In-plane distribution of huge local permittivity of KTa1-xNbxO3 estimated from local phase transition temperatures and spatially averaged permittivity.

Potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystals have a very large relative permittivity εr (>104) just above the paraelectric to ferroelectric phase transition temperature (TC). The quadratic electro-optic coefficient and the electro-strictive coefficient are also very large because of their proportionality to εr2. However, the local relative permittivity can easily vary spatially due to the incongruently melting nature of KTN. In this study, we quantitatively estimated the in-plane distribution of the huge local relative permittivity of KTN. First, we measured the spatial distribution of TC using scanning nonlinear dielectric microscopy, then deposited the electrodes and measured the temperature dependence of the spatially averaged permittivity using an LCR meter. Following that, we evaluated the spatial distribution of the huge local permittivity from the combination of the spatial distribution of TC and the spatially averaged permittivity. Finally, we measured the deflection angle of light to confirm the validity of the εr estimation procedure. The maximum error for the estimated permittivity was estimated to be around 3.3%.

Analyses of optical rays in KTN optical beam deflectors for device design.

The potassium tantalate niobate (KTN) optical beam deflector is an electro-optic deflector without any moving parts that works at frequencies higher than 200 kHz. In this paper, we discuss the performance parameters of this deflector. Optical beams are bent by the spatial distribution of the refractive index in the KTN crystal block used in this deflector. In addition to the deflection function, the index distribution operates as a cylindrical convex lens. Therefore, the deflector is often used with glass cylindrical lenses to cancel out the lens function. We analyzed optical rays curving in the block based on the graded index lens theory. We describe the way in which performance parameters, such as the deflection angle, change, depending on both the choice of the compensating lenses and the parameters of the KTN block, namely its size and charge density. We concentrate especially on methods designed to improve the resolvable spot number, which is the most important figure of merit for optical deflectors. One way to achieve improvement is to input a collimated beam or a slightly converging beam into the KTN block.

Anomalous index modulations in electrooptic KTa(1-x)Nb(x)O(3) single crystals in relation to electrostrictive effect.

KTa(1-x)Nb(x)O(3) is known for its huge Kerr effect, which is a second order electrooptic (EO) effect. By utilizing the large refractive index change Δn of this EO effect, a fast optical beam deflector has been realized. However, anomalous spatial distributions of Δn were observed with this beam deflector. This anomaly is ascribed to distortions caused by the electrostrictive effect that occurs when voltage is applied. We assumed a spheric distortion and used a variational method to deduce an analytic solution for the strains that accompany this distortion. The analytic solution coincides with numerical results obtained with the finite element method. In addition, the solution agrees well with the experimentally obtained Δn distribution.

Charge distributions in KTa1₋xNbxO3 optical beam deflectors formed by voltage application.

Controlling the space charge distributions in a crystal is indispensable for controlling a KTa1₋xNbxO3(KTN) optical beam deflector. The space charge is built up by applying a voltage and injecting electrons into the KTN crystal. Although a homogeneous distribution is preferable, we observed experimentally that the injected electrons concentrated in the vicinity of the cathode and for some samples the concentration was much lower around the anode. We investigated the electron dynamics theoretically and found that such inhomogeneity was caused by a freezing effect where the motion was very slow considering the duration of the practical voltage application. The depth of the space charge spread or the electron penetration depth from the cathode was proportional to the applied voltage and the permittivity, and inversely proportional to the density of traps or localized states that bind electrons. We believe that the trap density was too large for the samples with inhomogeneous charge distributions.

Trapped charge density analysis of KTN crystal by beam path measurement.

Because the function of a single crystal of potassium tantalate niobate (KTa(1-x)Nb(x)O(3), KTN) is largely decided by the trapped charge density inside it, it is essential to determine its value. We quantitatively estimate the charge density using two optical analysis methods, namely by investigating KTN's deflection angle when it is used as a deflector and by investigating KTN's focal length when it is used as a graded-index (GRIN) lens. A strobe technique is introduced with which to perform the measurement. The charge density values under different temperature conditions are shown. These results suggest that the charge density can be determined with both methods, and is constant in a specific temperature range. The charge density value is around 80 C/m(3) in our setup.

Polarization independent varifocal lens using KTN crystals.

We fabricated a polarization-independent varifocal lens using KTa(1-x)Nb(x)O3 (KTN) crystals. The polarization dependence of the KTN crystal is effectively compensated for by combining a pair of KTN lenses and a half-wave plate. This compensation is achieved by a total electro-optic effect, which consists of the Kerr effect and the elasto-optic effects via the electrostrictive and elastic strains in the KTN crystal.

Fast response varifocal lenses using KTa(1-x)Nb(x)O3 crystals and a simulation method with electrostrictive calculations.

We fabricated cylindrical varifocal lenses with fast responses by using the strong Kerr effect of KTa(1-x)Nb(x)O(3) (KTN) single crystals. We observed focus shifts of up to 87 mm with the assistance of a 250 mm focal length lens, which corresponds to a focus shift from infinity to 720 mm by the KTN lens itself. The response time was as fast as 1 µs. We also present a simulation method for calculating refractive index distributions in KTN single crystals, which is essential when designing the lens. The method is characterized by the strain contribution, which has not conventionally been typical of electro-optic simulations. We used this method to explain the refractive index modulations that are characteristic of the varifocal lenses.

Orthogonal aperture multiplexing for multilayered waveguide holographic read-only memories.

A multilayered waveguide holographic read-only memory is a promising candidate for the next generation of optical data storage systems. We improved the data density of the memory by using a multiplexing method with a set of orthogonal optical masks. We multiplexed as many as nine images into one waveguide hologram, and all the observed images had negligible cross talk. This made it possible to achieve a ninefold increase in data density. We provide experimental results for both metallic and liquid-crystal masks.