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Lanthanide upconverted luminescence for contactless optical thermometry.

Detailed optical properties of Tb3+-Yb3+ co-doped phosphate glasses were performed based on their emission spectra and decay measurements. Under blue excitation of Tb3+ at 488 nm, the intensity of Yb3+ emissions gradually enhanced upon increasing the Yb3+ content until 1 mol% indicated an energy transfer from Tb3+ to Yb3+. Otherwise, under near infrared excitation of Yb3+ at 980 nm, these glasses exhibit intense green luminescence, which led to cooperative sensitization of the 5D4 level of Tb3+ ions. A cooperative energy transfer mechanism was proposed on the basis of the study on the influence of Yb3+ concentration on up-conversion emission intensity, as well as the dependence of this up-conversion intensity on near infrared excitation power. Moreover, the temporal evolution of the up-conversion emissions have been studied, which was in positive agreement with a theoretical model of cooperative up-conversion luminescence that showed a temporal emission curve with rise and decay times of the involved levels.

Optical temperature sensors based on the morphology-dependent resonant modes of Ho3+ doped fibers and microspheres were investigated. Luminescent whispering-gallery-modes (WGMs) were detected for both the fibers and the microspheres in a contactless configuration using a modified confocal microscope to excite and detect the Ho3+ luminescence. The WGMs were simulated by numerical solution of the Helmholtz equations and using the geometrical approximation, achieving a good agreement with the experimental results in both cases. A red shift of the spectral position of the WGMs was detected when the microresonators were heated either by contact with a regulated thermal bath or under high power laser irradiation. The thermal resolution of the WGM microresonators was advantageously compared with results reported for other rare-earth doped optical sensors based on the fluorescence intensity ratio technique.

An optical temperature sensor suitable for label free liquid sensing has been designed and characterized. The sensor combines the photochemical stability of rare earth doped glasses and the high sensitivity of interferometric resonators. It is formed by a planar Fabry-Perot (FP) microcavity filled with the liquid to be monitored. A Ho3+ doped tapered optical fiber has been placed inside the microcavity surrounded by the fluid medium. An external laser is focused on the optical fiber inside the cavity to induce the luminescence of the Ho3+ ions, which couples to the FP optical resonances. The spectral position of the FP resonances is highly sensitive to the refractive index of the cavity medium. A second laser is co-aligned with the first one to locally heat the liquid medium around the optical fiber. An average blue shift of the FP resonances around 32 pm/°C is measured. The limit of detection of the laser induced heating of the liquid medium is about 0.3 °C in the biological temperature range. Alternatively, a hot-plate is used to heat the system. Interestingly, a red shift of the FP modes is observed with 75 pm/°C dependence and 0.12 °C limit of detection features.

0.5Ce3+-xSm3+ (x = 0.11, 0.68 and 1.05 mol%) co-doped phosphate glasses were successfully synthesized via melt-quenching method and analyzed using absorption, photoluminescence and decay curves. According to Judd-Ofelt formalism, the Ω2, Ω4 and Ω6 parameters were determined and used to obtain some radiative properties. With the increasing of Sm3+ concentration, it was noticed the decrease of emission peak intensity and decay curves of Ce3+ due to the energy transfer Ce3+→ Sm3+. Moreover, it was observed that the intensity of Sm3+ emission increases until 0.68 mol%, then decreases for the 1.05 mol% Sm3+ concentration. According to the decay curves of Sm3+, this decreasing can be explained by the cross relaxation between Sm3+ ions. The calculated lifetimes by Judd-Ofelt theory and the experimental lifetimes for the prepared phosphate glasses were obtained. The dependence of the emission intensity on the concentration of doping ions has been explained using energy transfer models in order to obtain the optimum Sm3+ concentration.

In several scientific and technical fields, contactless optical thermometry has been successfully used for precise local temperature sensing. One of the primary goals at the moment is to enhance sensitivity and resolution. In this work, we report the synthesis of crystalline Yb3+-Er3+ and Yb3+-Ho3+ co-doped Na3Gd(VO4)2 (NGVO) microparticles by a citrate-based sol-gel method. The phase purity, vibrational modes, morphology and luminescence characteristics were evaluated by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), and Photoluminescence (PL). Temperature sensing in the visible region is allowed by an effective upconverted emission from rare-earth ions (Yb3+, Er3+, Ho3+) codoped into matrix. The luminescence intensity ratio (FIR) technique is used to record temperature sensing behaviors based on UC luminescence in Yb3+/Ln3+-codoped NGVO phosphors and high sensitivities were obtained. The Sr value of the NGVO Er3+/Yb3+ as a function of temperature are calculated to be 0.83% K−1. For the NGVO: Yb3+/Ho3+ phosphors, the Sr values were determined for TCLs and non-TCLs and reaches 1.37% K−1, and 1.90% K−1, respectively. These particles have a sub-degree thermal resolution at 295 K, making them excellent materials for accurate temperature sensing. The temperature-induced multicolor emissions in the analyzed samples suggest that the Er3+, Ho3+/Yb3+ co-doped NGVO crystals could have promising applications as a safety sign in high-temperature environments.

