Vladimir Jerez

Optical spectroscopy and frequency upconversion properties of Tm3+ doped tungstate fluorophosphate glasses

Tungstate fluorophosphate glasses of good optical quality were synthesized by fusion of the components and casting under air atmosphere. The absorption spectra from near-infrared to visible were obtained and the Judd–Ofelt parameters determined from the absorption bands. Transition probabilities, excited state lifetimes and transition branching ratios were determined from the measurements. Pumping with a 354.7 nm beam from a pulsed laser resulted in emission at 450 nm due to transition 1D2→3F4 in Tm3+ ions and a broadband emission centered at ≈550 nm attributed to the glass matrix. When pumping at 650 nm, two emission bands at 450 nm (1D2→3F4) and at 790 nm (3H4→3H6) were observed. Excitation spectra were also obtained in order to understand the origin of both emissions. Theoretical and experimental lifetimes were determined and the results were explained in terms of multiphonon relaxation.

Optical properties and frequency upconversion fluorescence in a Tm3+-doped alkali niobium tellurite glass

Optical spectroscopic properties of Tm3+-doped 60TeO2−10GeO2−10K2O−10Li2O−10Nb2O5 glass are reported. The absorption spectra were obtained and radiative parameters were determined using the Judd–Ofelt theory. Characteristics of excited states were studied in two sets of experiments. Excitation at 360 nm originates a relatively narrow band emission at 450 nm attributed to transition 1D2→3F4 of the Tm3+ ion with photon energy larger than the band-gap energy of the glass matrix. Excitation at 655 nm originates a frequency upconverted emission at 450 nm (1D2→3F4) and emission at 790 nm (3H4→3H6). The radiative lifetimes of levels 1D2 and 3H4 were measured and the differences between their experimental values and the theoretical predictions are understood as due to the contribution of energy transfer among Tm3+ ions.

Optical transitions probabilities of Dy3+ ions in fluoroindate glass

Abstract Fluoroindate glasses of the following compositions: (40− x )InF 3 –20ZnF 2 –20SrF 2 –2GdF 3 – 2NaF–16BaF 2 – x DyF 3 with x =1.0, 2.0, 3.0 and 4.0 mol% were prepared in air atmosphere using ammonium process. Absorptions spectra of these glasses at room temperature in the spectral range 300–2500 nm were obtained. Each spectra obtained show identical characteristics and only a change in the intensity of the different bands as the concentration of Dy 3+ is changed. The oscillator strength is obtained from the area under the absorption band. Using the Judd–Ofelt theory, intensity parameters Ω λ ( λ =2, 4, 6) for f–f transitions of Dy 3+ ions as well as transition probabilities, branching ratios, and radiative lifetimes of each band were determined. The optical properties of the fluoroindate glasses doped with Dy 3+ are compared with those of other glasses described in the literature.

Dynamics of energy transfer and frequency upconversion in Tm3+ doped fluoroindate glass

Blue and ultraviolet upconversion (UC) emissions at 455 and 363nm were observed from Tm3+ doped fluoroindate glasses pumped at 650nm. The time behavior of the UC signals was studied for different Tm3+ concentrations. The measurements revealed the origin of the UC process as well as allowed to quantify the interaction between Tm3+ ions. The results indicate that a two-step one-photon absorption process is responsible for the UC emissions, and dipole-dipole interaction provides the main contribution for energy transfer (ET) among active ions. The critical distance between Tm3+ ions at which the ET rate is equal to the decay rate of noninteracting Tm3+ ions was determined.

Multi-wavelength emission through self-induced second-order wave-mixing processes from a Nd3+ doped crystalline powder random laser.

Random lasers (RLs) based on neodymium ions (Nd3+) doped crystalline powders rely on multiple light scattering to sustain laser oscillation. Although Stokes and anti-Stokes Nd3+ RLs have been demonstrated, the optical gain obtained up to now was possibly not large enough to produce self-frequency conversion. Here we demonstrate self-frequency upconversion from Nd3+ doped YAl3(BO3)4 monocrystals excited at 806 nm, in resonance with the Nd3+ transition 4I9/2 → 4F5/2. Besides the observation of the RL emission at 1062 nm, self-converted second-harmonic at 531 nm, and self-sum-frequency generated emission at 459 nm due to the RL and the excitation laser at 806 nm, are reported. Additionally, second-harmonic of the excitation laser at 403 nm was generated. These results exemplify the first multi-wavelength source of radiation owing to nonlinear optical effect in a Nd3+ doped crystalline powder RL. Contrary to the RLs based on dyes, this multi-wavelength light source can be used in photonic devices due to the large durability of the gain medium.

Observation of Lévy distribution and replica symmetry breaking in random lasers from a single set of measurements

Random lasers have been recently exploited as a photonic platform for studies of complex systems. This cross-disciplinary approach opened up new important avenues for the understanding of random-laser behavior, including Lévy-type distributions of strong intensity fluctuations and phase transitions to a photonic spin-glass phase. In this work, we employ the Nd:YBO random laser system to unveil, from a single set of measurements, the physical origin of the complex correspondence between the Lévy fluctuation regime and the replica-symmetry-breaking transition to the spin-glass phase. A novel unexpected finding is also reported: the trend to suppress the spin-glass behavior for high excitation pulse energies. The present description from first principles of this correspondence unfolds new possibilities to characterize other random lasers, such as random fiber lasers, nanolasers and small lasers, which include plasmonic-based, photonic-crystal and bio-derived nanodevices. The statistical nature of the emission provided by random lasers can also impact on their prominent use as sources for speckle-free laser imaging, which nowadays represents one of the most promising applications of random lasers, with expected progress even in cancer research.

