15 May 1997

OPTICS COMMUNICATIONS

Optics Communications 138 0997) 101-104

Photorcfractive effect in hydrogen-reduced BaTiO,

J.Y. Chang * , C.R. Chinjen, R.H. Tsou, C.Y. Huang, C.C. Sun, M.W. Chang Institute of Optical Sciences, National Central Unizxrsity, Chung-Li, Taiwan 32054, Taiwan, ROC

Received 17 September 1996; accepted 8 January 1997

Abstract

Optical and photore~a~tive properties of hydrogen-reduced BaTiO, are investigated. ~y~ogen-reduction induced a broad optical absorption around 620 nm. From two beam coupling, the electrooptic gain is highly dependent on intensi~, with electrons being the major carriers. When the annealing temperature increases, the electrooptic gain decreases, though the trap density increases. From light-induced erasure decay measurement, the response time has a little change with intensity at low intensity, though it is much faster than that of the as-grown sample. These properties can be attributed to high dark conductivity of the reduced sample. The dark conductivity increases about three orders after hydrogen-reduction. It is about 6.6 X lo-” l/(cm fl) for the reduced sample, compared with 2.3 X lo-l4 l/(cm a> for the as-grown.

~e~~~~~~~; Photo~~~c~ive BaTiO,; Hy~ogen-reduction; Dark condn~tivity

Photon&active BaTiO, is known for its high electroop- tie coefficient r,, and its application of self-pumped phase conjugation. Its applications include optical info~ation processing, optical computing, optical storage, etc. [I]. However, its slow speed prohibits it from further applica- tions. Usually, its properties can be modified by impurity- doping [2-51 and after-processing [6,7]. After reduction in CO/CO, atmosphere, the nominally undoped BaTiO, shows an absorption peak around 470 nm and an increase of the speed by a factor of 2.8 [7-91. The absorption peak of 470 nm also has a large photoindu~ed ~ansparency, though the origin of the peak is still not identified 181. In this report, we investigated the optical and photoreFractive properties of barium titanate reduced in a hydrogen atmo- sphere, which could induce high dark conduct~vi~ f lo].

The optical properties of hydrogen-reduced BaTiO, have been reported by Berglund and Braun [lo]. They found very high absorption (N 30 cm-“) in their thin sample (- 0.05 cm) with two energy bands near 2.6 eV and 2.1 eV in the visible range. In this report, they attributed the absorption to oxygen vacancies. To our

* Corresponding author. E-mail: jych~g~sp~2~.ncu.edu.~.

knowledge photorefractive properties of hydrogen-reduced BaTiO, have not been reported.

In this report, we successfully reduced BaTiO, in a hydrogen atmosphere at temperatures of 625°C and 650°C. The reduction induced a broad absorption peak around 620 nm. From two-beam coupling, the electrooptic gain is highly dependent on intensity. From the light-induced erasure decay measurement, the response time shows a different pattern as that ‘of conventional BaTiO,. After reduction, the response time decreases by a factor of about 700 at an intensity of 1 mW/cm~, though it decreases by a factor of four at an intensity of 2.5 W/cm’. It can be attributed to high dark conductivity due to reduction.

Using the raw materials of Cerac 99.999% BaCO, and 99.998% TiO,, BaTiO, was grown by the top-seeded solution method [ 1 I] using a resistance furnace. The boule was then X-ray oriented, and cut into four pieces. Three pieces were annealed in a hydrogen-atmosphere at tcmper- atures of 625°C 650°C and 7OO”C, respectively, and one piece was untreated. After annealing for about 70 hours, three samples were quenched to about 200°C and then the temperature was slowly decreased to room temperature. Then these four samples were poled to single domain crystals. Though the other three were in good conditions, the one annealed at 700°C had a crack inside the crystal

0030-401S/97/$17.00 Copyright 0 1997 Elsevier Science B.V. Ail rights reserved.

ffl s0030-401~(97~00036-9

102

Wavf&angth (nm)

Fig. I. Optical absorption spectra of the as-grown and hydrogen-

reduced BaTiO, at annealing temperatures of 625°C 650°C and

700°C. Curve 5 is the absorption difference between those of

annealing temperatures of 700°C and 625°C.

and was partially poled, and it was only used for the absorption measurement in the following experiments.

