Tallis Y. Chang

Optical phase conjugation by backscattering in barium titanate

We demonstrate optical-beam phase conjugation by the process of two-beam coupling in photorefractive barium titanate. The incident, image-bearing beam causes exponential gain for counterpropagating waves, which are fed by noise and emerge with a power of the order of 10% of the incident beam and phase conjugate to it. This is expected from the calculated plane-wave gain plus the analogy to the theory of phase conjugation of complex wave fronts by stimulated Brillouin backscattering. We conjugate beams at either 515 or 488 nm at between 10- and 50-mW power, and find, as expected, no frequency shift (<1 Hz) in the process.

Photorefractive-index gratings formed by nanosecond optical pulses in BaTiO 3

We write and read a refractive-index grating in a photorefractive crystal BaTiO(3) with a single 20-nsec laser pulse at 532 nm. The grating formed is erasable with similar pulses. Diffraction efficiency decreases exponentially with the cumulative erasing light energy for 20-nsec pulses at intensity levels of 5-30 MW/cm(2). Optical energies required to write and erase gratings with 20-nsec pulses are about an order of magnitude larger than for millisecond-to-to-second-long pulses at 515 nm, even though the grating is still formed by a one-photon process.

Photorefractive Measurement Of Anisotropy Of The Mobility Of Photo-Excited Holes In BaTiO3

We have measured the anisotropy of the mobility of photo-excited holes in nominally undoped single-crystal, poled, ferroelectric BaTiO3 (p-BaTiO3) at room temperature. In this crystal, a photorefractive grating results from the spatial rearrangement of trapped holes which have been excited by light-intensity beats and then drift and diffuse before recombining at similar empty traps. The resulting Coulomb field grating gives rise to a refractive index grating (the photorefractive grating) via the electro-optic effect. In this uniaxial crystal the mobility of the holes is different parallel (μ) or perpendicular (μ) to the crystal axis (c-axis). As is well known, a uniform light beam causes an established photorefractive grating to decay exponentially in time. We have measured this decay rate with the grating wavevector oriented at various angles to the c-axis and compared these rates with the prediction of the standard model which assumes simple drift and diffusion with direct recombination time T,1 as extended by us to allow different drift and diffusion rates parallel and perpendicular to the c-axis. The data is satisfactorily fit by this model if we use μ/μ = 18 ± 7 and = 5 x 10-1 cm2/Volt. We also require a ratio of dielectric constants el/e11 = 25 ± 15, which is consistent with the ratio measure-ments by other methods in other samples. To fit the data, we also needed values for the density of excitable traps (= 2 x 1016 cm-3) and photoconductivity parallel to the c-axis, which we took from previously published measurements on the same crystal. 2 Our measurements were made with 8 nsec pulses at A = 532 nm. We could see no effects of finite hole recombination time, which suggests that the recombination time must be less than 1 nsec. If this is true,and recombination is a direct process, then the mobility must be larger than 0.5 cm2/sec/Volt parallel to the c-axis.

Electro-Optic Power Limiter: Broadband, Self-Actuating Optical Limiter for Visible and Infrared Radiation

We present studies of an optical power limiter that uses a combination of the photoconductive and linear electro-optic effects. The electro-optic power limiter (EOPL), first demonstrated using CdTe in the near IR, possesses many desirable qualities including broadband wavelength responsivity, self-actuation, and low threshold power. One unique capability of EOPL, not found in any other nonlinear optical power limiter, is the fact that any light - coherent or incoherent - can be used. Our studies in CdTe, GaAs, and BSO crystals resulted in extending the wavelength range from the visible to MWIR wavelengths.

Digital holographic data storage with fast access

We report on the development and demonstration of two systems, one with 100 Mbytes and the other with more than 1 Gbyte of storage capacity. Both systems feature solid state design with the addressing mechanism realized with acoustooptic deflectors that are capable of better than 50 /spl mu/sec data seek time.

Erratum to “Continuous Color Reflective Display Fabricated in Integrated MEMS-and-TFTon- Glass Process”

In the above paper [1] , Figs. 1 – 3 , 10 , 15 , 28 , and 30 – 31 were inadvertently printed in black and white. The color figures are as follows.

Nonmechanical image rotation with an acousto-optic dove prism

Summary form only given. Image rotation is important for various applications such as rotational invariant pattern recognition, computer graphics, and beam steering of phased-array antennas. Most of the techniques rely on the mechanical rotation of a dove prism. However, such a mechanical process is slow and unreliable. We propose a new image rotation method that is both nonmechanical and programmable to permit rotation of an image to an arbitrary angle in a controllable manner.

Two-photon organic photochemistry for 2-D spatial multiplexing in volume holographic storage

The two-photon photochemistry described here is a potential candidate for 2-d spatial multiplexing in volume holographic storage. The initial diffraction efficiency obtained in these materials is promising. Issues remain with understanding and avoiding the decay of the initial diffraction efficiency, and with reducing the absorption of the exciting wavelength without simultaneously reducing the diffraction efficiency. The ease of manufacture of the two-photon photochemical system compared with the inorganic photorefractives makes them attractive for the holographic storage application.

Application of Laser and Related Materials to Demonstrate Large Nonlinear Optical Effects and Diffraction Efficiency

Long metastable lifetimes in laser and related materials combined with a large index change makes possible large nonlinear optical effects using cw lasers. We describe experiments demonstrating such effects using the solid state laser material Cr:YAlO3, and we show how studies of the spectroscopy and kinetics can be used to understand the results. Since the same optical nonlinearity will affect the propagation of beams, it should be considered in the design of lasers.