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Orthorhombic sodium niobate (NaNbO3) nanocrystals, transparent in the visible and near-infrared, were synthesized by using the Pechini's sol-gel method. Experiments were performed to measure the first hyperpolarizability of the nanoparticles by using a pulsed laser operating at 1064 nm. The hyper-Rayleigh scattering technique was used to analyze the second harmonic scattered light generated by the nanocrystals. By using the external reference method, with para-nitroaniline as the standard reference, we determined the NaNbO3 hyperpolarizability per nanoparticle volume (0.93 × 10−29 esu/nm3) for the first time, and show that the effective second-order susceptibility has the same order-of-magnitude as LiNbO3 and KNbO3 nanocrystals.

Recently, mixed-former glasses have attracted a large attention due to the appropriate combinations of the chemical, physical and optical properties of each glass former. In this scenario, the optical properties of fluoroborate glasses doped with rare earth ions is of importance due to their good transparency window from ultra-violet to near-infrared region, good rare-earth solubility, long excited state lifetime of such ions, and high mechanical strength. In this work, we had studied the optical properties of the Tb3+ doped fluoroborate glasses, focusing on the concentration dependent behavior of luminescence dynamics of 5D3 and 5D4 exited states of this ion. The blue-to-green intensity ratio, lifetime of the emitting levels under various excitation wavelengths, cross-relaxation rate, ion-ion critical distance and emission gain bandwidth are calculated.

Photon avalanche (PA) is a fascinating mechanism for exciting trivalent rare-earth ions (REI3+). It occurs when cross-relaxation mechanisms provide a non-zero population at a lower excited state. Then, photons that are nonresonant with ground-state transitions can be increasingly absorbed from this level. PA has been demonstrated with some REI3+-doped materials under specific excitation wavelengths and temperature regimes. Here the PA-like was observed on NdAl3(BO3)4 particles at some fixed temperature values (within 23 °C ≤ T ≤ 215 °C), while the excitation power was increased. In this study, the PA-like is associated to a cross-relaxation mechanism [4F3/2, 4I9/2] → [4I15/2, 4I15/2]. After multiphonon emissions, the population of Nd3+ ions at the 4I11/2 state doubles at every iteration, a process followed by a resonant excited-state absorption (4I11/2 → 4F3/2). In this case, the excitation power threshold decreased threefold. In a second round of experiments, we demonstrate for the first time the triggering the PA-like mechanism by increasing the temperature (27 °C ≤ T

Er3+/Tm3+/Yb3+ tri-doped YNO phosphors were obtained by the solid-state reaction method. X-ray diffraction, Raman spectroscopy, and FE-SEM were employed for characterization of the materials, and no significant structural changes have been noticed. The materials exhibited red, green and blue (RGB) upconversion emissions under 980 nm excitation. The increase of the Tm3+ concentration reduced the blue and green UC emissions due to possible Tm3+-Tm3+ and Er3+-Tm3+ energy transfer processes, respectively. White UC emission with CIE color coordinates equal to x = 0.31 and y = 0.35 has been obtained in the YNO:0.5%Er3+2.0%Tm3+20%Yb3+ composition under 64.5 mW/cm2. The tri-doped systems presented color tunability behavior indicating that multicolor, such as white light, can be easily achieved by adjusting of the Tm3+ and Yb3+ contents.

BaBi2Nb2O9:Li+/Tm3+/Er3+/Yb3+ phosphors were successfully synthesized by the high-temperature solid-state method and up-conversion white-light emission has been achieved by tuning of the Tm3+ and Li+ contents. X-ray diffraction and room temperature photoluminescence were used, respectively, to investigate the structural and optical properties of the luminescent systems. Decay dynamics of 4F9/2 levels in BBN:Tm3+/Er3+/Yb3+ and BBN:Er3+/Yb3+ phosphors have been analyzed to study the energy-transfer mechanism between Er3+ and Tm3+ ions. The introduction of Li+ ions into the BBN:0.5%Tm3+0.05%Er3+8%Yb3+ has induced an adjustment of tristimulus values shifting the chromaticity color coordinates of the phosphor towards to the ideal white region (x = 0.31 and y = 0.34). Furthermore, the high color purity white-light coming from the BBN:2%Li+0.5%Tm3+0.05%Er3+8%Yb3+ composition was obtained under moderate power density establishing the BBN phase as a host matrix able to generate RGB with energy saving.

