Antonio M. Brito-Silva

Random fiber laser

We investigate the effects of two-dimensional confinement on the lasing properties of a classical random laser system operating in the incoherent feedback (diffusive) regime. A suspension of 250 nm rutile (TiO2) particles in a rhodamine 6G solution was inserted into the hollow core of a photonic crystal fiber generating the first random fiber laser and a novel quasi-one-dimensional random laser geometry. A comparison with similar systems in bulk format shows that the random fiber laser presents an efficiency that is at least 2 orders of magnitude higher.

Theoretical and Experimental Studies of the Photoluminescent Properties of the Coordination Polymer [Eu(DPA)(HDPA)(H2O)2]·4H2O

We report on the hydrothermal synthesis of the [Eu(DPA)(HDPA)(H(2)O)(2)].4H(2)O lanthanide-organic framework (where H2DPA stands for pyridine-2,6-dicarboxylic acid), its full structural characterization including single-crystal X-ray diffraction and vibrational spectroscopy studies, plus detailed investigations on the experimental and predicted (using the Sparkle/PM3 model) photophysical luminescent properties. We demonstrate that the Sparkle/PM3 model arises as a valid and efficient alternative to the simulation and prediction of the photoluminescent properties of lanthanide-organic frameworks when compared with methods traditionally used. Crystallographic investigations showed that the material is composed of neutral one-dimensional coordination polymers infinity(1)[Eu(DPA)(HDPA)(H(2)O)(2)] which are interconnected via a series of hydrogen bonding interactions involving the water molecules (both coordinated and located in the interstitial spaces of the structure). In particular, connections between bilayer arrangements of infinity(1)[Eu(DPA)(HDPA)(H(2)O)(2)] are assured by a centrosymmetric hexameric water cluster. The presence of this large number of O-H oscillators intensifies the vibronic coupling with water molecules and, as a consequence, increases the number of nonradiative decay pathways controlling the relaxation process, ultimately considerably reducing the quantum efficiency (eta = 12.7%). The intensity parameters (Omega(2), Omega(4), and Omega(6)) were first calculated by using both the X-ray and the Sparkle/PM3 structures and were then used to calculate the rates of energy transfer (W(ET)) and back-transfer (W(BT)). Intensity parameters were used to predict the radiative decay rate. The calculated quantum yield obtained from the X-ray and Sparkle/PM3 structures (both of about 12.5%) are in good agreement with the experimental value (12.0 +/- 5%). These results clearly attest for the efficacy of the theoretical models employed in all calculations and create open new interesting possibilities for the design in silico of novel and highly efficient lanthanide-organic frameworks.

Random laser action in dye solutions containing Stöber silica nanoparticles

Random laser action is obtained in a diffusive weakly scattering regime using an alcohol solution of rhodamine 640 with silica nanoparticles prepared by the Stober method, with different average diameters of 49, 90, 219, and 490 nm. Particle size and concentration has proven to affect the random laser performance. It is also shown that silica nanoparticles contributes for a much slower photodegradation of the dye molecules than titania nanoparticles that has been used in the majority of the dye random lasers. This fact makes it advantageous to use silica nanoparticles for the fundamental studies of random laser, due to its increase in lifetime under pumping conditions.Random laser action is obtained in a diffusive weakly scattering regime using an alcohol solution of rhodamine 640 with silica nanoparticles prepared by the Stober method, with different average diameters of 49, 90, 219, and 490 nm. Particle size and concentration has proven to affect the random laser performance. It is also shown that silica nanoparticles contributes for a much slower photodegradation of the dye molecules than titania nanoparticles that has been used in the majority of the dye random lasers. This fact makes it advantageous to use silica nanoparticles for the fundamental studies of random laser, due to its increase in lifetime under pumping conditions.

