L. S. Kokhanchik

Periodic domain structures formed under electron-beam irradiation in LiNbO3 plates and Ti:LiNbO3 planar waveguides of the Y cut

Regular domain structures on the Y cuts of the LiNbO3 substrates and Ti:LiNbO3 waveguide structures based on these substrates have been fabricated under electron-beam irradiation. It has been revealed that the domains in undoped and titanium-doped LiNbO3 crystals are formed as a result of different processes. It has been demonstrated using chemical etching and waveguide second-harmonic generation that regular domain structures in the Ti:LiNbO3 waveguides are formed at a depth of approximately 8 μm from the surface, where the titanium concentration does not exceed 2 mol %. The quasi-synchronous waveguide optical second harmonic generation with an efficiency of 8.8% has been obtained using the fabricated structures.

Microdomain Patterns Recorded by an Electron Beam in He-Implanted Optical Waveguides on X-Cut LiNbO 3 Crystals

We present the results of studies in planar optical waveguides fabricated by He-ion implantation with the energy of 500 keV in X-cut LiNbO3 crystals. The thickness of the formed waveguide layer confined by the depth D of the implanted layer is of about 1.06 μm. The refractive indices as well as differences in refractive indices were evaluated for wavelengths λ = 445, 626.5, and 650 nm. Domain gratings with the period λ = 4 μm were recorded in these samples by electron beam irradiation with acceleration voltages U in the range from 5 to 25 kV. Gratings characteristics measured by the PFM method were obtained for different domain thicknesses Td determined by U. The optimum grating regularity is achieved when the domain growth occurs beyond the He-implanted damaged barrier, i.e., at Td ≤ D, which in the given case corresponds to U = 10 and 15 kV. Otherwise, (Td > D), the domain evolution is affected by the structurally damaged layer and the gratings become irregular.

Characterization of electron-beam recorded microdomain patterns on the nonpolar surface of LiNbO3 crystal by nondestructive methods

We report on characterization of the electron-beam fabricated planar domain gratings on the nonpolar (Y-) surface of LiNbO3 crystals performed with the use of AFM and confocal second harmonic generation (SHG) microscopy. The dependence of domain formation on the irradiation conditions was investigated. The relation of domain thicknesses to the electron penetration depth is experimentally proved. In particular, the possibility of controlling the thickness of planar domains by varying acceleration electron-beam voltages is demonstrated. The observed specificity of SHG is analyzed in the framework of the Kleinman-Boyd theory [G. D. Boyd and D. A. Kleinman, J. Appl. Phys. 39, 3597 (1968)] and Uesu approach [Kaneshiro et al., J. Appl. Phys. 104, 054112 (2008); Kaneshiro et al., J. Opt. Soc. Am. B 27, 888 (2010)] extended in our case to reflection geometry. The calculations performed predict the dependence of SHG conversion efficiency η on the domain thickness, which is in a qualitative agreement with the exper...

Recording of domains by an electron beam on the surface of +Z cuts of lithium niobate

Using an electron beam, spontaneous polarization on the surface of +Z cuts of lithium niobate crystals of various compositions is switched. Domains 100–200 nm in size are formed in a thin surface layer, with the thickness dependent on the primary-electron energy. The sizes of segments, on which submicron domains are formed, exceed the sizes of the irradiation region. The distributions of domains on the surface in crystals of various compositions are different and depend on the method of moving the electron beam and irradiation conditions. The results are discussed in the context of notions on the charging of dielectric materials by the electron beam. It is assumed that the spontaneous polarization in the surface layer of +Z cuts of lithium niobates is switched by the field of a double layer of charges formed as a result of the charging process near the surface.

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.

