The growth striations and ferroelectric domain structures in Czochralski-grown LiNbO3 single crystals

The rotational striations and power striations are studied in LiNbO₃ crystals and one-to-one correspondence between the striations and temperature fluctuations is demonstrated. The ferroelectric domain structures related to the rotational striations and the power striations have been observed. The distribution of solute concentration in rotational striations is measured by means of energy dispersive X-ray analysis in the scanning electron microscope, and it has been concluded that the ferroelectric domain structures depend on the solute concentration gradient.     

New optical configuration for nondestructive measurements of refractive index profiles of LiNbO(3) waveguides

A direct and nondestructive measurement technique for the determination of the refractive index profiles of one- and two-dimensional LiNbO(3) waveguides is presented for the first time to our knowledge. The technique generalizes the refracted near-field method, which is well known for optical fiber characterization. The spatial resolution and accuracy are 0.1 and 0.4 μm, respectively. The refractive-index calibration is done by an analysis of the near-field light power distribution, and its resolution is approximately 2 × 10(-4). The proposed experimental setup permits sample installation and data acquisition in a few minutes.     

Characterization of near-stoichiometric Ti:LiNbO(3) strip waveguides with varied substrate refractive index in the guiding layer

We have demonstrated the possibility that near-stoichiometric Ti:LiNbO(3) strip waveguides are fabricated by carrying out vapor transport equilibration at 1060 degrees C for 12 h on a congruent LiNbO(3) substrate with photolithographically patterned 4-8 microm wide, 115 nm thick Ti strips. Optical characterizations show that these waveguides are single mode at 1.5 microm and show a waveguide loss of 1.3 dB/cm for TM mode and 1.1 dB/cm for TE mode. In the width/depth direction of the waveguide, the mode field follows the Gauss/Hermite-Gauss function. Secondary-ion-mass spectrometry (SIMS) was used to study Ti-concentration profiles in the depth direction and on the surface of the 6 microm wide waveguide. The result shows that the Ti profile follows a sum of two error functions along the width direction and a complementary error function in the depth direction. The surface Ti concentration, 1/e width and depth, and mean diffusivities along the width and depth directions of the guide are similar to 3.0 x 10(21) cm(-3), 3.8 microm, 2.6 microm, 0.30 and 0.14 microm(2)/h, respectively. Micro-Raman analysis was carried out on the waveguide endface to characterize the depth profile of Li composition in the guiding layer. The results show that the depth profile of Li composition also follows a complementary error function with a 1/e depth of 3.64 microm. The mean ([Li(Li)]+[Ti(Li)])/([Nb(Nb)]+[Ti(Nb)]) ratio in the waveguide layer is about 0.98. The inhomogeneous Li-composition profile results in a varied substrate index in the guiding layer. A two-dimensional refractive index profile model in the waveguide is proposed by taking into consideration the varied substrate index and assuming linearity between Ti-induced index change and Ti concentration. The net waveguide surface index increments at 1545 nm are 0.0114 and 0.0212 for ordinary and extraordinary rays, respectively. Based upon the constructed index model, the fundamental mode field profile was calculated using the beam propagation method, and the mode sizes and effective index versus the Ti-strip width were calculated for three lower TM and TE modes using the variational method. An agreement between theory and experiment is obtained.     

Stress induced domain formation in LiNbO3 single crystals

The domain formation phenomenon of LiNbO₃ crystals was investigated in this study. A stress induced domain formation mechanism was proposed, and domain formation of the as-grown crystals and domain inversion of substrate crystals were explained. The strong piezoelectric effect of LiNbO₃ at elevated temperature, could be the direct driving force for inversion of the spontaneous polarization direction and could form domain walls. It was found that the tensile component of the internal stresses can inverse the original direction of the spontaneous polarization.     

Broadband ultrasonic transducers using a LiNbO3 plate with a ferroelectric inversion layer

Some heat treatment of a lithium niobate (LiNbO/sub 3/) plate induces domain inversion, thereby yielding a ferroelectric inversion layer. In such a piezoelectric plate with an inversion layer, even-order thickness-extensional modes, as well as odd-order modes, can be excited piezoelectrically. Therefore, the ultrasonic transducer using such a piezoelectric plate is expected to operate over a wide frequency range. In this paper, it is shown that broadband ultrasonic transducers can be obtained at a certain thickness ratio of inversion layer to plate, and that the transducer characteristics differ depending on whether the inversion layer is on the front side or on the backside. The broadband characteristics are experimentally demonstrated by fabricating transducers with 9 MHz or 75 MHz center frequency using 36/spl deg/ rotated Y-cut LiNbO/sub 3/ plates with a ferroelectric inversion layer.     

Optical index profile at an antiparallel ferroelectric domain wall in lithium niobate

Unexpected optical contrast at antiparallel domain walls is observed in non-stoichiometric lithium niobate. This is imaged using near-field scanning optical microscopy. A detailed modeling of the imaging process is performed, and a comparison of the experimental and simulation images is used to extract the index profile across a single antiparallel domain wall.     

