Search

Sessile droplets of a ferroelectric nematic liquid crystalline material were exposed to surface electric fields produced by pyroelectric and photogalvanic (photovoltaic) effects in X-cut iron-doped lithium niobate crystals. The resulting dynamic processes were monitored by polarization optical (video)microscopy (POM). During heating/cooling cycles, at first, the droplets change their shape from spherical to extended ellipsoidal. Then they start to move rapidly along the surface electric field, i.e., along the crystal's polar axis (c-axis). During this motion, several droplets merge into running streams (tendrils) extending towards the edges of the top surface area. Finally, practically all liquid crystalline material is transported from the top surface to the side surfaces of the crystal. At stabilized temperature, laser illumination of the assembly causes dynamic processes that are localized to the illuminated area. Also, in this case, the LC droplets merge into several tendril-like formations that are preferentially oriented along the c-axis of the crystal. The pattern of tendrils fluctuates with time, but it persists as long as the illumination is present. In this case, the LC material is transported between the central and the edge region of the illuminated area., 16 pages, 5 figures. arXiv admin note: text overlap with arXiv:2208.02318

The terahertz range of the electromagnetic spectrum reveals important insights when studying material properties. An ultra-broadband terahertz time-domain spectroscopy system based on state-of-the-art, high-stability organic nonlinear optical crystals used as both THz wave generator and detector is presented. In transmission geometry, a broad spectrum exceeding 20 THz and a high dynamic range of more than 80 dB is achieved using a compact 100 MHz femtosecond laser working at telecom wavelength 1560 nm. In the normal-incidence reflection geometry, a similar bandwidth with a dynamic range surpassing 60 dB is reported. The experimental results are supported by a complete theoretical model, which includes the pump pulse duration, THz phonon/vibrational modes of the organic crystals and optical/THz beam path optimizations. The effectiveness of the newly developed system is demonstrated by measuring pharmaceutical samples with distinct THz features in the ultra-broadband THz range and by measuring narrow water vapor lines at a spectral resolution of 2.7 GHz (0.090 cm−1), resulting in an excellent accuracy with a frequency deviation of less than 0.05% from the reference values.

DSTMS organic electro-optic crystal is a very promising and efficient THz-wave generation material. It allows for both very high THz electric fields generation, exceeding several GV/m using optical rectification of femtosecond laser pulses, as well as the possibility for extremely broad bandwidth extending well beyond 10 THz [1]. However, the usual laboratory THz systems based on these crystals exploit very bulky and expensive fs laser systems [2], which are not desired for industrial THz imaging and spectroscopy applications [3,4]. Recently, some compact system employing organic crystals were demonstrated [5], however they use a conventional photoconductive antenna as a THz-wave detector. Here we present a compact THz time-domain spectroscopy system based on a relatively low-cost and small-size femtosecond fiber lasers operating at 1560 nm. As the THz generation and detection material, we choose the organic crystal DSTMS (4-N,N-dimethylamino-4‘-N‘-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate), since it reaches the highest overall efficiency and the flattest spectrum among the presently known state-of-the-art organic crystals (Figure 1).

Second-harmonic generation was studied in III-metal-polar GaN films grown on sapphire substrates by metalorganic chemical vapor deposition and formed into ridge waveguides. Broadband near-IR femtosecond pulses of an optical parametric amplifier system were injected by end-fire coupling and the nonlinear response was measured while tuning the central wavelength. A prominent peak was found at 450 nm for 1140 nm thick and 10 &mu, m wide GaN waveguides. The measured second-harmonic peak was in agreement with the modal-dispersion phase matching condition calculated using the dispersion of the extraordinary refractive indices of GaN obtained by prism coupling.

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344 nm and 472 nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation.

Photobleaching of organic optical materials can be used to structure the surface layer for integrated optics applications. Linear optical properties and absorption were studied in 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) organic crystals within the absorption band from 260–700 nm in order to determine the depth-range of photobleaching. The results were obtained from reflectivity measurements and bleaching experiments. The depth range of photobleaching can be varied between 0.2 and 2.6 μm by selecting a suitable wavelength.

