15 July 1998

Ž .Optics Communications 153 1998 106–110

The optical and photorefractive properties of reduced Rh-dopedBaTiO at wavelength of 514 nm3

J.Y. Chang ), C.R. Chinjen, S.H. Duan, C.Y. Huang, R.H. Tsou, J.N. Cheng,C.C. Sun

Institute of Optical Sciences, National Central UniÕersity, Chung-Li 32054, Taiwan, ROC

Received 6 October 1997; revised 4 March 1998; accepted 20 April 1998

Abstract

Optical and photorefractive properties of reduced Rh-doped barium titanate are reported. Reduction eliminates energylevels of Rhq4 and Rhq5, and induces an absorption around 480 nm, which has a large light-induced transparency of ;4cmy1. After reduction, the speed increases by a factor of 30 though the electrooptic gain decreases by a factor of 1.6. Inaddition, it induces an increase of dark conductivity by three orders. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction

Ž .Rh-doped barium titanate BaTiO , a promising pho-3

torefractive material in the near infrared range, has at-w xtracted considerable attention 1,2 . Though it has a high

two beam coupling gain, its speed is slow. Recently,post-processing was commonly used in BaTiO to opti-3

w xmize its properties 3 . In this report, we describe opticaland photorefractive properties of reduced BaTiO :Rh at a3

wavelength of 514 nm.Reduction is a chemical process, which takes oxygen

atoms out of the crystal and leaves its vacancies andw xelectrons in the crystal 4,5 . This would transform the

crystal from p type to n type, and also change the valencew xstates of ions in the crystal. Wechsler and Klein 4 and

w x q2 q3Lahlafi et al. 5 reported that the densities of Fe , Feand Feq4 are redistributed by reduction and then thephotorefractive properties can be modified in BaTiO :Fe.3

w xFor BaTiO :Rh, Wechsler et al. 2 found that its blue3

color is from the charge-transfer transition involving Rhq4

q3 w xandror Rh . In addition, Kross et al. 6 suggested that

) E-mail: jychang@sparc20.cc.ncu.edu.tw

the charge-transfer band of Rhq4 had an absorption peakaround 650 nm and Rhq5 around 800 nm. Further, Steven-

w xdaal et al. 7 reduced BaTiO , doped with Rh and Fe, in3

the hydrogen environment. They found that an absorptionpeak around 470 nm evolves, and has a light-inducedtransparency of 1.5 cmy1 at a pumping intensity of 1.2Wrcm2.

In this report we investigate optical and photorefractiveproperties of an as-grown and two reduced BaTiO :Rh3

samples processed at 9008C in atmospheres of 10y10 atmand 10y14 atm oxygen partial pressure using COrCO2

mixture gases, respectively. We found that the reductionsignificantly modifies the optical and photorefractive prop-erties of BaTiO :Rh. Reduction induces an absorption3

peak around 480 nm and a large light-induced trans-parency. This peak significantly improves the speed with-out sacrificing much for the electrooptic gain. It has a fastresponse time of 35 ms and electrooptic gain of 3 cmy1

using the smallest electrooptic coefficient r at an inten-13

sity of 1 Wrcm2 and wavelength of 514 nm. In addition,the dark conductivity increases by three orders after reduc-tion.

Next, we describe experiments for crystal growth andpost-processing. Then absorption was measured to investi-gate the change of energy levels. Two beam coupling was

0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0030-4018 98 00213-2

( )J.Y. Chang et al.rOptics Communications 153 1998 106–110 107

conducted to determine its carrier type, magnitude ofelectrooptic gain, and speed. Dark conductivity was ap-proximated from the dark decay measurement. A discus-sion and conclusions then follow.

2. Experiment

A boule of Rh-doped BaTiO was grown by the top-3w xseeded solution growth method 8 . Rh was added by

dipping the Pt-Rh wire into the melt. The boule wasw xoriented by X-rays with edges at the 100 direction, and

Ž .three samples were extracted. One was the as-grown S1without post-processing, and the other two were reduced at

y10 Ž . y14 Ž .atmospheres of 10 atm S2 and 10 atm S3 oxy-gen partial pressures using COrCO mixture gases at2

9008C, respectively. The reduced samples were at 9008Cfor 12 hours and then quenched to about 3008C before theyslowly decreased to room temperature. These sampleswere then poled to single domain crystals. These threecrystals have very different colors: blue, transparent, anddeep red for S1, S2, and S3, respectively.

