Lili Tian

Domain reversal in stoichiometric LiTaO3 prepared by vapor transport equilibration

Domain reversal in stoichiometric lithium tantalate (LiTaO3) single crystals prepared by vapor transport equilibrium (VTE) method was studied. Starting from a virgin VTE crystal and using water electrodes, the coercive fields were found to be 1.39±0.01kV∕cm and 1.23±0.01kV∕cm for the first poling and the second poling, respectively, indicating a built-in internal field of 0.08kV∕cm. The spontaneous polarization, Ps was 55.2±0.5μC∕cm2 and the Curie temperature was Tc=701±2.5°C. The switching time, ts, exhibits an exponential dependence on the external field, E with activation energy of 10.84±0.22kV∕cm. No domain backswitching was observed. These properties are dramatically different from those of congruent and even near-stoichiometric compositions of lithium tantalate grown by Czhochralski method.

Multiferroic domain dynamics in strained strontium titanate

Multiferroicity can be induced in strontium titanate by applying biaxial strain. Using optical second harmonic generation, we report a transition from 4/mmm to the ferroelectric mm2 phase, followed by a transition to a ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film. Piezoelectric force microscopy is used to study ferroelectric domain switching. Second harmonic generation, combined with phase-field modeling, is used to reveal the mechanism of coupled ferroelectric-ferroelastic domain wall motion. These studies have relevance to multiferroics with coupled polar and axial phenomena.

Real-time study of domain dynamics in ferroelectric Sr0.61Ba0.39Nb2O6

Systematic studies of ferroelectric domain reversal of strontium barium niobate (Sr0.61Ba0.4Nb2O6 or SBN:61) single crystals by electric-field poling using liquid electrodes are reported. In contrast with previous studies using metal electrodes, it is shown that repeated domain reversal does not lead to polarization fatigue. Real-time optical imaging of domain nucleation and growth is reported in SBN:61 using electro-optic imaging microscopy. Measurements of hysteresis loops, switching times, and domain-wall mobility as a function of applied field are reported. The domain backswitching phenomenon was absent in SBN:61 crystals.

Nanoscale polarization profile across a 180° ferroelectric domain wall extracted by quantitative piezoelectric force microscopy

The structure of a single antiparallel ferroelectric domain wall in LiNbO3 is quantitatively mapped by piezoelectric force microscopy (PFM) with calibrated probe geometry. The PFM measurements are performed for 49 probes with the radius varying from 10 to 300 nm. The magnitude and variation of the experimental piezoelectric coefficient across a domain wall match the profiles calculated from a comprehensive analytical theory, as well as three-dimensional finite element method simulations. Quantitative agreement between experimental and theoretical profile widths is obtained only when a finite disk-type tip radius that is in true contact with the sample surface is considered, which is in agreement with scanning electron microscopy images of the actual tips after imaging. The magnitude of the piezoelectric coefficient is shown to be independent of the tip radius, and the PFM profile width is linearly proportional to the tip radius. Finally we demonstrate a method to extract any intrinsic material broadening ...

The influence of 180° ferroelectric domain wall width on the threshold field for wall motion

Unlike ideal 180° ferroelectric walls that are a unit cell wide (∼0.5 nm), real walls in ferroelectrics have been reported to be many nanometers wide (1–10 nm). Using scanning nonlinear dielectric microscopy of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) ferroelectrics, we show that the wall width at surfaces can vary considerably and even reach ∼100 nm in places where polar defects adjoin a wall. The consequence of such variable wall widths is investigated on the specific property of threshold field required for wall motion. Using microscopic phase-field modeling, we show that the threshold field for moving an antiparallel ferroelectric domain wall dramatically drops by two to three orders of magnitude if the wall was diffuse by only ∼1–2 nm, which agrees with experimental wall widths and threshold fields for these materials. Modeling also shows that wall broadening due to its intersection with a surface will influence the threshold field for wall motion only for very thin films (1–10 nm) whe...

Anomalous electro-optic effect in Sr0.6Ba0.4Nb2O6 single crystals and its application in two-dimensional laser scanning

We report an anomalous electro-optic effect that results in laser scanning along the polarization direction of strontium barium niobate (Sr0.6Ba0.4Nb2O6, or SBN:60) single crystals under external fields. The origin of such unusual phenomenon is believed to arise from inhomogeneous index gradients near the crystal surfaces, possibly arising from space-charge fields. Based on this effect, two-dimensional scanning is demonstrated by using a combination of SBN:60 single crystal (for vertical scanning; full scan angle of 0.79°±0.07°) and lithium tantalate (LiTaO3) (for horizontal scanning; full scan angle of 3.68°±0.14°).

Design and simulation of planar electro-optic switches in ferroelectrics

Conceptual design and numerical simulation of two polarization dependent planar optical switches based on the electro-optic effect in ferroelectrics operating at 1.55 μm wavelength are presented. The first design is a 3×3 optical switch based entirely on electro-optic beam steering (prism) elements and ion-exchanged lenses for collimation. The second design is a 1×N optical switch based on a combination of electro-optic beam steering and electro-optic focusing (lens) elements. The scalability of this device has been improved by compensating the in-plane divergence of the laser. Analytical expressions for the dependence of scalability are presented.

Phased-Array Electro-Optic Steering of Large Aperture Laser Beams Using Ferroelectrics

We present a device concept for scanning large (>1cm) laser beams using a domain microengineered ferroelectric device. In our design, the large input beam is divided into smaller beamlets, which are then individually deflected through an angle, and then recombined in the far-field to reconstruct the large beam. As a demonstration of this concept, a five-stage cascaded rectangular domain micropatterned scanner device with 13 beamlet channels was fabricated in LiTaO3 and was demonstrated to deflect a 1.064μm infrared laser beam by a total of 10.3° at 5.39kV∕mm.

Hybrid electrooptic and piezoelectric laser beam steering in two dimensions

With modern optical technology, there is a great demand for devices capable of steering a laser beam at high speed. A hybrid electrooptical scanner and an electromechanical scanner capable of two-dimensional (2-D) laser beam scanning are demonstrated. This device can scan a laser beam in 2-D. A 1/spl deg/ scanning angle in the horizontal axis at 50 Hz using electrooptic scanning and a 5.23/spl deg/ scanning angle in vertical axis at 25.6 kHz using piezoelectric scanning are shown.

Comparison of domain reversal and electro-optic properties of congruent and stoichiometric lithium tantalate

A change in the Li/(Li+Ta) ratio in LiTaO3 crystals from 0.485 (congruent) to ~0.5 (stoichiometric) results in a up to 130 times reduction in coercive fields for domain reversal and an elimination of the internal fields and domain backswitching. Dramatic differences in the fatigue behavior are also observed, that results in an electro-optic effect that is very sensitive to electrodes in stoichiometric compositions and rather insensitive in congruent compositions.