmetry sites in LGT but their spectra show inhomogeneous broadening due to structural disorder of the host and are markedly anisotropic. Results of the Judd-Ofelt treatment made it possible to assess emission cross sections for the 4F 9/2 → 6H 15/2 blue luminescence and for the 4F 9/2 → 6H 13/2 yellow luminescence employing Fuchtbauer-Ladenburg formula and reciprocity method. In particular, a spectral distribution of La 3 Ga 5.5 Ta 0.5 O 14 : Dy3+ σ-polarized luminescence band with an emission cross section σ em = 0.51 × 10−20 cm2 at 573.5 nm is favorable for a laser operation in the visible. Interpretation of the effect of temperature on the blue emission spectra leads to the conclusion that La 3 Ga 5.5 Ta 0.5 O 14 : Dy3+ is promising for application as luminescent thermometer. • Novel single crystal La 3 Ga 5.5 Ta 0.5 O 14 :Dy (LGT:Dy). • Detailed analysis of optical spectra and luminescence decay curves. • Potential visible laser operation at 573.5 nm. • Temperature sensitivity of LGT:Dy luminescence. [ABSTRACT FROM AUTHOR]

Single crystals of Gd 3 (Al,Ga) 5 O 12 with intentional stoichiometry Gd 3 Ga 2.5 Al 2.5 O 12 doped with 1%Tm3++x%Yb3+; (x = 1, 3, 5); were grown by the Czochralski method. Spectra and intensities of up-converted luminescence excited at 975 nm or 803 nm were measured as a function of temperature within 295 K–775 K region. It was found that for the two excitation wavelengths the growth of temperature brings about a decrease of an up-converted blue luminescence intensity. Thermally enhanced growth of nonradiative relaxation rates for excited states involved in the up-conversion process was determined based on an analysis of respective luminescence decay curves. Temperature-dependent change of fluorescence intensity ratio (FIR) for the 3F 3,2 – 3H 6 /3H 4 – 3H 6 up-converted bands was observed and resulting absolute temperature sensitivity of 0.058 K-1 was found for GAGG:1%Tm,3%Yb at 725 K. • Single crystals for high temperature applications. • Thulium thermally coupled excited states in novel co-doped GAGG:Tm, Yb garnet crystals. • Impact of temperature-activated excitation energy flow on optical thermometry sensitivity. • Effect of optically active ions interplays and energy transfer phenomena on temperature sensitivity. [ABSTRACT FROM AUTHOR]

Terbium ions in Gd 3 Ga 3 Al 2 O 12 (GGAG) single crystal exhibit strong luminescence originated in the 5D 3 and 5D 4 multiplets. The decay time of the 5D 3 state is 1072 μs and 724 μs at 5 K and 300 K, respectively. The time dependence of luminescence originated from the 5D 4 multiplet is consistent with single exponential decay, with lifetime values 3579 μs at low temperature and 2902 μs at 300 K. The non-exponential decay curves have been fitted in the frame of Inokuti-Hirayama model, indicating that the dipole-dipole interaction plays an important role in the fluorescence quenching of the 5D 3 level. The calculated R 0 parameter values are equal to 11.56 Å and 12.86 Å for 5 and K 300 K, respectively. The positions of multiplets up to 40000 cm−1 are determined employing the f-shell empirical programs. CIE 1931 coordinates are x = 0.298 , y = 0.664 (green) at 5 K and x = 0.360, y = 0.538 (yellowish green) at 300 K. • A new material has been obtained in the form of single crystal. • A starting stoichiometry is given to minimize segregation process. • Low temperature positions of 4f and 5d states have been determined. • Energy transfer processes have been examined. • Influence of temperature on the Tb transition ratio has been investigated. [ABSTRACT FROM AUTHOR]

