Multiline coherent oscillation in photorefractive crystals with two species of movable carriers

The coherent oscillation, because of nearly degenerate four-wave mixing in photorefractive crystals with two types of movable charge carriers, occurs at two spectral lines symmetrically shifted with respect to the pump frequency. Consequently the output oscillation exhibits the high contrast intensity modulation. The frequency separation of two oscillation modes (and modulation frequency of the output intensity) depend on the incident light intensity and spatial frequency of the developing grating. A model is presented explaining this type of oscillation by the two-maxima shape of the gain spectrum in crystals with sufficiently different relaxation times of two space-charge gratings, one formed by movable electrons and the other one by movable holes. The experimental data for coherent oscillator with tin hypothiodiphosphate (Sn₂P₂S₆) are in reasonable quantitative agreement with the calculations. Received: 12 November 1998 / Revised version: 11 January 1999 / Published online: 7 April 1999     

Holographic recording by excitation of metastable electronic states in Na2[Fe(CN)5NO]·2H2O: a new photorefractive effect

Elementary holographic phase gratings can be written in single crystals of Na₂[Fe(CN)₅NO]·2H₂O, sodiumnitroprusside, by excitation of metastable electronic states in the blue–green spectral range. For light polarized parallel to the crystallographic a and b axes of the orthorhombic crystal the light-induced modulation of the refractive index reaches Δn≈2×10^-3 at λ=514.5 nm. Although the largest population of the metastable states is reached for light polarized parallel to the crystallographic c axis, a photorefractive response is not observed. In contrast to electro-optic photorefractive materials the photorefractive effect depends mono-exponentially on the exposure and on the modulation of the incident light interference pattern. Beam-coupling experiments demonstrate that written gratings are in phase with the interference pattern in correspondence with the fact that the excitation of the metastable electronic states is local. The width of the rocking curve shows that the holographic gratings are written completely over the volume of the crystal. Variations of the wavelengths within the excitation range as well as of the crystal thickness do not influence the maximum photorefractive response. Investigations on the grating vector of the written gratings show unambiguously that charge migration is not responsible for the photorefractive effect. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Domain switching in strontium-barium niobate crystals investigated by photorefractive self-focusing

The domain switching behaviour of undoped and Ce-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) crystals is investigated for different experimental conditions. For the investigation the photorefractive self-focusing technique is used. This method is demonstrated to be well suited for determining the polarization state of SBN crystals spatially resolved. We find that the repoling behaviour of nominally pure SBN is spatially inhomogeneous. The crystals are well repoled at the positive electrode, but are depolarized at the negative electrode. The larger the Ce doping concentration is, the smaller the depolarized crystal area. The domain switching depends on the switching velocity, while no influence of the electrode material is observed. This revised version was published online in March 2005. In the previous version, the published online date was missing     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

Experimental determination of the electro-optic coefficients in Ba0.77Ca0.23TiO3

We propose an experimental method for the determination of the electro-optic coefficients of photorefractive crystals. Using anisotropic properties of the crystal and using the response of the crystal to different polarization states of the incident light, it is possible to determine the ratio between various electro-optic coefficients by measuring the gain in a two-wave mixing set-up. The accuracy of the measurements is estimated and the obtained electro-optic coefficients for a Ba_0.77Ca_0.23TiO₃ crystal show good agreement with corresponding values already known from the literature. Received: 24 January 2000 / Revised version: 23 May 2000 / Published online: 16 August 2000     

Simultaneous and sequential power transfer between coherent beams by photorefractive coupling

The power of several coherent laser beams can be transferred into a single diffraction-limited signal beam by photorefractive coupling. The efficiency of the power transfer is investigated experimentally in the case of two pump beams. Two different geometrical arrangements, one with overlapping (simultaneous) and the other with separate (sequential) pump beams are compared in a BaTiO₃:Ce crystal, with diffusion-driven photorefraction. We measured about the same power transfer efficiency in both arrangements for strong signal beams, but the efficiency was higher in the sequential arrangement for weak signals. A simple theoretical model of the power transfer process based on the standard linear two-wave-mixing theory is presented and the observed stationary beam-coupling behavior for different pump-to-signal intensity ratios is found to be in qualitative agreement with the theoretical predictions. Received: 10 November 1998 / Revised version: 22 December 1998 / Published online: 24 March 1999     

Effect of partial coherence on four-wave mixing in photorefractive materials via reflection grating approximation

