Double-doped KTN:Fe,Ti exhibits increased photorefractive sensitivity compared with single-doped KTN:Fe and KTN:Ti. Absorption and photoconductivity measurements correlate the increase with an increased concentration of Fe in the reduced Fe2+ valence state and to an increased fraction of Fe incorporated into the crystal from the flux

Agranat, Aharon

Leyva, Victor

Yariv, Amnon

English

Caltech Authors - Main

June 15, 1993 / Vol. 18, No. 12 / OPTICS LETTERS
Increased photorefractive sensitivity in double-doped
KTal-_,Nb03:Fe,Ti
Victor Leyva,* Aharon Agranat,t and Amnon Yariv
Department of Applied Physics, California Institute of Technology, Pasadena, California 91125
Received December 28, 1992
Double-doped KTN:Fe,Ti exhibits increased photorefractive sensitivity compared with single-doped KTN:Fe and
KTN:Ti. Absorption and photoconductivity measurements correlate the increase with an increased concentration
of Fe in the reduced Fe21 valence state and to an increased fraction of Fe incorporated into the crystal from the
flux.
Photorefractive materials have received increased
interest as volume holographic storage media.
Material limitations, including low photorefractive
sensitivity, remain to be overcome. In ferroelectric
oxides the photorefractive properties are strongly
dependent on the type, concentration, and valency
of transition-metal impurities,' all of which can
be modified either during or after the crystal
growth stage. Dopants added to the flux during
the growth stage determine the identity and the
total concentration of the photorefractive center.
Postgrowth oxidation/reduction treatments modify
the valence state of dopant ions through the creation
or filling of oxygen vacancies. 2' 3
Both the site occupancy and the valency of dopant
ions can also be modified in combination with
a double dopant. Electron-paramagnetic-resonance
results of Hannon in KTaO3 indicate that, in single-
doped samples, Fe3+ substitutionally occupies the Ta
site. By codoping with Ti, the Fe3+ is biased into the
K site.4 In LiNbO3 waveguides, Ti indiffusion leads
to a reduction of the valence state of Fe impurities
and to a resultant increase in photorefractive
damage.5 In this Letter we describe results in which
double doping of KTal-,Nb.O3 (KTN) with Fe and
Ti leads to an increased photorefractive sensitivity
compared with that of single-doped samples. The
increased sensitivity is a result of an increase of
Fe in the reduced Fe2+ valence state as determined
from absorption measurements. An increase in the
fraction of Fe that enters the crystal from the flux is
also observed.
Three KTN crystals with different dopant com-
binations were grown with the top-seeded solution
growth method.6 The first crystal was doped with
0.5 mol. % of Fe in the flux, the second with 0.5
mol. % of Ti, and the third with 0.25 mol. % of Fe and
0.25 mol. % of Ti. Samples were cut from each crys-
tal and polished along the [001] crystallographic axes.
Metal electrodes were evaporated onto each sample.
The KTN:Fe sample measured 5 mm X 5.7 mm X
3.7 mm, the KTN:Ti sample measured 6.4 mm X
6.7 mm X 2.9 mm, and the KTN:Fe,Ti sample mea-
sured 3.8 mm X 3.8 mm X 5.5 mm. The chemical
composition of each sample was determined with
electron microprobe analysis. The results are sum-
marized in Table 1. The Fe and Ti concentrations
were below the detection limit of 100 parts in 106 by
weight. The Curie temperatures were determined
from peaks in the dielectric constant to occur at T, =
-8CC for the KTN:Fe sample, T, = -15'C for the
KTN:Ti sample, and T, = -28'C for the KTN:Fe,Ti
sample. The variations in the Curie temperatures
are a result of the differing Ta/Nb concentration
ratios.7
The absorption spectrum of each sample was mea-
sured with a double-beam spectrophotometer, and all
the spectra are shown in Fig. 1. The double-doped
sample exhibits the largest absorption. The KTN:Ti
sample has low absorption, which suggests that the
absorption in the KTN:Fe,Ti sample is due primarily
to the Fe. The difference between the absorption in
the KTN:Fe,Ti sample and in the KTN:Fe sample is
shown in Fig. 2. Features in the difference spectra
should be a result of the dopant ions, with a magni-