Abstract: Spectroscopic measurements of the calcium niobate CaNb2O6 single crystal doped with samarium ions were performed. Polarized absorption and emission spectra as well as luminescence decay curves of this material were recorded as a function of temperature in the 5–300K temperature region. Analysis of low temperature spectra made it possible to determine energies of crystal field components of Sm3+ multiplets involved in the excitation and luminescence phenomena. Number of crystal field levels derived implies that Sm3+ ions in the CaNb2O6 host are accommodated in several different sites. Radiative lifetime of the 4G5/2 metastable level determined based on the Judd–Ofelt analysis of room temperature absorption spectra amounts to 777μs, a value close to that of luminescence lifetime determined from luminescence decay curve. Energy transfer between the NbO6 group and samarium ions was observed in wide temperature range. Intense luminescence related to the 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions centered at 610nm and 660nm, respectively, combined with strong absorption band near 404nm that matches perfectly the pump light provided by commercial InGaN/GaN diode lasers points at a potential of CaNb2O6:Sm3+ for the design of all-solid-state visible lasers. [Copyright &y& Elsevier]

Abstract: Spectroscopic properties of lutetium aluminum garnet Lu3Al5O12 (LuAG) single crystal doped with 1at.% of Pr3+ were investigated. The crystal was grown by the Czochralski method. Optical absorption, emission, excitation spectra and luminescence decay curves were recorded for this system in 5–500K temperature range. Excitation spectra in VUV–UV range (150–330nm) were also measured exploiting synchrotron radiation. It has been found that among bands of luminescence related to the 3PJ, 1D2 → 3HJ, 3FJ transition of Pr3+ in the visible region the 3P0 → 3H4 one, occurring ca. 488nm, is the most prominent. Luminescence lifetimes for the 3P0,1 and 1D2 levels were estimated to be 13 and 200μs respectively (at room temperature). VUV-excited emission spectrum stretches over the whole visible region. It has been concluded that the material under study is a promising phosphor that can be efficiently pumped by broad absorption bands centered at 240 and 280nm. [Copyright &y& Elsevier]

The Czochralski growth of Dy3+-doped (Lu1−xGdx)2SiO5 solid solution single crystals with different mixtures of Lu2SiO5 and Gd2SiO5 oxides (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) is reported. It was found that the (Lu1−xGdx)2SiO5 crystals retain structural, physical, and optical properties of Lu2SiO5 even for x = 0.8. Optical investigation of Dy:(Lu1−xGdx)2SiO5 revealed that the structural disorder of the host brings about a strong inhomogeneous broadening of Dy3+ spectral bands while influencing weakly rates of its radiative and nonradiative transitions. Dielectric characteristics of solid solution crystals are presented and discussed. Finally, (Lu1−xGdx)2SiO5 solid solution crystals seem to be an attractive alternative to a Lu2SiO5 host for the design of novel phosphors, radiation converters, and lasers owing to a significantly lower melting temperature and hence easier and less expensive manufacturing. Dy3+-doped (Lu1−xGdx)2SiO5 solid solution crystals (x = 0.2, 0.4, 0.6, 0.8) were grown by the Czochralski method and examined. The studied crystals retain structural, physical, and optical properties of Dy:Lu2SiO5 even for x = 0.8. Finally, (Lu1−xGdx)2SiO5 crystals are an interesting alternative to the Lu2SiO5 host for applications due to significantly lower melting temperatures and hence easier and less expensive manufacturing. [ABSTRACT FROM AUTHOR]