This work presents a spectroscopic investigation of tellurite glasses with the composition 65TeO2-15Li2O-20ZnO doped with different concentrations of Er2O3. Optical absorption and emission spectroscopy were performed at room temperature to determine emission properties of Er3+ in the visible and near-infrared regions. The samples presented intense green emission due to the H 11 / 2 2 → I 15 / 2 4 and S 3 / 2 4 → I 15 / 2 4 transitions. Emission intensities at 992 and 1550 nm exhibit a relative increase with increasing doping concentration. Luminescence decay curves for the S 3 / 2 4 → I 15 / 2 4 transition follows a non-exponential behavior, while the I 11 / 2 4 → I 15 / 2 4 and I 13 / 2 4 → I 15 / 2 4 transitions presented simple exponential behavior with high lifetime values. Under 975 nm excitation, upconversion luminescences in the green and red regions are observed, with a relative increase for red emission as function of doping concentration. The possible mechanisms considering multi-phonon relaxation, energy transfer and cross-relaxation processes were discussed for green and red emissions under excitations at 488 and 975 nm. The square dependence of green emission on the excitation power indicates two photons contribution to the upconversion emission. The full width at half-maximum values of emission at 1550 nm increased with increasing Er3+ concentration, showing the potential of the studied material as infrared amplifier.

Oxygen impurities played a pivotal role by affecting the optical and structural properties of Nd:YF3 phosphor, mainly when annealing procedure at high temperature is applied on it. In this work, it was demonstrated that luminescence decay, excitation and emission spectra of Nd:YF3 phosphor were greatly influenced by quenching sites and crystal bulk and surface defects induced by the presence of oxygen impurities incorporated into its structure. The excitation spectra presented a broad band from 200 to 550 nm, related to oxygen impurities, showing an alternative pathway to identify its presence. To overcome this recurrent problem, fluorine excess was used to reduce the oxygen contamination, allowing improvement of both crystallinity and luminescence efficiency of the Nd:YF3 phosphor. Despite this outstanding result, it was found that the fluorine content cannot be applied indiscriminately above of the stoichiometric proportion and its optimum quantity in excess was found to be at around 20%.

Rare earth ions doped transparent glasses are among promising solid-state materials for various optical applications. In this report, Eu3+ ions doped new fluoroborate glasses were prepared by conventional melt-quenching technique and their optical properties were characterized by absorption and luminescence spectroscopy as well as excited state lifetime measurements. The absorption spectra revealed the characteristic transitions between the 7F0 ground state and 7F1 first excited state to the upper-lying electronic states. The variation in the luminescence intensity of the radiative transitions occurring at 590 and 611 nm and the bi-exponential behavior of the fluorescence decay of the 5D0 level, with different trend for the amplitude of the decays, are discussed in terms of low and high symmetry sites depending on the EuF3 concentration and the possible energy transfer between ions at these sites.

Tm3+/Dy3+ co-doped LaF3 nanocrystals have been synthesized by the co-precipitation method and the luminescence properties under UV light excitation examined. Tunable light emission containing blue (450 nm), yellow (570 nm), and red (660 nm) light components was observed. The excitation spectrum was investigated and the optimum excitation UV wavelength determined. The excitation mechanism responsible for the generation of the multicolor emission indicated that Tm-Dy energy-transfer plays a major role in the process. Tuning of the overall white-light emission was accomplished via variation of the Tm and Dy active ions contents in the nano-powder samples. The CIE chromaticity coordinates show that the Tm3+/Dy3+ co-doped LaF3 may be an excellent candidate for white light emitting diode devices.

