| Potassium tantalate niobate(KTN)crystal has broad application prospects in the field of optical modulators,electro-optic switches,deflectors,sensors,actuators,optical communication and so on,because of its excellent electro-optic properties,photorefractive effect,field-induced-strain effect and ferroelectricities.In this paper,the field-induced-strain effect and photorefractive effect of the Mn:Fe:KTN co-doped crystal are mainly investigated.Main work is as follows:1 The ferroelectric domain configuration evolution of the tetragonal Mn:Fe:KTN crystal under an external electric field is observed in situ using optical microscope imaging system.An optical method is proposed to extract the displacement vector of domain wall sidewise motion from the ferroelectric domain configurations and then obtain the hysteresis loop.The method makes it possible for us to establish a relationship between local properties of the domain dynamics and macroscopic response i.e.,polarization hysteresis loop.And it provides an effective monitoring method for directly controlling the movement of domain walls in ferroelectrics and developing novel functional devices with corresponding performance.2 The ferroelectric hysteresis loops and the electrical strain curves in a tetragonal Mn:Fe:KTN crystal are studied with respect to the magnitude and frequency of electric field and temperature.The hysteresis loops show strong frequency dependence.The shape of the hysteresis loop experiences systemic f-dependent changes from an asymmetric double hysteresis loop change to a nearly symmetric double hysteresis loop and turn to a normal single loop,as the frequency from high to low(150-0.01Hz);At low frequencies(f<1Hz),the shape of hysteresis loop is dependent on the frequency.At high frequencies(f>5Hz),the hysteresis loop doesn't change with the frequency.Meanwhile,the maximum polarization(~12?C/cm~2)stays unchanged with frequency varying at the same temperature and amplitude of electric field.The frequency dependence characteristics of hysteresis loop stems from the forced vibration of two kinds of electric dipoles which are spontaneous polarization electric dipole and a dipole induced by the local composition gradient distribution.The electrical strain curves and the hysteresis loops corresponding to each other.A sprout-shaped electrical strain curve with large recoverable positive strain and zero negative strain is exhibited at high frequency while one typical butterfly-shaped electrical strain curve is shown at low frequency.The research results show that Mn:Fe:KTN crystals have potential applications in the development of precise actuators with large strain and variable frequency communication devices.3 Polar nanoregions(PNRs)are proved to exist in the Mn:Fe:KTN crystal at its paraelectric phase through its dielectric and electro-optic effect.Bragg gratings are written into the crystal by the two beam interference method.Both the writing process and dark decaying process of the gratings are visualized in situ and measured using digital holographic microscopy.Besides,the refractive index change of grating with respect to various experimental parameters(such as the writing electric field,the reading electric filed,temperature,the annealing rate,the grating spacing,the exposure time)during the writing and dark decay process.The experimental results are well consistent with the theoretical analysis.Experiments reveal that the dark storage time of the grating is strong dependent on the writing electric field.The dark storage life of the grating can be prolonged by applying electric field during the writing process.The reason of that is the PNRs are oriented by the external field in the crystal,and thus the grating is written on an ordered crystal,eventually prolonging the dark decay time.The research results provide an important basis for the development of electro-optic modulators and electro-holographic switches based on KTN crystals. |
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