Leopoldo L. Martin | Universidad Politécnica de Valencia (original) (raw)
Papers by Leopoldo L. Martin
Optics Express, 2016
We fabricate a liquid-core liquid-clad microcavity that is coupled to a standard tapered fiber, a... more We fabricate a liquid-core liquid-clad microcavity that is coupled to a standard tapered fiber, and then experimentally map the whispering-gallery modes of this droplet resonator. The shape of our resonator is similar to a thin prolate spheroid, which makes space for many high-order transverse modes, suggesting that some of them will share the same resonance frequency. Indeed, we experimentally observe that more than half of the droplet's modes have a sibling having the same frequency (to within linewidth) and therefore exhibiting a standing interference-pattern.
Optics Express, 2015
We experimentally demonstrate trapping a microdroplet by using an optical tweezer and then activa... more We experimentally demonstrate trapping a microdroplet by using an optical tweezer and then activating it as a microresonator by bringing it close to a tapered-fiber coupler. Our tweezers facilitated the tuning of the coupling from the under-coupled to the critically-coupled regime while the quality-factor [Q] is 12 million and the resonator's size is at the 80 μm scale.
ABSTRACT Export Date: 24 May 2012, Source: Scopus
Laser Refrigeration of Solids VIII, 2015
Improving the conversion efficiency of solar energy to electricity is most important to mankind. ... more Improving the conversion efficiency of solar energy to electricity is most important to mankind. For single-junction photovoltaic solar-cells, the Shockley-Queisser thermodynamic efficiency limit is extensively due to the heat dissipation, inherently accompanying the quantum process of electro-chemical potential generation. Concepts such as solar thermophotovoltaics and thermo-photonics, have been suggested to harness this wasted heat, yet efficiencies exceeding the Shockley-Queisser limit have not been demonstrated due to the challenge of operating at high temperatures. Here, we present a highly efficient converter based on endothermic photoluminescence, which operates at relative low temperatures. The thermally induced blue-shifted photoluminescence of a low-bandgap absorber is coupled to a high-bandgap photovoltaic cell. The high absorber's photo-current and the high cell's voltage results in 69% maximal theoretical conversion efficiencies. We experimentally demonstrate tenfold thermal-enhancement of useful radiation for the high-bandgap cell and 107% enhancement in average photon energy. This paves the way for introducing disruptiveinnovation in photovoltaics.
Photoluminescence (PL) is a fundamental light-matter interaction, which conventionally involves t... more Photoluminescence (PL) is a fundamental light-matter interaction, which conventionally involves the absorption of energetic photon, thermalization and the emission of a red-shifted photon. Conversely, in optical-refrigeration the absorption of low energy photon is followed by endothermic-PL of energetic photon. Both aspects were mainly studied where thermal population is far weaker than photonic excitation, obscuring the generalization of PL and thermal emissions. Here we experimentally study endothermic-PL at high temperatures. In accordance with theory, we show how PL photon rate is conserved with temperature increase, while each photon is blue shifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also show how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. Relying on these observations, we propose and theoretically study thermally enhanced PL (TEPL) for highly efficient solar-energy conversion, with thermodynamic efficiency limit of 70%.
Optical Materials, 2010
An appropriated thermal treatment of SrO-BaO-Nb 2 O 5 -B 2 O 5 glass leads to the formation of gl... more An appropriated thermal treatment of SrO-BaO-Nb 2 O 5 -B 2 O 5 glass leads to the formation of glass ceramics which are composed of Tm 3+ -Yb 3+ codoped strontium barium niobate (SBN) nanocrystals dispersed throughout an amorphous matrix. The glasses have been fabricated using a melt-quenching method and after a thermal treatment at 620°C have been obtained the glass ceramic samples. The formation of SBN nanocrystals has been confirmed by the X-ray diffraction patterns. Visible and NIR emission spectra have been reported and confirmed the incorporation of the Tm 3+ -Yb 3+ ions into the SBN nanocrystals. In the glass ceramics samples the decays of Tm 3+ levels for the Yb 3+ -Tm 3+ codoped samples are longer than in the Tm 3+ single doped samples. This surprising result could be explained in basis to the localisation of the active ions in the SBN nanocrystals.
