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In this chapter the results of scanning probe microscopic (SPM) investigations near ferroelectric (ferroic)—paraelectric phase transitions are presented. Submicroscopic investigations of domains near the transition temperature are of fundamental importance both for the basic understanding of the phase transitions itself but also for devices operating near the transition temperatures.

Summary from only given. The polarization and wavelength dependence of photogalvanic current and photoconductivity has been studied for LiTaO/sub 3/. Two different wavelengths (532 nm green and 350 nm UV lights) have been used. In order to study the optical damage, a focused beam of 532 nm green laser was used.

We present a compact, room temperature, and narrowband terahertz source, based on difference-frequency generation in the organic nonlinear optical crystals OH1 (2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile). The system employs a specific dual-wavelength infrared laser that emits coaxial, synchronous 10-ns pulses of similar energy and duration at wavelengths of 1064 nm and 1030 nm by using Nd:YAG and Yb:YAG crystals within the split laser cavity. The common part of the laser cavity comprises an acousto-optic Q-switch and an output coupler. The output is frequency-mixed in a stack of several OH1 crystals in a quasi-phase-matching configuration, which is determined on the basis of refractive index and absorption measurements in the 1–11 THz range. The system generates terahertz radiation in pulse trains with 1.0 μW average power and a near-Gaussian intensity profile.

The field of three-dimensional optical storage has lately witnessed a tremendous increase of interest from both the scientific and the industrial communities. Volume holographic storage was proposed by van Heerden [5.1] shortly after the introduction of the laser and interest in this technology has continued at varying degrees of intensity since. Holography potentially combines the advantages of high storage densities and ease of parallel data readout. The storage of multiple pages of information in the same volume and the parallel readout of many bits promise fast data access times and high data bandwidth. Furthermore, in holography the information is distributed throughout the storage medium and is therefore less sensitive to local material imperfections. If Fourier holograms are stored in holographic memories, the effects of imperfections in the medium are distributed throughout the page and are not linked to individual pixels; thus valuable bits of data are not lost, only the signal-to-noise ratio is diminished. This is in contrast to magnetic and conventional optical recording, where an individual bit of information is represented by a highly localized change in some physical property. This advantage may be less important in holographic digital data storage when additional redundancy of information storage and error correction can be easily implemented.

Light-induced changes in optical properties such as refractive index and absorption coefficients are always related to material excitation, i.e. deviation from (thermal) equilibrium [4.1]. Examples of material excitations are an increase in the number of conducting free electrons, the phonon density or the number of electrons occupying excited states. As a result of spatially inhomogeneous material excitation, the optical properties become spatially modulated in the interference region of two intense coherent light waves and create gratings that may be permanent or dynamic, depending on the mechanism and the material. Permanent gratings hold for longer times, while dynamic or transient gratings disappear after the inducing laser source is switched off. The latter have been produced in a large variety of materials and are usually detected by diffraction of a probe beam or by self-diffraction of the light waves inducing the grating.

We show that high-temperature reduction in a CO-CO(2) atmosphere increases the photorefractive sensitivity of KNbO(3):Rh at 860nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spacing of 0.15 mu;m is accelerated by a factor of 400. The grating buildup time at a grating spacing of 0.7 microm and an intensity of 1Wcm(-2) is 0.5 s, a value comparable with that of as-grown KNbO(3):Fe at visible wavelengths. The optical and photorefractive parameters of Rh-doped KNbO(3) subjected to reduction treatment are characterized for wavelengths of 0.48-1.064 microm .

The photorefractive properties of stoichiometric LiNbO(3) crystals with a small number of defect densities grown by the double-crucible Czochralski method are investigated and compared with the defect densities of commercially available congruent Fe-doped LiNbO(3) crystals. Two-wave-mixing experiments show that novel stoichiometric crystals exhibit larger photorefractive gain and considerably faster response times than congruent ones. The results indicate that the nonstoichiometry defect control of photorefractive crystals is of key importance for the improvement of their properties.

Optical waveguides of III-metal-polar and N-polar AlGaN are grown on sapphire substrates in order to test their use in integrated optics. The dispersion of the ordinary and extraordinary indices of refraction for films with aluminum mole fraction between 0.0 and 0.30 at four discrete wavelengths has been determined by the prism coupling method. The wavelength dependence of the refractive indices is described well by the first-order Sellmeier dispersion formula. The measurements show a small difference in the refractive indices between the two polarities, which is more pronounced at longer wavelengths.