The absorption coefficient was dete~ined by the trans- mission method [2], with optical polarization perpendicular to the c-axis of the crystal. Shown in Fig. 1 is the absorption spectrum from 430 nm to 1000 nm. For the as-grown, the absorption is from the band gap (- 400 nm) and diminished into the near-infrared region, For the hy- Hogan-reduced sample, the absorption of the ~n~Iing temperature at 625°C is similar to that of the as-grown. As the annealing temperature increases, a broad absorption band evolves, which can be deduced from the absorption difference between the annealing temperatures of 700°C and 625°C. A broad absorption peak at about 620 nm appears, which corresponds to that of 2.1 eV found by Berglund and Braun [lo]. At the wavelength of 620 nm, the absorption increases with annealing temperature. For the annealing temperature of 625”C, the absorption coeffi- cient is about 1 cm- ‘, and for 700°C it is about 3 cm- ‘. In

addition, the light-induced absorption [8] is determined. It is small, and about 0.05 cm-’ for the pump intensity of 3 W/cm’ at a wavelength of 514 nm. Since the reduction in hydrogen is heavy, Ti(II1) may be generated. The absorp- tion of 620 nm is then probably due to the electron excitation on the TifIII) ion within the TiO, resulting from crystal field transition splitting, which was found in Nb- doped BaTiO, [ 121.

Shown in Fig. 2 is the experimental setup of two-beam coupling, performed by an argon laser with a wavelength of 514.5 nm. The laser output is collimated and split into two writing beams with pump/signal intensity ratio of 800. These two o-polarized beams write a set of gratings with the grating vector parallel to the c-axis of the BaTiO,. The electrooptic gain is defined as 1131

Mirror

-IT--

PMT Lock-in Amp

Fig. 2. Experimental setup for two beam coupling, ~igIlt”induced

erasure decay, and dark decay measurements.

where y is the electrooptic gain, n is the interaction length, Z, is the signal intensity with the pump beam at steady state, and I,, is the signal intensity without pump beam. Shown in Fig. 3 is the electrooptic gain dependence of the incident intensity for the as-grown and reduced samples, at a grating period of 0.6 pm. For the as-grown, the electrooptic gain is about 3.7 cm- i and independent of the intensity within the range between 100 mW/cm” to 4 W/cm2. IIowever, for the hydrogen-reduced samples, the electrooptic gain is highly de~ndent on intensity. The electrooptic gain is about 0.5 cm-’ at 100 mW/cm’, and increases to about 3.5 cm-” at 4 W/cm’. In addition, the electrooptic gain decreases as the annealing temperature increases.

Shown in Fig. 4 is the electrooptic gain dependence of the grating wavevector at an incident intensity of 500 mW/cm2 for the reduced samples. The solid lines are the simulation fitted with the deep and shallow trap model [14]. It shows that the effective trap density increases as the annealing temperature increases. It is 8.5 X 1Or6 cmw3

Fig. 3. The electrooptic gain dependence on incident writing

intensity of the as-grown sample, and hydrogen-reduced BaTiO,

at annealing temperatures of 625°C and 650°C. respectively. The solid lines are fits with the equation y a (1 f a, /os)- ’ .

J.Y. Chang et al./Optics Communications 138 (19971 101-104 103

for the annealing temperature of 625°C and 11.8 X lOI cmw3 for 650°C.

The experimental setup of the light-induced erasure decay measurement is also shown in Fig. 2. The grating was written inside the crystal at a grating period of 0.6 +rn and with a total intensity of 1 W/cm’. It was monitored by a Bragg-matched 632.8 nm laser beam from a He-Ne laser. The reading beam power was about 200 nW with extraordinary polarization. This reading beam was modu- lated by a chopper connected to a lock-in amplifier, and the diffracted beam was detected by a photomultiplier. The time development of the diffracted beam was digitized and recorded by a personal computer. After the grating was steady, the writing beams were turned off, and the erasure beam was turned on. The diffracted beam intensity showed an exponential decay with time, giving a time constant. Shown in Fig. 5 is the light-induced erasure decay time constant versus the erasure beam intensity. For the as- grown sample, the time constant is followed with the sublinear intensity power law, r a I-“, with a power coef- ficient x of 0.85. The time constant is about 120 s for the intensity of 1 mW/cm* and decreases to 0.22 s for 2.5 W/cm’. After hydrogen-reduction, the time constant de- creases and shows a very different pattern with intensity as the conventional BaTiO, does. It changes very little with intensity from 1 mW/cm2 to 100 mW/cm*, and then decreases with intensity more significantly after 400 mW/cm2. For the sample at the annealing temperature of 650°C the response time is 0.18 s for the intensity of 1 mW/cm’ and 0.05 s for 2.5 W/cm*. Besides, the re- sponse time decreases as the annealing temperature in- creases.