We investigate two-color holography using Tb doped near-stoichiometric LiNbO 3 . We present the measurements of the hologram strength and sensitivity dependences on the wavelength and intensity of the writing near-IR laser light and on the gating UV light intensity. The diffraction efficiency of the holograms is satisfactory when the writing light wavelength is below 800 nm. The rapidly changing sensitivity around the photon energy of 1.6 eV is an indication that small polarons are involved in the recording mechanism. The recording speed mainly depends on the gating beam intensity, while the hologram strength is much more affected by the writing beam properties, both the intensity and wavelength. We show how to choose appropriate intensities to optimize the desired two-color performance that can be also applied to other two-color schemes.

The wavelength dependence of the space-charge field as induced in near-stoichiometric LiTaO3 crystals is investigated by measuring the bulk photogalvanic effect and the photoconductivity. LiTaO3 crystals of composition nearer to stoichiometry exhibit a pronounced reduction of the saturated space-charge field, which results from a larger increase in the photoconductivity relative to the photogalvanic effect. Especially, at near-UV wavelengths, the saturated space-charge field of the order of 0.1 kV/cm reaches a minimum that is two orders of magnitude lower than typical values observed in undoped near-stoichiometric LiNbO3.

The complete set of self-consistent parameters of nominally undoped LiNbO3 crystals of congruent composition that describe the electro-optic, piezoelectric, elasto-optic, elastic, and dielectric response has been determined by numerically evaluating available measurements. The parameters were determined at room temperature and consist of the low-frequency clamped dielectric constants eS ij, elastic stiffness constants at constant electric field CE ijkl, piezoelectric stress coefficients eijk, elasto-optic constants at constant electric field pE ijkl, and clamped electro-optic coefficients rS ijk. It is shown that the complete set is required for calculating the effective electro-optic coefficients and dielectric constants in photorefractive applications of LiNbO3.

We report on the investigations of electrically switchable polymer dispersed liquid crystal gratings. Diffraction properties, grating anisotropy, and temperature dependencies were analyzed to obtain information on the liquid crystal orientation inside the ellipsoidal droplets and the distribution of the droplets in a polymer host. We compare the samples of different compositions and grating spacings.

Bulk aluminium nitride single crystals grown by physical vapour transport were characterised for their nonlinear optical properties. The two independent nonzero optical nonlinear coefficients were determined by the Maker Fringe method, with a LiNbO3 plate as a reference. At a wavelength of 1030 nm, the measured coefficient |d33| = (4.3 ± 0.3) pm V−1 and |d31| = |d33|/(45 ± 5). Michelson interferometer was used to measure two unclamped electro-optic coefficients to give r33 = (1.16 ± 0.13) pm V−1 and r13 = (0.11 ± 0.02) pm V−1 at a wavelength of 633 nm. By calculating the dispersion relation for d33, we show that AlN is a very promising material for quasi phase-matched frequency conversion into UV.

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.

We determine the anisotropy of the mobility of photoexcited charge carriers in single crystals of KNbO3 and BaTiO3 by means of a purely optical method. In orthorhombic KNbO3 the mobility anisotropy is measured for both electrons and holes giving mamc 1.05 6 0.06, mbmc 2.9 6 0.3 for holes and mamc 1.15 6 0.09, mbmc 1.9 6 0.2 for electrons. In BaTiO3 we find for holes mamc 19.6 6 0.6. Our experiment also demonstrates that the acoustic phonon contribution to the dielectric permittivity is substantially different for spatially modulated electric fields as compared to the homogeneous case. [S0031-9007(96)02051-0] PACS numbers: 71.38.+i, 72.20.Fr, 72.20.Jv, 77.84.Dy The drift mobility of photoexcited holes in tetragonal barium titanate (BaTiO3) shows an anisotropy of about a factor of 20 between the two different crystallographic directions. A recent work [1] has pointed out that this anisotropy has the same temperature dependence as the anisotropy of the dielectric permittivity, a fact which lead to speculation upon a possible linear relation between the drift mobilities and the dielectric permittivities. Several hypotheses have been examined trying to explain these phenomena but none was completely satisfactory. The description of the charge transport by small-polaron hopping was not completely successful [1], either, although it was demonstrated that in BaTiO3 this process is dominant [2]. We now present new measurements of the anisotropy of the photoexcited charge carrier mobility in potassium niobate (KNbO3), another anisotropic polar material of the same family and with characteristics similar to those of BaTiO3. These two materials belong to the oxygen-octahedra ferroelectrics with perovskite structure and are particularly interesting for electro-optical, nonlinear optical as well as for photorefractive applications [3–5]. Although at different temperatures, both crystals have the same sequence of structural phase transitions, from the cubic to the tetragonal, orthorhombic, and finally to the low temperature rhombohedric phase. In the lower symmetry phases, both crystals are ferroelectric and show a spontaneous polarization PS associated with a weak lattice distortion. At room temperature KNbO3 is orthorhombic (point group mm2) with the crystallographic b axis in the pseudocubic [010] direction, while the a and c axes lie along the pseudocubic [101] and [101] directions. At room temperature, BaTiO3 has a tetragonal structure (point group 4 mm) with the axes parallel to the cubic ones. In both materials the c axis is oriented parallel to PS. We determine the anisotropy of the mobility in the bulk of the material from the photoconductivity measured by a purely optical method. Using light energies smaller than the material band gap, the photoconductivity is due to charge photoexcitation from energy levels introduced by impurities or intentional doping. In contrast to direct photoconductivity measurements, our results are not influenced by the quality of contacts between crystal and electrodes or by parasitic buildup of screening charges close to the electrodes themselves or by surface conductivity, all of which are particularly inconvenient effects in highly insulating materials like KNbO3 and BaTiO3.

We demonstrate efficient generation of narrowband (

Self-aligning of optical links and good wavefront matching in heterodyne detection can be achieved through the use of phase conjugation. We demonstrate an optical self-aligning link between two mutually incoherent laser sources using photorefractive Fe:KNbO3 as a double incoherent phase conjugate mirror. The performance of a heterodyne detection system for coherent optical communication based on double incoherent phase conjugation and four-wave mixing phase conjugation is also presented.

Efficient optical pumping of impurity levels in BaTiO3 is achieved using low intensity ultraviolet (UV) exposure. The resulting large photoinduced absorption in the visible is assessed in the context of interband charge transfer. An induced absorption of up to 12 cm−1 is observed at 633 nm. The effect, which takes place close to the UV‐exposed crystal surface, occurs on a millisecond time scale.

Intensity thresholding in a self-pumped phase conjugate mirror with a ring cavity is investigated in order to use it for an intensity thresholder in all optical data processing and associative memory system. The rise time and the saturated value of the phase conjugate reflectivity of the mirror depend on the incident intensity. Using these dependencies, two types of optical intensity thresholder with adjustable thresholding level are proposed.

Spatially modulated electric fields can be induced optically in some ferroelectric materials. We review some of our data obtained for crystals of BaTiO3, which indicate that such photo-induced fields may influence antiparallel domain patterns in such a way that the space charge becomes compensated by the bound charge created by domain walls. This leads to enhancement of the life-time and amplitude of photorefractive gratings. The effects are discussed primarily from the point of view of mechanisms of polarization reversal.

We relate the linear electro‐optic coefficients to the quadratic polarization‐optic and the elasto‐optic tensors by analyzing the dielectric phenomena driven by the spontaneous and induced polarization as well as the mechanical effects given by the deformation of the crystals. We determine the temperature dependence and the wavelength dispersion of the electro‐optic tensor in all the ferroelectric phases from a smaller set of data: the quadratic polarization‐optic tensor of the cubic phase, the dielectric permittivity, and the spontaneous polarization. Under the assumption that the elasto‐optic tensor at constant polarization is constant, the elasto‐optic coefficients at constant electric field in the ferroelectric phases are derived. Numerical evaluations of these coefficients for two perovskite crystals KNbO3 and BaTiO3 are presented. Modifications of the electro‐optic properties in doped crystals, mainly due to a change of the dielectric permittivity, are discussed as well.

We show that the three enigmatic lobes or petals observed in the far-field fanning pattern of photorefractive ${\mathrm{BaTiO}}_{3}$ crystals are correctly predicted by considering the piezoelectric and photoelastic contributions of crystal deformations to the photorefractive nonlinearity.

Periodically poled AlN thin films with submicron domain widths were fabricated for nonlinear applications in the UV-VIS region. A procedure utilizing metalorganic chemical vapor deposition growth of AlN in combination with laser interference lithography was developed for making a nanoscale lateral polarity structure (LPS) with domain size down to 600 nm. The Al-polar and N-polar domains were identified by wet etching the periodic LPS in a potassium hydroxide solution and subsequent scanning electron microscopy (SEM) characterization. Fully coalesced and well-defined vertical interfaces between the adjacent domains were established by cross-sectional SEM. AlN LPSs were mechanically polished and surface roughness with a root mean square value of ∼10 nm over a 90 μm × 90 μm area was achieved. 3.8 μm wide and 650 nm thick AlN LPS waveguides were fabricated. The achieved domain sizes, surface roughness, and waveguides are suitable for second harmonic generation in the UVC spectrum.

We show both theoretically and experimentally that the combined processes of optical rectification and the linear electro-optic effect lead to an effective nonlinear refractive index ${n}_{2}$ in noncentrosymmetric materials. This cascaded second-order nonlinear optical effect arises in addition to the well-known contribution due to second-harmonic generation and difference-frequency mixing and is of comparable magnitude. However, it has the advantage of a broad acceptance angle because no precise phase matching is needed. Experimental results in KNb${\mathrm{O}}_{3}$ crystals are presented.

AlN waveguides on sapphire substrates were fabricated from AlN films grown by metalorganic chemical vapor deposition. By tuning the wavelength of the pump light we demonstrated a second harmonic generation in the UV at 306, 331, and 356 nm using the d33 coefficient and modal dispersion phase matching (MDPM). A theoretical model for MDPM with two- and three-mode interaction in planar waveguides was used to explain the results. Its essential component describes the interaction of two, possibly different, waveguide modes at the fundamental frequency that excite the third mode at doubled frequency. The experimental results were found to agree well with the theoretical model. This work confirmed the application potential of high-quality AlN films as waveguides for nonlinear processes.

Nickel‐ and copper‐doped KNbO3 crystals were grown in order to investigate the photorefractive response in the visible and near infrared. Optical absorption, photoconductivity, and two‐beam coupling experiments between 515 and 1064 nm are reported. A two‐beam coupling constant at 860 nm of 1.4 and 3.2 cm−1 was measured in the nickel‐ and copper‐doped crystals, respectively.

The complete set of elastic, piezoelectric, dielectric, elasto-optic, and electrooptic tensors of potassium niobate crystals was determined by numerically evaluating the measurements published before and the additional measurements performed recently in our laboratory. Improvements in the accuracy of experimental data could be obtained because of the improved optical quality and larger size of KNbO3 crystal samples that are available nowadays. To illustrate the benefits of the newly determined complete set we calculate effective electro-optic coefficients and dielectric constants of KNbO3 in photorefractive experiments, where the elastic deformations associated with a periodic space-charge field have to be considered.

Near-ultraviolet light induces band-to-band electron transitions and is used to record interband photorefractive gratings in undoped ${\mathrm{KNbO}}_{3}$. The high absorption constant in the range from 500 to 6000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ dictates the use of thin crystals or of the crystal volume near the surface. The probing of the grating is usually performed in the visible. We measured a fast grating response of 10 \ensuremath{\mu}s at 1 W/${\mathrm{cm}}^{2}$ for refractive index changes of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$. The hologram diffraction efficiency reaches \ensuremath{\eta}=1.4% in a longitudinal recording geometry with a grating approximately 200 \ensuremath{\mu}m thick. In a transverse geometry we measured \ensuremath{\eta}=60% in 7.5-mm-long crystals. The photorefractive sensitivity of ${\mathrm{KNbO}}_{3}$ is similar to that of multiple-quantum-well structures while the resolution is better due to smaller carrier mobilities and because of shorter optical wavelengths used. Interband photorefractive gratings are highly robust with respect to intensive illumination in the visible. A theoretical model that describes interband electron transitions predicts the important role of free carriers in both bands. It explains the transition from a linear to a sublinear dependence of photoconductivity on light intensity which is observed already at intensities of a few mW/${\mathrm{cm}}^{2}$. The predictions are consistent with dynamic and steady-state holographic investigations performed under variation of grating spacing, applied electric field, ultraviolet and visible light intensities, and grating readout depth.

A tunable THz source for remote sensing has been realized using difference frequency generation of the signal and idler wavelengths of a BBO based optical parametric oscillator [1]. THz pulses with frequencies 1 – 20 THz and a bandwidth of less than 100 GHz are produced in two operational modes via difference frequency generation (DFG) process by mixing two infrared waves in novel organic crystals with extremely high optical nonlinearities (DSTMS and OH1) [2]. One operational mode is at a fixed THz wavelength of 1.25 THz for high output powers using OH1 as organic material for stand-off applications, and the second mode is for tuning between 1 – 20 THz using the organic crystal DSTMS for spectroscopy applications. The optimum wavelength range in view of the material properties of DSTMS and OH1 is centered at 1400 nm.