tude proportional to the difference in concentration
of the absorbing ions. Two features are visible in
the difference spectra. The band centered at 460 nm
(2.70 eV) is identified as that which produces a free
electron by photoexcitation of a Fe21 ion. The width
of the band in 0.66 eV. The Fe+2 charge-transfer
transition is broader and peaked at a lower energy
than that determined for Cu+ in KTN (peak, 3.0
eV; width, 0.38 eV). The other feature visible in
the difference spectra is the edge of an absorption
band visible at 400 nm. This is identified as the
edge of the band that generates a free hole by pho-
toexcitation of the Fe3+ ion. Hole photoconducivity
in LiNbO3:Fe has similarly been observed to occur
near the band edge at energies higher than that for
electron conductivity.8
The magnitude of the band at 460 nm in Fig. 2
is proportional to the difference in Fe2+ concentra-
tion between the samples. Similarly, the magnitude
of the band edge visible at 400 nm is proportional
to the concentration difference of Fe3+. By com-
parison with the total absorption at these wave-
lengths, the KTN:Fe,Ti sample is determined to have
a factor-of-3.7 increased Fe2 + (filled donor) concentra-
tion compared with the KTN:Fe sample. Similarly
the KTN:Fe sample is determined to have a factor-
of-1.3 increased Fe3" (electron trap) concentration
0146-9592/93/120959-03$6.00/0 © 1993 Optical Society of America
959
960 OPTICS LETTERS / Vol. 18, No. 12 / June 15, 1993
Table 1. Material Parameters of the KTN Samples Determined from Absorption
and Photoconductivity Measurements
Sample [Fe2 ] (a.u.) [Fe3 +] (a.u.) aATR [cm/(V s), X10- 10 ] ILTR [cm2/(V s), X10-10 ]
KTao.70Nb0.3 0 03 :Fe,Ti 3.7 1 6.24 4.99KTao68Nb0.32O3 :Fe 1 1.3 1.02 2.72KTao67Nb0 33O3 :Ti - - 0.86 1.72
E
.2 
_. 
___ 
___ 
___
.Q 4- Ti-- 
___--
3 -- 7'.____
2- Fe 
_ _ -
1- (Fei)
0 i....,.==.
350 450 550 650 750 850 950 1050
wavelength (nm)
Fig. 1. Absorption spectra of the KTN:Fe,Ti, KTN:Fe,
and KTN:Ti samples.
compared with the KTN:Fe,Ti sample. The addition
of Ti significantly increases the Fe that enters the
crystal in the reduced Fe2+ state. The effect is even
more pronounced when one considers that the flux
used to grow the KTN:Fe,Ti crystal had half as much
Fe as the flux used to grow the KTN:Fe crystal. In
addition, assuming that Fe enters primarily in the 2+
and 3 + valence states, the fraction of Fe incorporated
into the crystal from the flux is increased by codoping
with Ti. Transition-metal impurities substitution-
ally occupy lattice sites in perovskites. The bias of
Fe into the 2+ valence state when it is codoped
with Ti may be a result of charge compensation of
the lattice. Compensation occurs when a Ti4+ ion
occupies a (Ta, Nb)5 + site and a Fe2+ ion occupies
a K+ site.
The photocurrent of each sample was measured
under 514-nm illumination. Values for the products
aCtALTR and IlTOR determined from the measurements
are given in Table 1, where act is the coefficient for
absorption that results in a free carrier and is propor-
tional to the concentration of absorbing ions, IL is the
electron mobility, and rR is the free-carrier lifetime(recombination time) that is inversely proportional
to the trap concentration.3 The KTN:Fe,Ti sample
exhibits an increased photocurrent compared with
the KTN:Fe and KTN:Ti samples, while the mobility
lifetime products vary little. The results are consis-
tent with the relative Fe2+ and Fe3+ concentrations
determined from the absorption measurements.
Holographic diffraction measurements were made
on each sample in the paraelectric cubic phase, which
required the application of an external field. Diffrac-
tion gratings were written by using two expanded
beams from a 514-nm Ar laser. The beams hadintensities of 20 and 110 mW/cm2 (intensity modula-
tion m = 0.2) and were ordinarily polarized in order
to limit photorefractive wave coupling. A field of
1820 V/cm was. applied to the samples, and the writ-ing beams were adjusted to form a 5.1-gm grating
period. The diffracted intensity of an extraordinar-
ily polarized weak probe beam from a He-Ne laser
aligned at the Bragg angle monitored the strength of
the grating.