Measurement method of the refractive index of biotissue by total internal reflection

A simple method based on total internal reflection is presented for measuring the refractive index of biological tissue. Anarrow laser beam and a semicylindrical lens in contact with a tissue specimen are used in the experimental apparatus. The dependence of the internal reflectivity on the angle of incidence is measured to determine the critical angle and therefore the refractive index of tissue. The experimental results for several tissue samples show that the method is reliable and useful for studying tissue optics. The principal advantages of the method are its elimination of scattering effects, suitability to a small in vitro sample, and excellent accuracy.     

Measurement of group refractive index wavelength dependence using a low-coherence tandem interferometer

A technique for the measurement of the group refractive index wavelength dependence of optical materials using a low-coherence tandem interferometer and a spectrometer is proposed. Four channeled spectra resulting from interferences of light beams from different pairs of optical paths are used for the calculation of optical path differences. The group refractive index wavelength dependence is calculated from these optical path differences generated from the sample under measurement. No a priori information of the geometric thickness of a sample is required. The wavelength dependence of the group refractive index of the samples BK7 parallel plate of 5.200 and 10.025 mm from 675 to 850 nm is experimentally measured with an accuracy of the order of 10(-5) and a repeatability of the order of 10(-9).     

Extraordinary-mode refractive-index change produced by the linear electro-optic effect in LiNbO(3) and reverse-poled LiNbO(3)

To examine aspects of an integrated photonic electric-field sensor, we calculate electro-optically induced refractive-index change in regular and reverse-poled LiNbO(3). Specifically, for y-propagating extraordinary modes, we determine how index change depends on electric-field magnitude and direction. To accomplish this, changes in index-ellipsoid shape and orientation are found by the use of a numerical eigenvalue procedure to diagonalize the impermeability tensor; then, refractive index is calculated by the use of a vector reference-frame transformation and a small perturbation approximation. A general formula is inferred from calculations for specific field directions. Electro-optic coefficients for reversepoled LiNbO(3) are obtained by application of a tensor reference-frame transformation to those of LiNbO(3). The index-calculation procedure has utility beyond the problem that is considered.     

Matrix approach to quantitative refractive index analysis by Fourier domain optical coherence tomography

The rapid development in the field of optical coherence tomography has demanded increasingly sophisticated numerical models to enable the interpretation of image data and extract quantitative results. We use a matrix formulation of Fresnel's equations for multilayered media to extract layer-dependent thickness and refractive index directly from Fourier domain optical coherence tomography spectrograms. An eigenanalysis spectral decomposition approach is used to constrain the least squares fitting algorithm, avoiding the need for initial estimates of the parameter values. We demonstrate this novel quantitative analysis approach by using a multilayered phantom and show good agreement with the known layer parameter values. This approach introduces a powerful tool for the analysis of layer-dependent optical properties that could have an important role in the differentiation of healthy and diseased tissue.     

Measurement of the physical thickness and refractive index of thin epitaxial garnet films

A technique for the measurement of refractive index and physical film thickness of epitaxial garnet films is described which utilizes variable wavelength measurements. Experimental results are presented for gadolinium gallium garnet substrates and two different bubble domain film compositions. From these results, it is concluded that the technique is not applicable to wafers with films on both sides due to the differences in film thickness of the two films. For single sided wafers, the refractive index can be determined with an accuracy of ±0.3 percent.     

Measurement of the complex refractive index of isotropic materials with mueller matrix polarimetry

The complex refractive index of materials at infrared wavelengths is often determined when absorption measurements are made at selected wavelengths, and then the Kramers-Kronig relationship is used to calculate the real part of the index. Because many organic materials are highly absorbing in the infrared, absorption measurements require a short path length. We report on the use of an attenuated total internal reflection (TIR) method in combination with an infrared Mueller matrix spectropolarimeter to measure the Mueller matrix spectrum of samples from 3 to 14 mum. From the elements of the Mueller matrix the complex refractive index is determined for materials whose TIR interfaces are eigenstates of s and p polarization. The calculated index for water compares well with data found in the literature.     

Polarity determination of ferroelectric LiNbO3 crystals and BiFeO3 thin films by the convergent beam electron diffraction technique

The applicability of various convergent beam electron diffraction (CBED) methods to the polarity determination of LiNbO₃ crystals in the space group R3c was examined. We show that the method of zone-axis CBED combined with dynamical simulation is applicable to the polarity determination of LiNbO₃. The effects of the zone axis, the specimen thickness and the displacement of the homopolar atom on the polarity determination were studied. This method was also successfully applied to the polarity determination of a multiferroic BiFeO₃ film deposited on a (111)Pt/TiO₂/SiO₂/Si substrate by pulsed laser deposition. It shows that about 30% of the grains in the BiFeO₃ film are grown along the positive polar direction.     

Theoretical modeling and experiment of refractive index change in He+ ion-implanted KTP waveguide

A theoretical model is presented to explain the refractive index change in the ion-implanted KTP waveguide, which includes respective contributions of spontaneous polarization, molar polarization and molar volume, and photoelastic effect. Numerical calculations of refractive indices along different crystalline orientations (X, Y, and Z) as a function of the lattice damage level, determined by Rutherford back-scattering/channeling technique, are performed based on the results from a set of z-cut KTP crystals implanted by 300 keV He+ ions in doses ranging from 4×10(16) to 8×10(16) ions/cm2. The theoretical results show consistency with the experimental data. To our knowledge, this is the first model to comprehensively describe the ion-implanted KTP planar waveguide.