Tadayuki Imai

Space-charge-controlled electro-optic effect: Optical beam deflection by electro-optic effect and space-charge-controlled electrical conduction

This paper describes the experimental and theoretical studies of an anomalous optical beam deflection phenomenon based on electrooptic effect and space-charge-controlled electrical conduction. In the experiment, a large deflection angle of 250mrad (=14.3°) has been observed by applying ±250V to a 0.5‐mm-thick KTa1−xNbxO3 crystal with a short interaction length of 5.0mm. The crystal has a rectangular shape with uniform electrodes and there is no prism shape involved which is a common geometrical shape of crystal, electrode, or ferroelectric domain in the conventional electro-optic deflectors. The operating principle is investigated and it is found that the space-charge-controlled electrical conduction in the crystal plays an essential role in this deflection phenomenon. The electrical conduction is carried by electrons injected from the Ohmic contact of the electrode with the crystal. The injected electrons induce the space-charge effect and the electric field becomes nonuniform between the electrodes. The...

New Beam Scanning Model for High-Speed Operation Using KTa1-xNbxO3 Crystals

We propose a new beam scanning model that is applicable to electrooptic materials with electron traps. With this model, we can achieve both high-speed operation and wide-angle scanning, because the operating speed is limited not by the electron mobility but by the frequency limit of the electrooptic effect of the materials. The voltage dependence of the scanning angle at 100 kHz using a KTa1-xNbxO3 crystal is consistent with the property predicted by the proposed model.

Low-driving-voltage electro-optic modulator with novel KTa1-xNbxO3 crystal waveguides

We have successfully demonstrated a low-driving-voltage electro-optic (EO) modulator using newly developed KTa1-xNbxO3 (KTN) buried waveguides. We prepared high-quality and large KTN crystals. The crystals exhibited a large quadratic EO coefficient of 4.8×10-15 m2/V2 at 1.55 µm. The KTN crystals also exhibited a very large linear EO effect, for example 600 pm/V at a biased potential of 60 V/mm, which is twenty times larger than the r33 of LiNbO3. The waveguide layers for the core and cladding were formed by liquid phase epitaxy techniques on the KTN crystals. The propagation loss was less than 0.5 dB/cm and the polarization dependent loss was less than 0.1 dB/cm. The EO modulators had a low Vπ of <2.5 V (4.0-V biased potential, l=6 mm) and a fast response of 3 GHz.

PbTiO3 Thin Films Deposited by Laser Ablation

PbTiO3 thin films have been deposited on Si or Pt substrates by the laser ablation method. The ArF excimer laser (193 nm) and YAG laser (1064 nm) were used. Systematic investigations on the film deposition rates with its spacial distributions as functions of laser fluence have been made. As a result, it is suggested that an excimer laser having a shorter wavelength is better for preparing PbTiO3 thin films. The perovskite phase is obtained in the films deposited using such an excimer laser and under suitable conditions. A substrate temperature of over 400?C and appropriate ambient O2 gas pressure are needed.

Fast Response Variable Focal-Length Lenses Using KTa1-xNbxO3 Crystals

We fabricated variable focal length lenses with fast responses by using the strong Kerr effect of KTa1-xNbxO3 single crystals. We observed focus shifts of about 40 mm with the assistance of a 250-mm-focal-length lens, which corresponds to a focus shift from infinity to 1.56 m without the assisting lens. The time response was as fast as 1 µs.

Fast response varifocal lenses using KTa 1−x Nb x O 3 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.

Digital Holographic Memory Using Two-Dimensional Modulation Code

The degradation of reconstructed images from a digital holographic memory that uses highly sensitive strontium barium niobate as a recording medium was reduced using two-dimensional modulation codes that have low average intensity and high coding efficiency. The signal-to-noise ratios of the reconstructed images and hence the bit error rates of the data retrieved from the images were improved without reducing the coding efficiency.

Effects of Heat Treatment on Photorefractive Sensitivity of Ce- and Eu-Doped Strontium Barium Niobate

We investigated the effects of heat treatment on the data storage performance of strontium barium niobate doped with cerium and europium. The photorefractive writing sensitivity was increased by elevating the annealing temperature. When the annealing was undertaken at a high temperature, for example 900°C, the sensitivity reached a value about ten times that obtained at an annealing temperature below 600°C. This increase was caused by a photoconductivity increase which was probably due to an increase in the oxygen vacancy concentration. The page multiplexing performance of the crystal is also discussed in relation to the photorefractive trap density.

Preparation of PbTiO3 Thin Film on Si by ArF Excimer Laser Ablation

Deposition behavior of PbTiO3 thin films on Si substrates having native oxide has been investigated by the laser ablation method using an ArF excimer laser. Atomic ratio Pb/Ti in the film decreases with increase of substrate temperature and laser shot period, but only changes slightly with increase of laser fluence. Preferentially c-axis-oriented film on Si substrate is obtained during the deposition in O2 gas of 13 Pa at the substrate temperature of 500°C and low laser repetition rate. The change of Pb/Ti and the origin of the preferential orientation are explained by Pb reevaporation from the substrate and successive deposition of Pb and Ti layers. Dielectric constant and the remanent polarization are about 130 and 30 µC/cm2, respectively.

Measurement of charge density distributions in KTa 1-x Nb x O 3 optical beam deflectors

In a KTa1-xNbxO3 optical beam deflector, light rays are bent by a space charge formed by electrons that are injected through an electrode and captured by crystal defects. For complete device control, it is important to evaluate the space charge. We propose an optical method for measuring charge density distributions that utilizes the electrooptic (EO) effect of the material. With this method, electron accumulation caused by a screening effect was commonly observed near the cathode. The electron accumulation region extended toward the anode as the applied voltage and permittivity increased. This method can be applied to any EO materials that exhibit space charge distributions.