Journal of Crystal Growth 128 (1993) 880-885 j . . . . . . . . C R Y S T A L North-Holland G R O W T H

Growth and properties of a new TB type photorefractive crystal

H.C. Chen , D.L. Sun, Y.Y. Song and Q.Z . J i ang

Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, People's Republic of China

This paper is focused on the studies on the new composition of tungsten bronze photorefractive KNSBN:Cu crystals. By changing the A-site occupation of the crystals and doping Cu ions in them, their ferroelectric and photorefractive properties were modified. These new crystals have excellent polarization stabilities and a two-wave coupling gain coefficient as large as 20 cm i. The photorefractive sensitivity is of the order of 10 3 cm2/j and the cat-SPPC reflectivity can reach 65% with a response time lower than 10 s.

1. Introduction

During recent years, many related studies [1,5] have suggested that the photorefractive (PR) ma- terials are applicable in many aspects of optical image processing and optical computing, e.g. vol- ume holographic storage, coherent light amplifi- cation, incoherent -coherent transformation, opti- cal wave guide and optical phase conjugation. PR materials easily realize optical phase conjugation such as self-pumped phase conjugation (SPPC) [2] and multi-beam phase conjugation [3]. How- ever, some properties of the PR materials have given rise to problems in their practical applica- tions. For example, LiNbO 3 has relatively low PR sensitivity, and strong light scattering decreases the strength of the propagating beam consider- ably. KNbO 3 is easy to crack at phase transition 4 m m - m m 2 when it is cooled down to room tem- perature after growth. BaTiO 3 has excellent PR properties, but it is difficult to grow perfect single crystals. Furthermore, at room tempera- ture, there exists a phase transition 4 m m - m m 2 which causes a spontaneous polarization in [011] direction. SrxBa I xNb20 6 crystals have large electro-optic coefficients, but their Curie temper- atures are quite low (SBN: 75, T c = 39°C). So, in crystal processing or at room temperature , it will depolarize. In 1981, Chen and Xu [4] developed a new kind of ferroelectric crystals (KxNal_x)Zm (SryBa|_y) l_mNbzO 6 (KNSBN, 0 < x < 1, 0.2 < y

< 0.8, m = 0.2). These crystals have the advan- tages of high Curie temperature, large electro- optic coefficient, and good mechanical proper- ties, and they have no phase transition at room temperature. It is expected that these crystals will become one of the most promising PR materials.

There are two effective ways to improve the PR properties of a material: (1) by adjusting the compositions of the material, the dielectric and the electro-optic properties can be modified; (2) through doping new ions in the materials, new centers can be built up for free charge carrier stimulation and recombination. Fortunately, the structure of the KNSBN crystals provides the possibility for the above-mentioned two ways. Fig. 1 shows the structure of a KNSBN unit cell in the (001) plane. A-sites (A1 and A2) are completely occupied by K +, Na +, Sr e+ and Ba 2+ ions, B-sites (B1 and B2) are occupied by Nb 5+ ions, and C-sites are left to be occupied. In this paper, we report the Cu-doped crystals in which A-sites are not completely occupied, along with their crystal growth, the dielectric and the photorefractive properties.

2. Crystal growth

All the K N S B N : C u crystals were grown by pulling method with a Crystalox MCGS-3 system.

0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

H.C. Chen et al. / Growth and properties o f new TB type photorefractive crystal 881

Oxygen octahedron

j B 2

\ \

~ C "\)

\ • /

/

" j

Fig. 1. Schematic diagram showing a projection of the tungsten bronze structure (001) plane. The orthorhombic cell and the tetragonal cell are shown by solid line and dotted line, respectfully.

The starting materials are 99.99% BaCO3, SrCO3, K2CO3, Na2CO3, Nb20 5 and CuO. They were mixed according to the compositions, i.e. (KxNal_x)2m(SryBal_y)l_mNb206 (KNSBN, x = 0.5, y = 0.75, m = 0.1). A very small amount of 0.03, 0.04, 0.07 and 0.1 wt% CuO was added to each mixture. After ball-milling in pure alcohol for 4 h, the mixtures were dried and pressed into disks of 30 mm in diameter and 25 mm in thick- ness. The disks were placed in a 300 ml platinum crucible and heated to melt by induction heating in the air. After the melt was overheated 50°C for 15 min, it was cooled down to the temperature close to the melting point and the crystal was pulled out from the melt using a KNSBN seed. The growth conditions are: growth temperature of 1550°C, temperature gradient of 40°C/cm, pulling direction is [001], atmosphere is air and the ratio of pulling ra te / ro ta t ion is 0.2-0.4.

