Sub-THz Raman response in BaTiO3 and link with structural phase transition.

The THz and sub-THz polarized Raman response was measured in tetragonal and cubic phases of single domain BaTiO3 crystal. A large peak was detected at very low wavenumber, within a scattering geometry in which all phonon lines are Raman inactive. It lies below 700 GHz in the whole temperature range of the tetragonal phase, and is clearly distinct from the soft phonon band. This peak has relaxational behavior with a slowing down on approaching the phase transition from above and from below.

Time evolution of Symmetry-forbidden Raman lines activated by photorefractivity.

Transmission Raman spectroscopy experiments were performed on iron doped congruent lithium niobate within two -in principle equivalent- configurations, namely Y(ZX)Y and Y(XZ)Y. While the former respects the Raman selection rules, the other configuration gives a time dependent spectrum that, after a transient time of several minutes, finally results in a mixture of expected and forbidden modes. This breaking of Raman selection rules is caused by the spontaneous conversion of a part of the ordinarily polarized pump beam into an extraordinarily polarized beam by photorefractive anisotropic self-scattering. A numerical modelling of the phenomenon is developed and fairly reproduces the time dependence of conversion energy.

Experimental study of NaCl aqueous solutions by Raman spectroscopy: towards a new optical sensor.

Raman spectroscopy was used to study the NaCl aqueous solutions around the solid-liquid phase transition. Special attention was devoted to the modification induced by the salt on the OH stretching band of water. Investigations were carried out in the temperature range between -21 and 10 degrees C, for concentrations from 0 to 200 g/L. We demonstrated that micro-Raman spectroscopy can be used as a marker, allowing the determination of the salt concentration of an aqueous solution with an error close to +/-5%.

Generalized model for incoherent detection in confocal optical microscopy.

We develop a generalized model in order to calculate the point spread functions in both the focal and the detection planes for the electric field strengths. In these calculations, based on the generalized Jones matrices, we introduce all of the interdependent parameters that could influence the spatial resolution of a confocal optical microscope. Our proposed model is more nearly complete, since we make no approximations of the scattered electric fields. These results can be successfully applied to standard confocal optical techniques to get a better understanding for more quantitative interpretations of the probe.

Optical sensor for characterizing the phase transition in salted solutions.

We propose a new optical sensor to characterize the solid-liquid phase transition in salted solutions. The probe mainly consists of a Raman spectrometer that extracts the vibrational properties from the light scattered by the salty medium. The spectrum of the O-H stretching band was shown to be strongly affected by the introduction of NaCl and the temperature change as well. A parameter SD defined as the ratio of the integrated intensities of two parts of this band allows to study the temperature and concentration dependences of the phase transition. Then, an easy and efficient signal processing and the exploitation of a modified Boltzmann equation give information on the phase transition. Validations were done on solutions with varying concentration of NaCl.

New versatile and linear optical sensor based on electro-optical modulation and compensation.

This paper is devoted to the description and characterization of a new electro-optic sensor based on a polarimetric device operating in reflection mode, and on a closed-loop controller. The main features of the proposed sensor are versatility, insensitivity to the laser beam optical power fluctuation, and fully linear working mode. The resolution of the sensor, in terms of phase shift, is shown to be better than +/-20 mrad and its accuracy to +/-50 mrad.

Frequency dispersion of electro-optical properties over a wide range by means of time-response analysis.

We show that a Z-transform-based time-response analysis of the electro-optical response of a crystal to a step voltage with a short rise time allows one to obtain the dispersion of the electro-optical coefficients over a wide frequency range. We describe the method employed and present the results obtained for the main electroptic coefficients (r22, r61, and rc) of a standard LiNbO3 crystal. We also show that this method is able to provide even small values of the electro-optic coefficient as well as the dispersion within a wide frequency range, which is limited only by the rise time of the step voltage.