Lauro J. Q. Maia

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.

Relationship between crystal shape, photoluminescence, and local structure in SrTiO 3 synthesized by microwave-assisted hydrothermal method

This paper describes the effect of using different titanium precursors on the synthesis and physical properties of SrTiO3 powders obtained by microwave-assisted hydrothermal method. X-ray diffraction measurements, X-ray absorption near-edge structure (XANES) spectroscopy, field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HRTEM) were carried out to investigate the structural and optical properties of the SrTiO3 spherical and cubelikeshaped particles. The appropriate choice of the titanium precursor allowed the control of morphological and photoluminescence (PL) properties of SrTiO3 compound. The PL emission was more intense in SrTiO3 samples composed of spherelike particles. This behavior was attributed to the existence of a lower amount of defects due to the uniformity of the spherical particles.

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.

Toward a new generation of white phosphors for solid state lighting using glassy yttrium aluminoborates

We present a new family of highly emissive white phosphors. These phosphors are based on original glassy yttrium aluminoborates (g-YAB) compositions, obtained at low temperatures without any melting, from non-toxic and low cost precursors through the generic polymeric precursor method. Their photoluminescence (PL) arises from structural defects, whose energy levels are widely extended within the large bandgap of these materials. One of the main objectives of this work, is to enhance the point defect concentrations by accurately controlling the powder annealing (temperature and atmosphere) in order to generate intense and broadband PL. Moreover, these g-YAB powders exhibit excellent thermal and chemical stabilities and tunable PL properties, from bluish to warm white emissions, by simply adjusting the annealing temperatures. We determined the internal quantum yields of g-YAB powders using near ultraviolet excitations, which reached values above 90%. We quantified the relative amounts of point defects by electronic paramagnetic resonance and their distribution within the bandgap by means of thermally stimulated luminescence and we directly correlated this defect density to the PL properties. Finally, based on these promising lighting properties, a prototype was firstly developed to estimate the lighting performances of this new g-YAB phosphors family.

Tunable ultraviolet and blue light generation from Nd:YAB random laser bolstered by second-order nonlinear processes.

Ultraviolet and blue light were obtained by nonlinear frequency conversion in a random laser (RL) based on Nd0.10Y0.90Al3(BO3)4 nanocrystalline powder. RL operation at 1062 nm, due to the 4F3/2 → 4I11/2 transition of neodymium ions (Nd3+), was achieved by exciting the Nd3+ with a tunable beam from 680 to 920 nm covering the ground state absorption transitions to the 4F9/2, (4F7/2,4S3/2), (4F5/2,2H9/2), and 4F3/2 states. Light from 340 to 460 nm was obtained via the second-harmonic generation of the excitation beam while tunable blue light, from 417 to 486 nm, was generated by self-sum-frequency mixing between the excitation beam and the RL emission.

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.

Thermal sensitivity of frequency upconversion in Al4B2O9:Yb3+/Nd3+ nanoparticles

We report measurements of frequency upconversion (UC) processes in ytterbium (Yb3+) and neodymium (Nd3+) doped Al4B2O9 nanoparticles – labeled as Al4B2O9:Yb3+/Nd3+ – prepared by a polymeric precursor method. The nanoparticles investigated contained 1 mol%Yb3+ and 2 mol%Nd3+. The samples were characterized by X-ray diffraction, high-resolution transmission electron microscopy, optical diffuse reflectance and photoluminescence. UC emissions in the range of 770 to 940 nm were studied by exciting the samples with a continuous-wave diode laser operating at 977.7 nm, on resonance with the Yb3+ transition 2F7/2 → 2F5/2. The linear dependence of the UC signals versus the laser intensity was investigated and the thermal sensitivity of the UC process was measured by changing the sample temperature from 26 to 60 °C. The UC photoluminescence behavior was understood to be due to the energy transfer from the excited Yb3+ to Nd3+ initially in the ground state manifold, in a process involving phonon-annihilation. The potential of the Al4B2O9:Yb3+/Nd3+ nanoparticles to be used as optical thermometers was favorably evaluated in comparison with other materials. A large average thermal sensitivity of 2.6 × 10−2 K−1 was achieved.