Pr3+ and Yb3+ co-doped NaYF4 nanocrystals (NCs) were synthetized via a simple coprecipitation method and investigated their efficiency of near-infrared quantum cutting (down-converter) with possible application for photovoltaic solar cells. The effect of the Yb3+ concentration on the down-conversion and quantum cutting emissions was evaluated under ultraviolet/visible excitation. The Pr3+ emissions under excitation at 442 nm were observed in the range from 450 to 1550 nm. It was obtained that increasing the Yb3+ concentration all the Pr3+ emissions decrease, and it was associated to an efficient energy transfer process between Pr3+ and Yb3+ ions. In addition, the Yb3+ emission at 974 nm decreases with the Yb3+ concentration increasing, being the highest emission observed for the NC doped with 2 mol.% of Yb3+. The lifetime of the 3P0 state were measured and increasing the Yb3+ concentration the decay becomes faster leading to strong reduction of the lifetime from 18.6 (0 mol.% of Yb3+) to 2.2 μs (10 mol%Yb3+). A quantum cutting efficiency of 188% in NaYF4 co-doped with 0.5 mol.% of Pr3+ and 10 mol.% of Yb3+ was obtained, which is a relatively high value, close to the limit of 200%. Therefore, these investigated NCs, synthetized via a very simple and cheap coprecipitation route, have great potential for solar cells applications of silicon-based materials by down-converting the ultraviolet/visible part of the solar spectrum to ~1000 nm photons.

Phosphate glasses having composition 50P2O5–10B2O3–20ZnO–20PbO–xDy2O3 where x = 0.04, 0.18, 0.36, 1.10, 1.80, and 3.60 mol% were prepared by melt quenching technique. The FTIR spectra reveal the different structural groups of the phosphate glass induced by modifiers such as lead oxide and zinc oxide, which could be explained in terms of the distortion of PO4 tetrahedra and the network depolymerization process. The photoluminescence spectrum shows three prominent emission bands centered at 475, 586, and 675 nm corresponds to the 4F9/2 → 6HJ (J = 11/2, 13/2, 15/2) transitions, respectively, and the intensity ratio of yellow to blue emissions are enhanced as the concentration of Dy3+ ions increased up to 3.60 mol%. The 4F9/2 excited state's lifetime decreases by continuously increasing the amount of the trivalent lanthanide ion due to probable energy transfer between neighboring ions. The chromaticity diagram is studied as the rare-earth ion concentration, which gives the nearest condition to the white color at 0.36 mol% of the Dy3+ ions.

Pr2O3 doped 26.66B2O3–52.33PbO–16GeO2–4Bi2O3–1Pr2O3 (BPGBPr) glass was synthesized by melt quenching technique. X-ray diffraction confirmed the amorphous nature of the glass and the presence of different vibrational groups was identified by Raman spectroscopy. Differential thermal analysis indicated that the glass transition and crystallization temperature were ~354 °C and ~521 °C respectively, indicating good thermal stability. Visible and near infrared absorption spectra were measured and used to evaluate the Judd-Ofelt intensity parameters to calculate the radiative properties for the emission levels of Pr3+. Photoluminescence spectra were recorded in the visible and infrared regions at temperatures between 16 and 300 K. The spectroscopic results indicated that BPGBPr can be useful as a material for broadband optical amplifier in the region of ~1450 cm−1.

Nd 3+ ion doped LaF 3 dielectric nanoparticles have recently emerged as very attractive multifunctional nanoparticles capable of simultaneous sub-tissue heating and thermal sensing. Although they have been already used for selective photothermal treatment of cancer tumors in animal models, their real application as self-monitored photothermal agents require further optimization and development. Dynamic adjustment of the therapy parameters is mandatory for non-selective damage minimization. It would require real-time (sub-second) thermal sensing with a sub-degree thermal resolution. In this work we demonstrate that meeting this challenge is, indeed, possible by performing controlled thermal treatment on as-synthesized Nd 3+ doped LaF 3 nanoparticles. Temperature induced lattice ordering and defect re-combination have been concluded to induce, simultaneously, a line fluorescence narrowing, fluorescence brightness enhancement and a remarkable increment in thermal sensitivity. Ex-vivo experiments have demonstrated that, thanks to this multi-parameter optimization, Neodymium doped LaF 3 nanoparticles are capable of real time sub-tissue thermal reading with a temperature resolution as low as 0.7 °C.

The optical and structural properties of Nd:YF 3 phosphors, synthesized by precipitation reaction using ethylene glycol solvent, were investigated. The Y:F molar ratio of precursors, where NH 4 F was employed as the fluorine source, was varied to prevent oxygen contamination during nucleation of the particles and to improve the photoluminescence efficiency. Structural investigations were carried out by means of X-ray diffraction, Fourier transform infrared spectroscopy and energy dispersive X-ray techniques. Samples prepared using precursors in stoichiometric proportion presented orthorhombic YF 3 structure. The use of fluorine source in excess promoted formation of NH 4 Y 2 F 7 crystalline phase, which was decomposed into YF 3 after annealing at 400 °C for 1 h. Emission spectra of samples thermally treated enhanced by over 50 times compared with the as prepared. The results obtained show the role of fluorine excess associated with thermal annealing to obtain YF 3 with improved features.

