Significant enhancement of the optical second harmonic generation in a poled azopolymer thin grating.

Nonlinear optical properties of an electrically poled surface relief grating inscribed on a thin film of azopolymer have been investigated. The linear and nonlinear optical far-field diffraction patterns of the grating are compared, and they show a clear angular separation of the fundamental (lambda = 1.064 mum) and second harmonic generated beams that are diffracted at different angles. The intensity of the zero order transmitted second harmonic generation (SHG) beam from inscribed surface relief grating (SRG) areas has been recorded using the Maker fringe technique and compared to the response from flat areas that were only poled. Poled gratings exhibit a sharp second harmonic generation enhancement for coupling angles of theta(i) = +/-52 degrees due to a quasi-phase matching process. A simple phenomenological model allows one to explain both the second harmonic intensity generated from the thin polar film and the dispersion curve observed under "pp" polarization due to the mismatch of the angular coupling condition: this gives us a good indication of the intensity enhancement of the SHG beam in the forward direction.

Orientation distribution functions based upon both (P1), (P3) order parameters and upon the four (P1) up to (P4) values: application to an electrically poled nonlinear optical azopolymer film.

The most probable orientational distribution functions of rod-like polar molecules contained in a noncentrosymmetric uniaxial system are established using the first-rank and third-rank Legendre polynomials, (P1(cos theta)) and (P3(cos theta)) order parameters, and the maximum entropy method. Emphasis is put on the different domains of existence in the ((P1), (P3)) plane for the various shapes of the distributions: it is thus shown that, for any positive (P1(cos theta)) value and for decreasing (P3(cos theta)) values, the distribution function may exhibit either a distorted oblate form with an intense maximum at 0 degrees, or a three-leaved rose curve with maxima at 60 degrees, 180 degrees, and 300 degrees, and finally another markedly oblate shape with a strong maximum at 180 degrees. As an illustrative example, we have considered the azobenzene molecular orientations in an electrically poled p(DR1M) homopolymer thin film after a thermal process and several relaxation periods. We have made use not only of the (P1) and (P3) parameters determined from polarized second-harmonic generation (SHG) measurements, but also of the (P2) values extracted from UV-visible spectra and of the (P4) values adjusted according to the information entropy theory. In such a thin film with very large nonlinear properties (d33 coefficients were varying from 437.0 to 117.0 pm/V at 1064 nm) it is evidenced that a strong polar order is maintained even after a long relaxation period of 42 days. So, the distribution functions demonstrate that the poling treatment was quite efficient and they emphasize the importance in the determination of both couples of odd and even order parameters in such uniaxially oriented optical elements.

Confocal Raman microspectrometry: a vectorial electromagnetic treatment of the light focused and collected through a planar interface and its application to the study of a thin coating.

In-depth confocal Raman microspectrometry (CRM) studies through a planar interface between materials of mismatched refraction indices are known to be affected by a decrease of both the collected Raman intensity and the axial resolution as a function of the penetration depth. Following a previous model, which takes the refraction, diffraction, and spherical aberration effects into account when focusing a Gaussian incident laser beam with a high numerical aperture objective lens, a complete vectorial treatment of these phenomena is considered. It is demonstrated that off-axis refraction effects cannot be neglected and that the dimension of the confocal pinhole aperture plays a crucial role on the effective focal plane position and on the collection efficiency. We thus propose a more rigorous and complete approach to the problem, and we find a very good agreement between experimental and theoretical Raman intensity variations for a thick polyethylene sample as a function of the penetration depth. As compared with calculations where only refraction was considered, we confirm that the lengthening of the focus even for a large penetration depth is significantly reduced upon diffraction effects. As an illustrative example, the theoretical Raman responses for a thin coating of approximately 20 microns on a polymer substrate were investigated and compared to experimental results already published. Even though the interfacial region is spread over approximately 5-6 microns when using a 100x objective and a confocal pinhole of 200 microns diameter, it is definitively concluded that the apparent axial resolution is not drastically deteriorated with increasing depth and that the coating thickness may be directly estimated with a precision of approximately 1.0 micron (5%).

A vibrational study of various K2PdCl(4-x)Br(x) solid solutions.

Infrared and Raman vibrational spectra (700-30 cm(-1)) have been recorded on various K2PdCl(4-x)Br(x) (0 < or = x < or = 4) powdered samples at 300 K, including both K2PdCl4 and K2PdBr4 compounds and ten solid solutions with a bromide content varying from 5-95%. Characteristic variations of the sectra clearly demonstrate a wide domain of existence of the solid solutions and the only possible additional existence, due to no IR-Raman coincidence, of the 'trans' planar PdCl4Br2 centrosymmetric anion. A comparison of the vibrational data with results of valence force field calculations allows us to characterize this new anion, in Raman by two signals at 286 and 225 cm(-1) (v sym., Ag, PdCl2 and PdBr2, respectively), in Infrared by not only the two bands at 309 and 200 cm(-1) (v asym. B3u + B2u, of PdCl2 and PdBr2) but also intense deformation modes in the 178-140 cm(-1) wavenumber range. These results suggest slight modifications in the internal f(Pd-Cl) and f(Pd-Br) force constans on going from the PdCl4(2-) or PdBr4(2-) anions to the 'trans' PdCl2Br2(2-) anonic species.