Characterization of femtosecond pulses via transverse second-harmonic generation in random nonlinear media

We study, both experimentally and theoretically, the process of second-harmonic generation by two noncollinear beams in ferroelectric crystals with a disordered distribution of ferroelectric domains. We show that this parametric process results in generation of second-harmonic wave in the direction transverse to the propagation of the fundamental beam. We demonstrate how this effect can be used for the femtosecond pulse characterization enabling the estimation of both width and chirp of the pulse.     

Noncollinear optical frequency doubling in strontium barium niobate

The observation of a novel noncollinear optical second-harmonic generation mechanism is reported. In strontium barium niobate crystals, a circular cone of second-harmonic light is generated when a fundamental beam of intensive laser light is directed along the crystallographic c axis. It can be shown that the effect is caused by the nonlinear polarization of antiparallel ordered ferroelectric microdomains.     

Spatiotemporal toroidal waves from the transverse second-harmonic generation

We study the second-harmonic generation via transversely matched interaction of two counterpropagating ultrashort pulses in chi(2) photonic structures. We show that the emitted second-harmonic wave attains the form of spatially expanding toroid with the initial thickness given by the cross correlation of the pulses. We demonstrate the formation of such toroidal waves in crystals with random ferroelectric domains as well as in annularly poled nonlinear photonic structures.     

Near-field second-harmonic microscopy of thin ferroelectric films

We present a near-field optical technique for second-harmonic imaging by use of tapered optical fiber tips externally illuminated with femtosecond laser pulses. Enhancement of the electric field at the tip of the fiber results in enhanced second-harmonic (SH) generation from the sample region near the tip. This SH emission is collected by the same tapered fiber. The spatial distribution and polarization properties of SH generation from thin ferroelectric films and a poled single crystal of BaTiO(3) have been studied. A spatial resolution of the order of 80 nm was achieved. Symmetry properties of the near-field SH signal allow us to recover the local poling direction of individual ferroelectric domains in the film. Thus the technique provides a novel tool for nanometer-scale crystal analysis of polycrystalline samples.     

Analysis of ferroelectric 180 degrees -domain structures in BaTiO(3) by use of second-harmonic scattering

We present a method of determining the distribution of sizes of ferroelectric 180 degrees domains in BaTiO(3), using second-harmonic scattering. We find that the domains are rods with approximately square cross sections that extend from the -c to the +c face of the crystal, with widths ranging from 2 to 150 mum.     

Periodically poled potassium niobate for second-harmonic generation at 463 nm

We report on the fabrication and characterization of quasi-phase-matched potassium niobate crystals for second-harmonic generation. Periodic 30-mum -pitch antiparallel ferroelectric domains are fabricated by means of poling in an electrical field. Both birefrigence and periodic phase shift of the generated second harmonic contribute to phase matching when the d(31) nonlinear optical tensor element is used. 3.8 mW of second-harmonic radiation at 463 nm is generated by frequency doubling of the output of master-oscillator power-amplifier diode laser in a 5-mm-long crystal. The measured effective nonlinear coefficient is 3.7pm/V. The measured spectral acceptance bandwidth of 0.25 nm corresponds to the theoretical value.     

Dynamic properties of phase diagram in cylindrical ferroelectric nanotubes

The dynamic properties of phase diagrams in cylindrical ferroelectric nanotubes were investigated by utilizing the effective-field theory with correlations. Three different structure mechanisms are discussed by performing three different interaction functions. Effects of structure factors of the ferroelectric nanotubes are shown in phase diagrams. A linear relation between the crossover value of transverse field in inner-layer of nanotube and the exchange interaction are revealed. Moreover, for the first time, the phase transition characteristics are shown both in 2-D and 3-D phase diagrams by comparing the variations of the ferroelectric region and the morphology vividly. In comparison, results of the usual mean-field approximation are also presented in some phase diagrams, it suggest that the differential operator technique with correlations reduces some extent of the ferroelectric features of the ferroelectric nanotube.     

Bi3.25La0.75Ti3O12 (BLT) nanotube capacitors for semiconductor memories

We report results of fabrication and examination of Bi_3.25La_0.75Ti₃O₁₂ (BLT) ferroelectric nanotubes. BLT nanotubes are suggested for developing 3D ferroelectric nanotube capacitors which could be used in high-density memory applications. BLT nanotubes were prepared by template-wetting process using polymeric sources where anodic aluminum oxide had been used as a template. After annealing, tubular BLT structures were crystallized inside the pores of the template. By selective etching of the template, released BLT nanotubes have been obtained. Crystallization and nucleation of the nanotubes were analyzed by XRD and FE-SEM techniques.     

Second harmonic generation in a strontium barium niobate crystal with a random domain structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.     

Switching behaviour of ferroelectric liquid crystals probed by optical second-harmonic generation

Ferroelectric properties of thin (1.5–4 μm planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using electro-optic measurements, second-harmonic generation (SHG) and SHG interferometry. A switching behaviour of the FLC cells in external dc electric fields is observed. It is characterised by rotation of the polarisation plane of the transmitted light and by changes in the SHG intensity, phase and anisotropy dependences, which are attributed to a collective motion of the system as a ferroelectric uniform state with C₂ symmetry. Received: 16 October 2001 / Revised version: 18 March 2002 / Published online: 6 June 2002     

Phase-matched second-harmonic generation due to thermal expansion of TlGaSe2 layered crystal

Second-harmonic generation effects have been investigated in TlGaSe₂ layered crystal over a temperature range where ferroelectric phase exists. Pronounced periodical changes of the second-harmonic signal with temperature have been discovered. The observed effect is explained within the phase synchronism condition which changes with the temperature. The main mechanism of the thermal expansion of the sample in the observed phenomenon is explained.     