Optical recording mechanisms in undoped titanosillenite crystals

We show that optical recording in undoped photorefractive titanosillenite Bi12TiO20 (BTO) crystals involve electrons, holes and ions, depending on the experimental conditions. Holographic recording and erasure at higher than room temperature was carried out on an undoped crystal sample, showing the presence of a fast photosensitive electron-based hologram and a slower compensating one of nonphotosensitive nature that is responsible for hologram fixing in BTO and is likely to be based on H+ ions. The fixed grating showed a diffraction eficiency η≈0.002 and a characteristic activation energy of 0.85±0.05 eV. A strong light-induced (darkening) photochromic effect was also detected, that had to be accounted on for diffraction efficiency measurement. Holes are shown to participate in the optical recording process when selectively photoexcited with infrared light.

Structural and luminescence properties of Nd3+/Yb3+ codoped Al4B2O9 nanocrystalline powders

Morphological, structural and optical properties of Nd3+/Yb3+ codoped Al4B2O9 nanopowders prepared by the polymeric precursor method were investigated. The compounds previously heat-treated at 900 °C were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) techniques. The Al4B2O9 nanocrystals obtained possess an orthorhombic structure with grain dimensions between 10 and 20 nm. Photoluminescence (PL) spectra under excitation at 804 nm were collected and analyzed. The results indicate that the energy absorbed by Nd3+ (transition: 4I9/2 → 4F5/2 + 2H9/2) is efficiently transferred to Yb3+ in samples having 2 mol% of Nd3+ and 1 mol% of Yb3+, increasing the emission from 930 nm to 1130 nm by 12.2 times in comparison with the samples doped with 1 mol% of Nd3+ and 1 mol% of Yb3+. The dynamics of the PL process was also investigated. The PL decay at 975 nm from Yb3+ (2F5/2 → 2F7/2) and at 1080 nm from Nd3+ (4F3/2 → 4I11/2) was studied. The Yb3+ 2F5/2 level lifetime varies from 201 to 48 μs while the lifetime of the Nd3+ 4F3/2 level varies from 121 to 69 μs when the Nd3+ concentration is changed from 1 to 8 mol%. On the other hand, the Yb3+ 2F5/2 lifetime varies from 176 to 15 μs and the Nd3+ 4F3/2 lifetime varies from 97 to 44 μs when the Yb3+ concentration is changed from 1 to 8 mol%, due to interactions between the rare-earth ions. The present results show that Nd3+/Yb3+ codoped Al4B2O9 nanocrystalline powders have large potential to be tested in solar energy concentrators and IR laser devices.

Fixed photorefractive holograms with maximum index-of-refraction modulation in LiNbO3:Fe

We report the recording of a fixed good quality transmission hologram in LiNbO3:Fe with maximum index-of-refraction modulation using the simultaneous recording/compensation process at 120 °C in a specially designed setup with λ=514.5 nm. This process was shown to be reproducible and in good agreement with an already reported theoretical model. The analysis of this recording process showed that material saturation was reached so that the maximum possible fixed index-of-refraction modulation was achieved. From the comparison of theoretical and experimental recording/compensation process data some material parameters (dielectric relaxation time τMe≈15 min, saturation space-charge field Eq=18.8 kV/cm, and photovoltaic-to-saturation field ratio Eph/Eq=0.80) were determined. The diffraction efficiency of this grating was measured using a λ=633 nm probe laser beam in an independent setup and its actual value computed, taking into account the angular divergence of the probe beam. The good grating performance as a...

Silk fibroin as a biotemplate for hierarchical porous silica monoliths for random laser applications

Bombyx mori silk fibroin offers unlimited opportunities for functionalization, processing, and biological integration. We describe in this work the design of structured organic inorganic hybrids based on silica and silk fibroin, taking advantage of the relationship between the structure and processing of the latter. In situ self-assembly of fibroin nanofibers along with hydrolysis and polycondensation of tetraethyl orthosilicate was employed. Structural characterization was performed by Raman and solid state NMR spectroscopies. Our findings demonstrated that fibroin precipitates in the reaction medium with prevailing β-sheet conformation. The transition from amorphous to crystalline state was observed to be favored by the increase of the fibroin concentration in the samples. The samples were obtained as robust and biocompatible monoliths, making them candidates for several applications, particularly in the biomedical field. As a novel development, the fibroin nanofibers were used as pore biotemplates to create mechanically robust silica monoliths with a hierarchical macro-mesoporous network in an easy templating process. The template was removed by thermal treatment and the as obtained silica based materials displayed surface area values ranging from 704 to 1057 m2 g−1 and a maximum pore volume of 0.621 mL g−1. The porous silica monoliths were then doped with rhodamine 6G and typical random laser action could be observed, with a minimum laser threshold of 9.7 μJ per pulse and a linewidth narrowing from 40 to 4 nm. In addition, it was shown that two coupled gain mechanisms were taking place, the random lasing and the stimulated Raman scattering, allowing us to observe Raman Stokes lines due to vibrational modes of the dye molecule.