Optical Temperature-Sensing Performance of Nd3+ :MF2 (M=Ba, Ca, Sr) Crystalline Powders Prepared by Combustion Synthesis.

There is a large interest in luminescent materials for application as temperature sensors. In this scenario, we investigate the performance of neodymium-doped alkaline-earth fluoride (Nd3+ :MF2 ; M=Ba, Ca, Sr) crystalline powders prepared by combustion synthesis for optical temperature-sensing applications based on the luminescence intensity ratio (LIR) technique. We observe that the near-infrared luminescence spectral profile of Nd3+ changes with the temperature in a way that its behavior is suitable for optical thermometry operation within the first biological window. We also observe that the thermometric sensitivities of all studied samples change depending on the spectral integration range used in the LIR analysis. Nd3+ :CaF2 presents the largest sensitivity values, with a maximum absolute sensitivity of 6.5×10-3 /K at 824 K and a relative sensitivity of 1.71 %/K at human-body temperature (310 K). The performance of CaF2 for optical thermometry is superior to that of ß-NaYF4 , a standard material commonly used for optical bioimaging and temperature sensing, and on par with the most efficient oxide nanostructured materials. The use of thermometry data to help understand structural properties via Judd-Ofelt intensity standard parameters is also discussed.

UV random laser emission from flexible ZnO-Ag-enriched electrospun cellulose acetate fiber matrix.

We report an alternative random laser (RL) architecture based on a flexible and ZnO-enriched cellulose acetate (CA) fiber matrix prepared by electrospinning. The electrospun fibers, mechanically reinforced by polyethylene oxide and impregnated with zinc oxide powder, were applied as an adsorbent surface to incorporate plasmonic centers (silver nanoprisms). The resulting structures - prepared in the absence (CA-ZnO) and in the presence of silver nanoparticles (CA-ZnO-Ag) - were developed to support light excitation, guiding and scattering prototypes of a RL. Both materials were excited by a pulsed (5 Hz, 5 ns) source at 355 nm and their fluorescence emission monitored at 387 nm. The results suggest that the addition of silver nanoprisms to the ZnO- enriched fiber matrix allows large improvement of the RL performance due to the plasmon resonance of the silver nanoprisms, with ~80% reduction in threshold energy. Besides the intensity and spectral analysis, the RL characterization included its spectral and intensity angular dependences. Bending the flexible RL did not affect the spectral characteristics of the device. No degradation was observed in the random laser emission for more than 10,000 shots of the pump laser.

Managing optical heating via Al3+-doping in Er3+:SrF2 powder phosphors prepared by combustion synthesis.

Quenching of photoluminescence due to optical heating generated by high power laser sources has been identified as a major concern for photonics applications that relies on inorganic phosphor materials. Here we investigate how erbium-doped strontium fluoride (Er3+:SrF2) powders prepared by combustion synthesis respond to intense optical heating. We found that the near-infrared to visible photon up-conversion (UC) luminescence from Er3+ was quenched and the internal temperature of the sample increased from 298 to 695 K when the excitation power of a CW diode laser operating at 808 nm was increased from 0.1 to 2.1 W. However, when SrF2 was co-doped with Al3+, we observed an increase in the UC intensity and an unexpected internal temperature reduction of up to 155 K for an excitation power of 2.1 W. Our analysis suggests that Al3+ decreases the phonon energy and increases the local symmetry of the environment of the rare-earth ion in SrF2.

Enhancement of 1.5 µm fluorescence signal from Er3+ due to Yb3+ in yttrium silicate powders pumped at 975 and 808 nm.

The effect of the presence of ytterbium (Yb3+) on the near-infrared (NIR) emission profile of erbium (Er3+), more specifically the 4I13/2 â†’ 4I15/2 radiative transition around 1.5 µm, in yttrium silicate crystalline ceramic powders prepared by combustion synthesis was investigated under different NIR laser excitation wavelengths (λ = 808 and 975 nm). Enhancement of fluorescence around 1.5 µm due to the presence of Yb3+ was observed, which has potential use in medicine (NIR-III biological window) and optical communications (C-band transmission window). Two different excitation channels involving energy transfer between Er3+ and Yb3+ were studied: one involving the sensitization of Er3+ by Yb3+ (for λ = 975 nm laser light excitation) and the other involving direct excitation of Er3+ with Yb3+ acting as an energy reservoir (for λ = 808 nm laser light excitation). The energy transfer mechanisms of both processes are discussed in the text.

Cooling of Er(3+) with Tm(3+) for accurate temperature sensing using yttrium silicate compact powders.

Er(3+) doped nanocrystalline powders are extensively used for thermometry based on luminescence spectral analysis. The luminescence from Er(3+) is produced by a nonlinear (two-photon) absorption process which may generate strong internal heat by activation of nonradiative relaxation channels. If the heat dissipation is not efficient, as is the case for compact powders, there will be inaccurate readings of the temperature. Our proposed solution is to cool down Er(3+) by transferring part of its accumulated energy to another rare-earth element in the lattice. Here, we show our results for Er(3+)-Tm(3+) co-doped yttrium silicate powders prepared by combustion synthesis.

Nd³âº-Yb³âº doped powder for near-infrared optical temperature sensing.

Er³âº doped powders are generally used for fluorescence-based temperature sensing application when near-infrared lasers are the excitation sources of choice. The fluorescence of Er³âº is produced by nonlinear (upconversion) processes, which generate strong internal heat. Lowering the excitation power causes drastic reduction of the fluorescence signal, and as a consequence the sensor applicability of Er³âº doped powders becomes compromised. Here we propose the use of the downconverted fluorescence of Yb³âº produced by efficient energy transfer from Nd³âº as an alternative temperature sensing system. Our results are presented for yttrium silicate powders prepared by combustion synthesis.

Red to blue tunable upconversion in Tm3+-doped ZrO2 nanocrystals.

The effect of dopant concentration on the blue upconversion (UPC) emission of Tm(3+) -doped ZrO(2) nanocrystals under different excitation wavelengths in the red region is reported. The UPC emissions are due to the f-f electronic transitions from excited states (1)G(4) and (1)D(2) of Tm(3+). We observed a chromatic change in the UPC with tuning the excitation wavelength. The UPC emission bands at 475, 488, and 501 nm are observed under excitation at 649 nm, but bands centered at 454 and 460 nm are observed when the excitation wavelength is tuned to 655 nm. The UPC emission could be tuned from 501 to 454 nm ( approximately 47 nm) by changing the excitation wavelength from 649 to 655 nm ( approximately 6 nm). The pump power dependence of the emission bands at 475, 488, and 501 nm were investigated on excitation intensity at 649 nm, and the emission bands at 454 and 460 nm are investigated on excitation intensity at 655 nm, which confirms that all of these UPC emission lines are a two-photon absorption process.