Efeitos nÃo lineares em lÃquidos orgÃnicos e meios nanoestruturados

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

In this thesis, we report studies of nonlinear transverse effects in organic liquids, carbon disulfide (CS2) and dimetil sulfoxide (DMSO), and of fluorescent emission effects in nanostructured media, more specifically, frequency upconversion in erbium doped barium titanate nanocrystals (BaTiO3:Er3+) and laserlike emission from inverted opal nanostructure of silica infiltrated with rhodamine. The transverse effects are discussed in the first part of this work. Initially, we studied a laser induced conical diffraction when two light beams propagate co-linearly in a nonlinear medium, DMSO, at the pump-probe configuration. We attributed the origin of this effect to the third order nonlinearity and described it theoretically using the Fresnel-Kirchhoff integral of diffraction. Following this line of research, we observed a conical diffraction when two intense laser beams, with different or the same frequency, interact with a nonlinear medium, CS2 or DMSO, at the noncolinear configuration. Light is emitted through the surface of cones, which are centered at the propagation directions of the beams, and with an angular extent, which is equal to two times the angle between the incident beams. A model was developed, which explains this effect by the noise amplification that comes from the Rayleigh scattering of the incident beams, due to a combination of the nonlinear effects of modulational instability, two-beam emission and nonlinear Bragg diffraction. The second part of the work is related to the frequency upconversion in BaTiO3:Er3+ nanocrystals. The samples studied present dimensions smaller than the light wavelength. Three experiments were performed. In the first experiment, frequency upconversion was excited using a tunable dye laser, emitting at the range of 638-660 nm. It was observed green emission due to transitions from the Er3+ levels 2H11/2 and 4S3/2 to the 4I15/2 multiplet. This phenomenon was modeled by a rate equation system for the electronic density of population, including a saturation effect when the laser is resonant with the 4I15/2 → 4F9/2 transition. In another experiment, we used a diode laser emitting in the infrared (980 nm) and observed emission bands centered at the wavelengths of 525, 547 and 650 nm, that correspond to the transitions from the excited states 2H11/2, 4S3/2 and 4F9/2 to the fundamental multiplet, respectively. We also studied the thermal coupling between the 2H11/2, 4S3/2 levels, monitoring the temperature dependence of the fluorescence intensity ratio of the emissions corresponding to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2. We also observed that this effect depends on the size of the nanocrystals. Based on these studies, we proposed this system as a temperature nanosensor. In the third experiment, we investigated the possibility of using this sensor in biological environments. We performed measurements in different media (air, water and glycerol) and observed that the sensor sensitivity depends only on the particles size. Our analysis suggests that the electron-phonon coupling sensed by the erbium ions in the surface of the nanocrystals change with the nanocrystal size. In the final part of this work, we studied the laserlike emission from inverted opal of silica infiltrated with rhodamine. This kind of structure consists of a periodic distribution of nanometric holes in a solid material. At some special conditions of the refractive index and geometry, this structure may present a photonic band gap. However, the optical frequencies in our experiment are outside the band gap of the samples. The dye fluorescence was excited using the second harmonic of a Nd:YAG laser and it was observed linewidth narrowing of the emission as well as a nonlinear growth of the fluorescence intensity as the pump intensity was increased. This effect was explained due to the multiple scattering of fluorescence photons by the structural defects of the material, which increases the optical path of these photons and the number of dye molecules that interact with them. Besides this fact, local field effects may play a role in this process, contributing to the light amplification due to stimulated emission in the medium

ASSUNTO(S)

fisica lÃqudos orgÃnicos - efeitos nÃo lineares

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