For weak coupling the diffraction efficiency 77is given by that from a thick plane holographic
grating9:
= exp(--al)sin2 [ A cos(0/2) (
where a is the total absorption, I is the thickness of
the sample, An is the magnitude of the index grating,
A is the wavelength of the illuminating radiation,
and 0 is the angle between the writing beams. Wedetermined the erase time r by monitoring the expo-
nential decay of An under 110-mW/cm 2 illumination
after one of the writing beams was blocked. The
photorefractive sensitivity, defined as the refractive-
index change per absorbed unit of energy, is given
by' 0
_ dn An
d(aIt) (2Ir
where I is the intensity.
We placed the samples on a thermoelectric mount
and measured the refractive-index change and the
erase rate at several temperatures above each
sample's Curie temperature. Both the diffraction
efficiency and the response time increase as the
Curie temperature is approached. The photorefrac-
tive sensitivity has a much weaker temperaturedependence than do An and r near the phase tran-
sition, which allows for comparisons among samples
with differing transition temperatures. Results for
the temperature dependence of the photorefractive
2-
0.5 - ------
C.,
C. 1- -0__.5--_- .5 - ____________
-1.5 --- --- _ -
i! 0 T450"55'0'650 750 .850 9501050
wavelength (nm)
Fig. 2. Difference in the absorption spectra of the
KTN:Fe,Ti and KTN:Fe samples.
(1)
(2)
June 15, 1993 / Vol. 18, No. 12 / OPTICS LETTERS 961
.5
R5)
0
_I
-15.00 0.00 15.00 30.00
temperature (IC)
Fig. 3. Phoitorefractive sensitivity measurements of the
K-TN:Fe,Ti, KTN:Fe, and KTN:Ti samples above each
sample's Curie temperature (A = 514 nm, A = 5.1 s.im,
E = 1820 V/cm, and m = 0.2).
sensitivity for each sample are given in Fig. 3. The
KTN:Fe,Ti sample exhibits more than a factor-of-3
increased sensitivity compared with the single-doped
samples. The higher sensitivity is due mainly to a
faster response time for the double-doped sample.
The photoconductivity measurements indicate that
the faster response is due to a larger absorption
from the increased concentration of the Fe2+ ions
rather than to an increased free-carrier lifetime. An
increased photorefractive sensitivity is also observed
in samples in which.the photorefractive center is
reduced through the creation of oxygen vacancies.3
The double-dopant approach described here does not
require any postgrowth processing. It can also be
applied to other host crystals, including KNbO3,
LiTaO3, and LiNbO3, in which the effect has been
observed in Ti-indiffused waveguides.5
The photorefractive properties of Fe-doped, Ti-
doped, and Fe- and Ti-double-doped KTN samples
were investigated. Absorption measurements indi-
cate that the double-dopant combination of Fe and
Ti increases the fraction of Fe in the reduced Fe2+
valence state and increases the fraction of Fe that
enters the crystal from the flux compared with
that of single-doped Fe sample. The double-dopant
combination of Fe and Ti leads to a factor-of-3
increased photorefractive sensitivity compared with
single-doped samples.
The authors acknowledge the support of the U.S.
Army Research Office.
*Present address, Accuwave Corporation, 1653
19th Street, Santa Monica, California 90404.
tPresent address, Division of Applied Physics, He-
brew University, Jerusalem 91904, Israel.
References
1. W. Phillips, J. J. Amodei, and D. L. Staebler, RCA
Rev. 33, 94 (1972).
2. W. Phillips and D. L. Staebler, J. Electron. Mater. 3,
610 (1974).
3. V. Leyva, A. Agranat, and A. Yariv, J. Opt. Soc. Am.
B 8, 701 (1991).
4. D. M. Hannon, Phys. Rev. 164, 164 (1967).
5. P. Gericke, P. Hertel, E. Kratzig, J. P. Nisius, and R.
Sommerfeldt, Appl. Phys. B 44, 155 (1987).
6. A. Agranat, V. Leyva, K. Sayano, and A. Yariv, Proc.
Soc. Photo-Opt. Instrum. Eng. 1148, 52 (1989).
7. S. Treibwasser, Phys. Rev. 114, 63 (1959).
8. E. Kratzig and R. Orlowski, Ferroelectrics 27, 241
(1980).
