We present what is to our knowledge the first report on the photorefractive properties of Rh-doped Sr0.6Ba0.4Nb2O6 and experimental results showing a reduction of the photorefractive two-beam coupling response time by more than an order of magnitude with an external dc field of 10 kV/cm

Neurgaonkar, Ratnakar R.

Sayano, Koichi

Yariv, Amnon

English

Caltech Authors - Main

January 1, 1990 / Vol. 15, No. 1 / OPTICS LETTERS 9
Order-of-magnitude reduction of the photorefractive response
time in rhodium-doped Sr0 *6Ba0 *4Nb206 with a dc electric field
Koichi Sayano and Amnon Yariv
California Institute of Technology, Department of Applied Physics, 128-95, Pasadena, California 91125
Ratnakar R. Neurgaonkar
Rockwell International Science Center, P.O. Box 1085, Thousand Oaks, California 91360
Received June 26, 1989; accepted October 26, 1989
We present what is to our knowledge the first report on the photorefractive properties of Rh-doped Sr 0o 6Ba0 .4Nb 2 06
and experimental results showing a reduction of the photorefractive two-beam coupling response time by more than
an order of magnitude with an external dc field of 10 kV/cm.
Doped SrU.6BaO.4Nb2 0 6 (SBN:60) using Fe, Ce, Ce-Ca,
and Cr has been shown to be photorefractive with high
gain and real-time response.1-3 Cr-doped SBN:60
crystals have been shown to have response times of
near 0.2 sec at 0.25 W/cm2 , which is approximately an
order of magnitude faster than what was measured in
Ce-doped samples, but their photorefractive gain
ranged from 3 to 6 cm-1 compared with more than 10
cm-1 in Ce-doped SBN:60. Applying an electric field
enabled the gain of Cr-doped SBN:60 to approach 10
cm-', but its response time increased to over 1 sec. In
this Letter, we present the first report to our knowl-
edge on the photorefractive properties of Rh-doped
SBN:60 and, in addition, results showing significant
reduction of the photorefractive response time of this
material by applying a dc electric field of up to 10 kV/
cm in the direction of the electro-optic axis.
The crystal was grown by the Czochralski method,
poled by cooling it through its cubic-to-ferroelectric
phase-transition temperature of approximately 750C
with an applied electric field of 8 kV/cm, and cut and
polished to an optical-quality cube measuring 6.5 mm
X 6 mm X 5.5 mm. The crystal was doped with ap-
proximately 1.5% by weight of Rh in the flux. When
exposed to coherent light of 514.5 nm, strong beam
fanning was observed, indicating high two-beam cou-
pling gain, which resulted in amplification of scattered
light.
The two-beam coupling experiment illustrated in
Fig. 1 has been used to characterize the photorefrac-
tive properties of various materials and to develop a
new generation of phase-conjugate and image-pro-
cessing applications.4 7 A weak signal beam with in-
tensity I2(0) is mixed with a strong pump beam I1(O)
coherent to the signal beam in the photorefractive
medium, which sets up a periodic space-charge grating
that follows the light interference pattern from the
drift and diffusion of charges. The space-charge grat-
ing is converted to a 900 phase-shifted index grating
from the electro-optic effect, which results in the pho-
torefractive two-beam coupling amplification of I2 in
the direction of the electro-optic or c axis. This can be
characterized by
Il(z) = I(O)exp[-(r + a)Z,
I 2(z) = I 2(0)exp[(r - am, (1)
where a is the absorption coefficient and r is the two-
beam coupling constant, which is given by
r = Re( 1 no3reffEsc)
c 03 ffmE E0 + iEd
=4c oefmNE 0 + i(Ed + EN) [1 - exp(-t/T)]
(2)
for the one-carrier, one-species case of electrons being
the dominant charge carrier and only one donor type
being present. The response time of the material is
given by
EO + i(Ed + E,1 )
EO + i(Ed + EN) (3)
M
M
Fig. 1. Experimental configuration of the two-beam cou-
pling experiment. I,(l) and I2(1) were measured simulta-
neously and in real time to obtain the photorefractive cou-
pling constant and response time. The beam polarization
was in the same plane as the c axis, with Il(0) + I2(0) - 0.25
W/cm2 and m < 0.01. BS, beam splitter; M's, mirrors.
0146-9592/90/010009-03$2.00/0 © 1990 Optical Society of America
10 OPTICS LETTERS / Vol. 15, No. 1 / January 1, 1990
E0
9 (gm)
Fig. 2. Photorefractive gain coeffi
SBN:60 as a function of grating wavelen
peak at approximately 1.2 5 Am indicateE
2 X 1022 m 3.
6
5
4
C3
o~3
k-
2
1
_0
Fig. 3.
field.
0
0
0 0
1 2 3
_ 
1 2 3
g (tR m)
Response timer of Rh-doped S.
where
the electron charge, kB is Boltzmann's constant, NA is
the trap density, ND is the donor density, and s is the
photoionization cross section.
The photorefractive gain and response time of the
Rh-doped SBN:60 crystal are shown in Figs. 2 and 3
for various grating periods and no applied electric
fields. The 514.5-nm line of an argon laser was used,
El polarized in the plane of the c axis to take advantage of
the large r 33 electro-optic coefficient. I1(O) was 0.25
W/cm 2 , with m = I2(0)1I1(O) < 0.01 to avoid pump
depletion. Both beam intensities Il(l) and I2(1) were
measured simultaneously in real time to obtain r as a
function of time and r. As shown in the plots, the
4 5 6 maximum gain of the Rh-doped crystal is 10.5 cm-' at