![Research paper thumbnail of Transfer and back transfer processes in Yb[sup 3+]–Er[sup 3+] codoped fluoroindate glasses](https://attachments.academia-assets.com/53405592/thumbnails/1.jpg)
](Journal of Applied Physics, 1999
The kinetics of forward and backward energy transfer processes in fluoroindate glasses double dop... more The kinetics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb 3ϩ and Er 3ϩ has been studied. It was possible to excite selectively Yb 3ϩ ions and detect emission due only to these ions or combined with the emission coming from the Er 3ϩ ions. At low concentration of Yb 3ϩ ͑0.1 mol %͒ the emission decay of these ions is nonexponential when there is back transfer from Er 3ϩ ions; the dynamics is well described using the fluorescence ''transfer function'' model. Moreover, the evolution of the emission from the 4 I 11/2 (Er 3ϩ ) level is in good agreement with the behavior predicted by the model. The critical radii obtained from the fitting for forward and backward energy transfer are compared with those calculated with the Dexter formula. When the Yb 3ϩ concentration is increased, the migration among these ions is very important and the emission decay of the Yb 3ϩ ions is nearly exponential with the lifetime of these ions. This behavior is explained considering that, at high Yb 3ϩ concentration and due to the migration, the transfer from Yb 3ϩ to Er 3ϩ ions is restricted to very closed couples of ions with high back transfer probability. The limitations of the fluorescence ''transfer function'' model associated with the correlation effect in the excitation of Er 3ϩ ions by energy transfer are discussed.
Lasers rely on stimulated electronic transition, a quantum phenomenon in the form of population i... more Lasers rely on stimulated electronic transition, a quantum phenomenon in the form of population inversion. In contrast, phonon masers 1-3 depend on stimulated Raman scattering and are entirely classical. Here we extend Raman lasers 1-3 to rely on capillary waves, which are unique to the liquid phase of matter and relate to the attraction between intimate fluid particles. We fabricate resonators that co-host capillary 4 and optical modes 5 , control them to operate at their non-resolved sideband and observe stimulated capillary scattering and the coherent excitation of capillary resonances at kilohertz rates (which can be heard in audio files recorded by us). By exchanging energy between electromagnetic and capillary waves, we bridge the interfacial tension phenomena at the liquid phase boundary to optics. This approach may impact optofluidics by allowing optical control, interrogation and cooling 6 of water waves.
2015 International Conference on Optical MEMS and Nanophotonics (OMN), 2015
Optics Express, 2016
This publisher's note amends a re... more This publisher's note amends a recent publication [Opt. Express24(3), 2850-2857 (2016)] to include Acknowledgments.
Droplets, particularly water droplets, are abundant in both natural and artificial systems. Their... more Droplets, particularly water droplets, are abundant in both natural and artificial systems. Their capillary oscillations are governed by surface tension and are therefore distinguished from acoustic oscillations. These capillary oscillations play a major role in droplet coalescence, for example, and are also an important phenomenon in interface theories. Here, we experimentally and theoretically analyze the capillary oscillation within an optical cavity with walls of water. Our droplet benefits from an optical finesse of 520 that, accordingly, boosts its sensitivity in recording Brownian capillaries with amplitudes of 1 0.025 Å and kilohertz rates in agreement with natural-frequency calculations. Our hybrid device allows resonantly enhanced interactions between electromagnetic and capillary waves that could potentially lead to optical excitation or the cooling of droplet capillary oscillations.
We experimentally demonstrate, for the first time, binding of aerosols of various sizes and shape... more We experimentally demonstrate, for the first time, binding of aerosols of various sizes and shapes in white light. The optomechancial interaction between particles is long range and is in the underdamped regime. Incoherency allows mitigation of interference fringes to enable monotonically changing the distance between particles from 60 μm to contact, constituting a parametrically controlled testbed for transition studies at new scales.
In submerged microcavities there is a tradeoff between resonant enhancement for spatial water and... more In submerged microcavities there is a tradeoff between resonant enhancement for spatial water and light overlap. Why not transform the continuously resonating optical mode to be fully contained in a water microdroplet per se? Here we demonstrate a sustainable 30-mm-pure water device, bounded almost completely by free surfaces, enabling 41,000,000 re-circulations of light. The droplets survive for 416 h using a technique that is based on a nano-water bridge from the droplet to a distant reservoir to compensate for evaporation. More than enabling a nearly-perfect optical overlap with water, atomic-level surface smoothness that minimizes scattering loss, and B99% coupling efficiency from a standard fibre. Surface tension in our droplet is 8,000 times stronger than gravity, suggesting a new class of devices with water-made walls, for new fields of study including opto-capillaries.