9. H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969).
10. G. C. Valley and M. B. Klein, Opt. Eng. 22, 704 (1983).


Increased photorefractive sensitivity in double-doped KTa1-xNbxO3:Fe,Ti

June 15, 1993 / Vol. 18, No. 12 / OPTICS LETTERS
Increased photorefractive sensitivity in double-doped
KTal-_,Nb03:Fe,Ti
Victor Leyva,* Aharon Agranat,t and Amnon Yariv
Department of Applied Physics, California Institute of Technology, Pasadena, California 91125
Received December 28, 1992
Double-doped KTN:Fe,Ti exhibits increased photorefractive sensitivity compared with single-doped KTN:Fe 
and
KTN:Ti. Absorption and photoconductivity measurements correlate the increase with an increased concentration
of Fe in the reduced Fe
2
1 valence state and to an increased fraction of Fe incorporated into the crystal from the
flux.
Photorefractive materials have received increased
interest as volume holographic storage media.
Material limitations, including low photorefractive
sensitivity, remain to be overcome. In ferroelectric
oxides the photorefractive properties are strongly
dependent on the type, concentration, and valency
of transition-metal impurities,' all of which can
be modified either during or after the crystal
growth stage. Dopants added to the flux during
the growth stage determine the identity and the
total concentration of the photorefractive center.
Postgrowth oxidation/reduction treatments modify
the valence state of dopant ions through the creation
or filling of oxygen vacancies.
2' 3
Both the site occupancy and the valency of dopant
ions can also be modified in combination with
a double dopant. Electron-paramagnetic-resonance
results of Hannon in KTaO3 indicate that, in single-
doped samples, Fe3+ substitutionally occupies the Ta
site. By codoping with Ti, the Fe3+ is biased into the
K site.4 In LiNbO3 waveguides, Ti indiffusion leads
to a reduction of the valence state of Fe impurities
and to a resultant increase in photorefractive
damage.5 In this Letter we describe results in which
double doping of KTal-,Nb.O3 (KTN) with Fe and
Ti leads to an increased photorefractive sensitivity
compared with that of single-doped samples. The
increased sensitivity is a result of an increase of
Fe in the reduced Fe2+ valence state as determined
from absorption measurements. An increase in the
fraction of Fe that enters the crystal from the flux is
also observed.
Three KTN crystals with different dopant com-
binations were grown with the top-seeded solution
growth method.6 The first crystal was doped with
0.5 mol. % of Fe in the flux, the second with 0.5
mol. % of Ti, and the third with 0.25 mol. % of Fe and
0.25 mol. % of Ti. Samples were cut from each crys-
tal and polished along the [001] crystallographic axes.
Metal electrodes were evaporated onto each sample.
The KTN:Fe sample measured 5 mm X 5.7 mm X
3.7 mm, the KTN:Ti sample measured 6.4 mm X
6.7 mm X 2.9 mm, and the KTN:Fe,Ti sample mea-
sured 3.8 mm X 3.8 mm X 5.5 mm. The chemical
composition of each sample was determined with
electron microprobe analysis. The results are sum-
marized in Table 1. The Fe and Ti concentrations
were below the detection limit of 100 parts in 106 by
weight. The Curie temperatures were determined
from peaks in the dielectric constant to occur at T, =
-8CC for the KTN:Fe sample, T, = -15'C for the
KTN:Ti sample, and T, = -28'C for the KTN:Fe,Ti
sample. The variations in the Curie temperatures
are a result of the differing Ta/Nb concentration
ratios.7
The absorption spectrum of each sample was mea-
sured with a double-beam spectrophotometer, and all
the spectra are shown in Fig. 1. The double-doped
sample exhibits the largest absorption. The KTN:Ti
sample has low absorption, which suggests that the
absorption in the KTN:Fe,Ti sample is due primarily
to the Fe. The difference between the absorption in
the KTN:Fe,Ti sample and in the KTN:Fe sample is
shown in Fig. 2. Features in the difference spectra
should be a result of the dopant ions, with a magni-
tude proportional to the difference in concentration
of the absorbing ions. Two features are visible in