The as-grown crystals display good symmetry under the above growth conditions (fig.2) with a size of 35 × 35 × 35 mm 3. The groups of facets {100}, {110} and {210} predominate. Usually there are sixteen facets around the crystal, but as the concentration of the CuO dopant increases, the color of the crystals changes from yellow to red and the facets decrease to twelve. The lattice parameters checked with X-rays and compared

with pure KNSBN show no difference. The point group is 4mm at room temperature. Moh's hard- ness scale is 6.5. Its density is 5.3 g / c m 3. The specific heat Cp of the crystal is 0.33 × 103 J k-1 K-1. The index gradient along the growth direc- tion is 0.5 x 10 -4. The transmission spectra of doped and undoped crystals are shown in fig. 3. In the spectra of Cu-doped crystals (KNSBN : Cu), there are two obvious absorption peaks which

Fig. 2. The photograph of the as-grown crystals.

882 H.C. Chen et al. / Growth and properties of new TB type photorefractive crystal

T(%)

60

4 0

2O

0 400 500 600 (rim)

Fig. 3. The transmission spectra of doped and undoped KNSBN crystals: (a) KNSBN; (b) KNSBN:Cu, 0.03 wt% CuO; (c) KNSBN:CuO, 0.04 wt% CuO; (d) 0.07 wt% CuO;

(e) KNSBN:Cu, 0.1 wt% CuO.

A-site all-occupied KNSBN. The electro-optic co- efficients have the following relation with the constants:

r51 = 2g44P3~llEO, r33 = 2g33P3E33E0,

where P3 is the c-axis polarization; gu is the quadratic electro-optic coefficient and obviously cannot be changed. So, the new KNSBN:Cu crystals would have larger electro-optic coeffi- cients than the A-site all-occupied KNSBN crys- tals.

There is the intriguing problem that the p-E loops of such ferroelectrics as BaTiO 3 and SBN will be reduced when electrical fields are applied to them repeatedly [6]. Fig. 5 shows the p-E loops of KNSBN and KNSBN:Cu. The p-E loops of KNSBN crystals appeared reduced un- der a repeated electrical field while those ofKNSBN : Cu crystals were very stable.

correspond to the energy levels formed by Cu ions in the band gap. This means that the Cu ions are able to act as centers for free charge carrier stimulation and recombination, and that the PR properties will be modified.

In order to get high quality, the following aspects should be taken into consideration: (1) the purities of the reagents; (2) the temperature gradient at the solid-liquid interface and (3) the ratio of the puling rate to rotation.

The as-grown crystals were annealed at 1050°C for 24 h, and then cut into samples with a size of 5 mm × 6 mm × 6 mm along the a, b and c axes. The samples were polished and polarized at 100°C with a 6 k V / c m DC electric field.

3. Ferroelectric properties

The dielectric properties at 1 kHz for the KNSBN and Cu : KNSBN crystals in which A-sites are completely occupied are shown in fig.4 as a function of temperature for c-axis crystals. As some A-sites are unoccupied, the Curie tempera- ture becomes lower. At room temperature, the dielectric constants are etl = 550 and e33 = 580, approximately two times larger than those of the

4. Two-wave coupling in the KNSBN:Cu crystals

The predominant feature of the PR effect is that there exists, between an interference pattern and the grating, a phase shift that results in the

10 s , • ,

Z 0

10 3

(b ~J

~ 10 2

i 0 , . . - ~ , '

0 80 160 240 3'20 360

TEMPERATURE ( o C )

Fig. 4. The dielectric constants as a function of temperature

of (a) thc new KNSBN crystal and (b)the A-site all-occupicd KNSBN.

H.C. Chen et aL / Growth and properties of new TB type photorefractiue crystal 883

Fig. 5. The hysteresis loops of (a) KNSBN and (b) KNSBN: Cu.

energy transformation (two-wave coupling) be- tween the two coherent beams. The variation of the two-wave coupling properties can quantita- tively reflect the influence of the doping ions on the PR properties of KNSBN.

The usual experimental set-up for two-wave coupling was used in our experiments. The ex- traordinary polarized beam emitted from an Ar ÷ laser (A = 488 nm) was split into two beams to form an interference pattern in a sample. A photo-electric detector connected to a X - Y recorder was used to record the variation of the signal beam. Fig. 6 shows the relation between the gain and the angle 20 formed by the two beams outside the sample. The effective acceptor density of KNSBN:Cu is calculated to be 4.6 x 1016 cm 3. This density is higher than that of KNSBN and as a result, the PR electric field is

3o

2O

O

IO

0 , I , I , I 0 2 0 4 0 6 0

2 e ( O e ~ )

Fig. 6. Thetwo-wavecouplinggaincoefficients F a sa ~nc- t ionof20ofKNSBN(e)andKNSBN:Cu(o) .

increased and the gain becomes larger. Since the gain is positive, the electrons play the dominant role in the electron-hole competition. Although they can decrease the electron mobility [7], Cu ions have improved the response speed for the larger number of optical induced free electrons (fig. 7).