The absorption was conducted by the transmissionmethod with optical polarization perpendicular to the c-axisof the crystal. Shown in Fig. 1 is the absorption spectrafrom 420 nm to 1100 nm for these three crystals. S1 hasblue color, with an absorption spectrum similar to the other

w xRh-doped BaTiO samples 1,2 . It has an absorption peak3

around 650 nm with an absorption coefficient of 7.2 cmy1

and 800 nm of 2 cmy1. These two absorption peaks areq4 q5 w xfrom energy levels of Rh and Rh , respectively 6 .

w xKaczmarek et al. 9 found that 2–4 ppm of Rh induce anabsorption coefficient of 1.5 cmy1 at 650 nm for theas-grown BaTiO :Rh. From comparison of the absorption3

Fig. 1. Absorption spectra for the as-grown, and the two reducedcrystals processed in atmospheres of 10y10 atm and 10y14 atmoxygen partial pressure at 9008C, respectively.

Fig. 2. Light-induced absorption versus intensity for the as-grown,and the two reduced crystals.

coefficients, we estimate that there are 10–20 ppm of Rhin S1. After reduction, there is a significant change in theabsorption spectra. S2 has a small absorption in the visiblerange. The absorption coefficient is 0.4 cmy1 at 650 nmand almost zero at 800 nm. However, S3, reduced in ahigh reducing atmosphere, has a large absorption peakaround 480 nm. The absorption coefficient is 11 cmy1.This peak is similar to those found by Schunemann et al.w x w x w x8 , Chang et al. 10 , and Stevendaal et al. 7 . The originof this absorption peak is still unknown.

Light-induced absorption, observed first by Motes et al.w x11 , is a function of wavelength and intensity in

w xBaTiO :Rh 12,13 . We found that the light-induced ab-3

sorption significantly depends on the reduction level inRh-doped BaTiO . Shown in Fig. 2 is the light-induced3

absorption dependence on the pump intensity for the threecrystals. The pump is from an argon ion laser with awavelength of 514 nm, and the signal is from a monochro-mator also with a wavelength of 514 nm. Both pump andsignal have ordinary polarizations. For S1, it is light-in-duced transparent which is similar to that of the other

w xas-grown BaTiO :Rh 12 , and increases with pump inten-3

sity. It is y0.2 cmy1 for 10 mWrcm2 and y0.5 cmy1

for 1000 mWrcm2. For S2, it changes to light-inducedabsorption, which also increases with pump intensity. It is0.15 cmy1 for 10 mWrcm2 and 0.5 cmy1 for 1000mWrcm2. For S3, it changes back to light-induced trans-parency, and has a large amplitude. It still increases withpump intensity. It is y1.6 cmy1 for 10 mWrcm2 andy4.1 cmy1 for 1000 mWrcm2. This large light-inducedtransparency is similar to those of the reduced samples

w x w xprepared by Chang et al. 10 and Stevendaal et al. 7 ,though they have smaller amplitudes.

Two beam coupling was performed by an argon laserwith a wavelength of 514.5 nm, and its experimental setupis shown in Fig. 3. The laser output is collimated and split

( )J.Y. Chang et al.rOptics Communications 153 1998 106–110108

Fig. 3. Experimental setup for two beam coupling, photoinduced erasure decay, and dark decay measurements.

into two writing beams with pumprsignal intensity ratio of800. These two o-polarized beams write a set of gratingswith the grating wavevector parallel to the c-axis ofBaTiO . It uses the smallest electrooptic coefficient r so3 13

that the noise from fanning is minimum. The electroopticw xgain is defined as 14

gs 1rd ln I rI ,Ž . Ž .s s0

where g is the electrooptic gain, d is the interactionlength, I is the signal intensity with the pump beams

present at steady state, and I is the signal intensitys0

without pump beam. To separate the light-induced absorp-tion, absorption coupling and electrooptic gain, the ex-

w xchange method is used 15 . Shown in Fig. 4 is theelectrooptic gain dependence on the intensity at a gratingperiod of 0.6 mm. Positive electrooptic gain, defined as thegain direction toward the qc-axis, indicates that the domi-nant carrier are holes, and negative electrooptic gain is dueto electrons. For the as-grown, the carrier are holes, andchanges to electrons for crystals reduced at atmospheres of

Fig. 4. Electrooptic gain versus intensity at a grating period of 0.6mm for the as-grown, and the two reduced crystals.