Emission of visible and NIR frequency upconversion luminescence light mainly within the first biological window spectral region using excitation at 1319 nm in the second biological window in Tm3+/Er3+-codoped TeO2-based glass, is demonstrated. For Tm3+/Er3+-codoped tellurite glass samples, green(525, and 550 nm), red(660 nm) and NIR(980 nm) owing to erbium ions in addition to the main 800 nm signal originating from thulium ions is readily observed. Here, energy-transfer Tm3+[3F4]+Er3+[4I15/2]→Tm3+[3H6]+Er3+[4I9/2] producing the NIR emission, followed by excited-state absorption of 1.319 µm photons populating 2H11/2, 4S3/2, and 4F9/2 visible emitting levels. Erbium single-doped samples did not exhibited visible detectable emissions for excitation powers as high as 1.8 W of cw radiation at 1.319 µm. In Tm3+/Nd3+-codoped samples, energy-transfer from Tm3+ (3H6–3F4) to Nd3+ ions populates the 4F3/2 excited-state level producing NIR emission around 880 nm.

A highly transparent Sm3+ glass with composition B2O3–PbO–Bi2O3–GeO2 was obtained by the traditional melt quenching technique and characterized from structural and spectroscopic points of view. Analysis by X-ray diffraction and Raman spectroscopy confirmed the amorphous nature of the sample and revealed the expected low phonon energy. Differential thermal analysis was also carried out to obtain the glass transition and the crystallization temperatures, related to the thermal stability of the sample. Judd–Ofelt theory was applied to evaluate phenomenological intensity parameters Ωλ (λ=2, 4 and 6) from the optical absorption measurements. The transition probabilities, radiative lifetimes, branching ratio and stimulated emission cross-section were also calculated. Photoluminescence spectra recorded in the visible and infrared regions revealed intense green, orange, red and near infrared emission bands providing a new trace to develop tunable laser and optoelectronics devices.

Photo-thermal therapies, based on the light-induced local heating of cancer tumors and tissues, are nowadays attracting an increasing attention due to their effectiveness, universality, and low cost. In order to avoid undesirable collateral damage in the healthy tissues surrounding the tumors, photo-thermal therapies should be achieved while monitoring tumor’s temperature in such a way that thermal therapy could be stopped before reaching the damage limit. Measuring tumor temperature is not an easy task at all and novel strategies should be adopted. In this work it is demonstrated how luminescent nanoparticles, in particular Neodymium doped LaF 3 nanoparticles, could be used as multi-functional agents capable of simultaneous heating and thermal sensing. Advantages and disadvantages of such nanoparticles are discussed and the future perspectives are briefly raised.

In this work, we study energy transfer (ET) processes between Mn2+ and Nd3+ ions in Nd2O3-doped P2O5–ZnO–Al2O3–BaO–PbO glass system, nanostructured with Zn1−xMnxTe diluted magnetic semiconductor nanocrystals (NCs), aiming nanotechnology applications. Transmission electronic microscopy (TEM), X-ray diffraction (XRD), optical absorption (OA), photoluminescence (PL), and lifetime measurements were performed for a complete characterization of the investigated materials. The TEM and XRD data confirm the formation of Zn1−xMnxTe NCs in the host glass system. The overlap between the Mn2+ ion emission band and the Nd3+ ion absorption bands suggests that an efficient resonant type energy transfer from Mn2+ to Nd3+ ions is expected. PL spectra show that the characteristic emission relative intensities of Nd3+ ions increase systematically with increasing Mn concentration. Indeed, the 4T1(4G) state lifetime of Mn2+ ions gradually decreases with rising Mn concentration. This result confirm the evidence on ET from Mn2+ ions to Nd3+ ions. In addition, the increase in the 4F3/2 state lifetime is an evidence of persistent luminescence due to the presence of Mn2+ ions.

