Regular Domain Structure Research Articles

Third-harmonic generation in regular domain structures

The process of frequency conversion in regular domain structures is studied using the constant-intensity approximation. The investigations are carried out at values of complex amplitudes of the fundamental radiation and third harmonic at the output of each domain equal to the values of the corresponding complex amplitudes at the input of the subsequent domain. We show that the optimum length of each domain depends on the input pump intensity in the given domain. Thus, it is possible by choosing the optimum lengths of domains, phase mismatch, and pump intensity even at a low number of periods of nonlinear susceptibility modulation of the lattice to reach considerable values of conversion efficiency at the structure output in comparison with the traditional case of homogeneous nonlinear media.

Cascade parametric light amplification with low-frequency pumping

Cascade parametric amplification in a regular domain structure is examined theoretically taking the variations in phase of all the interacting waves in a dissipative medium into account. Analytic expressions are obtained for the conversion efficiency of the laser energy with low-frequency pumping. The dependence of the parametric amplification efficiency on the number of layers is given. Ways of increasing the frequency conversion efficiency are discussed. The presence of a nonzero input intensity at the sum frequency is found to cause a nonlinear increase in the high-frequency signal at the output of the structure. As the losses of the interacting waves increase, both the frequency conversion efficiency and the optimum domain length decrease.

Periodic micro- and nanostructures in LiNbO3〈Gd〉 single crystals grown under unsteady-state conditions

Gd-doped lithium niobate single crystals having a regular domain structure with a period from 100 to 1 μm have been grown under highly unsteady thermal conditions. Examination by atomic force microscopy shows that such lithium niobate single crystals contain periodic nanostructures with a period from 10 to 100 nm in the region of regular domain structures. Raman scattering data suggest that the cation sublattice of the lithium niobate crystals contains a superstructural sublattice of defect clusters with a step of 1–2 nm. We conclude that lanthanide-doped lithium niobate single crystals contain periodic fractal structures in a range of length scales, from ≃ 1 nm to 100 μm.

Efficiency of sum frequency generation by regular domain structures

We theoretically examine generation of sum-frequency radiation in a regular domain structure, taking into account phase changes for all the interacting waves in a dissipative medium. We show that in contrast to the constant-field approximation, in the constant-intensity approximation there is an optimal value for the longitudinal dimension (length) of the domains at which maximum conversion of the signal wave energy to the sum-frequency wave energy occurs. As the pump intensity increases, the optimal domain length decreases. As the losses of the interacting waves increase, both the conversion efficiency and the optimal domain dimensions decrease.

Formation of regular domain structures and peculiarities of switching of the spontaneous polarization in lithium tantalate crystals during discrete electron irradiation

The processes of the formation of domain structures in lithium tantalate crystals (≈300 μm thick) under electron irradiation in a scanning electron microscope were studied. Successive discrete quasi-point irradiations with an electron beam over the —Z-cut surface were used. This method of drawing by an electron beam made it possible to control the implanted charge and to examine the influence of the distances between conditionally point charges on the domain structure formed during switching. The effect of change in the direction of the beam displacement under irradiation on the domain-line width was investigated. The domain lattice with a period of 12 μm was formed on the —z face of the crystal, which transformed into a 2D-type structure deep in the crystal.

10.1007/s11490-008-1008-z

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO3 crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO3 crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.

FRACTAL STRUCTURES IN SINGLE CRYSTALS OF FERROELECTRIC LITHIUM NIOBATE GROWN UNDER STRONGLY UNSTABLE CONDITIONS

Atomic force microscopy studies of lithium niobate single crystals containing heterogeneously distributed lanthanide (Gd) admixture and a regular domain structure of 100 nm to 1 μm steps obtained under conditions of severe thermal instability have revealed fractal structures of the size of 10 to 100 nm within regions of the regular domain structures. A super-structure of clustered defects with 1–2 nm steps explaining results of Raman spectra analysis is supposed to exist in the cation sub-lattice and formation of periodic fractal structures of the size of ∼1 nm–100 μm is suggested to take place in lithium niobate single crystals containing lanthanide admixture.

X-Ray Diffraction on a LiNbO3 Crystal with a Short Period Regular Domain Structure

The dynamical theory based on Takagi-Taupin equations was used to model x-ray diffraction by the Z-cut of a LiNbO 3 crystal with short period regular domain structure. It found that the presence of even satellites on the x-ray diffraction spectra caused by the crystal lattice deformation. A proposed model of the deformation allowed us to achieve a very good agreement with the experimental data.

Study of LiNbO3 and LiTaO3 ferroelectric domain structures using high-resolution x-ray diffraction under application of external electric field

Regular domain structures formed in ferroelectric LiNbO3 and LiTaO3 crystals by the aftergrowth thermoelectrical treatment technique were studied by high-resolution x-ray diffraction and topography under an external electric field applied to the crystals. The application of an external electric field to the crystal causes longitudinal and shift deformations due to the reverse piezoelectric effect and, as a result, inverse changes in the Bragg angle values in adjacent domains. The changes in the Bragg angle values lead to an increase in the x-ray topographic contrast of the domain structure images and allow the direction of the polar and piezoelectrically active axis in adjacent domains to be unambiguously determined. In addition, it was shown that the x-ray diffraction technique can be used to study the behavior of elastic deformations near domain boundaries under an external electric field applied to crystals.

The surface domain structure and off-diagonal magnetoimpedance in glass-coated amorphous microwires

We have studied the response voltage in a coil wound on a glass-coated amorphous microwire, through which an ac current is passed. The amplitudes of the first and second harmonics of the response voltage have been studied as functions of the applied magnetic field. The first harmonic amplitude, which corresponds to a linear off-diagonal magnetoimpedance, is negligibly small, while the second harmonic is highly sensitive to the magnetic field. The behavior of harmonics in the frequency spectrum of the response voltage can be explained by the existence of a regular domain structure in the surface layer of the microwire exposed to a weak magnetic field.

MICRO- AND NANO-STRUCTURES IN SINGLE CRYSTALS OF LITHIUM NIOBATE CONTAINING LANTHANIDE ADMIXTURES

ABSTRACT Micro-size domain structures in lithium niobate single crystals containing lanthanide (Gd, Tm, Sm, Lu) admixture, grown under stationary and non-stationary conditions are studied by efficient and flexible Thixomet® image analyser. Regular fractal-type domain structures observed along the three Y-axes of the hexagonal crystallographic set-up are assigned to pinning of the domain walls at defects created under conditions of imbalanced crystallization of the doped melt. Characteristic types of domains have been found analysing the configurations of domains on optical and atomic force microscopic images of surfaces of different crystallographic orientations.

Research on Peculiarities of Growth Domain Structure of Doped LiNbO3 Single Crystals Depending on Growth Regimes

Domain structure images of Gd doped lithium niobate single crystals (LiNbO3:Gd) grown under both steady and strongly unsteady conditions have been studied using image analyzer Thixomet. R As a result, the technological regimes were formulated for growth of doped LN crystals: a) with periodic spatial distribution of impurity leading to periodic domain structure, which period is controlled by the ratio of pulling and rotation rates and increases essentially with lowering of the melt level; b) with periodic domain structure, which period is independent on the melt level; c) without regular domain structure and homogeneous impurity distribution along the crystal growth direction.

Investigation of piezoelectric effect in lithium tantalate crystals by high-resolution X-ray diffractometry

Lithium tantalate crystals under the action of an external electric field have been studied by x-ray diffractometry. Both single-domain samples and crystals with a regular domain structure have been investigated. In the case of a single-domain crystal, the angular position of the Bragg peak varies under the action of an external electric field due to the reverse piezoelectric effect. According to this variation, the corresponding tensor component of piezoelectric constants can be calculated. In the crystal with a regular domain structure, a surface relief appears under the action of an external electric field. Piezoelectric constants can also be calculated using this relief.

Second harmonic generation by femtosecond laser pulses in the Laue scheme

Results of the study of second harmonic generation by femtosecond laser pulses in a lithium niobate crystal with a regular domain structure in the Laue scheme are presented. The angular distribution of the intensity and the polarization properties of the second harmonic radiation are studied. The orders of the corresponding phase-matching conditions and the angular width of the second harmonic maxima are calculated theoretically. The theoretical results are compared with experimental data.

Polar exit angle distributions of slow H+ ions, related to the presence of a regular domain structure, calculated within the diffuse scattering approximation upon interaction of ions with the sample surface

The angular distributions of slow H ions scattered from the surface of a magnetic film with a stripe domain structure have been calculated. The calculations were performed within the models of specular and diffuse scattering upon collisions of ions with surface. It is shown that the magnetic fields of the stripe domain structure block the ions scattered at small exit angles. It is established that, within the more adequate model of diffuse scattering, the anisotropy of interaction of ions with the magnetic field of the stripe structure manifests itself in a wider angular range.

Periodic current domains in a system of carbon nanotubes

The dynamics of propagation of periodic electromagnetic waves and the current induced by them in a system of carbon nanotubes is investigated. The study is performed on the basis of the analysis of the coupled equations for the classical distribution function of electrons in carbon nanotubes and the Maxwell equations for an electromagnetic field. An effective equation describing the electromagnetic field dynamics is obtained. Periodic changes in the form of an electromagnetic wave during its propagation are revealed. This effect is related to the energy exchange between periodic vibrations with different periods in the system under consideration. The corresponding regions of the current through carbon nanotubes form a regular periodic domain structure. The results of numerical calculations are reported, which make it possible to consider this effect at different problem parameters.

Effect of growth conditions on the domain structure of LiNbO3〈Gd〉 single crystals

The domain structure of LiNbO3〈Gd〉 single crystals grown under steady-and unsteady-state conditions has been studied using Thixomet, a high-speed, flexible image analysis system. The results have been used to formulate the growth conditions of (1) doped lithium niobate single crystals with a modulated dopant distribution and, accordingly, a regular domain structure, whose period is determined by the ratio of the pull rate to the crystal rotation rate, and (2) doped lithium niobate single crystals with a sufficiently uniform dopant distribution along the growth direction, without any regular domain structure.

Experimental study of surface domain structure effects on off-diagonal magnetoimpedance in glass-coated Co-based microwires

The off-diagonal magnetoimpedance in glass-coated Co-based amorphous microwires is studied using a pick-up coil wound around the sample. The first and second harmonics in the pick-up coil voltage were measured as a function of the external magnetic field. It was observed that the first harmonic in the voltage corresponding to the linear off-diagonal magnetoimpedance was very small. This fact is attributed to the existence of the regular bamboo domain structure within a surface layer of the microwire. On the contrary, the second harmonic in the voltage differed from zero, which is related to the domain-walls motion.

Degenerate parametric frequency conversion of phase-modulated femtosecond laser pulses in crystals with a regular and chirped domain structure

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO3 crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO3 crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 µm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO3 crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO3 PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.