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.     

Phase effects at second harmonic generation in the layer media

An analysis is made of quasi-phase-matched interaction at second harmonic generation in a regular domain structure with accounts for losses and change of phases of all the interacting waves. The constant-intensity approximation of basic radiation is applied for this purpose, not to regular domain structure as a whole, but to each separately taken domain. It allows to carry out more strict analysis of quasi-synchronous interaction during the frequency conversion to the polydomain consisting of n layers, forming “grating” periods of modulation of the nonlinear susceptibility. With this, the values of complex amplitudes of basic radiation and second harmonic wave at the outlet of each domain are entrance values of the corresponding complex amplitudes to the following domain. The analytical expression is given for the case of n domains and the factors limiting the efficiency of the process of frequency conversion are analyzed. In the constant-intensity approximation, in contrast to the constant field approximation, the coherent length of domain depends on pump intensity. With increasing pump intensity the optimum length decreases. In a regular domain structure at frequency conversion from a layer to a layer, intensity of the basic radiation changes. Also the optimum length of domains at which conversion efficiency is maximal therefore changes. Thus, it is possible to obtain the high values of conversion frequency at the outlet of a regular domain structure by choice of optimum parameters of a task (length of domains, phase mismatch, pump intensity), as well as using the layers-domains of high quality.     

Domain kinetics in the formation of a periodic domain structure in lithium niobate

The evolution of the domain structure in LiNbO₃ with polarization switching in an electric field is investigated experimentally. Special attention is given to the formation processes of a regular domain applicable to nonlinear optical devices. A new method based on the spontaneous backswitching effect is proposed for creating a regular structure with a period of 2.6 μm in LiNbO₃ with a thickness of 0.5 mm. Fiz. Tverd. Tela (St. Petersburg) 41, 1831–1837 (October 1999)     

Generation of squeezed light upon frequency self-doubling in active nonlinear crystals with regular domain structure

A quantum theory of self-convers of the lasing frequency in active nonlinear crystals placed in a cavity is developed. The generation of nonclassical (quadrature squeezed) light upon self-doubling of the laser radiation frequency is studied in detail, with no limitations on the relations between the relaxation times of the inverse population and the polarization of the medium and the lifetime of a photon in the cavity. The fluctuation spectra of the quadrature components of the fields of the laser radiation and its second harmonic are studied in dependence on the pumping parameters and the parameters of the cavity. Numerical calculations are performed for a Nd:Mg:LiNbO₃ crystal with a regular domain structure.     

Nonclassical light generation in quasi-phase-matched parametric self-frequency conversion

We present a quantum theory of the parametric self-conversion of the laser radiation frequency in active nonlinear crystals with a regular domain structure. Such crystals feature simultaneous lasing and quasi-phase-matched parametric conversion of the laser radiation frequency. These processes are described using the Heisenberg-Langevin equations in two regimes of the subharmonic generation: super-and subthreshold. The spectral properties of the quadrature components of the laser frequency and its subharmonic and the photon statistics have been studied as dependent on the pump power, crystal length, and reflectance of the laser cavity output mirror. Using the obtained analytical expressions, these characteristics are calculated for a active nonlinear Nd:Mg:LiNbO₃ crystal with a regular domain structure. In the subthreshold regime, the maximum decrease in the spectral density of fluctuations in the subharmonic quadrature component relative to the standard quantum limit may reach 90%; in the above-threshold regime, these fluctuations are virtually not suppressed. A decrease in the spectral density of fluctuations of the laser frequency quadrature does not exceed 10%. In the subthreshold excitation regime, the subharmonic photons obey a super-Poisson statistics; in the above-threshold regime, the photon statistics is Poisson-like.     

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 ??m) are formed in both types of stoichiometric crystals.     

Photochemistry of solid C60 with tunable infrared radiation

60 with trains of picosecond infrared (IR) pulses, tuned over the 8–15 μm range, is studied. At some specific wavelengths, white-light emission as well as ejection of ionic species from the solid is observed. The spectral characteristics of the white-light emission resemble those of a black body. The mass distribution of the ejected ionic species shows substantial amounts of C₆₀ coalescence products. Unexpectedly, all these processes only occur at wavelengths where solid C₆₀ is relatively transparent. No white-light emission nor ejection of ionic species is observed when being resonant with an IR-allowed transition of C₆₀. It is concluded that regular C₆₀ is not the chromophore for the observed processes, and that sequential absorption of single photons by a strong absorber that is dilute in the crystal takes place. Plausible chromophores are sites that are intercalated with alkali metals. Accumulation of energy at these sites leads to fullerene coalescence in the solid, ion ejection, and white-light emission, ultimately resulting in the destruction of the C₆₀ molecules. Received: 14 April 1998/Accepted: 22 April 1998     