Tm,Ho:YAG lasers operating at 2.1 μm are becoming increasingly important finding their way into different applications ranging from medicine to remote sensing and range finding. Much of the interest on the Tm,Ho:YAG lasers was perpetuated by their inherent eye safety. In particular, the safety of the remote sensing and range finding applications can be substantially improved by replacing currently widely used 1.06 μm Nd:YAG lasers which are potentially very harmful to the human eye.

Photorefractive crystals are those in which the refractive index can be electro‐optically varied by an internal, light‐induced space‐charge field. Many photorefractive crystals are also ferroelectric, which means that they posess a spontaneous polarization that can be reversed by an electric field. The same light‐induced space‐charge field that alters the refractive index of the material, as well as the charge distribution that produces the field, can also selectively reverse the direction of the spontaneous polarization, leading to effects such as ferroelectric domain grating fixing of photorefractive gratings, space‐charge field enhancement, and periodic domain structures which can be used for quasi‐phase matched second harmonic generation.

Measurements of the space-charge grating buildup in the dark are performed in potassium niobate using short pulse excitation at a wavelength of 532 nm. The experiments allow the separate determination of the lifetime of the photoexcited electrons and of their mobility in a number of ${\mathrm{KNbO}}_{3}$ crystals. The lifetime of (3.9\ifmmode\pm\else\textpm\fi{}0.2) ns is shown to be an intrinsic property of ${\mathrm{KNbO}}_{3}$. Estimates of the electron mobility are refined to (0.5\ifmmode\pm\else\textpm\fi{}0.1) ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ in reduced ${\mathrm{KNbO}}_{3}$. From saturation effects we derive an electron donor photoexcitation cross section of (2.6\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$.

Measurements on high quality samples of top-seeded-solution-grown crystal BaTiO3 have been performed in order to complete the set of data, which is necessary to describe the optic response of this material to electric fields and elastic deformation.1 Improvements in the accuracy of some previously published results could be obtained because of better optical quality and larger size of the crystal samples that are available nowadays. The most important results are the values and the signs of the piezoelectric coefficients d311 and d333, the absolute values of the elasto-optic coefficients, and all the electro-optic measurements. Both low frequency electro-optic measurements in a stress-free sample and the measurements in the inertia-clamped samples using a step-like electric field were used to correct the previous results. The complete set of material parameters of the BaTiO3 crystals at room temperature has been afterwards determined by a numerical fitting procedure. The signs of the elasto-optic coefficients could also be determined with great confidence. We propose that the new calculated values are used as a consistent set of material parameters of the BaTiO3 crystal, in particular for describing its photorefractive properties, that is, to calculate its effective electro-optic and dielectric properties.

The determination of the complete set of material parameters of KNbO3 relevant for photorefractive experiments has been recently accomplished.

We report for the first time to our knowledge optical waveguiding in an organic crystalline waveguide produced by ion implantation. Using H+ ions a refractive index barrier suitable for waveguiding has been realized in the highly nonlinear optical organic crystal 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). The refractive index changes in the waveguiding region as a function of the distance from the surface have been measured. Maximal refractive index changes of up to -0.2 and -0.1 at wavelengths of 633nm and 810nm have been realized, respectively. The waveguide refractive index profiles as a function of the ion fluence have been determined. Planar waveguiding has been demonstrated by polishing sharp edges and using conventional end-fire coupling. The measured losses are approximately 7 dB/cm at 1.57mum.

We present experimental and theoretical results of photorefractive one-dimensional self-focusing in an Fe-doped KNbO3 crystal. The self-focusing is investigated by Z-scan experiments with and without an applied electric field. We observe the strongest focusing effect for a ratio Iˆ/ID≈6 of the beam peak intensity to the so-called dark intensity (the intensity at which photoconductivity is equal to the conductivity in the dark). The good agreement between the presented theory and experiment in the Z-scan technique allows the precise determination of dark intensity, the photovoltaic field, and the electric field inside the crystal.

The photorefractive sensitivity of potassium niobate crystals doped with Ce, Co, Cu, Fe, Mn, Ni, and Rh and double-doped with Mn and Rh is investigated over an extended spectral range. We present experimental evidence on extrinsic properties important for the photorefractive effect, such as absorption and effective trap density. Photorefractive gratings are investigated with two-wave mixing experiments. Results on exponential gain, response time, and photorefractive sensitivity at near-infrared wavelengths are reported. The best photorefractive sensitivities at 860 and 1064 nm were obtained in crystals doped with Rh, Fe, Mn, and Mn–Rh. This makes them suitable for applications at laser-diode wavelengths; at 1064 nm, however, Rh:KNbO3 shows a better photorefractive sensitivity than the others.