We investigate the effect of beam coherence on four-wave mixing via reflection gratings in photorefractive media. For the case of phase conjugation, the results of our theoretical analysis indicate that partial coherence always leads to a drop of signal gain and phase conjugate reflectivity in non-depleted cases. In general, the mutual coherence of the signal beam and the pump beam can be enhanced due to the process of wave mixing. The mutual coherence of the phase conjugate beam and one of the pump beams depends on the beam intensity ratio as well as the optical path difference. This is distinctly different from the four-wave mixing case with a transmission grating. Received: 15 October 1999 / Revised version: 26 June 2000 / Published online: 7 February 2001     

Wavelength-stable, narrow-spectral-width oscillation of an AlGaInP diode laser coupled to a BaTiO3:Co stimulated photorefractive backscattering phase conjugator

3 ) stimulated photorefractive backscattering (SPBS) phase conjugator. The SPBS process was successfully achieved by the combination of the short-wavelength diode laser and the highly doped photorefractive BaTiO₃:Co crystal. The spectral width of the diode laser is reduced to 7.2 pm because of the narrow spectral bandwidth of the gratings [with wave number of K=2k(2k gratings)] formed in the SPBS phase conjugator, compared with the transmission grating formed in the conventional cat conjugator, which was used previously. The SPBS phase conjugator was successfully used to suppress self-frequency scanning, and wavelength-stable oscillation of the SPBS phase-conjugator-coupled diode laser was achieved. Received: 4 November 1996/Revised version: 12 February 1997     

Incoherent all-optical NAND-, NOR-, and XOR-gate using photorefractive fanning effect

An all-optical computing device is proposed to perform incoherent digital operations, including the NAND, NOR, and XOR gates. Instead of using interferometer-based methods, making use of self-generating noise gratings and the erasure of them by another beam in two photorefractive self-pumped phase conjugators (SPPC), the device is able to process mutually incoherent as well as mutually coherent inputs. Changing the orientations of two polarizers and a photorefractive crystal, we are able to perform incoherent parallel NAND, NOR, and XOR operations with this proposed setup. Received: 18 November 1998 / Revised version: 10 February 1999 / Published online: 12 April 1999     

Photorefractive gratings generated by band-gap excitation: Application to KNbO3

A simple model was developed to account for the generation of thin space–charge gratings induced by band-gap illumination in photorefractive crystals. It includes the high-intensity regime where the contribution of free carriers to the space–charge field cannot be neglected. The model was specifically applied to explain the occurrence of two mutually π-phase-shifted gratings below the surface of KNbO₃ as well as diffraction features that have been previously observed experimentally. Received: 11 October 2000 / Revised version: 22 January 2001 / Published online: 20 April 2001     

Molecular orientations and photorefractive effects in C60-doped liquid crystals with capillary-flow-induced alignment

Molecular orientations and photorefractive effects of C₆₀-doped nematic liquid crystals were investigated with layer-structured liquid crystal cells where a nematic phase was injected. Regardless of the surface treatments of the layers, liquid crystals injected with nematic phase showed a higher degree of director orientation and photorefractivity than those injected with an isotropic phase. The nematic alignment of liquid crystals in an untreated cell is generated by a capillary flow of liquid crystal during the nematic injection. However, it was observed that the grating formation was delayed a little longer with injection of nematic phase than with injection of isotropic phase. Received: 11 March 2002 / Revised version: 22 April 2002 / Published online: 12 July 2002     

Periodic poling of magnesium-oxide-doped stoichiometric lithium niobate grown by the top-seeded solution method

Magnesium-oxide-doped stoichiometric lithium niobate has been produced using the technique of top-seeded solution growth from a lithium-rich melt. Optical tests, performed with a combination of argon-ion laser lines, have confirmed a previously published result (at 532 nm) that this material has superior resistance to photorefractive damage. This material has been shown, for the first time, to be amenable to periodic poling. Optical parametric oscillator tests have shown that this material maintains the advantages of periodically poled, congruent, un-doped lithium niobate while showing no evidence of photorefractive damage under typical operating conditions. Operating wavelengths as a function of quasi-phase-matching period and temperature have been measured for the optical parametric oscillator, providing useful new information about refractive-index dispersion in this material. This work establishes periodically poled, magnesium-oxide-doped stoichiometric lithium niobate as a viable material for nonlinear optics. Received: 28 June 2000 / Revised version: 12 September 2000 / Published online: 7 February 2001     