Oxyfluoride glass microspheres doped Nd3+ were synthesized and calibrated to function as optical pressure sensors. When the microspheres were placed in a vacuum chamber and excited at 532 nm, Nd3+ typical emission spectrum was recorded. Due to the specific geometry of the microspheres, that allow them to act as an optical resonator, it was also possible to record Whispering Gallery Modes (WGM) superimposed to the Nd3+ typical emission spectrum. Furthermore, when decreasing pressure, changes in the Fluorescence Intensity Ratio (FIR) of the 2H9/2,4F5/2 → 4I9/2 (820 nm) and 4F3/2 → 4I9/2 (890 nm) Nd3+ transitions as well as a red-shift of the WGM was observed. Both changes were calibrated as a function of pressure. Relative sensitivity and limit of detection were estimated for both techniques as a way of comparing their performance, also taking into account the previous results found in the literature. Due to the high thermal expansion and thermo-optical coefficient of oxyfluoride glasses and the high energy gap of Nd3+ thermally coupled levels, the sensor presented groundbreaking levels of relative sensitivity and limit of detection.

Trivalent erbium-doped tellurite glasses have been prepared and investigated with an aim to improve the efficiency of a Silicon (Si) solar cell through photon upconversion processes. The upconversion emissions at 548 nm (4S3/2, 2H11/2 → 4I15/2), 671 nm (4F9/2 → 4I15/2), 800 nm (4I9/2 → 4I15/2) and 975 nm (4I11/2 → 4I15/2) of Er3+ ions have been observed under 1500 nm laser excitation. The 3.0 mol% Er2O3-doped glass exhibits high upconversion emission at 975 nm compared to 0.01 and 0.5 mol% Er2O3–doped glasses. Temporal evolution studies confirm that energy transfer upconversion is responsible for observed upconversion emissions, which is in good agreement with a simple rate-equations model. The upconversion intensity and photocurrent of solar cell are found to increase with the increase of pump power of the laser. A significant enhancement in photocurrent of a solar cell has been observed, when the 3.0 mol% Er2O3-doped glass is placed over a solar cell under 1500 nm laser excitation. All the results indicate that 3.0 mol% Er2O3-doped glass could be suitable for enhancing the solar cell efficiency.

This work shows the utility of the carbon dots, prepared from candle shoot, as promising candidates for nanothermometry devices based on temperature dependence of their emission lifetimes and of the changes of the full width at half maximum of the emission spectrum. The results indicate that both parameters show appreciable changes over the physiological temperature range (20–60 °C).

Single crystals of Nd 3+ -doped Co-picromerite has been synthesized by the slow evaporation method. After two weeks on hold, the crystals obtained were characterized by different techniques, indeed X-ray powder diffraction (XRPD) shows that these compounds crystallize in the monoclinic system with space group P21/a. Complete dehydration of the double salt was obtained before reaching 200 °C with a phase transition of KS around 570 °C. The IR spectroscopic study confirms the vibrational modes of the sulfate groups and water molecules. Exciting at 473 nm, the emission peaks associated with the 4F 5/2 →4I 9/2 , 4F 3/2 →4I 9/2 and 4F 3/2 →4I 11/2 transitions were observed. The luminescence decay curve was analyzed in the frame of the Inokuti-Hirayama model in order to obtain the intrinsic lifetime and the energy transfer parameter among Nd 3+ ions.

