Sr0.59Bao.41Nb2O6 and Sro.75Bao.25Nb2O6 exhibit an intensity-dependent absorption in the visible spectral range. We perform pump (green)-probe (red) measurements and find variation as high as 0.45 cm-' in the absorption coefficient, which depends significantly on the polarization of the probe light beam. Our results indicate a nonexponential dark decay time of the induced absorption and suggest that the trapping potential of the shallow traps varies significantly from one trap to another (multiple isolated shallow traps)

Neurgaonkar, Ratnakar R.

Orlov, Sergei

Segev, Mordechai

Yariv, Amnon

English

Caltech Authors - Main

September 1, 1994 / Vol. 19, No. 17 / OPTICS LETTERS 1293
Light-induced absorption in photorefractive
strontium barium niobate
Sergei Orlov, Mordechai Segev,* Amnon Yariv, and Ratnakar R. Neurgaonkart
California Institute of Technology, 128-95, Pasadena, California 91125
Received February 10, 1994
Sr0.59Bao.41Nb2O6 and Sro.75Bao.25 Nb2O6 exhibit an intensity-dependent absorption in the visible spectral range.
We perform pump (green)-probe (red) measurements and find variation as high as 0.45 cm-' in the absorption
coefficient, which depends significantly on the polarization of the probe light beam. Our results indicate a
nonexponential dark decay time of the induced absorption and suggest that the trapping potential of the shallow
traps varies significantly from one trap to another (multiple isolated shallow traps).
Photorefractive (PR) strontium barium niobate (SBN)
has been the subject of extensive research in re-
cent years. Large PR nonlinearities and relatively
fast response times at low power levels' make this
material a preferred candidate for a variety of appli-
cations, such as optical data storage. Earlier mod-
els of the PR effect2 assume that charges are given
off and trapped by donor and acceptor levels corre-
sponding to different ionization states (for example,
Fe2 ' and Fe3+) of the same atom and consider only
a single level of each type. Numerous phenomena,
such as intensity-dependent absorption,3'4 the nonlin-
ear dependence of the PR response time on the light
intensities,5 and the intensity dependence of the PR
response, can be understood only in the framework
of a more-complicated model4 -6 that involves mul-
tiple donor levels and additional shallow traps.
Intensity-dependent absorption and related phe-
nomena were discovered and investigated extensively
in different PR crystals and at different optical light
intensities in the range of 1 mW/cm2 to 108 W/cm2 .
A recent study7 of this effect examined pulsed record-
ing in nominally undoped and Fe-doped SBN:60. In
this Letter we investigate light-induced absorption
through pump-probe measurements at relatively low
optical power densities in PR SBN:60 and SBN:75.
We employ high-optical-quality 6 mm X 6 mm x
6 mm crystal samples of Cr-doped SBN:60 and Ce-
doped SBN:75. In our experimental setup (Fig. 1)
we used a polarized expanded beam (A = 488 nm)
of an argon-ion laser as a pump. We measured the
light-induced changes in the absorption coefficient by
monitoring the transmitted power of a weak, loosely
focused probe beam (A = 633 nm) from a low-power
(=1-ttW) He-Ne laser. The polarization of both
beams is controlled independently by appropriate
half-wave plates. The incident power of the pump
beam is controlled by a variable attenuator and is
measured with a power meter in the beam path.
The pump beam is expanded by a cylindrical lens
followed by a spherical lens, which results in a uni-
form elliptical spot of an area of 0.1 cm2 inside the
crystal. The probe beam occupies a much smaller
area and is enclosed entirely by the pump inside
the crystal. The SBN sample is placed on a bulk
copper plate to minimize potential heating by the
intense pump laser light. Our measurements are
taken in a single-pulse regime, i.e., the pump beam
is blocked by an electromechanical shutter (with a
response time of -50 /s) and is switched on only
for short periods of time (0.2 to 20 s). The power of
the transmitted probe is monitored and recorded by
a storage oscilloscope.
The light-induced absorption coefficient a, is
defined4 by
Is (with IP on)
Is(with Ip off) = exp(- aid), (1)
where Is and Ip are the probe and the pump intensi-
ties at the crystal output, respectively, and d is the
length of interaction (crystal thickness). For evalua-
tion of some important material parameters, such as
the density of the shallow traps in the crystals and
the dark ionization rate, from our experimental data,
several significant (though commonly used for ferro-
electric PR materials) assumptions need to be made.
First, we assumes that all the shallow traps have
the same thermal ionization rate ,82. Second, we as-
sume that the density of the free electrons reaches
its steady state much faster than does the concentra-
tion of charged impurities (adiabatic approximation).