Electron beam recording of microdomains on the nonpolar LiNbO3 crystal surface at different SEM accelerating voltages

The effect of the accelerating voltage U of the SEM electron beam on the characteristics of microdomains recorded by the electron beam method on the nonpolar Y-surface of LiNbO3 crystals has been studied. The thickness Td of domains in the Y direction is determined by the depth range Re of primary electrons, which depends on U. This makes it possible to define Td in the range of 0.2–4 μm at U = 5–25 kV, respectively. The electron emission coefficient σ is estimated for different values of U exceeding the second equilibrium point U2 (σ = 1) in the σ(U) diagram. These data are used to construct the dependence σ(U) for LiNbO3. Based on the exposure characteristics of the length Ld of domains growing along the polar Z axis, the dependence of the space charge field controlling the domain planar growth along Z on the surface electron emission σ is found.

Characteristics of microdomains and microdomain patterns recorded by electron beam irradiation on Y-cut LiNbO3 crystals

We present the results of investigations of planar domain patterns (isolated domains and domain gratings) fabricated by irradiation of the nonpolar Y-surface of LiNbO3 crystals by an electron beam (EB) incident normally onto the surface. The EB recorded domains were investigated using atomic force microscopy, confocal second harmonic generation microscopy, and chemical etching as an auxiliary method. The dependence of the domain characteristics on irradiation conditions (acceleration voltage U, EB current I, and irradiation time tirr) were determined. The length Ld of both isolated domains and domain gratings along the polar axis Z grows linearly with tirr (at U, Iu2009=u2009const) with no tending to saturation. The plots Ld(tirr) obtained for Uu2009=u200910 and 15u2009kV are practically identical, whereas the values of Ld for Uu2009=u20095u2009kV are essentially lower. The domain thickness Td along the Y-direction, i.e., the depth of the switched layer grows with acceleration voltage U. These results are discussed in terms of space-cha...

Investigation of Periodic Domain Structures in Lithium Niobate Crystals

Bulk periodic domain structures created in LiNbO 3 crystal by the aftergrowth thermoelectrical treatment were studied by SEM and x-ray technique. Z and Y cuts, which have “tail-to-tail” and “head-to-head” domain walls, were comparatively investigated. In the SEM, a periodic domain contrast of the Z and Y cuts was studied and charge amount for equalization of the contrast was experimentally estimated. Special surface charging by an e-beam allowed the observation of domain contrast of highly distorted “tail-to-tail” domain walls. The SEM investigations were supported by x-ray results on crystal distortions near different domain walls.

Electron beam domain writing in reduced LiNbO3 crystals

Electron-beam domain writing (DEBW) on the nonpolar surfaces of reduced LiNbO3 crystals was studied. Essential distinctions were found as compared with DEBW in as-grown congruent LiNbO3 crystals (CLN). The threshold dose Dthr of domain emerging in reduced LiNbO3 (RLN) exceeds essentially Dthr in CLN, which is related to a decreased space-charge Qsc in the irradiated area due to the increase of the bulk leakage current I L bulk . The shape of domain gratings in RLN differs from that in CLN because of an increased dark conductivity. The velocity of domain-wall frontal motion in RLN is more than by a factor of 20 lower than in CLN, which is due both to a decreased Qsc and to an essential reduction of the domain-wall mobility. All these effects related to a fundamental change of the charge transport in RLN are discussed in the framework of the current model of the LiNbO3 defect structure. The results obtained show that the only path to realize DEBW in crystals with varied conductance is the adjustment of EB c...

Regular domain structures fabricated by an electron beam in stoichiometric LiNbO3 crystals

Electron beam writing of regular domain structures in Z-cuts 0.75 mm thick of stoichiometric and close to stoichiometric LiNbO3 crystals has been carried out. Crystals have been grown by the Czochralski method from a melt with excess Li2O (58.6 mol %) and from a congruent-composition melt in the presence of 6 wt % K2O alkali solvent (flux). In both crystals, threshold charge doses required to form individual domains have been determined, and the optimal conditions of periodic structure patterning by sequential local irradiations have been found. Domain gratings of similar type (with periods of 6.5, 7, and 10 μm) are formed in both types of stoichiometric crystals.