the difference spectra. The band centered at 460 nm
(2.70 eV) is identified as that which produces a free
electron by photoexcitation of a Fe21 ion. The width
of the band in 0.66 eV. The Fe+2 charge-transfer
transition is broader and peaked at a lower energy
than that determined for Cu+ in KTN (peak, 3.0
eV; width, 0.38 eV). The other feature visible in
the difference spectra is the edge of an absorption
band visible at 400 nm. This is identified as the
edge of the band that generates a free hole by pho-
toexcitation of the Fe3+ ion. Hole photoconducivity
in LiNbO3:Fe has similarly been observed to occur
near the band edge at energies higher than that for
electron conductivity.8
The magnitude of the band at 460 nm in Fig. 2
is proportional to the difference in Fe2+ concentra-
tion between the samples. Similarly, the magnitude
of the band edge visible at 400 nm is proportional
to the concentration difference of Fe
3+. By com-
parison with the total absorption at these wave-
lengths, the KTN:Fe,Ti sample is determined to have
a factor-of-3.7 increased Fe2 + (filled donor) concentra-
tion compared with the KTN:Fe sample. Similarly
the KTN:Fe sample is determined to have a factor-
of-1.3 increased Fe3" (electron trap) concentration
0146-9592/93/120959-03$6.00/0 © 1993 Optical Society of America
959
960 OPTICS LETTERS / Vol. 18, No. 12 / June 15, 1993
Table 1. Material Parameters of the KTN Samples Determined from Absorption
and Photoconductivity Measurements
Sample [Fe2 ] (a.u.) [Fe3 +] (a.u.) aATR [cm/(V s), X10- 10 ] ILTR [cm2/(V s), X10-10 ]
KTao.70Nb0.3 0 03 :Fe,Ti 3.7 1 6.24 4.99KTao68Nb0.32O3 :Fe 1 1.3 1.02 2.72KTao67Nb0 33O3 :Ti - - 0.86 1.72
E
.2 _. ___ ___ ___
.Q 4- Ti-- ___--
3 -- 7'.____
2- Fe _ _ -
1- (Fei)
0 i....,.==.
350 450 550 650 750 850 950 1050
wavelength (nm)
Fig. 1. Absorption spectra of the KTN:Fe,Ti, KTN:Fe,
and KTN:Ti samples.
compared with the KTN:Fe,Ti sample. The addition
of Ti significantly increases the Fe that enters the
crystal in the reduced Fe2+ state. The effect is even
more pronounced when one considers that the flux
used to grow the KTN:Fe,Ti crystal had half as much
Fe as the flux used to grow the KTN:Fe crystal. In
addition, assuming that Fe enters primarily in the 2+
and 3 + valence states, the fraction of Fe incorporated
into the crystal from the flux is increased by codoping
with Ti. Transition-metal impurities substitution-
ally occupy lattice sites in perovskites. The bias of
Fe into the 2+ valence state when it is codoped
with Ti may be a result of charge compensation of
the lattice. Compensation occurs when a Ti4+ ion
occupies a (Ta, Nb)5 + site and a Fe2+ ion occupies
a K+ site.
The photocurrent of each sample was measured
under 514-nm illumination. Values for the products
aCtALTR and IlTOR determined from the measurements
are given in Table 1, where act is the coefficient for
absorption that results in a free carrier and is propor-
tional to the concentration of absorbing ions, IL is the
electron mobility, and rR is the free-carrier lifetime
(recombination time) that is inversely proportional
to the trap concentration.3 The KTN:Fe,Ti sample
exhibits an increased photocurrent compared with
the KTN:Fe and KTN:Ti samples, while the mobility
lifetime products vary little. The results are consis-
tent with the relative Fe2+ and Fe3+ concentrations
determined from the absorption measurements.
Holographic diffraction measurements were made
on each sample in the paraelectric cubic phase, which
required the application of an external field. Diffrac-
tion gratings were written by using two expanded
beams from a 514-nm Ar laser. The beams had
intensities of 20 and 110 mW/cm2 (intensity modula-
tion m = 0.2) and were ordinarily polarized in order
to limit photorefractive wave coupling. A field of
1820 V/cm was. applied to the samples, and the writ-
ing beams were adjusted to form a 5.1-gm grating
period. The diffracted intensity of an extraordinar-
ily polarized weak probe beam from a He-Ne laser
aligned at the Bragg angle monitored the strength of
the grating.