Finally, we assume that at low light intensities (i.e.,
S2I << «P2, where s2 is the photoionization cross sec-
tion for occupied shallow traps) most of the shallow
HeNe
Laser
Mechanical L
1/2 Shutter
Aperture q1
Oscillo- A
scope D
Fig. 1. Experimental setup: L's, lenses; A/2's, half-
wave plates; PRC, photorefractive crystal; D, photode-
tector.
0146-9592/94/171293-03$6.00/0 © 1994 Optical Society of America
1294 OPTICS LETTERS / Vol. 19, No. 17 / September 1, 1994
0.2s
E
0.2
U 0.15
U 0.1
8 0.05
0In 5 10 15 20
Pump Light Intensity, W/cmZ
Fig. 2. Light-induced absorption coefficient at A =
633 nm versus the intensity of the pump beam
(A = 488 nm) in SBN:60:Cr. R (red) and G (green)
denote the polarizations of the probe and the pump,
respectively. The linear absorption of the probe beam
is all = 0.4 cm-' for extraordinary polarization and
a 1 = 0.7 cm-' for ordinary polarization.
0.8
E 0.6
0.4
I .
'S o. Z
0 5 10 15
Pump Light Intensity, W/cm2
20
Fig. 3. Buildup rate of the induced absorption at A =
633 nm versus the intensity of the pump beam (A =
488 nm) in SBN:75:Cr.
traps are ionized, their density remains constant, and
the density of the free electrons in the conduction
band n is dominated primarily by the photoionization
and the recombination involving the primary deep PR
centers (although n is much smaller than the den-
sity of the PR impurities). At intensities far below
the saturation of the light-induced absorption effects
(s 21 << /2), the rate at which the population of the
shallow traps, and consequently the light-induced ab-
sorption coefficient, relaxes to a given equilibrium
state is given by9
1-= /32 + 821 ± Y~n
-I
probe beam may be given if we account for the nature
of the material defects that act as the shallow traps.
If we assume that these traps are primarily struc-
tural defects (e.g., vacancies) rather than impurities
and that mutual interaction between them is negli-
gible, the traps and the associated wave functions
possess a crystal-imparted symmetry, leading to
strong polarization effects. Intuitively, one may
view the shallow traps as isolated potential wells
with their associated wave functions localized differ-
ently along the a and c axes. This may result in
a larger dipole moment (and, consequently, a larger
photoionization cross section) for the light polar-
ized along the c axis. This explanation, if correct,
may also provide useful information about the actual
atoms whose absence leads to the shallow traps.
Next we investigate the buildup rate of the induced
absorption and define it as the inverse of the time
required for the signal to reach the (1 - le) level
of its saturation value. Our results are presented
in Fig. 3. The buildup rate curves are significantly
nonlinear with respect to the pump intensity with
a pronounced saturation at high pump intensities.
Notice that for both polarizations of the probe we
observe shorter response times for the extraordi-
narily polarized pump, a result of the higher
absorption of the extraordinarily polarized light
(all = 2.0 cm-' at A = 488 nm) than of the ordinar-
ily polarized one (a, = 1.5 cm-' at A = 488 nm).
The linear behavior at the low-intensity branch of
curves permits [Eq. (2)] an estimate of the effec-
tive dark decay rate /32 of the population of the
shallow traps as a result of the thermal ionization:
A2 = 30 ± 5 s-1. This value is very close to sim-
E
0.4
2 0.3
U
<0
X 0.2
to1
.~ 0(2)
where Y2 is the recombination constant (for electrons)
on the shallow traps. Note that at low intensities n
is proportional to 1.
We now measure both the light-induced absorption
coefficient a1 and its response time r. First, we in-
vestigate the absorption of a weak red He-Ne probe
induced by a strong green pump in a SBN:60:Cr
crystal sample for the different polarizations of both
the pump and the probe beams. The steady-state
values of a1 are shown in Fig. 2. The light-induced
absorption coefficient saturates at high pump in-
tensities and appears to be fairly insensitive to the
polarization of the pump (488-nm) beam. The de-
pendence of a, on the probe-beam polarization is
much more dramatic. The data in Fig. 2 suggest
that, for a given pump intensity and polarization,
ac(extraordinary)/aj(ordinary) = 1.95 ± 0.10 (see
also Ref. 10). A possible explanation of this strong
asymmetry with respect to the polarization of the
J 5 10 15
Pump Light Intensity, W/cm2
20
Fig. 4. Light-induced absorption coefficient at A =
633 nm versus the intensity of the pump beam (A =
488 nm) in SBN:75:Ce. The probe beam is ordinarily
polarized in both cases, and its linear absorption
coefficient is 0.6 cm-l.
0.3
0.25
E 0.2
6
t 0.1
c3 0.05
0
0 5 10 15
Pump Light Intensity, W/cm2
20
Fig. 5. Buildup rate of the induced absorption at A =
633 nm versus the intensity of the pump beam (A =
488 nm) in SBN:75:Ce. The probe beam is ordinarily