In a crystal without inversion symmetry there exist two-step indirect contributions to third-order nonlinear optical processes (cascading). Contributions to optical four-wave mixing occur through optical rectification and linear electro-optic effects. In contrast to cascading by second-harmonic generation, which has to satisfy strict phase-matching conditions, optical rectification is always allowed. In polar KNbO3 crystals we measured four-wave mixing in several geometries to evaluate the direct contribution of the third-order polarizabilities and the cascaded contribution. We present a theoretical model and show experimentally that the cascading effect is large and that contributing polarization gratings must be transversely polarized.

Recently determined complete sets of materials parameters describing the dielectric, elastic, piezoelectric, elasto-optic, and electro-optic properties of BaTiO3 and KNbO3 crystals at room temperature are used to calculate the effective electro-optic coefficients and dielectric constants required for describing photorefractive phenomena. We show a substantial deviation of the new values from the electro-optic coefficients for homogeneously applied electric fields that were used previously in describing the photorefractive effects. We derive angular dependences of the effective electro-optic coefficients and the effective dielectric constants relevant for grating recording in both crystals and verify them experimentally.

Characterization and optimization of KNbO 3 crystals for photorefractive applications over an extended spectral range is described. Intrinsic properties are used to describe the refractive index response to space-charge field gratings. Extrinsic properties that are important for the photorefractive effect, such as absorption, photoconductivity, charge transport, and trapping, are discussed. Some results on the extended IR response of crystals doped with Fe, Ni, Cu, Ce, Mn, and Co are presented. Post growth annealing at temperatures of 400 to 900°C in a controlled atmosphere is shown to modify the charge-transport parameters, therefore also modifying the response time of photoinduced refractive index changes.

We treat the photorefractive effect in oxide crystals at elevated temperatures where charge compensation occurs in the absence of photoexcitation of the compensating species. These species can be either mobile ions or holes in the valence band. Two models are presented that take into account particularities of ion and hole transport. In the small-modulation approximation, solutions for the steady state and the dynamic evolution of the photorefractive effect are given. The maximum space-charge field Eq that can be reached depends on the effective number of electron traps in the crystal. However, in the steady state, while the component of the space-charge field that is due to electrons and the one that is due to the compensating carriers both approach the value Eq, an almost complete compensation of these two components occurs. The speed of compensation is slower for larger grating spacings than for smaller grating spacings and can be increased by applying an electric field. Applying an external electric field also produces a phase shift between the two gratings, therefore increasing the total space-charge field. Experiments performed in KNbO3 confirm the theoretical predictions and indicate that the ionic model is more appropriate for this crystal. Implications of these compensation effects for quasi-permanent hologram storage are discussed.

We discuss the writing of a photorefractive grating with pulses much shorter than the diffusion time of photoexcited charge carriers and compare the results with experiments performed in reduced KNbO3 crystals. From the grating-spacing dependence of the observed buildup time and diffraction efficiency we show that the lifetime of the photoexcited electrons is approximately 4 ns and that the mobility of these electrons before trapping is 0.4 ± 0.2 cm2/(V s).

In a usual photorefractive experiment neither the unclamped nor the clamped linear electro-optic coefficients should be used for calculating the refractive-index change. We show that a crystal deformation in a form of a static plane wave is produced by the space-charge electric-field grating. The total refractive-index change is then calculated by adding together the strain-free electro-optic contribution and the elasto-optic contribution, properly taking into account also the rotation of the axes of the optical indicatrix that is produced by shear deformations. This roto-optic contribution to the photorefractive effect has been overlooked until now.

