In the present work a modification of the well-known z-scan technique is presented. It is based on the direct measurement of the beam dimensions in the far field rather than the transmittance of the irradiance through an aperture. More specifically, the quantity measured in the case of a circular Gaussian incident beam is the beam radius in the far field, while in the more general case of an elliptic Gaussian beam the measured quantities are the lengths of the principal semiaxes of the beam. It is worth emphasizing that the latter case is more interesting in practice since even a very small misalignment of the resonator of an actual laser system can easily induce astigmatism to the beam. The measurements were performed through a CCD camera in connection with a laser beam profiler. The advantages of the proposed modification compared to the classic z-scan technique are the elimination of the sensitivity in beam pointing instability as well as the drastic reduction of the sensitivity in energy fluctuations. Furthermore, the application of the standard z-scan technique in the case of an elliptic Gaussian incident beam is extremely difficult. The technique has been tested using the standard nonlinear optical material CS2 as a sample.
The linear electro-optic coefficients r33 and r13 of a highly nonlinear organic chromophore were measured via an experimental method based on waveguide sample geometry. The technique used is simple and accurate and the interpretation is straightforward. Viscous solution of chromophore and PMMA were prepared in chlorophorm (CHCl3) and films were made by drop casting of these solutions onto metallic substrates. A DC poling electric field Vp was applied, in order to polarize the doped molecules and a linearly polarized beam from a He-Ne laser was left to pass through the film. A polarizer placed in front of a photodiode was used to determine the polarization state of the beam, allowing us to measure the magnitude of the induced birefringence Δn. The electro-optic coefficients difference (Δn=r33-r13) was calculated via the induced birefringence. The dependence of the coefficients r33 and r13 on the concentration of chromophore in PMMA was also investigated. By applying an AC electric field onto the film the dynamic electro-optic effects can be studied.
A series of pyrylium dyes is studied concerning their Non-Linear Absorption (NLA) in order to assess their applicability as optical limiters. The research aims at combining the selective material preparation through the systematic change of the molecules' substituents and the linear and nonlinear optical characterization. The open z-scan technique at various excitation intensities was used for the nonlinear absorption characterization. With this method, the third and fifth order contributions to the susceptibility can be separated and quantified. Femtosecond pulses at 760, 790 and 840 nm were used to induce the NLA. All wavelengths lay in the two-photon absorption spectral region of the dyes. In most of the cases studied an efficient Excited State Absorption (ESA), fifth order nonlinearity, following Two-Photon Absorption (TPA), third order nonlinearity, was observed above a threshold of the excitation intensity (Ithr). This was indicated through an increase of the value of the NLA coefficient above Ithr. If TPA was the only nonlinear mechanism, the NLA coefficient would be intensity independent. The third and fifth order NLA coefficients as well as the ESA cross section values were extracted. High values of the ESA cross sections of the order of 10-15 cm2 were calculated indicating that the pyrylium dyes are potential candidates in optical limiting applications.
Molecular two-photon absorption (TPA) has gained great interest over recent years owning to each application in various fields, including spectroscopy, microscopy, 3-D optical data storage, optical power limitation and microfabrication. The requirement for organic molecules with large TPA cross-sections is therefore essential. In the present work the two-photon absorption properties of a series of pyrylium-based chromophores were investigated. The molecules of this series were synthesized by systematically changing the chemical structure of a specific substituent of an initial, strong two-photon absorbing, molecule. Very large TPA cross-sections (1800 GM) and high quantum yields were achieved in the visible red and NIR region of the spectrum. The relation between the chemical structure of the molecules and their TPA performance was discussed. Furthermore, the photobleaching efficiency of these molecules doped in polymer matrices was investigated. Parallel successive patterns were recorded in a polymer matrix via photobleaching. The accuracy of recorded patterns reveals the great potential of these chromophores as memory materials.
A new blue emitting conjugated polymer is studied for Amplified Spontaneous Emission (ASE) and laser action in liquid solutions and solid matrices. Both processes were observed simultaneously when the liquid polymeric sample is placed into a resonator consisting of a mirror and a grating. This observation permits the distinguishing between the main differences of ASE and laser action such as tunability and coherence. The stimulated emission efficiency can be strongly affected by the presence of polymer chain aggregates into the sample. In order to study this effect mixtures of the polymer with good and poor liquid solvents were prepared and the ASE action was examined. The polymer molecules tend to aggregate when they approach the poor solvent molecules. Thus, by varying the poor solvent quantity into the liquid samples, the aggregates concentration can be systematically controlled. It is shown that the ASE behavior exhibits a rapid suppress with the aggregates concentration. Performing spectroscopic and stimulated emission cross section measurements in isolated and aggregated polymer solutions, it is proved that in the latter media ASE is caused by the intrachain excitons while the spontaneous emission is caused by the aggregates. The suppress of the ASE is due to the increase of aggregates at the expense of isolated chains.
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