Photoluminescence (PL) is a fundamental light–matter interaction that conventionally involves the... more Photoluminescence (PL) is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a redshifted photon. Conversely, in optical refrigeration, the absorption of a low-energy photon is followed by endothermic PL of an energetic photon. These two quantum processes are, in contrast to thermal emission, governed by photon-rate conservation. Thus far, both aspects of PL have been studied under thermal population that is far weaker than the photonic excitation, hiding the role of rate conservation when thermal excitation is significant. Here we theoretically and experimentally study endothermic PL at high temperatures. In contrast to thermal emission, we find that the PL photon rate is conserved with temperature increase, while each photon is blueshifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrate how endothermic PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. These new findings show that endothermic PL is an ideal optical heat pump. This opens the way for a proposed novel device that harvests thermal losses in photovoltaics with record efficiency.
.N− Thermo−optic effects 78.55.−m Photoluminescence Properties and materials a b s t r a c t We a... more .N− Thermo−optic effects 78.55.−m Photoluminescence Properties and materials a b s t r a c t We analyze the suitability of Nd 3+ doped phosphate glasses as optical temperature sensors based on the response of their luminescence bands to temperature. At room temperature, laser excitation of the sample at 532 nm results in an emission spectrum with an intense band centered at 880 nm and an adjacent weak broad band centered at 810 nm, corresponding to the Nd 3+ ion transitions: 4 F 3/2 → 4 I 9/2 and 4 F 5/2 → 4 I 9/2 respectively. Because of thermalization effects between the next 4 F 3/2 and 4 F 5/2 energy levels ( E ≈ 908 cm −1 ), a detectable change on the relative intensities of these emission bands occurs when temperature is increased, affording the experimental calibration of the fluorescence intensity ratio with temperature in the range 300-850 K. It has been shown that the radiative transfer processes favored by the content of Nd 3+ ions are also responsible for changes to the spectra shapes and so play an important role in the fluorescence intensity ratio. A reliable thermal sensing operation was obtained in a 0.1 mol% doped Nd 3+ phosphate glass with relative sensitivity values ranging from 153 × 10 −4 K −1 at 300 K to 22 × 10 −4 K −1 for 850 K, demonstrating good prospects for optical temperature measurements compared to other reported results. However, the reabsorption measurements seen in the samples with a higher Nd 3+ content reveal that the reliability of the sensing performance is compromised with the doping concentration and the experimental conditions of the measurement. (C. Pérez-Rodríguez). transparent medium without the need for wires or coupling devices . Regarding their construction and the region measured, these systems are designed to be small sized or distributed . As with several other sensing devices, temperature optical sensors are widely proposed in the form of optical fibers since, apart from their inherent light guiding of the luminescence, this construction also confers light weight, small size and low cost . Linked with the optical fibers, the trivalent Rare Earth (RE) elements have been one the most reported luminescence centers in the literature. Several fluorescence emissions from RE 3+ in adequate hosts have exhibited spectral changes associated with a temperature variation. Some remarkable examples are the variations in the intensity of the emission lines from Eu 3+ in La 2 O 2 S [7], or the thermal induced evolution of the Ce 3+ emission decays in YAG nanocrystals . Uniquely, the RE 3+ luminescence emitters -Pr 3+ , Nd 3+ , Sm 3+ , Ho 3+ , Er 3+ , Dy 3+ , Er 3+ and Yb 3+ -undergo modifications in the http://dx.
La 2 TeO 6 nanocrystals doped with Eu 3 þ ions have been prepared by the Pechini sol-gel process.... more La 2 TeO 6 nanocrystals doped with Eu 3 þ ions have been prepared by the Pechini sol-gel process. A total of seven samples obtained with different Eu 3 þ concentrations (1-7%). The Eu 3 þ ions are usually taken as probe ions to test the local structure of the lanthanide in solids. Analyzing the luminescence has been shown two different sites for the Eu 3 þ ions (in good agreement with the crystallographic analysis). Moreover, the luminescence properties have been analyzed as function of the Eu 3 þ doping concentration in order to study the interaction between these ions. Under direct excitation into the 5 D 0 level (at 578 nm) the corresponding decay curves show a pure exponential character independently of the Eu 3 þ concentration. However, the decay curves obtained for the 5 D 1 level becomes non-exponential for the higher doped nanocrystals samples indicating that the energy transfer processes are important.