The elastic, piezoelectric, dielectric, elasto‐optic, and electro‐optic tensors have been determined by numerically evaluating the measurements published until now and using the additional measurements presented in this work. The dependence of the speed of two elastic waves on the applied electric field is measured yielding one electroelastic constant g13133=(95±10)NV−1 m−1. The complete set of parameters consisting of the low‐frequency clamped dielectric constants eSij, refractive indices, elastic stiffness constants at constant electric field CEijkl, piezoelectric coefficients eijk, elasto‐optic tensor at constant electric field pEijkl, and clamped electro‐optic coefficients rSijk is used to calculate effective electro‐optic coefficients and effective dielectric constants that have to be used in photorefractive experiments where the elastic deformations associated with a periodic space‐charge field have to be considered.

We give a short overview over pulsed photorefractive experiments and present measurements performed in orthorhombic KNbO3 with nanosecond and picosecond pulses that allowed the investigation of some material parameters. We discuss the dark decay of the grating after pulsed illumination and show that from the temperature dependence of the decay the existence of a shallow trap level with a thermal excitation energy of approximately 0.3 eV can be inferred. From the dynamics of the grating build-up on a microsecond time scale we deduce a recombination time of the order of microseconds. After illumination with 70 ps pulses the photorefractive grating builds-up in reduced KNbO3 in about 5 nanoseconds. This build-up time is typical for reduced KNbO3 and does not change by more than 20%if the grating-spacing is changed by a factor of ten or the temperature is increased to 165°C.

In a crystal without inversion symmetry there is a direct and an indirect contribution to the third-order nonlinear optical processes. In polar KNbO3 crystals degenerate four-wave mixing is measured in several geometries in order to evaluate the direct contribution and the contribution of the second-order effects to the observed third-order hyperpolarizabilities. Contributing second-order processes are mostly related to optical rectification and to linear electro-optic effect. Cubic KTaO3 is investigated as a reference material in which only pure third-order processes are allowed.

We present time resolved measurements of photorefractive grating build-up in reduced and unreduced KNbO3 crystals. After writing with 70 ps laser pulses at 532 nm the photorefractive diffraction efficiency reaches a stationary value in about 5 ns. The dynamics of grating build-up for different configurations exploiting isotropic and anisotropic Bragg diffraction and for different crystal temperatures is reported. We show that the build-up time is remarkably constant over many different experimental conditions.

A novel mechanism is used to record photorefractive gratings in dielectric crystals. The photon energy of near-ultraviolet recording beams is larger than the energy gap, and the absorbed photons excite electrons from the valence to the conduction band. These interband photorefractive gratings in undoped KNbO3 show a faster response than gratings recorded in the same material with visible light, i.e., below band-gap excitation. We measured a response time τ = 5 μs for an intensity I = 1 W/cm2 and a refractive-index change of 2 × 10−6. In a transversal readout geometry, large diffraction efficiencies of 60% in 7.5-mm-long crystals are demonstrated. Interband photorefractive gratings are only weakly affected by visible light.

Single 75-ps-long pulses at a wavelength of 532 nm are used to write phase gratings in KNbO(3) crystals. Two grating buildup processes are resolved. The faster and generally weaker one is instantaneous on a time scale of the pulses used and is attributed to intrinsic third-order processes. In a reduced crystal that shows preferentially electronic charge transport and that is oriented to show a space-charge-induced photorefractive effect, a higher diffraction builds up in some nanoseconds after the illumination of the crystal. The rise time of the photorefractive grating does not seem to depend on the grating period. Two tentative explanations are proposed: (i) a high electron mobility and a limited response time owing to ferroelectric relaxation processes and (ii) fast electron recombination.

Build-up and decay of transient photorefractive gratings written with 200 ns pulses (intensity up to 20 kW/cm2) in reduced KNbO3 were studied. Grating build-up times of about 1 μs were measured. Saturation effects and fast transients preceding a millisecond dark decay were observed. From an accurate measurement of the temperature dependence of the dark decay we could identify a component with an activation energy of 0.3 eV that may be attributed to a shallow trap level. The measurements were compared with the results of computer simulations based on the band transport model. The observed phenomena can be described by a single carrier model assuming that an additional shallow electron trapping level is present below the conduction band.