High-quality single crystals of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN) were grown from dimethylsulfoxide solutions. Crystal platelets with dimensions as large as 10 mm × 5 mm × 2 mm were produced and optically characterized. An interferometric method for the measurement of refractive indices of platelike samples was developed and used for platelet surfaces parallel to one principal axis of the indicatrix. Dispersion data of the refractive indices and the birefringence were used to calculate the optical angle and the propagation directions for phase-matched frequency doubling of Nd:YAG laser radiation as well as the dependence of phase-matching angles in the x–z plane on different wavelengths. Loci of directions for type I and type II collinear phase-matched second-harmonic generation were predicted and experimentally verified. By measuring the effective nonlinear-optical susceptibilities in several phase-matched configurations we were able to evaluate all different nonlinear-optical susceptibility coefficients allowed by Kleinman’s symmetry, the highest one being d23 = (50 ± 15) pm/V with Miller’s coefficient δ23 = (0.49 ± 0.1) m2/As.

Two hydrodynamic modes of the orientational fluctuations were investigated in a homeotropically aligned sample of a lyotropic uniaxial nematic mixture of potassium decanoate, potassium heptyloxybenzoate, potassium chloride, decanol and deuteriated water, which forms discotic micelles. The relaxation times, measured with a photon correlation light scattering technique, show a large anisotropy over the whole nematic range: the pure splay relaxation time is much larger than the pure twist and pure bend relaxation times. This is caused by the anisotropy of the effective viscosity, probably resulting from a backflow effect. The relaxation time exhibits an approximately exponential temperature dependence, caused by the temperature dependence of the viscosity coefficients, with activation energies 0·54eV and 0·40 eV for the deformations close to pure bend and pure splay.

We have measured Raman and far-infrared spectra of YBa2 Cu3 O7 as a function of temperature. No significant changes are observed in the Raman spectra as with temperature. Far-infrared transmission measurements show a band at 100 cm −1 broadening at Tc .

The composite transmission function of a multipassed plane parallel Fabry-Perot interferometer is calculated for a realistic experimental setup and compared with experiment. The amount of losses between the passes has a decisive influence on the transmission properties, most notably on the contrast which is always lower than the appropriate power of the single pass contrast. The ensuing design criteria are discussed.

The hydrodynamic fluctuations in the lyotro-pic system which exhibits a discotlc nematic phase with positive magnetic anisotropy were measured with photon correlation light scattering technique. Th...

The behaviour of some elastic constants of diglycine nitrate across the ferroelectric phase transition were measured with Brillouin scattering. The results were compared with low frequency behaviour of one elastic constant. Results were analysed in the framework of Landau free energy expansion. Discrepancy was found between the relaxation time for the spontaneous polarization from dielectric measurements and our results.

The Brillouin scattering spectra of KH 2 PO 4 under an applied uniaxial stress have been studied in the neighborhood of the ferroelectric transition. Dependence of shear acoustic phonon on the uniaxial stress is similar to dependence on electric field. The acoustic phonon is overdamped near T c , but become underdamped under the applied shear stress. In the case of Δ T(= T — T c ) K , application of stress caused a transition not to have any soft acoustic phonon.

A method of growing high quality 4‐(N,N‐dimethylamino)‐3‐acetamidonitrobenzene (DAN) single crystals is presented. Crystal plates with dimensions of up to 10×5×2 mm3 were produced and optically characterized. The transparency region extends from 485 to 2270 nm. Large birefringence and strong dispersion of the refractive indices were measured allowing two branches of type I and two of type II phase‐matched second‐harmonic generation. From effective second‐order nonlinear optical susceptibilities measured along the branches all nonlinear susceptibilities were evaluated, the highest being d23 =(50±15) pm/V.

In order to investigate the dynamics of members of a novel series of ferroelectrics we have measured the Raman spectra of PbHPO4, as well as the Brillouin spectra in the paraelectric and ferroelectric phases. Mode assignment has been made. A weak underdamped soft mode has been observed below 100 cm-1. The Brillouin spectra revealed low frequency dispersion below Tc which could be fitted very well to the simple relaxation type formula. By comparison with far infrared reflectivity data we conclude that a low frequency relaxing mode and not the mode which is observed in Raman is responsible for the transition.

The ferroelectric transition in CsH2PO4 (P21/m - P21) seems to be of the 1D type from about 90 K to 3 K above the transition with the chain direction along the ferroelectric b axis. We have measured the temperature and frequency dependence of the electrooptic effect. The linear electrooptic coefficient (r22 - r23) is in the paraelectric phase proportional to the square of the dielectric constant. The bias field dependence of (r22- r23) shows saturation effects at relatively low bias fields, which are attributed to the existence of long range correlations along the chains. The frequency dependence of the effect shows a lorentzian shape with a half width of about 300Hz. Above the transition this relaxation frequency is temperature independent.