As I R=0.53 W/cm z and I s=0.44 W/cm 2, the response time (at 1 - 1 /e level) is 0.3 s with a saturation diffraction efficiency of 41%, and the transmission is 53%.So the PR dynamic range is 3.9 x 10 -4 and the PR sensitivity is of the order of 10 -3 c m Z / J .

21.1 I , '4

1

0 0

~IPKNSBN:Cu

~ ~ " N S BN

n • • 1

12 24 36 48 60 T i m e ( s )

Fig. 7. The time response in two-wave coupling of KNSBN and KNSBN:Cu.

884 H.C. Chen et al. / Growth and properties of new TB type photorefractive crystal

6 2 v

13.

- ( a )

0 10 20 30 40 T i m e (s )

60

40

20

(b)

i

0

o

I h I i I

20 40 60 @ (De 9)

"~ 5O

E

c 0 el.

ID

(c)

1 I i i i I

0.1 1 5 ! (w /cm 2)

Fig. 8. The basic SPPC properties of K N S B N : C u : (a) the time response; (b) the SPPC reflectivity versus the incidence angle; ( o )

K N S B N : C u , 0.03 w t % , (o) K N S B N : C u , 0.1 w t % , a n d (c) response time (at l / e ) versus input energy.

5. Cat self-pumped phase conjugation in KNSBN : Cu

Cat SPPC has been demonstrated by using K N S B N : C u at different wavelengths. The exper- imental setup is the same as that reported in ref. [2]. The mechanism is very similar to that re- ported in ref. [2], but at the saturation state in the sample, there is only one straight line heading toward one edge of a sample instead of a loop. This phenomenon can be attributed to the shrinkage of the loop for the coupling between the fanning beams.

The basic SPPC properties of KNSBN : Cu are given in fig. 8: (a) The time response properties. Initially,there is a pumping beam forming process with no SPPC output. (b) The relation between the SPPC reflectivity and the incident angle at A = 514.5 nm. The variation of the incident angle strongly affects the coupling gain, the beam fan-

ning as well as the SPPC output. (c) The relation between the response time at e-~ level and the input energy. An increase of the input energy can greatly reduce the response time.

The SPPC output at a certain wavelength has a strong dependence on the doping concentration (table 1). At A = 514.5 nm, increasing the doping concentration greatly reduces the SPPC output.

Table 1

The dependence of SPPC reflectivity R (%) on wavelength and doping concentration N

A (nm) N ( w t % )

0.03 0.04 0.07 0.1

632.8 - 40 55 63

514.5 62 65 40 29

448.0 42 38 - -

496.5 47 47 - -

467.5 - 21 - -

H.C. Chen et al. / Growth and properties o f new TB type photorefractive crystal 885

This is due to the enhanced absorpt ion at this wavelength from the amount of the doping ions. However, at A = 632.8 nm, as the concentra t ion of the doping ions increases, the SPPC output is improved. This p h e n o m e n o n suggests that, at this wavelength, which is far f rom the intrinsic wave- length of Cu ions in KNSBN, the Cu ions only act as the electron recombinat ion centers in the PR process, and increasing the amount of the Cu ions improves the number of the effective recom- bination centers.

6. Conclusion

High-quali ty K N S B N : C u crystals with A-sites incompletely occupied have been grown by a pulling method. The Cu ions can form two energy levels in the band gap and can act as centers for free electron stimulation and recombinat ion. The

high coupling gain coefficient, short response time, as well as the high SPPC reflectivity of these materials will enable them to be a promis- ing PR material that can be used in optical infor- mat ion processing.

References

[1] P. Giinter and J.-P. Huignard, Eds., Photorefractive Mate- rials and Their Applications, I and II, Topics in Applied Physics, Vols. 61 and 62 (Springer, Berlin, 1989).

[2] J. Feinberg, Opt. Letters 7 (1982) 486. [3] M. Cronin-Golomb and A. Yariv, Opt. Letters 12 (1988)

714. [4] H.C. Chen and Y.H. Xu, Physics 10 (1981) 729 (in Chi-

nese). [5] P. Giinter, Phys. Rept. 93 (1982) 199. [6] J.C. Burfoot and G.W. Taylor, Polar Dielectrics and Their

Applications (University of California Press, 1979) ch.3. [7] D.C. Look and J.S. Blakemore, Eds., Semi-Insulating III-

V Materials, Kah-nee-ta, Warm Springs, OR, 1984 (Shiva, Nantwich, 1984) section IV.