A. Grunnet-Jepsen

Investigation of the internal field in photorefractive materials and measurement of the effective electro-optic coefficient

An experimental investigation of the electric field in the bulk of a Bi12SiO20 crystal is carried out, and a two-region model is developed that can account for the buildup of screening charges near the electrodes. In light of our results, a simple method is proposed for the determination of the effective electro-optic coefficients based on applying a sufficiently high-frequency square-wave voltage to prevent screening charge buildup. A demonstration of this method for Bi12SiO20 leads to a value of 4.4 pm/V for the stress-free (unclamped) coefficient, and a subsequent consideration of piezoelastic contributions allows the strain-free (clamped) coefficient to be estimated at 3.7 pm/V.

Fundamental space-charge fields for applied alternating electric fields in photorefractive materials

The fundamental component of the space-charge field in a photorefractive crystal is studied in the presence of an applied alternating electric field. A general equation for the amplitude of the fundamental space-charge field is derived, and a solution in integral form is given for the linear-modulation approximation. Exact and approximate analytical solutions are presented for the case in which the applied field varies in time as a square and a sine wave, respectively. The temporal variation of the amplitude of the space-charge field is investigated for both waveforms of the applied field, and the oscillation amplitude dependence on repetition frequency and interbeam angle is studied.

Subharmonic instabilities in photorefractive crystals for an applied alternating electric field: theoretical analysis

The spatial subharmonic instability arising from the material equations is investigated theoretically for both square and sinusoidal applied alternating electric fields. The threshold conditions are given for the amplitude of the applied field as a function of the detuning frequency, contrast of fundamental interference pattern, and interbeam angle by the use of the temporal solutions for the amplitude of the space-charge field presented elsewhere [ J. Opt. Soc. Am. B.10, 124 ( 1993)] The different effects of the square wave and sinusoidal applied fields for subharmonic generation are discussed.

Space-charge fields in photorefractive materials enhanced by moving fringes: comparison of electron–hole transport models

Space-charge fields in photorefractive materials enhanced by moving fringes are studied for simultaneous electron–hole transport. Two distinct models, one with a single set of impurity sites and the other with two independent sets, are discussed. It is shown that the latter model gives rise to a new space-charge field resonance that depends on the relative concentration of electrons and holes. The space-charge wave concept is used in the mathematical analysis of both models.

Effect of subharmonics on two-wave gain in Bi 12 SiO 20 under alternating electric fields

It is demonstrated that the emergence of subharmonics during two-wave mixing in a Bi(12)SiO(20) photorefractive crystal under applied alternating electric fields will markedly reduce two-wave gain. The repetition frequency at which the observed drop in two-wave gain appears is correlated with threshold conditions for the subharmonic instability for a wide range of experimental parameters.

GENERATION AND INHIBITION OF DOMAIN MOVEMENT IN SEMI-INSULATING GAAS:CR

Electric‐field domains in semi‐insulating GaAs:Cr are investigated both by imaging the locally induced birefringence using a transverse electro‐optic configuration and by measuring the dc current flowing through the crystal. It is shown that the formation of domains may be controlled by optical means.

Subharmonic domains in a bismuth germanate crystal.

It is shown that a subharmonic grating may have two different phase relationships relative to the fundamental grating, leading to the appearance of domains. The subharmonic beam is generated in a Bi12GiO20 crystal illuminated by two beams in the presence of an applied ac square-wave voltage. The domain structure and the relative phases of the gratings contained in them have been observed with the aid of an interferometric technique.

High-frequency resonances in photorefractive crystals.

Two-wave mixing in photorefractive materials is investigated for the case of applied dc electric fields and large detuning frequencies and for high-frequency alternating electric fields. The appearance of high-frequency resonances is demonstrated.

Can trap density limitations be overcome in photorefractive two-beam coupling?

Abstract Two-wave mixing gain is investigated in the presence of a temporally varying (pulsed or sinusoidal) field. It is shown that, contrary to former claims, the trap density limitation cannot be overcome.

Effect of piezoelectricity on the photorefractive gain in a bismuth silicate crystal

The application of a voltage, V0, across a photorefractive crystal in the presence of detuning is well known as a means of enhancing two-wave gain. By mounting a crystal of bismuth silicate, with faces cut parallel to the (001), (110) and (110) planes, in a rotating holder, measurements of two wave gain as a function of (theta) , the angle between the electric field vector E0 (in the [001] direction) and the grating vector, K (in the (110) plane), have been made. Our results show that by having a non-zero value of (theta) the signal gain can be improved by a factor of two. Furthermore the experimental results showed that the (theta) dependent gain peaks differ in magnitude according to whether the crystal is rotated in an anti-clockwise or clockwise direction. In order to model the experimental results the steady-state expression for the space-charge field is extended to two dimensions and the usually neglected piezoelectric effect is included in the analysis of the modulation of the refractive index by the presence of this field inside the crystal. The asymmetry of the gain peaks is found to be caused by the optical activity of bismuth silicate and is predicted by the theory.