The effects of pressure on the behavior of optical microspherical resonators, prepared from Nd 3 ... more The effects of pressure on the behavior of optical microspherical resonators, prepared from Nd 3 doped, barium titanium silicate glass, have been studied up to 5 GPa inside a diamond anvil cell using silicone oil as the hydrostatic transmission medium and ruby emission lines as the pressure gauge. The optical resonances, known as whispering gallery modes, were observed within the broad emission band of the Nd 3 ions, and the resonances were identified as a function of pressure. By means of simulations, it was found that the average wavelength position of both transverse electric and magnetic modes depended on the radius and refractive index of the sphere, but not on the refractive index of the pressure transmitting medium under the experimental conditions. This was used to define the average sensitivity of the resonant modes with the pressure. Therefore, a value of 6.5 × 10 −4 GPa −1 has been obtained for this sensitivity, which is higher than the value for ruby, the most conventional pressure sensor.
In this work, the generation of photocurrent in a commercial solar cell has been achieved under e... more In this work, the generation of photocurrent in a commercial solar cell has been achieved under excitation at 1480 nm in fluoroindate glass samples codoped with Yb 3+ and Er 3+ ions. These samples were placed over a solar cell and under excitation at 1.48 mm, upconversion emissions coming from the Yb 3+ and Er 3+ ions were obtained. In the upconversion emission spectra, several emission bands at 545 nm, 660 nm and 975 nm have been detected for all the analyzed samples (doped with different Yb 3+ and Er 3+ concentrations). The dependence of the external quantum efficiency to produce photocurrent with the Er 3+ and Yb 3+ concentration and with the pump power excitation at 1480 nm has been analyzed. As conclusion, the maximum generation of photocurrent was obtained with the highest and lowest concentrations of Er 3+ and Yb 3+ , respectively.
Laser action using non-coupled excitation and detection of microspheres made of Nd 3+ doped bariu... more Laser action using non-coupled excitation and detection of microspheres made of Nd 3+ doped barium-titanium-silicate glass has been demonstrated and measured. The microspheres have also been successfully deposited over Si 3 N 4 strip waveguides with a SiO 2 separation layer, thus enabling the laser emission extraction onto a CMOS compatible photonic circuit. The dynamics of the lasing wavelength and intensity has been studied as a function of the pump power and interpreted in terms of thermal effects generated through non-radiative recombination of the excited ions.
Optics Express, 2016
We fabricate a liquid-core liquid-clad microcavity that is coupled to a standard tapered fiber, a... more We fabricate a liquid-core liquid-clad microcavity that is coupled to a standard tapered fiber, and then experimentally map the whispering-gallery modes of this droplet resonator. The shape of our resonator is similar to a thin prolate spheroid, which makes space for many high-order transverse modes, suggesting that some of them will share the same resonance frequency. Indeed, we experimentally observe that more than half of the droplet's modes have a sibling having the same frequency (to within linewidth) and therefore exhibiting a standing interference-pattern.
Optics Express, 2015
We experimentally demonstrate trapping a microdroplet by using an optical tweezer and then activa... more We experimentally demonstrate trapping a microdroplet by using an optical tweezer and then activating it as a microresonator by bringing it close to a tapered-fiber coupler. Our tweezers facilitated the tuning of the coupling from the under-coupled to the critically-coupled regime while the quality-factor [Q] is 12 million and the resonator's size is at the 80 μm scale.
ABSTRACT Export Date: 24 May 2012, Source: Scopus
Laser Refrigeration of Solids VIII, 2015
Improving the conversion efficiency of solar energy to electricity is most important to mankind. ... more Improving the conversion efficiency of solar energy to electricity is most important to mankind. For single-junction photovoltaic solar-cells, the Shockley-Queisser thermodynamic efficiency limit is extensively due to the heat dissipation, inherently accompanying the quantum process of electro-chemical potential generation. Concepts such as solar thermophotovoltaics and thermo-photonics, have been suggested to harness this wasted heat, yet efficiencies exceeding the Shockley-Queisser limit have not been demonstrated due to the challenge of operating at high temperatures. Here, we present a highly efficient converter based on endothermic photoluminescence, which operates at relative low temperatures. The thermally induced blue-shifted photoluminescence of a low-bandgap absorber is coupled to a high-bandgap photovoltaic cell. The high absorber's photo-current and the high cell's voltage results in 69% maximal theoretical conversion efficiencies. We experimentally demonstrate tenfold thermal-enhancement of useful radiation for the high-bandgap cell and 107% enhancement in average photon energy. This paves the way for introducing disruptiveinnovation in photovoltaics.