Improved Synthesis of Gold and Silver Nanoshells

Metallic nanoshells have been in evidence as multifunctional particles for optical and biomedical applications. Their surface plasmon resonance can be tuned over the electromagnetic spectrum by simply adjusting the shell thickness. Obtaining these particles, however, is a complex and time-consuming process, which involves the preparation and functionalization of silica nanoparticles, synthesis of very small metallic nanoparticles seeds, attachment of these seeds to the silica core, and, finally, growing of the shells in a solution commonly referred as K-gold. Here we present synthetic modifications that allow metallic nanoshells to be obtained in a faster and highly reproducible manner. The main improved steps include a procedure for quick preparation of 2.3 ± 0.5 nm gold particles and a faster approach to synthesize the silica cores. An investigation on the effect of the stirring speed on the shell growth showed that the optimal stirring speeds for gold and silver shells were 190 and 1500 rpm, respectively. In order to demonstrate the performance of the nanoshells fabricated by our method in a typical plasmonic application, a method to immobilize these particles on a glass slide was implemented. The immobilized nanoshells were used as substrates for the surface-enhanced Raman scattering from Nile Blue A.

High-order nonlinearity of silica-gold nanoshells in chloroform at 1560 nm.

The nonlinear response of silica--gold nanoshells (SGNs) in chloroform was studied using laser pulses of 65 fs at 1560 nm. The experiments were performed using the thermally managed Z--scan technique that allows measurements of the electronic contribution for the nonlinear response, free from thermal influence. The results were analyzed using an analytical approach based on the quasi--static approximation that allowed extraction of the nonlinear susceptibility of a SGN from the data. High third--order susceptibility, χsh((3)) = - 1.5 x 10(-11) m(2)/V(2), approximately four orders of magnitude larger than for gold nanospheres in the visible, and large fifth--order susceptibility, χsh((5)) = - 1.4 x 10(-24) m(4)/V(4), were obtained. The present results offers new perspectives for nonlinear plasmonics in the near--infrared.

Bichromatic random laser from a powder of rhodamine-doped sub-micrometer silica particles

We studied the random laser (RL) bichromatic emission (BCE) from a powder consisting of silica particles infiltrated with Rhodamine 640 (Rh640) molecules. The BCE is attributed to Rh640 monomers and dimers. Because of the efficient monomer-dimer energy transfer, we observed RL wavelength switching from ≈ 620 nm to ≈650 nm and the control of the emitted wavelength was made by changing only the excitation laser intensity. None of external parameters such as excitation laser spot size or radiation detector position was changed as in previous experiments. Two laser thresholds associated either to monomers or dimers were clearly observed. Moreover, an effect analog to frequency-pulling among two coupled oscillators was identified measuring the RL spectra as a function of the excitation laser intensity. A wavelength shift, Δλ, was measured between the monomer and dimer resonance wavelengths, changing only the excitation laser intensity. The maximum value of Δλ ≈ 16 cm−1 was obtained for laser pulses of 7 ns wit...

Random Laser Action in the Core of a Photonic Crystal Fiber

Lasers: We have created lasers with operating properties that we never would have dreamed of when the technology was invented 50 years ago.

Estudos espectroscópicos e estruturais dos polímeros de coordenação 2D, ∞[Tb(DPA)(HDPA)] e ∞[Gd(DPA)(HDPA)]

This paper presents the synthesis of the coordination polymers ∞[Ln(DPA)(HDPA)] (DPA=2,6-pyridinedicarboxylate; Ln= Tb and Gd), their structural and spectroscopic properties. The structural study reveals that the ∞[Ln(DPA)(HDPA)] has a single Ln+3 ion coordinated with two H2DPA ligands in tridentade coordination mode, while two others H2DPA establish a syn-bridge with a symmetry-related Ln3+, forming a two-dimensional structure. The spectroscopic studies show that ∞[Tb(DPA)(HDPA)] compound has high quantum yield (qx≈ 50.0%), due to the large contribution of radiative decay rate. Moreover triplet level is localized sufficiently over the emitter level 5D4 of theTb3+ ion, avoiding a retrotransference process between these states.

Creating and fixing a metal nanoparticle layer on the holes of microstructured fibers for plasmonic applications

Methods to insert, keep and fix silver and gold nanoparticles to the inner walls of microstructured fibers are demonstrated. Fibers optimized for evanescent field interaction were employed and can result in efficient plasmonic chemical sensing.