For weak coupling the diffraction efficiency 77
is given by that from a thick plane holographic
grating9:
= exp(--al)sin2 [ A cos(0/2) (
where a is the total absorption, I is the thickness of
the sample, An is the magnitude of the index grating,
A is the wavelength of the illuminating radiation,
and 0 is the angle between the writing beams. We
determined the erase time r by monitoring the expo-
nential decay of An under 110-mW/cm 2 illumination
after one of the writing beams was blocked. The
photorefractive sensitivity, defined as the refractive-
index change per absorbed unit of energy, is given
by' 0
_ dn An
d(aIt) (2Ir
where I is the intensity.
We placed the samples on a thermoelectric mount
and measured the refractive-index change and the
erase rate at several temperatures above each
sample's Curie temperature. Both the diffraction
efficiency and the response time increase as the
Curie temperature is approached. The photorefrac-
tive sensitivity has a much weaker temperature
dependence than do An and r near the phase tran-
sition, which allows for comparisons among samples
with differing transition temperatures. Results for
the temperature dependence of the photorefractive
2-
0.5 - ------
C.,
C. 1- -0__.5--_- .5 - ____________
-1.5 --- --- _ -
i! 0 T450"55'0'650 750 .850 9501050
wavelength (nm)
Fig. 2. Difference in the absorption spectra of the
KTN:Fe,Ti and KTN:Fe samples.
(1)
(2)
June 15, 1993 / Vol. 18, No. 12 / OPTICS LETTERS 961
.5
R5)
0
_I
-15.00 0.00 15.00 30.00
temperature (IC)
Fig. 3. Phoitorefractive sensitivity measurements of the
K-TN:Fe,Ti, KTN:Fe, and KTN:Ti samples above each
sample's Curie temperature (A = 514 nm, A = 5.1 s.im,
E = 1820 V/cm, and m = 0.2).
sensitivity for each sample are given in Fig. 3. The
KTN:Fe,Ti sample exhibits more than a factor-of-3
increased sensitivity compared with the single-doped
samples. The higher sensitivity is due mainly to a
faster response time for the double-doped sample.
The photoconductivity measurements indicate that
the faster response is due to a larger absorption
from the increased concentration of the Fe2+ ions
rather than to an increased free-carrier lifetime. An
increased photorefractive sensitivity is also observed
in samples in which.the photorefractive center is
reduced through the creation of oxygen vacancies.3
The double-dopant approach described here does not
require any postgrowth processing. It can also be
applied to other host crystals, including KNbO3,
LiTaO3, and LiNbO3, in which the effect has been
observed in Ti-indiffused waveguides.5
The photorefractive properties of Fe-doped, Ti-
doped, and Fe- and Ti-double-doped KTN samples
were investigated. Absorption measurements indi-
cate that the double-dopant combination of Fe and
Ti increases the fraction of Fe in the reduced Fe2+
valence state and increases the fraction of Fe that
enters the crystal from the flux compared with
that of single-doped Fe sample. The double-dopant
combination of Fe and Ti leads to a factor-of-3
increased photorefractive sensitivity compared with
single-doped samples.
The authors acknowledge the support of the U.S.
Army Research Office.
*Present address, Accuwave Corporation, 1653
19th Street, Santa Monica, California 90404.
tPresent address, Division of Applied Physics, He-
brew University, Jerusalem 91904, Israel.
References
1. W. Phillips, J. J. Amodei, and D. L. Staebler, RCA
Rev. 33, 94 (1972).
2. W. Phillips and D. L. Staebler, J. Electron. Mater. 3,
610 (1974).
3. V. Leyva, A. Agranat, and A. Yariv, J. Opt. Soc. Am.
B 8, 701 (1991).
4. D. M. Hannon, Phys. Rev. 164, 164 (1967).
5. P. Gericke, P. Hertel, E. Kratzig, J. P. Nisius, and R.
Sommerfeldt, Appl. Phys. B 44, 155 (1987).
6. A. Agranat, V. Leyva, K. Sayano, and A. Yariv, Proc.
Soc. Photo-Opt. Instrum. Eng. 1148, 52 (1989).
7. S. Treibwasser, Phys. Rev. 114, 63 (1959).
8. E. Kratzig and R. Orlowski, Ferroelectrics 27, 241
(1980).
9. H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969).
10. G. C. Valley and M. B. Klein, Opt. Eng. 22, 704 (1983).


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Increased photorefractive sensitivity in double-doped KTa1-xNbxO3:Fe,Ti

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