Photoluminescence (PL) is a fundamental light-matter interaction, which conventionally involves t... more Photoluminescence (PL) is a fundamental light-matter interaction, which conventionally involves the absorption of energetic photon, thermalization and the emission of a red-shifted photon. Conversely, in optical-refrigeration the absorption of low energy photon is followed by endothermic-PL of energetic photon. Both aspects were mainly studied where thermal population is far weaker than photonic excitation, obscuring the generalization of PL and thermal emissions. Here we experimentally study endothermic-PL at high temperatures. In accordance with theory, we show how PL photon rate is conserved with temperature increase, while each photon is blue shifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also show how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. Relying on these observations, we propose and theoretically study thermally enhanced PL (TEPL) for highly efficient solar-energy conversion, with thermodynamic efficiency limit of 70%.
Optical Materials, 2010
An appropriated thermal treatment of SrO-BaO-Nb 2 O 5 -B 2 O 5 glass leads to the formation of gl... more An appropriated thermal treatment of SrO-BaO-Nb 2 O 5 -B 2 O 5 glass leads to the formation of glass ceramics which are composed of Tm 3+ -Yb 3+ codoped strontium barium niobate (SBN) nanocrystals dispersed throughout an amorphous matrix. The glasses have been fabricated using a melt-quenching method and after a thermal treatment at 620°C have been obtained the glass ceramic samples. The formation of SBN nanocrystals has been confirmed by the X-ray diffraction patterns. Visible and NIR emission spectra have been reported and confirmed the incorporation of the Tm 3+ -Yb 3+ ions into the SBN nanocrystals. In the glass ceramics samples the decays of Tm 3+ levels for the Yb 3+ -Tm 3+ codoped samples are longer than in the Tm 3+ single doped samples. This surprising result could be explained in basis to the localisation of the active ions in the SBN nanocrystals.
Journal of Applied Physics, 1999
The kinetics of forward and backward energy transfer processes in fluoroindate glasses double dop... more The kinetics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb 3ϩ and Er 3ϩ has been studied. It was possible to excite selectively Yb 3ϩ ions and detect emission due only to these ions or combined with the emission coming from the Er 3ϩ ions. At low concentration of Yb 3ϩ ͑0.1 mol %͒ the emission decay of these ions is nonexponential when there is back transfer from Er 3ϩ ions; the dynamics is well described using the fluorescence ''transfer function'' model. Moreover, the evolution of the emission from the 4 I 11/2 (Er 3ϩ ) level is in good agreement with the behavior predicted by the model. The critical radii obtained from the fitting for forward and backward energy transfer are compared with those calculated with the Dexter formula. When the Yb 3ϩ concentration is increased, the migration among these ions is very important and the emission decay of the Yb 3ϩ ions is nearly exponential with the lifetime of these ions. This behavior is explained considering that, at high Yb 3ϩ concentration and due to the migration, the transfer from Yb 3ϩ to Er 3ϩ ions is restricted to very closed couples of ions with high back transfer probability. The limitations of the fluorescence ''transfer function'' model associated with the correlation effect in the excitation of Er 3ϩ ions by energy transfer are discussed.
Lasers rely on stimulated electronic transition, a quantum phenomenon in the form of population i... more Lasers rely on stimulated electronic transition, a quantum phenomenon in the form of population inversion. In contrast, phonon masers 1-3 depend on stimulated Raman scattering and are entirely classical. Here we extend Raman lasers 1-3 to rely on capillary waves, which are unique to the liquid phase of matter and relate to the attraction between intimate fluid particles. We fabricate resonators that co-host capillary 4 and optical modes 5 , control them to operate at their non-resolved sideband and observe stimulated capillary scattering and the coherent excitation of capillary resonances at kilohertz rates (which can be heard in audio files recorded by us). By exchanging energy between electromagnetic and capillary waves, we bridge the interfacial tension phenomena at the liquid phase boundary to optics. This approach may impact optofluidics by allowing optical control, interrogation and cooling 6 of water waves.
2015 International Conference on Optical MEMS and Nanophotonics (OMN), 2015
Optics Express, 2016
This publisher's note amends a re... more This publisher's note amends a recent publication [Opt. Express24(3), 2850-2857 (2016)] to include Acknowledgments.