Role of stoichiometric point defect in electric-field-poling lithium niobate

Non-congruent LiNbO₃ crystals has been used to determine the influence of the intrinsic defect density in the ferroelectric domain inversion mechanism. The poling processes have been carried out either at high temperature or at RT throughout the electric-field-poling technique. It is shown that the coercive field of LiNbO₃ crystals is strongly influenced by the intrinsic defect density in the crystals. Moreover it is shown that it is possible to realign the ferroelectric domain structure of near-stoichiometric LiNbO₃ crystals to prepare periodic poled structures which allows second harmonic generation at shorter wavelengths than with congruent crystals at a fixed wavelength. Received: 6 July 1999 / Accepted: 24 November 1999 / Published online: 24 March 2000     

Superradiant emission of LaF<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> in resonator

Experimental data concerning superradiant emission from the LaF₃ medium doped with impurity praseodymium ions are presented in the cases of a free medium and when the medium is placed into an optical resonator. The spike structure of superradiance is registered and studied. When the medium is placed inside a cavity, a new channel of energy removal by superradiance related to the cavity mode appears; the old noncavity channels are preserved. The duration of superradiance in the cavity channel is decreased, and regular modulation arises. These peculiarities of superradiance induced by the presence of an optical resonator are explained.     

Polarization-optical study of the Barkhausen effect and domain structure of lead germanate

A method of localized polarization-optical measurement of dielectric hysteresis loop parameters on microscopical areas of ferroelectric samples combined with simultaneous observation of their domain structure is proposed. A sensitive two-beam differential polarization-optical hysteresisgraph is built for performing the experiments. Examples are given of the study of slow domain structure relaxation processes and dielectric hysteresis curves of lead germanate Pb₅Ge₃O₁₁.     

Observation of the Photoinduced Conductivity in a Regular Domain Structure with Tilted Walls in MgO:LiNbO3 at a Wavelength of 632.8 nm at Bragg Diffraction

A 632.8-nm radiation-induced change in the conductivity of a regular domain structure (RDS) formed in a 5% MgO:LiNbO₃ crystal has been detected for the first time. As a result, the relaxation rate for the Bragg diffraction efficiency on the RDS, which is observed after the application of an external electric field, increases with the intensity of a probe beam. This dependence is linear in the initial stage of relaxation caused by the screening of the external field because of the redistribution of charges over tilted conductive domain walls of the RDS. For the probe beam with an intensity of 49 mW/mm², the induced effective conductivity of the RDS, which is estimated as σ_eff = 3.5×10⁻⁹Ω⁻¹m⁻¹, is more than four orders of magnitude higher than the dark conductivity of the single-domain MgO:LiNbO₃ sample.

     

Magnetization processes in the narrow domain structures of amorphous ferromagnetic alloys

The magnetization processes within the narrow domain laminae of amorphous ferromagnetic alloys have been investigated by means of the magneto-optical Kerr effect. Changes of the domain width of the closure domains by a magnetic field applied perpendicular to the laminae have been determined for the alloys Fe₈₀B₂₀ and Fe₄₀Ni₄₀P₁₄B₆. These results are compared with theoretical calculations, assuming that wall displacements within the closure domains and rotations of the magnetization in the bulk domains take place simultaneously and a stray field free domain structure is developed. It turned out, that the closure domain structure on the surface of the sample vanishes at the same magnetic field where magnetic saturation is approached.     

Mode-hopping-free scanning over 2 THz by means of quasi-synchronous tuning

We report mode-hopping-free tuning (MHF) over 2 THz (6 nm) of a Littman-Metcalf-type external-cavity diode laser by means of quasi-synchronous tuning, wherein the pivot point is displaced 65 mm away from the conventional rigorous synchronous rotation center. An experimental method for judging the relative position between actual pivot location and the quasi-synchronous tuning line is developed. An adjustment structure with only one freedom of movement is enough to accurately locate the pivot point, and the precise requirement is relaxed to more than 300 μm for achieving a MHF tuning range of over 1THz in our configuration.     