Non-centrosymmetric crystals of new N-benzylideneaniline derivatives as potential materials for non-linear optics.

Three new N-benzylideneaniline derivatives [p-nitrobenzylidene-p-phenylamineaniline (I), 2,4-dinitrobenzylidene-p-phenylamineaniline (II) and p-dinitrobenzylidene-p-diethylamineaniline (III)] containing electron-push-pull groups have been prepared. They present a planar N-benzylideneaniline core and neighbouring functional atoms, which are related through an efficient intramolecular charge transfer (CT). Two of the derivatives crystallize in non-centrosymmetric space groups, a necessary condition for non-linear optical (NLO) responses. The NLO properties were calculated for the molecular conformations determined by single-crystal X-ray diffraction as well as for the four molecules packed into each corresponding unit cell, using a quantum-chemical method at the cam-B3LYP/NLO-V level of theory. As expected from antiparallel face-to-face stacking through centrosymmetry, the main NLO descriptors - namely, the first hyperpolarizability (βtot) and its projection on the dipole moment direction (βvec) - are almost zero for the tetramer of derivative III. Interestingly, the calculated first hyperpolarizability decreases in the non-centrosymmetric unit-cell content of derivative II when compared to its single molecule, which may be related to its molecular pillaring, similar to that observed in derivative III. On the other hand, a desirable magnification of the NLO properties was found for packed units of derivative I, which may be a consequence of its parallel face-to-tail stacking with the CT vectors of all molecules pointing in the same direction. Moreover, the CT vector of compound I makes an angle of θ = 33.6° with its crystal polar axis, resulting in a higher-order parameter (cos(3)θ = 0.6) compared with the other derivatives. This is in line with the higher macroscopic second-order NLO response predicted for derivative I, βtot = 120.4 × 10(-30) e.s.u.

Solvatomorphs of 25,26,27,28-tetrahydroxycalix[4]arene and 5,11,17,23-tetramino-25,26,27,28-tetrabutoxycalix[4]arene: quenching photoluminescence through switching the guest

Here, we have disclosed two solid state forms of 5,11,17,23-tetramino-25,26,27,28-tetrabutoxycalix[4]arene (1) and two solid state forms of the non-functionalized 25,26,27,28-tetrahydroxycalix[4]arene (2). This is the first structural knowledge of a tetra-amino functionalized calixarene derivative, even though this compound is well known and used as a precursor of several other functionalized calix[4]arenes. The two solid forms of 1 differ by the presence of either water or water/dimethylsulfoxide (DMSO) solvent molecules entrapped in the major calixarene cavity, even though the pinched conformation is adopted in both forms as a consequence of the contacts between solvent molecules and phenyl rings. Likewise, the switch from water to DMSO in the cone cavity of 1 has abrogated the photoluminescence (PL) found only in the dihydrate form. Frontier molecular orbital calculations at the B3LYP/6-31G* level of theory support a short-range electron transfer between guest (DMSO) and host (1) molecules quenching the solid state photoluminescence when DMSO is entrapped in the cone. This solvatomorphism approach for PL search in calixarenes opens a perspective on tuning and even increasing the performance of calixarenes through changing the guest solvent molecule. Similarly, the two crystal forms of 2 entrap either methyl alcohol or DMSO in their cones, which, as well as both crystal forms of 1, are packed into sheets through different fashions and contact patterns.

Physicochemical/photophysical characterization and angiogenic properties of Curcuma longa essential oil

This study analyzed the physicochemical and photophysical properties of essential oil of Curcuma longa and its angiogenic potential. The results showed that curcumin is the main fluorescent component present in the oil, although the amount is relatively small. The experimental chorioallantoic membrane model was used to evaluate angiogenic activity, showing a significant increase in the vascular network of Curcuma longa and positive control groups when compared to the neutral and inhibitor controls (P <0.05), but no significant difference was found between Curcuma longa essential oil and the positive control (P >0.05). Histological analysis showed extensive neovascularization, hyperemia and inflammation in the positive control group and Curcuma longa when compared to other controls (P <0.05), characteristic factors of the angiogenesis process. In conclusion, Curcuma longa oil showed considerable proangiogenic activity and could be a potential compound in medical applications.