In the present work, we exploited the capability of the trivalent thulium (Tm3+) ions doped sodium niobate (NaNbO3) nanocrystals as an optical nanothermometer with a giant relative thermal sensitivity (Sr) of up to 12%°C−1. Tm3+:NaNbO3 nanoparticles, under excitation at 1064 nm using a Q-switched and mode-locked Nd:YAG laser, provided emissions at multi-wavelengths from 460 nm to 840 nm through non-parametric transitions in Tm3+ ions, and parametric second-harmonic generation (SHG) at 532 nm (I532) of the excitation beam thanks to the large nonlinear response of the crystalline matrix. The proposed nanothermometer is based on the intensity ratio between the upconverted fluorescence at 695 nm (I695), associated with the 3F2,3 → 3H6 Tm3+ transitions, and I532. Due to the opposite behaviors of these two emissions with the temperature in the range of 20–80 °C, a super sensitive nanothermometer was demonstrated with a record of Sr ~12% oC−1 around 40–50 °C (the biological temperature regime). In addition, an efficient pump-induced self-heating system was reported and assigned to multiphonon relaxations in Tm3+ ions. These properties make this dual emitting system a nanoprobe for many potential applications, e.g., for studying dynamical biological and pump-induced self-monitored photothermal events. The combination of SHG in a wide spectral range (e.g., in complete first biological window) and emissions of lanthanide ions greatly increases the range of applications using this system, including as a multifunctional nanoprobe.

Transparent Y 2 O 3 ceramics singly-doped with either Eu 3+ or Tm 3+ were fabricated by means of sequential consolidation steps at high pressure and temperature. These ceramics were characterized for their luminescence and thermal lensing behaviors, and the results compared to data on single crystals reported in literature. Thermal diffusivity, D , and conductivity, K , values of D =26×10 −3 cm 2 /s and K =5.8 W/m K, respectively, for 1.0 mol% Eu 3+ and 0.5 mol% Tm 3+ singly-doped Y 2 O 3 transparent ceramics were obtained. These values are about half of those for single crystal analogs. A small temperature coefficient of the optical path length change, ds / dT =3×10 −6 K −1 , was determined, making these materials suitable for applications requiring nearly athermal response. Selected spectroscopic properties were obtained by means of Judd–Ofelt analysis and together with thermal lens results provided absolute values for the fluorescence quantum efficiency of several levels, particularly 62% for the 5 D 0 level of Eu 3+ and 84% for the 3 F 4 level of Tm 3+ .

In this work we report on the ability of Er/Yb co-doped NaYF4 nano-crystals and CdTe Quantum Dots as two-photon excited fluorescent nano-thermometers. The basic physical phenomena causing the thermal sensitivity of the two-photon excited emission bands have been discussed and the maximum thermal resolution achievable in each case has been estimated. The practical application of both systems for thermal sensing at the micro-scale in biological systems is demonstrated. In particular, they have been used to evaluate the thermal loading induced by tightly focused laser beams in both living cells and fluids.

The propagation of an optical beam through dielectric media induces changes in the refractive index, Δn, which causes self-focusing or self-defocusing. In the particular case of ion-doped solids, there are thermal and non-thermal lens effects, where the latter is due to the polarizability difference, Δα, between the excited and ground states, the so-called population lens (PL) effect. PL is a pure electronic contribution to the nonlinearity, while the thermal lens (TL) effect is caused by the conversion of part of the absorbed energy into heat. In time-resolved measurements such as Z-scan and TL transient experiments, it is not easy to separate these two contributions to nonlinear refractive index because they usually have similar response times. In this work, we performed time-resolved measurements using both Z-scan and mode mismatched TL in order to discriminate thermal and electronic contributions to the laser-induced refractive index change of the Nd3+-doped Strontium Barium Niobate (SrxBa1−xNb2O6) laser crystal. Combining numerical simulations with experimental results we could successfully distinguish between the two contributions to Δn.

We present highly efficient 480 and 800 nm upconversion emissions in Tm 3+ /Yb 3+ co-doped water-free low silica calcium aluminosilicate glass under excitation at 976 nm. As a result of this efficient upconversion process, a luminescent switching with the excitation intensity has been observed. The switching is explained and discussed using rate equations analysis and saturation effects. By means of fitting of the experimental data point, it was possible to obtain the value of the energy transfer parameter related to the transition 2 F 5/2 , 3 H 4 → 2 F 7/2 , 1 G 4 . The value of this parameter is higher than that of materials like YLF. This switching mechanism could be used in the development of sensors and networks for optical processing and optical communications.