Detailed spectroscopic investigation of YVO 4 :5 at% Eu 3+ , 1 at % Tm 3+ single crystal encompassing analysis of absorption, excitation and time resolved emission spectra recorded at 5 K and 300 K made it possible to determine energies, intensities and bandwidths of transitions within 4f n configurations of trivalent thulium and europium. Transients recorded upon femtosecond pulse excitation made it possible to determine dynamics of population build-up and relaxation of excited states involved in luminescence phenomena. It was found that in YVO 4 :Eu, Tm system the metastable 5 D 0 level of Eu 3+ and 1 G 4 level of Tm 3+ can be populated by energy transfer from excited VO 4 3− group and respective intra-ion relaxation processes from higher energy levels. Recorded excitation spectra of the 5 D 0 luminescence of Eu 3+ and of the 1 G 4 luminescence of Tm 3+ do not evidence the Eu 3+ ↔Tm 3+ energy transfer. Nevertheless, an examination of acquired luminescence decay curves, performed with reference to available information on relaxation dynamics of YVO 4 :Tm 3+ system, reveals that the incorporation of Eu 3+ accelerates decays of the 1 G 4 and 3 H 4 levels of Tm 3+ . Spectroscopic parameters characterising the donor–acceptor interaction involved in the 3 H 4 decay were evaluated applying the Inokuti–Hirayama model. Relevance of donor and acceptor transition peculiarities for energy transfer processes contributing to the 1 G 4 and 3 H 4 decays was assessed in the framework of phenomenological model proposed by Kushida and discussed. It was concluded that the intensity ratio of the blue 1 G 4 luminescence to orange–red 5 D 0 luminescence; hence the colour of visible emission in UV-excited YVO 4 :Eu 3+ , Tm 3+ phosphor, may be adjusted by appropriate choice of Eu 3+ and Tm 3+ concentrations. [ABSTRACT FROM AUTHOR]

Abstract: Illumination of single crystal samples of erbium-doped YVO4 and LuVO4 by infrared femtosecond pulses brings about an intense green luminescence assigned to the 4S3/2 - 4I15/2 transition of Er3+ ions. When lowering the sample temperature an additional broad band in the blue related to the VO4 3− emission appears and next grows steadily. It has been observed that the variation of wavelength of incident infrared femtosecond pulses in the region 800nm–1600nm affects weakly the intensity of both the blue and green luminescence bands. Analysis of luminescence dynamics made it possible to reveal that upon infrared excitation the rise time of the 4S3/2 luminescence is longer by a factor of four, roughly as compared to that recorded upon direct excitation into high energy levels of Er3+. Observed temperature dependence of up-converted blue and green luminescence implies that the energy transfer from vanadate groups cannot be considered as a main mechanism involved in the excitation erbium ions. It has been supposed that erbium ions are likely to be excited by energy transfer from free electrons created in the conduction band of the host by multiphoton excitation and /or by non-resonant multi-step ETU process involving absorption of infrared light in multiphonon side bands of electronic transitions. Creation of free electrons has been corroborated by changes of electrical conductivity of crystals induced by an illumination with infrared femtosecond pulses. [Copyright &y& Elsevier]

In this work, spectroscopic properties of (Lu 0.25 Gd 0.75) 2(1-x) Tm 2x SiO 5 (x = 0.04 and 0.005) single crystals were investigated. The material was found to be biaxial, exhibiting strong optical anisotropy, as clearly demonstrated in polarized absorption and emission spectra. Data from room temperature polarized absorption spectra were used in calculations within the phenomenological Judd-Ofelt model. The intensity parameters Ω t were established at Ω 2 = 3.24 (±0.58), Ω 4 = 2.96 (±0.24) and Ω 6 = 0.86 (±0.14) [10−20 cm2]. The values of fluorescence lifetimes of the 1D 2 and 1G 4 levels were measured and subsequently compared with those calculated using the Judd-Ofelt theory. The strong absorption bands located at wavelengths around 355, 460 and 790 nm, related to the 3H 6 → 1D 2 , 1G 4 , 3H 4 transitions, were found suitable for optical pumping of the investigated material. The characteristics of emission spectra recorded in the blue (ca. 455 nm) and IR (ca. 1750 nm) regions indicate that further investigations of stimulated emission within those wavelength ranges are well justified and can be considered promising. • Spectroscopic investigations of Tm doped (Lu 0.25 Gd 0.75) 2 SiO 5 single crystals were conducted. • Material shows high potential for lasing in IR region ca. 1900 nm. • Influence of Anisotropy and temperature on spectroscopic properties is reported. [ABSTRACT FROM AUTHOR]