10y10 atm and 10y14 atm oxygen partial pressure. For S1,the electrooptic gain increases with intensity, and saturatesat high intensity. It is 3 cmy1 for the intensity of 20mWrcm2 and saturates at about 4.5 cmy1 around 500mWrcm2. After reduction, the electrooptic gain decreases.For S2, the electrooptic gain changes little with intensity.It is about y2 cmy1 to y3 cmy1 between an intensity of20 mWrcm2 and 5 Wrcm2. For S3, the electrooptic gainis similar to those of S2.

The electrooptic gain dependence with gratingwavevector was also investigated and the result is shownin Fig. 5. The writing intensity is 200 mWrcm2. The solidlines are the simulation fitted with the deep and shallow

w xtrap model 16 . It shows that the effective trap densityvaries with the reduction level. For S1, the effective trapdensity is 22.8=1016 cmy3 and S2 has a smaller effectivetrap density of 5.1=1016 cmy3. With further reduction,S3 has a higher effective trap density of 12.4=1016 cmy3

though still smaller than that of S1.

Fig. 5. Electrooptic gain versus grating wavevector at an intensityof 200 mWrcm2 for the as-grown, and the two reduced crystals.

( )J.Y. Chang et al.rOptics Communications 153 1998 106–110 109

The response time was measured by the photoinducederasure decay measurement. The experimental setup is alsoshown in Fig. 3, with wavelength of 514 nm as writing anderasure beams. A He-Ne laser is used for the readingbeam. The diffracted beam is detected by a photomultiplierconnected with a lock-in amplifier. The total writing beamintensity was 400 mWrcm2 with a beam intensity ratio of1 and the grating period is 0.6 mm. The response timedependence on the intensity is shown in Fig. 6. Theresponse time decreases with increasing intensity. For S1,the response time is 55 s at an intensity of 5 mWrcm2,and 1 s for 1000 mWrcm2. It shows a sublinear intensitydependence of tA Iyx with xs0.72, where t is theresponse time and I is the intensity. After reduction, theresponse time decreases. For S2, the response time is 12 sfor 5 mWrcm2 and 450 ms for 1000 mWrcm2. It alsoshows a sublinear intensity dependence with response time,with xs0.66. For S3, it does not have the sublinearpower dependence between response time and intensity. Itchanges little within intensities from 5 mWrcm2 to 100mWrcm2, and has a larger change after 200 mWrcm2.This is probably due to high dark conductivity as we willdiscuss later. However, its response time has a much lowervalue. The response time is 250 ms for 5 mWrcm2, andabout 35 ms for 1000 mWrcm2. It improves by a factor of30 when compared with that of the as-grown.

Further, we conducted the dark decay measurement.The experimental setup and conditions are the same asthose in the photoinduced erasure decay measurementshown in Fig. 3, except that there is no erasure beam. Thepower of the reading beam is 200 nW and had no measur-able influence on the decay rate as we turn on and off thereading beam. From the diffracted beam intensity depen-

Fig. 6. Photoinduced decay time constant versus intensity at agrating period of 0.6 mm for the as-grown, and the two reducedcrystals.

dence with time, S1 showed two subsequent decays indi-cating the existence of deep and shallow traps. The slowdecay was exponentially dependent on time with a timeconstant of 2000 s. S2 also showed two subsequent decaysand the slow decay had a slower time constant of 230 000s. However, S3 only showed one exponential decay with amuch faster decay of 1 s. The dark decay time constant canbe used to approximate the thermal-equilibrium dark con-ductivity s by using the dark dielectric relaxation timed

equation, s s´ ´rt , where ´ is the permittivity ofd 0 d 0

vacuum, ´ is the dielectric constant and t is the darkd

decay time constant. The calculated dark conductivity ofy15 Ž . y17 Ž .S1 is 6.6=10 1r cm V , 5.7=10 1r cm V for

y11 Ž .S2, and 1.3=10 1r cm V for S3. After reduction, thedark conductivity increases by about three orders.