Droplets, particularly water droplets, are abundant in both natural and artificial systems. Their... more Droplets, particularly water droplets, are abundant in both natural and artificial systems. Their capillary oscillations are governed by surface tension and are therefore distinguished from acoustic oscillations. These capillary oscillations play a major role in droplet coalescence, for example, and are also an important phenomenon in interface theories. Here, we experimentally and theoretically analyze the capillary oscillation within an optical cavity with walls of water. Our droplet benefits from an optical finesse of 520 that, accordingly, boosts its sensitivity in recording Brownian capillaries with amplitudes of 1 0.025 Å and kilohertz rates in agreement with natural-frequency calculations. Our hybrid device allows resonantly enhanced interactions between electromagnetic and capillary waves that could potentially lead to optical excitation or the cooling of droplet capillary oscillations.
We experimentally demonstrate, for the first time, binding of aerosols of various sizes and shape... more We experimentally demonstrate, for the first time, binding of aerosols of various sizes and shapes in white light. The optomechancial interaction between particles is long range and is in the underdamped regime. Incoherency allows mitigation of interference fringes to enable monotonically changing the distance between particles from 60 μm to contact, constituting a parametrically controlled testbed for transition studies at new scales.
In submerged microcavities there is a tradeoff between resonant enhancement for spatial water and... more In submerged microcavities there is a tradeoff between resonant enhancement for spatial water and light overlap. Why not transform the continuously resonating optical mode to be fully contained in a water microdroplet per se? Here we demonstrate a sustainable 30-mm-pure water device, bounded almost completely by free surfaces, enabling 41,000,000 re-circulations of light. The droplets survive for 416 h using a technique that is based on a nano-water bridge from the droplet to a distant reservoir to compensate for evaporation. More than enabling a nearly-perfect optical overlap with water, atomic-level surface smoothness that minimizes scattering loss, and B99% coupling efficiency from a standard fibre. Surface tension in our droplet is 8,000 times stronger than gravity, suggesting a new class of devices with water-made walls, for new fields of study including opto-capillaries.
Photoluminescence (PL) is a fundamental light–matter interaction that conventionally involves the... more Photoluminescence (PL) is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a redshifted photon. Conversely, in optical refrigeration, the absorption of a low-energy photon is followed by endothermic PL of an energetic photon. These two quantum processes are, in contrast to thermal emission, governed by photon-rate conservation. Thus far, both aspects of PL have been studied under thermal population that is far weaker than the photonic excitation, hiding the role of rate conservation when thermal excitation is significant. Here we theoretically and experimentally study endothermic PL at high temperatures. In contrast to thermal emission, we find that the PL photon rate is conserved with temperature increase, while each photon is blueshifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrate how endothermic PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. These new findings show that endothermic PL is an ideal optical heat pump. This opens the way for a proposed novel device that harvests thermal losses in photovoltaics with record efficiency.
.N− Thermo−optic effects 78.55.−m Photoluminescence Properties and materials a b s t r a c t We a... more .N− Thermo−optic effects 78.55.−m Photoluminescence Properties and materials a b s t r a c t We analyze the suitability of Nd 3+ doped phosphate glasses as optical temperature sensors based on the response of their luminescence bands to temperature. At room temperature, laser excitation of the sample at 532 nm results in an emission spectrum with an intense band centered at 880 nm and an adjacent weak broad band centered at 810 nm, corresponding to the Nd 3+ ion transitions: 4 F 3/2 → 4 I 9/2 and 4 F 5/2 → 4 I 9/2 respectively. Because of thermalization effects between the next 4 F 3/2 and 4 F 5/2 energy levels ( E ≈ 908 cm −1 ), a detectable change on the relative intensities of these emission bands occurs when temperature is increased, affording the experimental calibration of the fluorescence intensity ratio with temperature in the range 300-850 K. It has been shown that the radiative transfer processes favored by the content of Nd 3+ ions are also responsible for changes to the spectra shapes and so play an important role in the fluorescence intensity ratio. A reliable thermal sensing operation was obtained in a 0.1 mol% doped Nd 3+ phosphate glass with relative sensitivity values ranging from 153 × 10 −4 K −1 at 300 K to 22 × 10 −4 K −1 for 850 K, demonstrating good prospects for optical temperature measurements compared to other reported results. However, the reabsorption measurements seen in the samples with a higher Nd 3+ content reveal that the reliability of the sensing performance is compromised with the doping concentration and the experimental conditions of the measurement. (C. Pérez-Rodríguez). transparent medium without the need for wires or coupling devices . Regarding their construction and the region measured, these systems are designed to be small sized or distributed . As with several other sensing devices, temperature optical sensors are widely proposed in the form of optical fibers since, apart from their inherent light guiding of the luminescence, this construction also confers light weight, small size and low cost . Linked with the optical fibers, the trivalent Rare Earth (RE) elements have been one the most reported luminescence centers in the literature. Several fluorescence emissions from RE 3+ in adequate hosts have exhibited spectral changes associated with a temperature variation. Some remarkable examples are the variations in the intensity of the emission lines from Eu 3+ in La 2 O 2 S [7], or the thermal induced evolution of the Ce 3+ emission decays in YAG nanocrystals . Uniquely, the RE 3+ luminescence emitters -Pr 3+ , Nd 3+ , Sm 3+ , Ho 3+ , Er 3+ , Dy 3+ , Er 3+ and Yb 3+ -undergo modifications in the http://dx.