Electrode geometries for periodic poling of ferroelectric materials

We have realized novel electrode configurations for efficient periodic poling of ferroelectric crystals by applying a simple patterning method based on laser-induced material ablation. The new geometries were theoretically investigated and compared with conventional configurations. A systematic optimization of the patterning according to the ablated profile led to high-quality quasi-phase-matched structures in lithium niobate with a grating period of 20 mum . The periodically poled samples were characterized by use of third-order second-harmonic generation of a Nd:YAG laser.     

Second harmonic generation in layered TunS2 in the vicinity of the low-temperature phase transitions

Second-harmonic generation in the ternary layered semiconductor TlInS₂ excited with the wavelength λ = 1.06 μm of YAG: Nd^{3 +} laser is investigated in the temperature range corresponding to the low-temperature phase transitions. It is shown that the intensity of the second-harmonic signal corresponding to the non-linear coefficients d_eff, d₂₁, d₂₂ reveals peculiarities close to the commensurate-incommensurate phase transitions. The temperature hysteresis of the second-harmonic signal in the low-temperature region (below and close to T_i2 = 206K) is explained assuming that an incomplete lock-in transition in TlInS₂ takes place at T_c1 =204K within the temperature range between a ferroelectric (T_c2 = 201K) and an incommensurate (T_i2 = 206 K) phases. It is shown that the monoclinic point group symmetry C₂ is preserved also at temperatures lower than the phase transition temperature Tc4 = 79K to a weak ferroelectric state.     

Switchable nonlinear two-dimensional ferroelectric photonic crystal

A two-dimensional photonic crystal with a system of electrodes providing interaction of the incident and transmitted light waves has been developed on the basis of an epitaxial ferroelectric BaSrTiO₃ film. Investigation of the spectral characteristics has revealed the presence of a photonic band gap near 1.5 eV. A possibility of spatial frequency reconstruction of the second-harmonic wave generated by the photonic crystal is shown. The switching efficiency is as high as 6000%.     

Transverse second-harmonic generation from disordered nonlinear crystals

We investigate the transverse second-harmonic generation in as grown strontium barium niobate (SBN) crystals with a random structure of anti-parallel ferroelectric domains. We consider both, single and counter-propagating pulse geometries. We investigate polarization properties of the second harmonic signal and discuss applications of this process for short pulses characterization. Presented at 9-th International Workshop on Nonlinear Optics Applications, NOA 2007, May 17–20, 2007, Świnoujście, Poland     

Photorefractive properties of a nanocomposite based on a ferroelectric polymer

The photorefractive effect is demonstrated for the first time for the composite consisting of a ferroelectric material (polyvinylidenefluoride-trifluoroethylene) and single-walled carbon nanotubes that serve as nonlinear optical chromophores and, simultaneously, spectral sensitizers for the laser radiation with a wavelength of 1064 nm. It is demonstrated that the internal field in the ferroelectric material makes it possible to measure the photorefractive effect in the absence of the external field.     

碳纳米管--一种新的巨电致变形纳米智能材料

纳米机电系统的发展要求智能材料具有大的应变和功率密度，以获得优良的能量转换能力．文章介绍了作者的最新发现：在电场的作用下，碳纳米管能产生巨大电致变形；而且与传统智能材料如铁电体等相比，其体积功率密度和质量功率密度预计可分别提高3个和4个量级，是一种极具潜力的新型智能材料．     

Photovoltaic properties of polymeric ferroelectric with various dopings

The photovoltaic properties and the bulk photovoltaic effect have been studied in a polyvinylidene difluoride–trifluoroethylene polymeric ferroelectric doped by single-walled nanotubes and a ruthenium-based dye. The dopants serve as spectral sensibilizers that improve sensitivity to 532-nm laser radiation.     

Nondestructive remote imaging of ferroelectric domain distributions with high three-dimensional resolution

The domain distribution in the bulk and at the surface of ferroelectric crystals is imaged by second-harmonic microscopy with three-dimensional far-field optical resolution. The nondestructive technique allows us to investigate the success of a poling procedure in a fabrication state of an integrated optical device in which the poling electrodes have not been removed. Additionally, a confocal linear optical imaging technique is introduced, which reveals the surface topography with ±0.7 nm height sensitivity. At the surface of a periodically poled specimen, we detect unexpected and unwanted surface topographies that correspond to the domain structure. The search for improved fabrication parameters that guarantee smooth surfaces could be substantially promoted with our topography-detection technique. Received: 16 May 2001 / Published online: 23 October 2001     

Tunable ultraviolet radiation by second-harmonic generation in periodically poled lithium tantalate

We describe electric-field poling of fine-pitch ferroelectric domain gratings in lithium tantalate and characterization of nonlinear-optical properties by single-pass quasi-phase-matched second-harmonic generation (QPM SHG). With a 7.5-microm-period grating, the observed effective nonlinear coefficient for first-order QPM SHG of 532-nm radiation is 9 pm/V, whereas for a grating with a 2.625-microm period, 2.6 pm/V was observed for second-order QPM SHG of 325-nm radiation. These values are 100% and 55% of the theoretically expected values, respectively. We derive a temperature-dependent Sellmeier equation for lithium tantalate that is valid deeper into the UV than currently available results, based on temperature-tuning experiments at different QPM grating periods combined with refractive-index data in the literature.