ULTRASHORT DOMAIN PERIODICALLY POLED

3 INTRODUCTION . 3 EXPERIMENT . 4 3D analysis of the dom ain patterns . 5 STAGES OF THE DOMAIN EVOLUTION DURING POLING . 9 M A IN A PPR O A C H . 10 MECHANISMS OF THE DOMAIN EVOLUTION . 11 N ucleation of new dom ains at the surface . 11 Forward growth and merging of spike-like domains under the electrodes . 12 Dom ain wall m otion out of electrodes . 14 "Finger assisted" mechanism of domain wall motion . 16 Evolution of the domain structure during backswitching process . 17 Stabilization of the domain structure after poling . 19 Frequency m ultiplication . 19 ANALYSIS OF THE SWITCHING CURRENTS . 22 Switching in linear increasing field . 22 Field dependence of the wall motion velocity . 24 Switching current during domain patterning . 25 APPLICATION OF BACKSWITCHED POLING FOR 4 AND 2.6 gtm PATTERNING . 28 RECO M M EN D A TION . 30 R E F E R E N C E S . 3 1 PR E SE N T A T IO N S . 32 3 ABSTRACT In nonlinear optics applications employing quasi-phasematching, short-pitch domain gratings are generally required for the efficient generation of visible and ultraviolet light. Here we introduce an enhanced electric-field poling technique which incorporates spontaneous backswitching and leads to uniform short-pitch bulk domain structures. The total volume of backswitched material, and hence the duty cycle of the backswitched domain grating, can be controlled. Backswitched domain periods down to 2.6 ýim are demonstrated in 0.5-mm-thick LiNbO 3 substrates. INTRODUCTION Recently a new branch of ferroelectric technology and science named domain engineering has been developing rapidly. It is aimed at production of desired reproducible domain structures in commercially available ferroelectric materials. Ferroelectric domain engineering is important for quasi-phasematched (QPM) nonlinear optics which requires the short-period domain gratings [1]. Electric field poling at room temperature, first applied to bulk LiNbO 3 by Yamada et al. [2], has become conventional due to its repeatability and applicability over a wide range of nonlinear optical materials. To date the creation of the domain period of 3-4 ýim in 200 300 ptm thick substrates have been reported for electric field poled LiNbO3 [3], LiTaO3 [4,5] and KTP [6] which shows promise for blue and even UV light applications. Nevertheless, in thicker substrates, remain difficulties to prepare QPM materials for visible and UV wavelengths. The field induced development of the domain structure in LiNbO 3 is practically unstudied due to extremely high value of the threshold field (more than 21 kV/mm at room temperature). As all domain patterns in ferroelectrics obtained during switching in external field are of kinetic nature [7], the comprehensive study of domain evolution at different stages of poling process is required to improve the conventional technique. We investigate formation and stabilization of the bulk domain patterns produced in LiNbO 3 by application of a high field to photolithographically-defined periodic electrodes. Visualization of the domain patterns by SEM and optical microscope, mathematical treatment of the poling current data and computer simulation of domain kinetics are used for analyzing of the stages of domain patterning. The domain evolution during spontaneous backswitching after removing/decreasing of the poling field is explored. We demonstrate that controlled backswitching can be used as a powerful tool for producing the high quality short-pitch domain patterns with periods up to 2.6 gtm in 0.5-mm-thick LiNbO 3 substrates needed for quasi-phase matching devices.