La 2 TeO 6 nanocrystals doped with Eu 3 þ ions have been prepared by the Pechini sol-gel process.... more La 2 TeO 6 nanocrystals doped with Eu 3 þ ions have been prepared by the Pechini sol-gel process. A total of seven samples obtained with different Eu 3 þ concentrations (1-7%). The Eu 3 þ ions are usually taken as probe ions to test the local structure of the lanthanide in solids. Analyzing the luminescence has been shown two different sites for the Eu 3 þ ions (in good agreement with the crystallographic analysis). Moreover, the luminescence properties have been analyzed as function of the Eu 3 þ doping concentration in order to study the interaction between these ions. Under direct excitation into the 5 D 0 level (at 578 nm) the corresponding decay curves show a pure exponential character independently of the Eu 3 þ concentration. However, the decay curves obtained for the 5 D 1 level becomes non-exponential for the higher doped nanocrystals samples indicating that the energy transfer processes are important.
The effects of pressure on the behavior of optical microspherical resonators, prepared from Nd 3 ... more The effects of pressure on the behavior of optical microspherical resonators, prepared from Nd 3 doped, barium titanium silicate glass, have been studied up to 5 GPa inside a diamond anvil cell using silicone oil as the hydrostatic transmission medium and ruby emission lines as the pressure gauge. The optical resonances, known as whispering gallery modes, were observed within the broad emission band of the Nd 3 ions, and the resonances were identified as a function of pressure. By means of simulations, it was found that the average wavelength position of both transverse electric and magnetic modes depended on the radius and refractive index of the sphere, but not on the refractive index of the pressure transmitting medium under the experimental conditions. This was used to define the average sensitivity of the resonant modes with the pressure. Therefore, a value of 6.5 × 10 −4 GPa −1 has been obtained for this sensitivity, which is higher than the value for ruby, the most conventional pressure sensor.
In this work, the generation of photocurrent in a commercial solar cell has been achieved under e... more In this work, the generation of photocurrent in a commercial solar cell has been achieved under excitation at 1480 nm in fluoroindate glass samples codoped with Yb 3+ and Er 3+ ions. These samples were placed over a solar cell and under excitation at 1.48 mm, upconversion emissions coming from the Yb 3+ and Er 3+ ions were obtained. In the upconversion emission spectra, several emission bands at 545 nm, 660 nm and 975 nm have been detected for all the analyzed samples (doped with different Yb 3+ and Er 3+ concentrations). The dependence of the external quantum efficiency to produce photocurrent with the Er 3+ and Yb 3+ concentration and with the pump power excitation at 1480 nm has been analyzed. As conclusion, the maximum generation of photocurrent was obtained with the highest and lowest concentrations of Er 3+ and Yb 3+ , respectively.
Laser action using non-coupled excitation and detection of microspheres made of Nd 3+ doped bariu... more Laser action using non-coupled excitation and detection of microspheres made of Nd 3+ doped barium-titanium-silicate glass has been demonstrated and measured. The microspheres have also been successfully deposited over Si 3 N 4 strip waveguides with a SiO 2 separation layer, thus enabling the laser emission extraction onto a CMOS compatible photonic circuit. The dynamics of the lasing wavelength and intensity has been studied as a function of the pump power and interpreted in terms of thermal effects generated through non-radiative recombination of the excited ions.