Manifestation of Curie–Weiss law for optical phase transition

Considerable slowing down is observed for both the temporal development of the coherent oscillation slightly above the threshold and the refractive index grating decay slightly below the threshold for a semilinear photorefractive oscillator with two counter-propagating pump waves. It is shown that in the vicinity of the threshold the reciprocal characteristic time is a linear function of deviation from the threshold coupling strength. This behaviour is similar to an empirical Curie–Weiss law and points to the analogy of the oscillation threshold to a second-order phase transition. Received: 2 April 2001 / Revised version: 23 August 2001 / Published online: 7 November 2001     

Enhanced performance of a mutually pumped phase-conjugation setup using two photorefractive crystals

A mutually pumped phase-conjugator (MPPC) consisting of two photorefractive crystals is investigated experimentally at a wavelength of about 807 nm. In addition to a Rh:BaTiO₃ crystal, which is used in a modified bridge configuration for phase-conjugation, a Co:BaTiO₃ crystal is used to amplify the phase-conjugate beam by wave-mixing. The phase-conjugate power is at least doubled compared to that using only one crystal, and the beam quality of the phase-conjugate beam is distinctly improved. Moreover, the temporal stability of the phase-conjugate output using two crystals is much higher than that using only one crystal. Received: 7 December 2000 / Revised version: 12 January 2001 / Published online: 27 April 2001     

Reflection-type photorefractive gratings in tin hypotiodiphosphate

Reflection photorefractive gratings recorded by nearly counterpropagating light waves in the near infrared are studied in tin hypothiodiphosphate. The ratios are established for certain electrooptic tensor components responsible for reflection grating recording, and the Debye screening length is evaluated. Reflection holograms of binary objects are recorded. Received: 26 June 2001 / Revised version: 3 September 2001 / Published online: 29 November 2001     

Incoherently coupled bright–dark hybrid soliton families in biased photovoltaic–photorefractive crystals

The propagation of multiple mutually incoherent spatial solitons in biased photovoltaic–photorefractive crystals is investigated under steady-state conditions. It is shown that incoherently coupled bright–dark hybrid screening–photovoltaic soliton families can be established in the crystals provided that the mutually incoherent incident beams have the same polarization and the same wavelength. Relevant examples are presented where the photovoltaic–photorefractive crystal is taken to be lithium niobate. Received: 15 May 2000 / Revised version: 27 July 2000 / Published online: 5 October 2000     

Performance improvement of a holographic double-exposure interferometer by heating of the photorefractive recording crystal

A holographic double-exposure interferometer for quasi real-time analysis is realized, utilizing a photorefractive BTO:Cd crystal for hologram recording. Heating of the crystal reduces the time required to reach the steady-state working situation. Light-induced absorption losses are reduced by heating as well. Additionally, the diffraction efficiency is increased by this method. A sixfold increase in the light intensity of the reconstructed interferogram is achieved. Received: 2 December 1998 / Revised version: 28 January 1999 / Published online: 7 April 1999     

Off-Bragg-angle light diffraction and structure of dynamic interband photorefractive gratings

Steady-state and time-resolved off-Bragg-angle diffraction experiments are used to determine the structure and the dynamics of photorefractive gratings induced by interband photoexcitation. In potassium niobate, we identify in such gratings basically a two-layer structure. Close to the surface, we find a space-charge electric field generated by a charge modulation stored directly in the bands. This grating component is typically 50 μm thick, the amplitude of the refractive index modulation is larger than 10^-4, and the response time is a few μs for resonant intensities of 100 mW cm^-2. This component is also robust under non-resonant illumination. Deeper in the crystal, a second holographic layer extends over a few hundreds of μm, its amplitude is smaller, and its slower response time is in the ms range. The mutual phase shift between the grating components is also determined. Received: 23 November 1998 / Revised version: 14 January 1999 / Published online: 12 April 1999     

Semilinear coherent oscillator with reflection-type photorefractive gratings

The saturation intensity, oscillation spectrum, and temporal dynamics of oscillation are studied for a semilinear coherent oscillator with two counterpropagating pump waves and a photorefractive crystal with dominating reflection gratings. The instability of the single-frequency oscillation spectrum is revealed, similar to that known for an oscillator with dominating transmission gratings. The experimental manifestation of this transition in the output characteristics of the oscillator is favourably compared with numerical calculations. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf     

Photorefractive properties in homogeneous and heterogeneous polymer/liquid crystal composites

The photorefractivity of a photorefractive composite consisting of poly(N-vinylcarbazole) (PVK) doped with a non-room-temperature liquid crystal, 4-butyloxy-4’-cyanobiphenyl (4OCB), and C₆₀ was studied by means of measuring the two-beam coupling coefficients. The results show that the photorefractivity is enhanced by phase separation. Received: 6 December 2000 / Revised version: 5 March 2001 / Published online: 9 May 2001