This work highlights some physical properties on langbeinite K 2 Mg 2 (SO 4 ) 3 :Dy 3+ sulfates which were prepared by high temperature solid state reaction technique. X-ray powder diffraction confirmed the high purity of the obtained phases. These sulfates crystallize in the cubic phase with space group P2 1 3 . Several spectroscopic measurements on the synthesized samples were carried out: Fourier transform infrared spectroscopy showed the different vibration modes of sulfate group, the ultraviolet – visible – near infrared absorption spectroscopy revealed Dy 3+ transitions from the ground state 6 H 15/2 to different excited levels and the emission spectra in the visible region revealed four emission bands originated from the 4 F 9/2 level to the 6 H 15/2 , 6 H 13/2, 6 H 11/2 and 6 H 9/2 + 6 F 11/2 levels of Dy 3+ ions. Moreover, the temperature dependence lifetime of this emitting level can be explained in terms of the thermalization effect.

Yb3+-doped langbeinite salts were prepared by the solid solution method. X-ray diffraction patterns and vibrational spectroscopy confirmed that all obtained phases are highly pure, iso-structural and they crystallize in the cubic system with the space group P213. The emission luminescence comes from the 2F5/2 → 2F7/2 transition of Yb3+ ions. Moreover, intense blue cooperative emission was observed at 476 nm under excitation in the near infrared at 975 nm.

In this work, a new strategy to increase the temperature working range of rare-earth based optical thermometers is demonstrated. Nanocrystalline yttrium gallium nanogarnets codoped with 0.1% Er3+ and 0.1% Ho3+ were synthesized. The codoped samples combine the high performance of erbium ions at high temperatures and that of holmium ions at low temperature, providing an overall relatively high sensor performance in a temperature range from 30 to 540 K, broader than that achievable by single Er3+ or Ho3+ doped nanogarnets. Measurements were carried out using 406 and 473 nm commercial diode lasers to excite the erbium and holmium ions, respectively. The emission spectra were analyzed as a function of temperature, using the fluorescence intensity ratio technique for each ion. Relative sensitivity and limit of detection were obtained in order to compare the efficiency of the sensor with optical thermometers reported up to date, capable of working in a similar range from cryogenic to high temperatures. The sensor presents a maximum relative sensitivity of about 1.3% K−1 at 200 K when exciting the erbium ions and around 0.4% K−1 at 200 K for the holmium ions. The limit of detection is under 0.5 K for most of the range from 30 K to 540 K using this combination of ions.

Poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is a semiconducting optically active polymer widely used in optoelectronics research. MEH-PPV can be commercially acquired in a large range of molecular weights. However, the influence of this property on the optical performance of the polymer is often disregarded. In this paper, the thermal dependence of the refractive index of MEH-PPV thin films prepared from high and medium molecular weight polymers is investigated. Thus, monolithic Fabry-Perot (FP) microcavities are fabricated, in which the active polymer film is part of their defect layer. It is found that when these devices are used as optical temperature sensors, the position of the emission band of the microcavities excited with a blue diode laser shifts to lower wavelengths when temperature increases with sensitivities in the 0.2–0.3 nm/°C range. This effect is ascribed to the variation in the refractive index of the polymer active layer within the resonator with temperature. According to theoretical simulations of optical transmittance by classical transfer-matrix method and the evaluation of the optical eigenmodes by finite element methods of the manufactured FP resonator cavities, it is found that the MEH-PPV films present negative thermo-optic coefficients of about −0.018 K−1 and −0.0022 K−1 for high and medium molecular weight polymers, respectively, in the temperature range between 20 and 60 °C. These values are about the highest reported so far, to the best of our knowledge, and points to high performance thermal sensor applications.

We experimentally demonstrate the high sensitivity of a novel liquid state, whispering-gallery-mode optical resonator to humidity changes. The optical resonator used consists of a droplet made of glycerol, a transparent liquid that enables high optical quality factor, doped with fluorescent material. As glycerol is highly hygroscopic, the refractive index and radius of the droplet change with ambient humidity. This produces a shift on the whispering gallery mode's wavelengths, which modulates the emission of the fluorescent material. This device shows an unpreceded sensitivity of 10sup-3/supper relative humidity percent.

Brain is arguably the most complex organ in biology. It shows a heterogeneous structure, in which different areas are responsible for different functions. New non-invasive techniques to monitor the structure and/or composition of the brain can help to understand this organ. In this paper, we show successful random lasing from a mouse brain tissue impregnated with a dye laser and demonstrate that it can be used to detect changes in structure and composition. We systematically observe that the lowest random lasing threshold and highest intensity are recorded in the corpus callosum anatomical region of the brain. This area is formed by a large number of axon fibers coated by myelin, which is a lipid rich substance. Both the fiber structure and the lipid rich composition contribute to increase the scattering strength, which improves random lasing. A thorough photoluminescence characterization of the tissues as a function of the staining concentrations is also included in the study.