Organic molecules comprising strong electron donors and acceptors groups connected by a π-conjugated system (often designated as donor-π-acceptor or “push-pull” chromophores) are of fundamental importance in materials chemistry due to their wide application in modern technology, such as nonlinear optical (NLO) devices, poled polymers, photovoltaic cells, organic light-emitting diodes (OLEDs), semiconductor materials and in optical data storage devices. Several push-pull molecules containing benzothiazole as an (auxiliary) electron-withdrawing group have already been reported, and typically exhibit favorable fluorescence, electrochemical, solvatochromic, photochromic, and NLO properties. We have reported the synthesis and characterization of the UV–vis, solvatochromic, thermal and second-order NLO properties of benzothiazole derivatives containing bithienyl, arylthienyl and thienylpyrrolyl heterocyclic π-spacers in order to evaluate the effect of different π-excessive donor moieties/π-bridges on their optical properties. As an extension of our earlier work and in order to improve the thermal stability and NLO properties of heterocyclic chromophores of the benzothiazole type, we decided to synthesize arylthienyl-benzothiazoles bearing a vinyl spacer between the donor π-excessive heterocycle (thiophene) and the acceptor π-deficient benzothiazole moiety. In this work we report the synthesis and characterization of the optical and thermal properties of a series of vinyl-benzothiazole derivatives as second harmonic generators (SHG) for NLO applications.

Amorphous materials present macroscopic inversion symmetry. As a consequence, their even order nonlinearities are null. The technological interest in vitreous medium with second order nonlinearity ( χ(2) ) to make photonic devices has stimulated the development of techniques to induce this property. Among them, the thermal-electric field poling is one of the most investigated due its experimental simplicity and results reproducibility. In this work the investigations were directed to the thermal-electric field poling dynamics in soda-lime glasses in view of identify the mainly contributions to the induced nonlinearity stability. Initially were identified the mainly contributions to the induced electrical current. It was observed during the polarizations electroluminescence emission which was shown to be due to ionic displacement and air ionization outside the sample. By using a simplified ionic conduction model was determined two activation energies: ~0,60 and ~3,8eV. The first one, determined from the induced electrical current, was attributed to the sodium conduction; while the second ones, determined by the electroluminescence, attributed to the calcium displacement. Besides the small contribution to the electrical current, the calcium ions, due their small diffusion coefficient, were identified as responsible to the possibility of induce stable second order nonlinearity in soda-lime glasses. Based on these evidences it was used an alternative procedure in which the electrical current flux is controlled. This procedure enabled induce stable χ(2) . This effect was demonstrated to exist 12 months after the polarization procedure, even with a ~50% decrease. The second harmonic generation efficiency was increased in until three times by pumping continuously the polarized samples with infrared radiation from an Nd-YAG laser operating at QSML regime. The threshold applied voltage to observe χ(2) was determined through a modulated applied voltage procedure to be due to the mobility difference between sodium and calcium.

Second harmonic generation (SHG) in nonlinear optical crystal is well-established technique for the measurement of the pulse-width of ultrafast femtosecond laser pulses. However, only selected nonlinear crystalline materials are suited for this purpose due to the limitations in transparency cut-off, limited phase-matching wavelength range, and large difference between the group velocities of the fundamental and harmonic waves. It is found that some newly discovered crystals, such as CsLiB6O10 (CLBO), K2Al2B2O7 (KABO), Li2B4O7 (LB4), and KBe2BO3F (KBBF) have some advantageous characteristics for use in ultrafast nonlinear optical applications. Here we have presented several linear and nonlinear optical parameters including phase-matching angle, angular and spectral acceptance bandwidths, walk-off angle, nonlinear coupling coefficient, and group-velocity mismatch for generation of second harmonic of ultrafast laser radiations by employing type-I and type-II SHG techniques in these crystals.