3. Discussion

Ž .Reduction is a chemical reaction of O ™ 1r2 O ≠O 2

qVq2 q2ey, where O is the oxygen atom at the oxygenO O

site in the crystal, O the oxygen gas, Vq2 the vacancy at2 O

the oxygen site with charge q2, and ey the electron withcharge y1. The reaction takes oxygen atoms out of thecrystal and forms the oxygen gas, and generates oxygenvacancies and electrons in the crystal. The concentration ofthe generated electrons increases as the oxygen partialpressure decreases. S1 has a dark conductivity of 6.6=

y15 Ž .10 1r cm V with holes as dominant carrier. It has aFermi level below the mid band gap. When it is reduced,generated electrons compensate the holes, and dark con-ductivity decreases. The Fermi level rises towards the midband gap. If the oxygen partial pressure goes on decreas-ing, the dark conductivity further decreases until it reachesthe smallest value at the compensation point. At thismoment, the Fermi level is located at the mid band gap. Asthe oxygen partial pressure further decreases, the Fermilevel is beyond the mid band gap, and the dark conductiv-ity begins to rise with electrons as carrier. For S2, reducedat an atmosphere of 10y10 atm oxygen partial pressure, theFermi level already passes beyond the mid band gap, andthereby it is n type and has a smaller conductivity of

y17 Ž .5.7=10 1r cm V . S3, reduced at an even loweroxygen partial pressure, has a much higher dark conductiv-

y11 Ž .ity of 1.3=10 1r cm V . Its Fermi level is more closeto the conduction band, resulting in a much higher electronconcentration.

High dark conductivity may play an important role inthe photoinduced erasure decay measurement. The re-sponse time is contributed from conductivity combinedfrom both dark conductivity and photoconductivity. For S1and S2, dark conductivity is small compared with photo-conductivity, and can be ignored in the contribution of theresponse time. However, S3 has a dark conductivity com-

( )J.Y. Chang et al.rOptics Communications 153 1998 106–110110

pared to the photoconductivity at low intensity, leading toa relative small change of the response time with intensity.

Reduction not only transforms the crystal from p typeto n type, but also changes the valence state of ions in thecrystal. Absorption spectra indicate that Rhq4 and Rhq5

are present in S1. In S2, most of the absorption from Rhq4

and Rhq5 disappears. It is probable that Rhq4 and Rhq5

are reduced to Rhq3, which is suggested to have now xabsorption below the band gap of ;3.1 eV 6 . This

results in a very small absorption in the visible range. Withfurther reduction, an absorption peak around 480 nm ap-pears in S3. The origin of this absorption is still unknown,but it has some interesting properties. It has a largeabsorption peak around 480 nm, a large light-inducedtransparency of 4 cmy1, and a fast response time of 35 msat a pump intensity of 1 Wrcm2 and wavelength of 514nm. Apparently, electrons are easily photoinduced to theconduction band from this level. It is likely that it has alarge excitation cross section, and then is a good candidatefor improving the slow speed of BaTiO .3

4. Conclusion

The optical and photorefractive properties of the anas-grown and two reduced BaTiO :Rh samples were inves-3

tigated. We found that the optical and photorefractiveproperties are strongly dependent on the reduced atmo-sphere. It first reduced Rhq4 and Rhq5 to Rhq3, and thengenerated an absorption around 480 nm at an atmosphereof lower oxygen partial pressure. After reduction, thecarrier type changes from holes to electrons. For thecrystal reduced at an atmosphere of 10y10 atm oxygenpartial pressure, the dark conductivity decreases by twoorders, electrooptic gain decreases by a factor of 1.6 andresponse time increases by a factor of 2. However, for thecrystal reduced at an atmosphere of 10y14 atm oxygenpartial pressure, the dark conductivity increases by threeorders, electrooptic gain decreases by a factor of 1.6 andresponse time is much improved by a factor of 30.

Acknowledgements

This research was supported by the National ScienceCouncil of the Republic of China.

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