Orientationa order in smectic phases of a ferroelectric liquid-crystalline material 4'-(2S,3S)-2″-chloro-3″-methylpentanoyloxyphenyl-4-decyloxy thiobenzoate, abbreviated 10.S.C1Isolleu, was examined by IR absorption spectroscopy. Oriented samples were prepared in both bookshelf and homeotropic alignment geometry. Several stronger IR vibrations were selected characterizing different parts and orientations of the molecules. Results show that the molecular tilt of the core parts of the molecules changes very little from the S A to the S C * phase implying that the core parts of the molecules are already tilted in the S A phase. Axes of the core parts, that are disordered on a cone in the S A phase, start to align with each other but remain on this cone on lowering the temperature into the S C * phase. The model agrees with some previous results On a etudie par spectroscopie d'absorption infrarouge l'ordre orientationnel dans les phases smectiques d'un materiau cristal liquide ferroelectrique, le decyloxy-4-thiobenzoate de [(2S,3S)-chloro-2-methyl-3-pentanoyloxy]-4'-phenyle, en abrege 10.S.C1Isoleu. Deux types d'echantillons orientes, l'un homeotrope, l'autre en configuration Clark-Lagerwall, ont ete prepares et plusieurs bandes intenses en infrarouge, caracteristiques des differentes parties et orientations des molecules, ont ete choisies. Les resultats montrent que l'inclinaison des parties centrales (cœurs) des molecules varie tres peu de la phase S A a la phase S C *, ce qui implique que les cœurs des molecules sont deja inclines dans la phase S A . Leurs axes, qui sont desordonnes sur un cone dans la phase S A , s'alignent mais restent sur ce cone quand on passe en phase S C * en abaissant la temperature. Ce modele est en accord avec quelques resultats anterieurs

We report on measurements of the refractive indices and the absorption in bulk single crystals of aluminum nitride, in the region from 1 to 8 THz. The birefringence is approximately 0.2 and is larger than in the optical frequency range. Both indices exhibit normal dispersion with no pronounced absorption resonances. Optical power loss coefficients are approximately 2 cm−1 and 4 cm−1 and the estimated static dielectric constants are 7.84 and 9.22, for the ordinary and extraordinary polarization, respectively.

Dispersion of the extraordinary and ordinary refractive indices of Al- and N-polar AlN waveguides is measured by multiple angle-of-incidence and spectroscopic ellipsometry techniques. The polarity-controlled AlN layers are grown by metal–organic chemical vapor deposition on (0001)-sapphire substrates. Taking into consideration the different surface morphologies of the Al- and N-polar AlN waveguides, we propose two optical models to describe the measured ellipsometry data. The results indicate that there is no difference between the refractive indices of the AlN grown in opposite directions, which confirms the potential of the AlN lateral polar structures for use in nonlinear optical applications based on quasi phase matching.

In this chapter, we summarized the effects of the various anisotropic material properties on light diffraction, space-charge field formation, and photorefractive two-wave mixing interactions. These properties include the birefringence, the dielectric constants, the dichroism, the electro-optic constants, the elastic and photoelastic constants, the piezoelectric constants, the mobility tensor, and the cross-sections for free carrier photoexcitation. All these material properties have a substantial influence on the observable quantities. In particular, the role played in photorefractive materials by an anisotropy of the photoexcitation process with respect to light polarization, often over-looked, was discussed in detail here. Also, light diffraction in anisotropic media was treated in deeper detail. While the coupled wave model presented in Section 4.3 does apply to photorefractive volume gratings, it is equally important for any holographic materials having strong birefringence and/or containing volume gratings with strongly anisotropic properties.

Photostructuring by photobleaching and femtosecond laser ablation are promising techniques tor surface structuring with one processing step. We report our results of applying these two different structuring techniques to the highly nonlinear optical crystal DAST, where standard photolithography can not be easily applied.

Optical waveguiding in the highly nonlinear optical crystal DAST has been demonstrated. Planar waveguides have been realized by H+ ion implantation whilst fs ablation has been used to pattern channel waveguide structures.

It is well known that the extraordinary large linear electro-optic effect in inorganic crystals of the perovskite class (e.g., BaTiO31, KNbO32) is significantly influenced by acoustic phonon contributions. The electro-optic response of these materials shows a strong dependence on the frequency of the applied electric field. The largest of the clamped electro-optic coefficients r s (valid at high modulation frequencies) is typically reduced to half of the unclamped effect (r T ) occurring at low frequencies.