Electrical domain fixing of photorefractive index gratings in „K 0.5 Na 0.5 … 0.2 „Sr 0.75 Ba 0.25 … 0.9 Nb 2 O 6 crystals Xiaonong Shen, a) Jianhua Zhao, b) Ruibo Wang, c) Zhenxiang Cheng, Shujun Zhang, and Huanchu Chen State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong, 250100, People’s Republic of China ~Received 10 April 2000; accepted for publication 27 June 2000! Electrical domain fixing in Cu- and Mn-doped (K 0.5 Na 0.5 ! 0.2 ~Sr 0.75 Ba 0.25 ! 0.9 Nb 2 O 6 ~KNSBN! crystals is presented. The relation between the applied field and the revealed grating in single and multidomain crystals is investigated. In multidomain Mn-doped KNSBN, a revealing efficiency of over 233% is obtained. The mechanism of domain fixing in single and multidomain crystal is discussed. © 2000 American Institute of Physics. @S0003-6951~00!02034-9# Photorefractive ~PR! crystals have many applications in nonlinear optical information processing and holographic storage, etc. One problem is that the PR gratings recorded in a crystal can be erased during the readout process. Experi- ments have shown that nondestructive readout can be real- ized by several methods, such as thermal fixing, 1 electrical fixing, 2–7 two-step recording, 8,9 etc. Electrical fixing, which is widely used in the fixing of PR grating in ferroelectric crystals, is known as domain fixing. Due to the partial switching of the ferroelectric domains, the boundary charges located on the domain walls compensate the light-induced space charges, so that the storage lifetime of the PR grating is enhanced. This effect was first observed in the 1970’s, 2 and was subsequently developed in Sr x Ba 1 2x Nb 2 O 6 ~SBN!, 3,4 BaTiO 3 , 5 and KNbO 3 ~KN!~Ref. 7! crystals. The (K 0.5 Na 0.5 ! 0.2 ~Sr 0.75 Ba 0.25 ! 0.9 Nb 2 O 6 ~KNSBN! crystal, being first grown by Chen and Xu, 10 has a larger electro-optic co- efficient and better mechanical properties than SBN, BaTiO 3 , and KN. It is a promising PR material. Due to its higher Curie temperature, it is not easy to depole, thus the storage lifetime should be longer than for SBN if it is fixed. In addition, this kind of crystal has only 180° domains, so that it is easier to pole than BaTiO 3 , and is more suitable for achieving domain switching in electrical fixing. In this letter, the results of the electrical domain fixing in Cu- and Mn- doped KNSBN crystals ~Cu:KNSBN and Mn:KNSBN! are presented and discussed. The hysteresis of a ferroelectric material can provide much insight into the domain fixing of photorefractive index gratings. Before the experiment of domain fixing, the P – E loops of Cu:KNSBN and Mn:KNSBN were measured, and are shown in Fig. 1. Compared with the P – E loops of pure KNSBN crystals, 11 in both crystals the ion doping enhanced the stability of polarization. The standard experimental setup for two-wave mixing was employed to carry out the domain fixing experiment. An ordinarily polarized beam generated from an Ar 1 laser ( l 5514.5 nm) was split into two beams by a beam splitter, which were used to record a PR index grating in the crystal. Another weak beam from a He–Ne laser ( l 5632.8 nm) was used to monitor the diffraction efficiency incident from the back of the crystal at a Bragg-matching angle. In the experi- ment, a high-voltage pulse was applied along the c axis of the crystal. A non-Bragg-matching beam from an Ar 1 laser ( l 5514.5 nm) with an ordinary polarization was used for erasure. The crystals used in the experiment were Mn:KNSBN and Cu:KNSBN, with dimensions of 5 35 35 mm 3 . 12,13 The intensity of each incident recording beam was 10.5 mW, and that of the reading beam was 1.004 mW. Most previous researchers investigated the domain fixing re- sults with respect to the grating period and found that a higher diffraction efficiency of the revealed grating can be obtained with a large grating period. 3 Therefore, the angle between the two incident recording beams was chosen to be 2 u 52°, which corresponds to a grating period L 56.4 m m. A typical process of domain fixing can be found in Refs. 3 and 5. The amplitude of the revealed grating is equal to that of the compensated grating. 14 From the ratio of the dif- fraction efficiency of the revealed grating to that of the pre- recorded electrical grating, which was referred to as the re- vealing efficiency, we can monitor the fixing results. The influence of the amplitude of the negative applied a! Also with the Department of Physics, Jinan University, Jinan, Shandong, 250100, China. b! Permanent address: Department of Optoelectronic and Information Engi- neering, Shandong University, Jinan, Shandong, 250100, China. Present address: Departments of Chemical and Mechanical Engineering, Univer- sity of California at Santa Barbara, Santa Barbara, California 93106; elec- tronic mail: jianhua@engineering.ucsb.edu c! Department of Electrical and Computer Engineering, University of Cali- fornia at Santa Barbara, Santa Barbara, California 93106. FIG. 1. P – E loop of ~a! Cu:KNSBN and ~b! Mn:KNSBN. APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 8 21 AUGUST 2000 1206 0003-6951/2000/77(8)/1206/3/$17.00 © 2000 American Institute of Physics Downloaded 14 Jul 2001 to 146.186.113.240. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp