Y. Le Grand

Orientation fields of nonlinear biological fibrils by second harmonic generation microscopy.

The orientation of fibrils within biological tissues is of primary importance. In this study, we propose a simple method based on second harmonic generation (SHG) microscopy to map, pixel by pixel, the orientation of the symmetry axis of the second‐order nonlinear susceptibility tensor of fibrils that produce SHG. The method uses only four images acquired at specific polarizations of the input laser beam, and can be easily and cheaply implemented on a confocal microscope. In addition to orientation informations, the method also provides polarization independent images and estimations of the ratio of the nonlinear susceptibility components. We demonstrate the relevance of our concept by studying the orientation fields of the collagen meshwork in a healthy rat liver that provides well separated fibrils. By correlating the mean orientation of the nonlinear susceptibility to the fibril orientation itself for many fibril segments, and using circular statistics, it is shown that both orientations are truly parallel at the fibril scale. Our polarimetric method allows to map fibril orientation fields, independently of individual fibril contrast in the SHG image.

Second-harmonic scattering by domains in RbH2PO4 ferroelectrics

Abstract Second-harmonic scattering (SHS) of light by laminar ferroelectric domains of the orthorhombic phase of RbH 2 PO 4 below T c =147 K is studied in detail. CCD images of the far-field SHS pattern reveal up to six off-axis distinct intensity maxima in a plane normal to the ferroelectric axis c . These scattering maxima are interpreted on the basis of the quasi-phase matching (QPM) conditions for the three sets of polarizations allowed by the nonvanishing elements of the second-order nonlinear susceptibility tensor. On this assumption scattering angles of the three SHS modes calculated from crystal optics are shown to fit well the experimental data for any incidence of the pump beam normal to c . Scattering intensities of the two most intense modes are also measured versus the component of the wave vector perpendicular to the modulated structure. A stochastic model giving the intensity as a function of the density probability of domain widths is developed and is shown to give a good fit of the scattered intensity. Moreover, it permits the evaluation of the mean and dispersion of the ferroelectric domain widths, which are found to be, respectively, 2.6 and 0.7 μm in the crystal under study. This is consistent with the domain size of the ferroelastic counterpart revealed by polarization microscopy.

Measurement of small rotations by eigenvector flips in a quasi-isotropic Fabry-Perot

A novel small rotation or small birefringence measurement scheme based on the dynamics of the helicoidal eigenstates of a passive Fabry–Perot cavity with two quarter‐wave plates is described. When the system is inserted between two crossed polarizers, the sensitivity of the method is shown to be proportional to the square of the finesse of the cavity. For a finesse of 50, the observed noise floor corresponds to an estimation of the minimum detectable small rotation of 3×10−8 rd. We discuss possible improvements and potential possibilities of the method.

Spatially distributed gains in semiconductor lasers

A careful analysis of the gain using the amplified spontaneous emission in semiconductor lasers concludes that there is a spatial distribution of the gain. A twofold distribution is observed in the planes parallel and perpendicular to the junction with different decreasing variations from the laser axis to the edge of the radiation lobe. Collecting the entire emitted light beam gives usually too low gain values. Gain measurements restricted to the region around the laser beam axis lead to an improvement of the gain value determination being within 96% of the actual value determined by the laser threshold.

Nonlinear optical properties and domain microstructure of epitaxial SrBi2Nb2O9 thin films on SrTiO3 and on MgO substrates studied by second-harmonic generation

Abstract The nonlinear optical properties and domain microstructure of ferroelectric thin films of SrBi 2 Nb 2 O 9 epitaxially grown by pulsed laser deposition on SrTiO 3 and MgO substrates are studied using second-harmonic generation (SHG). The input light polarization dependence of the measured SHG intensities along the principal crystallographic directions of the film is fully interpreted on the basis of a simple nonlinear model considering four possible domain variants. Ratios of the nonlinear optical coefficients and of the net populations of antiparallel ferroelectric domains of SrBi 2 Nb 2 O 9 are determined for the first time to our knowledge.

Second-Harmonic Scattering by Ferroelectric Domains in RbH 2 PO 4

Second-Harmonic Scattering (SHS) of light by laminar ferroelectric domains of the low temperature phase of RbH 2 PO 4 is reported. CCD images of the far-field SHS pattern reveal up to six-off axis intensity maxima normal to the ferroelectric axis. These maxima are identified from the quasi-phase matching conditions with the three SHS modes allowed by crystal symmetry. Scattering angles of these modes are then calculated from crystal optics and are shown to fit well the experimental data for any scattering geometry. Scattering intensities of the two most intense modes are also measured for different scattering geometries so as to evaluate the mean domain width.