Rudolf Hofmeister

Growth and characterization of the perovskite K1-yLiyTa1-xNbxO3:Cu

Abstract We report the growth of doped potassium lithium tantalate niobate (KLTN) crystals and characterization of their physical properties. The top seeded solution growth method is described. The specifics of a particular KLTN:Cu growth are presented, and the resultant crystal is described. The dielectric properties are compared to those of KTN; the addition of lithium is demonstrated to change the character of the phase transitions as well as increase the transition temperature. Phase diagrams for the KLTN system are illustrated as a function of lithium and niobium concentration.

Simple methods of measuring the net photorefractive phase shift and coupling constant

We report measurements of the photorefractive phase shift and coupling constant of several photorefractive materials. We solve the problem of beam coupling and diffraction in a material with a dynamically written grating for arbitrary input beams. These solutions are used to determine the beam coupling as a function of the photorefractive phase ø and coupling constant g when one beam is either sinusoidally phase modulated or ramped in phase. Experimental results are obtained for LiNbO(3), BaTiO(3), and for paraelectric potassium lithium tantalate niobate as a function of applied electric field.

Vibration detection using dynamic photorefractive gratings in KTN/KLTN crystals

We demonstrate a sensitive, all‐optical, self‐aligning holographic microphone/vibration sensor utilizing the zero external electric field photorefractive (Zefpr) effect. The device relies on the unique phase relationship, φ=0, between a spatially periodic intensity standing wave and the resultant index grating created with the Zefpr effect. Under this zero phase condition, the transmitted intensity of interfering beams in a two beam coupling geometry varies linearly with displacement of either the index grating or one of the interfering beams. In this way, vibrations are sensed remotely without any electrical signals in the vicinity of the sensor. The sensitivity of the microphone was determined as a noise equivalent power of 15 dB sound pressure level relative to 0.0002 μbar across the range 1.6–15.5 kHz.

Spectral response of fixed photorefractive grating interference filters

We report a theoretical investigation of the frequency response of optical interference filters written in photorefractive materials. Counterpropagating coherent beams interact in the volume of a photorefractive crystal through two-beam coupling. The resulting hologram is fixed. The reflectivity of the hologram is calculated as a function of frequency. An analytic solution is obtained for arbitrary grating phase φ in the lossless case, α = 0. Numerical solutions are performed for α > 0. Experimental results are compared favorably with the theory.

Fixing of volume holograms in ferroelectric K_(1-y)Li_yTa_(1-x)Nb_xO_3

We report on the fixing of photorefractive volume holograms in potassium lithium tantalate niobate with ionic gratings and also with ferroelectric domain-reversed gratings. A diffraction efficiency of 55% is obtained with domain reversal in a 2-mm-thick ferroelectric phase K(1-y)Li(y)Ta(1-x)Nb(x)O(3) crystal doped with Co, V, and Ti. We measured the decay rate of the domain gratings and also of the initial electron gratings and ion gratings. The domain grating decay agrees with Vogel-Fulcher fits. The activation energies for ionic and electronic conductivity are 0.76 and 0.12 eV, respectively.

Near infrared absorption and dark conductivity of K1−yLiyTa1−xNbxO3 crystal

Absorption bands in the wave‐number range of 3525–3470 cm−1 have been observed in K1−yLiyTa1−xNbxO3 (KLTN) doped crystals except in crystals doped with Cu and V. These absorption bands are attributed to the O–H vibration band. The hydrogen concentration in KLTN doped crystal is controllable either by doping or by heat treatment. The activation energy of hydrogen ion migration is between 0.6 and 0.7 eV in KLTN doped crystals. The [H +] ion is identified as responsible for fixing (screening) the holographically produced electronic grating.

FERROELECTRIC DOMAIN GRATINGS AND BARKHAUSEN SPIKES IN POTASSIUM LITHIUM TANTALATE NIOBATE

The observation of Barkhausen current spikes during the recording of volume phase holograms in potassium lithium tantalate niobate is reported on. These spikes are due to the ferroelectric domain reversal induced by photorefractive space charge fields. Both “small” (1 nA) and “large” (100 nA) spikes are observed, which correspond to micro and macro domain reversal, respectively. The diffraction efficiency can change as much as 50% during a single macrodomain switching.