its optimum grating wavelength of 1.25 itm, which is
comparable with that of Ce-doped SBN:60. By differ-
cient of Rh-doped entiating Eq. (2) with respect to Xg and using the value
igth for E0 =0 . The for ;g, max obtained from Fig. 2, the trap density was
a trap density NA of determined to be approximately 2 X 1022 m- 3 . The
exponential time constant of the crystal at its opti-
mum grating period was approximately 2 sec for a total
beam intensity of 0.25 W/cm 2 .
When an external dc field is applied, we can see from
Eq. (3) that in the limit of large E0, r approaches to,
which is the time required to generate NA photoexcita-
tions per unit volume from energy deposition consid-
erations.8 In the EA, > EN case, T approaches its limit
o from above as the dc field is increased. For typical
values of NA = 2 X 1022 m- 3 , f = 570Eo, T = 295 K, Xg =
1.51 ,um, and y/,u = 2 X 10-10 V-m, EN = 1.55 kV/cm,
Ed = 1.06 kV/cm, and E, = 10 kV/cm.9 Figure 4 shows
the expected behavior of r/to as a function of Eo under
these conditions.
The experimental results of the gain and response
time of Rh-doped SBN:60 with an applied electric
4 5 6 field are shown in Figs. 5 and 6 for Xg = 1.51 and 0.84
Am. The electric field, which varied from 0 to 10 kV/
;BN:60 for no applied cm, was applied in the direction of the c axis since high
*N:6 for no applied reverse voltages would tend to reduce the polarization
and ultimately depole the crystal. Fields significantly
beyond 10 kV/cm were not attempted owing to consid-
erable arcing across the surface of the crystal. For
to = hs NA
°sIO(ND -NA) (4)
The imaginary part of the complex r indicates an oscil-
latory factor in r in addition to the exponential decay.
In Eqs. (2) and (3), EO is the externally applied dc
electric field, m is the modulation index, and the char-
acteristic fields are given by
EN= eNA (1 _NA) NAforNA<<ND,
kBTK
Ed =
E yNA
E, = (5)
where K = 27r/Ag, is the wave number corresponding to
the grating period, y is the electron recombination
rate, y is the electron mobility, e is the magnitude of
Fig. 4. Expected response time as a function of applied
field for Ed = 1.06 kV/cm, EN = 1.55 kV/cm, and E, = 10 kV/
cm, showing n/to approaching unity for large E0 .
1Z-
10- O °
8
0
6 -
4 -
2-
0 l I I
o 1 2 3
n
5
0
4 6
Eo (kV/cm)
January 1, 1990 / Vol. 15, No. 1 / OPTICS LETTERS 11
12;
10 5
0 L0
0 0 O0 0
2 4 6
E0 (kV/cm)
8 10
Fig. 5. Measured gain coefficient as a function of E0 for kg
= 1.51 Asm (circles) and 0.84,um (squares).
3.0
2.5
1.5 I-
E
C1
0
1.0 I-
0.5 H
0.0
0
9
2 4 6
E0 (kV / cm)
8 10
Fig. 6. Experimental response time of the SBN crystal for
kg = 1.51 Am (circles) and 0.84 ,m (squares), which de-
creased from 2 sec atE 0 = 0 to approximately 0.1 sec for Eo =
10 kV/cm.
both grating periods, the gain drops from r = 10 cm-'
at E0 = 0 to near 8 cm- 1 at Eo = 10 kV/cm. Although
Eq. (2) predicts that r increases with E0, we can attri-
bute the small decline in this case to causes such as
fanning losses from the large gain and increased scat-
tering due to the electric field.
The advantage of this procedure, however, is in the
more than an order-of-magnitude reduction of the re-
sponse time, which was observed to decrease from 2.0
sec at E0 = 0 to approximately 0.1 sec at E0 = 10 kV/
cm. Strong beam fanning was also observed to set in
faster as the time constant was reduced through in-
creased E0, making this material especially suited for
high-gain, fast-response applications such as field-en-
hanced optical limiters and real-time image process-
ing. According to the theoretical model, the photoex-
cited charges are transported through drift and diffu-
sion in the conduction band. The presence of the dc
field in the background would tend to drive the free
electrons into the traps, contributing to the faster
grating formation that was observed.
In summary, we have demonstrated a significant
improvement in the response time of Rh-doped
SBN:60 through the application of an external dc
field. A reduction in n by a factor of 20 with approxi-
mately a 20% loss in gain has been observed. This
gives a significant advantage in the development of
photorefractive devices that rely on real-time response
and introduces another element of control of the pho-
torefractive effect.
This research has been supported by grants from
the U.S. Air Force Office of Scientific Research and
the U.S. Army Research Office. Koichi Sayano also
acknowledges partial support from a U.S. Air Force
Weapons Laboratory Graduate Fellowship.
References
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Neurgaonkar, Appl. Phys. Lett. 53,1465 (1988).
3. K. Sayano, A. Yariv, and R. R. Neurgaonkar, Appl. Phys.
Lett. 55, 328 (1989).
4. K. Sayano, A. Yariv, and R. R. Neurgaonkar, J. Appl.
Phys. (to be published, 1990).
5. N. V. Kukhtarev, V. B. Markov, and S. G. Odulov, Opt.
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Order-of-magnitude reduction of the photorefractive response time in rhodium-doped Sr0.6Ba0.4Nb2O6 with a dc electric field

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Order-of-magnitude reduction of the photorefractive response time in rhodium-doped Sr0.6Ba0.4Nb2O6 with a dc electric field

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