Domain structure and magnetization processes in permalloy propagation elements

A review of domain structures and magnetization processes in permalloy overlays is given, together with some new results. The simplest domain structure for a given element consists of a loop of magnetic flux, but in elements with irregular geometry the circulating flux is not constant. More complex structures arise when an element contains internal closure domains. In-plane anisotropy in permalloy affects the distribution of closure domains but with decreasing bar width the influence of anisotropy is reduced. Reversible wall motion in weak fields gives way to hysteresis effects when the applied field exceeds a certain level, H_s. In particular magnetization buckling may occur. Some details of buckling in asymmetric chevrons and half-discs are given and compared with the behaviour in an I-bar. The proximity of a bubble medium containing stripe domains is shown to reduce considerably the applied fields needed for buckling in overlay components. Following saturation, changes in the demagnetized state are usually apparent. On a simple level, the spin structure and polarity of Bloch walls is altered. More noticeably the wall pattern itself can change when closure domains are created or annihilated in pairs. The significance of these fluctuations for bubble propagation is assessed by considering the intrinsic stray field profile of a Bloch wall segment. A simple wall model is employed. It is demonstrated that a curved domain wall provides a reasonable basis for modelling the field of a magnetized bar up to saturation. Calculated values of H_s agree qualitatively with experiment. The external field of the bar is rather insensitive to the exact distribution of free-pole density. Together with the observed complexities of domain behabiour this reaffirms the validity of the continuum approach to modelling.     

Transition from thermally activated to regular flow of magnetic flux vortices in HTSC

The effect of structural inhomogeneities in a superconductor on a vortex medium flow in weak magnetic fields at temperatures varying from 78 to 83 K for various bias current densities is investigated by using transport measurements of Bi₂Sr₂CaCu₂O_8+x thin-film microbridges. The results obtained are analyzed on the basis of the theories of flux creep and the regular flow of vortices. It is shown that the current dependences of the effective potential for vortex pinning can be satisfactorily described in the framework of two statistical models, one of which was proposed earlier by the authors. Both models cover the regimes of thermally activated and regular flow of vortices as limiting cases. The wide transition region in which the creep and regular vortex flow processes simultaneously occur due to a large dispersion in the pinning energy distribution. It is found that when the magnetic field exceeds a certain value, the average value and dispersion of the pinning potential decrease sharply, so that the conditions of regular flow set in even for small values of the bias current. This fact is attributed to the destruction of vortex lines into two-dimensional segments.     

Diffraction of Light on a Regular Domain Structure with Inclined Walls in MgO:LiNbO3

JETP Letters - Features of the Bragg diffraction of a Gaussian light beam on a regular domain structure with inclined 180° domain walls in a 5% MgO:LiNbO3 crystal have been studied...     

Narrow domain wall propagation and metamagnetic properties of a Tb0.5Y0.5Ni single crystal

The non-collinear magnetic structure of Tb_0.5Y_0.5Ni shows a ferromagnetic component associated with an antiferromagnetic one. At very low temperatures the domain walls are narrow and frozen by the anisotropy. Magnetization processes studied on a spherical monocrystalline sample provide the experimental evidence that, when the anisotropy is strong enough, the narrow domain wall propagation as well as the metamagnetic process depends only on exchange interactions.     

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.     

Selective removal and patterning of a Co/Cu/Co trilayer created by femtosecond laser processing

The selective removal and patterning of a typical pseudo-spin-valve structure, consisting of a Co(20 nm)/ Cu(6 nm)/Co(3 nm) trilayer, by femtosecond laser has been examined in terms of irradiation parameters and layer structure. Ablation thresholds of the individual Co and Cu thin films and the SiO₂/Si substrate have been measured for single-shot irradiation with a 200 femtosecond (fs) laser pulses of a Ti:sapphire laser operating at 775 nm. Ablation of the entire trilayer structure was characterized by a sequential removal of the layers at a threshold level of fluence of 0.28 J/cm². Atomic Force Microscopy, optical microscopy, profilometry and Sputtered Neutral Mass Spectroscopy were employed to characterize the laser-induced single-shot laser selective removal and patterned areas. As a result, two phenomena were found to characterize the laser process: (i) selective removal of the Co and Cu layer due to the change of the laser fluence and (ii) regular pillars’ area of Co/Cu/Co could be achieved in a regular manner with the lowest pillar width size of 1.5 μm. Ablation through the layers was accompanied by the formation of bulges at the edges of the pillars, which was the biggest inconvenience in lowering the pillar size through the femtosecond laser process.     

On the theory of the second optical harmonic in nonlinear crystals with a regular domain structure

The theory of the second-harmonic generation in crystals with a regular domain structure (RDS) is developed in the approximation of the fixed intensity of the fundamental radiation. An analytical expression for the second-harmonic intensity is derived. The results are compared with the data obtained in the fixed-field approximation and the numerical solution. The maximum number of layers n _max at which the theory remains valid is determined. Numerical estimates of n _max are presented for the frequency doubling of an ultrashort light pulse in the LiNbO₃ RDS crystal.