In light scattering experiments on superconducting Y-Ba-Cu-O ceramic oxides we observe an unexpectedly intense Stokes-shifted spectrum from about 100 cm/sup -1/ (--13 meV) to 4000 cm/sup -1/ (--0.5 eV). We propose that this is due to light scattered from energy-density fluctuations of quasiparticle states excited above the superconducting ground state. The temperature dependence of the scattering intensity shows strong hysteresis, not observed in other experiments on the oxide superconductors.

Photorefractive gratings can be fixed in ferroelectric crystals if a complimentary “grating" of ferroelectric domains is somehow created,1-4 i.e., the direction of the spontaneous polarization Ps of the crystal is selectively reversed such that the bound charge located on the domain walls electrically compensates the light-induced space-charge distribution of the original grating. Usually the selective reversal of the direction of Ps is accomplished by first creating a space-charge distribution by the usual manner (interfacing two light beams in the crystal) and then applying an electric field in the direction opposite to Ps.

Interband photoexcitation in photorefractive ABO3 perovskites has only recently attracted the attention of researchers. Band to band excitation of electrons can lead to fast photorefractive response times [1] and, through trapping, to the sensitisation of impurity levels in the band gap. Strong effects are observed that lead to cw photorefractive response times on the microsecond time scale [1]. Absorption changes at visible wavelengths induced by uv illumination have been observed in BaTiO3 [2].

We report observations of photorefractive grating fixing in potassium niobate at room temperature by means of ferroelectric domains. A grating is recorded while the crystal is simultaneously depoled. By repoling the crystal, the presence of a latent grating is revealed. This fixed grating diffracts light with an efficiency which increases when an electric field is subsequently applied and when it is uniformly illuminated. We explain these observations qualitatively using a previously developed model that involves charge compensation by domain walls, and a new aspect that invokes the creation of new trap sites at the ferroelectric domain walls.

It is known that the optical properties of oxide crystals, such as BaTiO3, LiNbO3 and KNbO3, are strongly influenced by the presence of impurity levels in the bandgap of the material. The distribution and population of these levels have been the focus of much recent research into the tailoring of photorefractive materials to respond to near infrared wavelengths compatible with compact solid-state laser diodes [1]. The impurity levels must have a sufficiently high occupation to provide significant absorption at a wavelength of interest. However, in thermal equilibrium most levels are not occupied by an excitable charge carrier and therefore do not contribute directly to the optical nonlinearity. The creation of a nonequilibrium state by optical illumination may produce a more convenient charge distribution in the material band gap levels.

We study the mechanism of operation of a ring-cavity self-pumped phase-conjugate mirror and focus our interest on the influence of reflection gratings on the response time and the reflectivity of the mirror. Our ring-cavity self-pumped phase-conjugate mirror is constructed around an Fe-doped KNbO3 crystal. In the analysis we consider six possible two-wave-mixing processes in the photorefractive crystal and select the crystal orientation that maximizes these couplings in the desired two-wave-mixing processes. By use of a vibrating mirror in the ring cavity or a decrease in the coherence length of the laser below the length of the ring cavity, the response time becomes short and the reflectivity increases. The results show that the buildup of reflection gratings in a ring-cavity self-pumped phase-conjugate mirror presents an obstacle to good performance.

We give experimental and theoretical evidence that two-wave mixing light amplification in electro-optic and photoconductive materials is dramatically influenced by an anisotropic photoexcitation cross section of mobile charge carriers. An enhancement of the two-wave mixing gain is predicted and an increase of 90% is observed in a dichroic Ni doped ${\mathrm{KNbO}}_{3}$ crystal in a geometry with large angles between the two wave propagation directions. The enhancement is possible because in anisotropic materials the photogeneration can lead to higher modulation depths as compared to the depth of light intensity modulation.

This study aims to fabricate optical planar waveguide structures by H+ ion implantation and direct electron beam patterning. Waveguide losses of ~20 dB are measured by light scattering detection. A reduction of the refractive index of the organic crystal is observed which is due to reduced polarizability of some molecules.

LiNbO3 single crystals with a composition close to stoichiometry are grown by using a double crucible Czochralski method equipped with an automatic powder supply system. The photorefractive properties are compared with those of LiNbO3 crystals grown from the congruent composition melt. Two beam coupling experiments show that both nondoped and Fe-doped stoichiometric crystals exhibit stronger photorefractive effect than the Fe-doped congruent crystals.