Since there is only a little change of the response time with intensity at low erasure intensity, a dark decay mea-

em-‘)

Y

2.0 , , , , , , , , , , , , , ) ,

_ . H,625C

. H,650C

1.5 -

0.0 1 ’ 4 ’ ’ ’ ’ ’ ’ ’ 1 g 0 2 4 6 8 10 12 14 16

ks (pm-‘)

Fig. 4. The electrooptic gain dependence on grating wavevector of the hydrogen-reduced BaTiO, at annealing temperatures of 625°C

and 650°C.

1 . . l . l * l .

.

A A A AA A 0:

0.1 ’ *A 3

Fig. 5. The light-induced erasure decay time constant dependence

on erasure beam intensity of the as-grown and hydrogen-reduced

BaTiO, at annealing temperatures of 625°C and 650°C.

surement was then performed which has no erasure beam present to determine the dark decay time constant. The experimental setup and condition are the same as those in the light-induced erasure decay measurement shown in Fig. 2, except that there is no erasure beam. After the grating was written and steady, the writing beams were turned off and we let the grating decay in the dark. The reading beam power was still 200 nW and had no measur- able influence on the decay rate as we turned on and off the reading beam. From the diffracted beam intensity dependence with time, the as-grown sample shows two subsequent decays indicating the existence of deep and shallow traps. The first decay drops about 10% within 10 s, and is followed by an exponential decay to zero with a time constant of about 565 s. However, the hydrogen-re- duced sample showed a single exponential decay with a much smaller time constant. For the sample at the anneal- ing temperature of 625°C the time constant is 0.7 s and for that of 650°C is 0.25 s. The time constant can be used to approximate the conductivity by the dielectric relaxation time equation, u = E,,E/~, where u is the conductivity, e. is the permeabillity of vacuum, E is dielectric constant and T is a time constant. The calculated dark conductivity of the as-grown is 2.3 X lo-l4 l/(cm a>, and 1.9 X lo-” l/(cm fin) and 6.6 X lo-” l/(cm fin) for the hydrogen- reduced samples with annealing temperatures of 625°C and 650°C respectively. It increases about three orders of

magnitude after hydrogen-reduction. From the above investigation including beam coupling

gain, light-induced erasure and dark decay measurements, it can be found that the dark conductivity plays an impor- tant role in the photorefractive properties of the hydrogen- reduced BaTiO,. For the as-grown sample, the electrooptic gain is intensity independent, the intensity power coeffi- cient with the response time is 0.85 and the dark conduc-

tivity is low. Apparently it is a type A crystal in which the deep dopant is dominating [ 153. Hydrogen-reduction gener- ates oxygen vacancies [lo] and leave electrons inside crystal, which was transformed from p-type to n-type, as measured from two-beam coupling. Besides, the hydrogen-reduction is so heavy that the dark conductivity is high. High dark conductivity can result in a reduction of the etectrooptic gain according to the equation y a (1 + ad/g)-” [16], where od is the dark conductivity and an is the photoconductivity, and both conductivities can be approximated from the time constants. In Fig. 3, the sohd lines are fits with the equaiion for the hydrogen-redn~ed BaTiO,, in which it shows that the high dark conductivity can result in a highly elcctrooptic gain dependent on intensity. In addition, high dark conductivity also plays an irnpo~~t role in the response time of the light-induced erasure decay measurement. The response time is con- tributed from both the dark conductivity and photoconduc- tivity. At low intensity, the photoconductivity is low and the response time is controlled by the dark conductivity which results in little change of the response time with intensity.

In conclusion, we successfully produced hydrogen-re- duced BaTiO,, and investigated the optical and photore- fractive properties. ~y~o~en”reduction induces an absorp- tion peak around 620 nm. It results in a carrier change from hoie to electron and a high dark conductivity of 6.6 X 10~~” I/(cm &I> at the annealing t~m~eratnre of 650°C. Consequently, the electrooptie gain is highly de- pendent on the writing intensity from 0.5 cm-’ to 3.5 cm-’ for the intensity of 100 mW/cm’ to 2.5 W/cm’. Furthermore, the electrooptic gain decreases as the anneal- ing ternp~~t~~ increases, though the trap density in- creases as the annealing temperature increases for hydro- gen-reduction. The response time decreases by a factor of 700 after hydrogen-reduc~on at an intensity of I mW/cm’, though only by a factos of 4 at 2.5 W/cm’. This can be attributed to high dark conductivity after hydrogen-reduc- tion.

Acknowledgements

This research was supported by the National Science Council of the Republic of China through contract NSC 83-0208-M-008-037.

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