Lowell T. Wood

Method for measuring off-diagonal Kerr coefficients by using polarized light transmission

We report a method for measuring the off-diagonal coefficients of the quadratic electro-optic (Kerr) tensor by using polarized light transmission. The method relies on designing an experimental configuration in which the linear electro-optic (Pockels) effect does not contribute to the data. Our method can be used to obtain off-diagonal Kerr coefficients for all but two of the 20 crystal point groups for which the Pockels effect and the Kerr effect coexist. Our theoretical model includes effects from transmission, multiple reflections, and electrostriction but neglects absorption in the crystal. To verify the method, we used it to measure the R12 and R13 Kerr coefficients for a (100)-type single crystal of ferroelectric barium titanate (BaTiO3) at room temperature (23.5 degrees). To our knowledge, this is the first time this method has been used and the first time these coefficients have been measured for the unclamped crystal in the tetragonal state. The mean values obtained with this method are R12 = -3.5+/-0.3 x 10(-17) m2/V2 and R13 = -8.0+/-0.7 x 10(-17) m2/V2.

Diffraction from tunable periodic structures: application for the determination of electro-optic coefficients.

We discuss a method for measuring electro-optic coefficients by measuring diffraction from a tunable grating. The method involves measuring the changes in the diffraction pattern of a reflection grating, where applied electric fields of alternating direction induce changes in the index of refraction through the electro-optic effect. For certain geometries, these applied fields cause period-doubling effects that produce new peaks in the diffraction pattern. Numerically calculated diffraction patterns are presented for the assumptions of both homogeneous and inhomogeneous fields. Peak splitting, as a function of both the number of slits illuminated and the induced change in the index of refraction, is observed and discussed. Finally, the usefulness of our method for the measurement of electro-optic coefficients is discussed.

Laser-array generators produced by patterned ion irradiation of acrylic films

Ion irradiation of polymer films is a promising process technology for photonics applications that require flexible, lightweight devices resistant to selected environmental variables. Crossed phase gratings that may serve as laser-beam array generators are fabricated using the dry process of irradiation of acrylic (PMMA) films with various doses of high-energy alpha particles through a stencil mask. The gratings are examined with the aid of AFM and SEM images, and Raman-Nath diffraction analysis is applied to estimate the generated refractive-index modulation as a function of the dose. SEM images of a stained grating cross-section suggest a mechanism of unsaturated bond formation and accompanying contraction of the irradiated polymer. Post-irradiation baking is shown to increase the contraction or generated surface relief by around an order of magnitude. Since the index modulation and surface relief due to irradiation tend to cancel, the overall diffraction effciencies of unbaked gratings do not surpass 67%, although baked gratings can provide higher diffraction effciencies.

An ellipsometric method for the determination of the dielectric tensor of an optically uniaxial material suited for in-situ measurements

We introduce a photometric ellipsometric method to determine the dielectric tensor of an optically uniaxial material suited for in-situ measurements. The technique employs a rotatable polarizer to provide a series of polarization states, eliminating the need for sample rotation or changes in angle of incidence. A method is presented that enables extraction of the dielectric tensor using standard rotating analyzer methods. As an example, the method is used to determine the indices of refraction and the orientation of principal axes for a calcite crystal.

Improved sensitivity in sensor fibers by the proper source/coating selection

The dependence of the optical attenuation on the fiber coating has been measured for a sensor fiber subjected to periodic microbending. The dependence on the spectral characteristics of the selected light sources is also addressed and the proper source/coating combination is discussed. Results indicate that the proper selection of this combination is of crucial importance in the design, development, and performance of fiber optic sensors in which intensity modulated microbending transducers are considered. A combination of a short wavelength for the light source and a fiber coated with a hard, elastomeric material seems to be the optimal selection for enhancing the sensitivity in controlled microbending experiments with applications to fiber optic sensors. The highest sensitivity to microbending was obtained for a nylon coated fiber and using a He-Ne laser at 632.8 nm.

Determining selected quadratic coefficients in noncentrosymmetric crystals

We present a systematic theoretical examination concerning the practical possibility of measuring components of the quadratic symmetry tensors for solid materials lacking inversion symmetry. We show that information concerning the quadratic effects in noncentrosymmetrical crystals can be obtained directly by methods that allow for the bypassing of the coexisting dominant linear effects. Here, we deal only with the particular case of the electro-optic effect; however, the results and conclusions are also valid for the elasto-optic effects.

Fraunhofer diffraction to determine the twin angle in single-crystal BaTiO3

We present a new method for determining the electrically induced twin angle alpha of a (100) bulk single crystal of barium titanate (BaTiO3) using a nondestructive optical technique based on Fraunhofer diffraction. The technique required two steps that were performed simultaneously. First, we analyzed the diffracted light intensity captured with a line camera. Second, we measured the size of the diffracting element by analyzing images of crystals surface taken with a CCD camera. The value obtained for the twin angle is 0.67 degrees +/- 0.05 degrees, which compares favorably with the theoretical value of 0.63 degrees.

An experimental method to investigate the structure and kinetics of patterned surfaces using laser light diffraction

We describe a novel experimental method using the diffraction of a He–Ne laser beam to study surfaces patterned with structures on mesoscopic to macroscopic length scales. The technique provides high spatial and temporal resolution; it is not limited to periodic, artificial structures, but is also well suited to study the development of self-organized surface relief. Measurements can be performed under in situ conditions in a diffraction mode or an imaging mode, providing (1) qualitative and quantitative information on the surface structures, (2) information on time-dependent surface changes with a resolution of 10 μs or better, (3) observation of incubation processes (including determination of incubation time) in first-order, displacive phase transformations, and (4) observation of the surface in real space, in particular, the pattern evolution as a function of temperature or other parameters. As an example we show results of the application of our method to a Ni0.63Al0.37 single crystal undergoing a ma...

Electric-field imaging using optical techniques

We describe an optical technique using divergent laser beams and sample scanning for measuring electric-field distributions in the spaces between interdigital electrodes. We present experimental results using a LiNbO3 sample and compare them to calculated electric-field distributions. The basic structure of the fields and the locations of the maxima show excellent agreement between experiment and theory.

Optimization of conventional ellipsometric setups for determining the dielectric tensor of arbitrarily anisotropic systems

We present a general theoretical development to optimize the setup of a conventional ellipsometer to determine dielectric tensors of arbitrarily anisotropic systems. Typical conventional ellipsometers permit changing several variables: angle of incidence, azimuth angle of the sample, and the azimuth angles of the polarizer, compensator, or analyzer. We illustrate the optimization with two examples, one using a rotating analyzer ellipsometer where we show that our theory requires changing at least one less variable than required by the Jones matrix approach. In the second example, a new ellipsometric approach is proposed to determine the dielectric tensors and film thicknesses of an arbitrarily anisotropic system. With this approach, only the sample is rotated and all other parts are fixed. Numerical calculations were performed to simulate several common multi-layer structures. Potential applications for in situ quality monitoring of MBE growth or of space growth are also discussed.