polarized in both cases.
SBN-60: Cr o 0 e a a
Ops
* i i W , ji i i d IIGlc RM c0 =;E i j * Gtic, Ric
,.,1 W 0 GlIc, Ric
o Gic, RIec
* GlIc, RI~c
SBN-60: Cr
* 0 0 0 Gc.0|: 0 ° C El3 Gic, RIlc
jO C * Gllc, Rle
0 Glic, RIIc
I I R
SBN-75:Ce X X *
o Gic, Ric
* GlIc, Ric
- SBN-75:Ce 0000
0
00 *e
0 *-
OOZ O Gic, Ric 0.
* 0 GS~c, Rico
o
1.
September 1, 1994 / Vol. 19, No. 17 / OPTICS LETTERS 1295
Fig. 6. Nonexponential dark decay of the induced ab-
sorption in SBN:75:Ce, after an initial 200-ms pump
light pulse. The vertical axis corresponds to the induced
absorption (0.025 cm-'/division), and the horizontal scale
is 5 s/division.
ilar data obtained in the BaTiO3 crystals.4 How-
ever, even at moderate intensities (>5 W/cm2) the
response time is approximately 1.5 to 2 ins, which
is significantly shorter than both the PR and the
thermal response times for SBN crystals." This ob-
servation raises the possibility of fast recording of
photochromic gratings by light-induced absorption.
We used the same setup to investigate the light-
induced absorption in a Ce-doped SBN:75 sam-
ple. In this crystal strong PR coupling is present
for extraordinarily polarized light. Therefore the
light-induced absorption measurements are per-
formed only with ordinary polarization of the probe
beam. Our experimental data, which are shown in
Figs. 4 and 5, indicate that even low pump inten-
sities (<5 W/cm2 ) induce a large increase in the
absorption at the probe wavelength. The linear
absorption of the crystal at the pump wavelength
(A = 488 nm) is nearly equal for the extraordinary
(al = 4.7 cm-') and the ordinary (a, = 4.8 cm-') po-
larizations. The pump intensity at which the light-
induced absorption coefficient exhibits saturation
appears to be much smaller for SBN:75. This may
explain the intensity-dependent diffraction efficiency
and intensity-dependent wave coupling observed in
some SBN:75 PR crystals. It should be noted that
the dark decay (shown in Fig. 6) and the buildup of
the light-induced absorption at low light intensities
(below saturation, S2I << /2) both exhibit a nonex-
ponential temporal behavior. This phenomenon is
attributed' 2 to the fact that the trapping potential of
the shallow traps may differ significantly from one
isolated trap to another (inhomogeneous broaden-
ing) rather than having a single energy value. This,
in turn, implies that the effective relaxation time
depends on the initial distribution of the occupied
shallow traps and that the decay toward equilib-
rium obeys a logarithmic, rather than exponential,
law within a certain time window.'3 Our obser-
vations of the dark decay indicate that the relax-
ation time for the light-induced absorption in SBN:75
may be as long as 3.0 to 5.0 s. This indicates that
in this material the secondary traps are relatively
deep. However, even at moderate pump intensities
(-10 W/cm2) the buildup of the absorption is very
fast (<5 ms).
Finally, we obtained a lower estimate for the den-
sity of the shallow levels for the crystals that we stud-
ied by using the absorption cross section evaluated4
for BaTiO3: S2 = 5 X 10-8 cm2 . The lower limit for
the density of the secondary traps Nsh is given by
Nh a 1 (max)/s 2 = 5 x 1016 to 5 x 1017 cm-3. The
actual value may be at least 2-10 times larger, since
the approximate formula that we use assumes that
all the traps are occupied.
In conclusion, we have presented experimental
evidence for light-induced, polarization-dependent
absorption in PR SBN:60 and SBN:75. Large
changes in the crystal absorption can be induced even
at moderate cw light intensities in the visible range.
This fact has a significant effect on applications, such
as optical data storage, for which SBN is a poten-
tial candidate.
This research was sponsored by the U.S. Army Re-
search Projects Agency.
*Present address, Department of Electrical Engi-
neering, and Advanced Technology Center for Pho-
tonics and Optoelectronic Materials and Princeton
Material Institute, Princeton University, Princeton,
New Jersey 08544.
tPermanent address, Rockwell International Sci-
ence Center, 1049 Camino dos Rios, Thousand Oaks,
California 91360.
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Light-induced absorption in photorefractive strontium barium niobate

After this Letter was submitted, a similar observation that indicated an identical ratio between light-induced absorption coefficients of extraordinarily and ordinarily polarized light in SBN:60:Ce was reported by

Bluelight-induced infrared absorption in KNbO3 &quot;

Phys. Status Solidi A 113,

This assumption is equivalent to assuming that the shallow traps occupy a single level and is valid for SBN:60 but, as shown below, is not valid for SBN:75.

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Light-induced absorption in photorefractive strontium barium niobate

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