Fast transient bleaching in Rh-6G functionalized T1O2 nanoparticles: Charge transfer dynamics

Summary form only given. The charge-transfer mechanisms from dye to semiconductor nanoparticles and the time scales involved in these processes are a matter of interest for diverse applications, such as dye-sensitized solar cells (DSC) [1] and photocatalysis [2]. In DSC, the primary mechanism encompasses light-induced electron excitation following electron transfer from excited LUMO states of the dye to the conduction band (CB) of the semiconductor. To take place this electron transfer, the semiconductor CB must have a lower energy than the LUMO states of the sensitizer. Titanium dioxide (TiO2) often meets this requisition and therefore is the most widely semiconductor employed in DSC. Fast electron transfer is mandatory to avoid exciton recombination before charge-transfer and therefore, improve the efficiency of a DSC. Conversely, slow backward charge transfer (from the semiconductor to the dye) increases the efficiency of a DSC.the dye) increases the efficiency of a DSC.Charge transfer from rhodamine 590 (Rh6G) to amorphous TiO2 nanoparticles in colloidal suspension is investigated here via pump-probe transient absorption (TA). The TA kinetics is analyzed taking into account the relevant energy levels of the hybrid nanocomposite and electron transfer rates are estimated from the TA absorption signals. The pump-probe TA experimental apparatus used, as pump beam, a regeneratively amplified Ti:Sapphire laser (800 nm, 100 fs, 1 kHz repetition rate), which was frequency doubled (λ = 400 nm) by a 1 mm long nonlinear BiBO crystal. The pump beam was modulated by an optical chopper operating at 461 Hz and its optical path was controlled by a mechanical delay line. The probe beam was derived from an optical parametric amplifier (OPA) pumped by the Ti:Sapphire laser whose output beam wavelength was tuned to λ = 530 nm at the maximum of the S0-S1 transition of Rh6G. The photodiode signals of the reference and signal beams were sent to two different boxcar averagers. The analog processor output voltage was sent to a lock-in amplifier locked at the chopper frequency. With this system, absorbance changes as low as 10-5 could be accurately measured. To functionalize the TiO2 nanoparticles with rhodamine 6G, a modified methanolic dye solution (Si:Rh6G) was prepared by reacting the dye with a silane molecule (3-isocyanatepropyltriethoxysilane) according to [3]. After the reaction, 10 mL of the Si:Rh6G solution was mixed to 5 mL of the TiO2 colloidal solution. The dye is proposed to anchor to the TiO2 nanoparticles via OH groups at the particles surface formed during the hydrolysis reaction. The colloid was kept under reflux at 80 °C for 24 hours. The resulting colloid was washed several times and redispersed in ethanol. The normalized transient bleaching signal shows curves that could be fitted by exponential functions. In this case, the bleaching recovery time is ≈ (2.9 ± 0.5) ns, which is smaller than the rhodamine 590 lifetime in ethanol (≈ 4.0 ns). However, for the dye binded to TiO2, the signal could be fitted by a sum of two exponential functions plus an offset. The time constants derived from the exponential fits to the TA kinetics for the Rh6Gfunctionalized TiO2 particles were τ1 = (160 ± 70) ps and τ2 = (1.0 ± 0.2) ns. The faster decay is assigned as charge transfer from thermalized excited states of the dye to the TiO2 conduction band, while the slower one corresponds to the back electron transfer from TiO2 to the dye. Direct exciton recombination and triplet states also contribute to the slower decay dynamics. The offset is a signature that the bleaching recovery is not completed during the time scale investigated due to trapping of electrons at in-gap states of the semiconductor or trapping of molecules in triplet states. Using the values of extracted from the TA data, the charge transfer rate is in the range (0.5-6.0)x109 s-1. This value is smaller than the charge transfer rates for other classes of dyes, such as ruthenium complexes or donor-pi-aceptors, which are in the range 109-1013 s.