The dynamic range, sensitivity, and dark decay of holographic recording of wavelength 458nm in LiNbO3 crystals doped with 0.2-at.% Mn with different oxidation states have been measured. The measured sensitivity is 0.5 cm/J and is found to be independent of the oxidation state, and the largest M/# obtained is 12/mm (extraordinary light polarization; light wavelength, 458nm). This combination of very large M/# and high sensitivity is in strong contrast with results for LiNbO3:Fe for which a direct trade-off exists between M/# and sensitivity. The activation energy of the dark decay of holograms stored in these LiNbO3:Mn crystals is 1.0 eV , which is characteristic of proton compensation and leads to a projected lifetime of holograms of three years at room temperature

Buse, Karsten

Psaltis, Demetri

Yang, Yunping

English

Caltech Authors - Main

158 OPTICS LETTERS / Vol. 27, No. 3 / February 1, 2002Photorefractive recording in LiNbO3:Mn
Yunping Yang
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125
Karsten Buse
Physikalisches Institut, Universität Bonn, Wegelerstrasse 8, D-53115 Bonn, Germany
Demetri Psaltis
Department of Electrical Engineering, California Institue of Technology, Pasadena, California 91125Received October 3, 2001
The dynamic range, sensitivity, and dark decay of holographic recording of wavelength 458 nm in LiNbO3
crystals doped with 0.2-at. % Mn with different oxidation states have been measured. The measured sensitiv-
ity is 0.5 cmJ and is found to be independent of the oxidation state, and the largest M# obtained is 12mm
(extraordinary light polarization; light wavelength, 458 nm). This combination of very large M# and high
sensitivity is in strong contrast with results for LiNbO3:Fe for which a direct trade-off exists between M#
and sensitivity. The activation energy of the dark decay of holograms stored in these LiNbO3:Mn crystals
is 1.0 eV, which is characteristic of proton compensation and leads to a projected lifetime of holograms of
three years at room temperature. © 2002 Optical Society of America
OCIS codes: 090.2900, 090.7330, 160.3730, 210.0210, 210.4810.Photorefractive LiNbO3 crystals have been of intense
interest for applications such as holographic data stor-
age1 and narrow-band wavelength filters for optical
telecommunications.2 – 4 Two of the most important
properties of LiNbO3 crystals are dopant and doping
level. Usually, transition-metal dopants, such as Fe,
Cu, Ce, and Mn, are added to the melt as oxides to
improve the photorefractive effect. Among all kinds
of dopants, Fe has been investigated extensively,
whereas Mn has been less popular and its role in
LiNbO3 is not understood completely. It is known
that in LiNbO3 crystals the Mn center is deeper than
the Fe center, and doubly doped LiNbO3:Fe:Mn crys-
tals have been used to achieve nonvolatile holographic
storage.5
We performed holographic experiments with a
LiNbO3 crystal doped with 0.2-at. % Mn, with dimen-
sions of 1.0 mm 3 4.0 mm 3 4.5 mm. The c axis of
the crystal is parallel to its longest direction. The
crystal was proton reduced by annealing treatment.
In our experiments, in which we measured M#
(Ref. 6) and sensitivity, an argon-ion laser beam with
wavelength 458 nm was used to record and erase holo-
grams. The crystal was placed on a rotation stage.
The laser beam was split into two equal-intensity ex-
traordinarily polarized beams that were expanded to
cover the whole crystal during recording and erasure.
The intensity of each beam was 10 mWcm2, and
the grating vector was aligned along the c axis with a
period of 1.1 mm. During recording, we blocked one
beam from time to time to measure the holographic
diffraction efficiency. We used Bragg-mismatched
erasure; i.e., during erasure the sample was rotated
away from the Bragg-matched position and illumi-
nated by the same two beams that were used to record
holograms. To avoid building another strong holo-
gram, we rotated the sample 0.02± every 10 s during0146-9592/02/030158-03$15.00/0erasure. At the end of each period of erasure, we
measured the diffraction efficiency by scanning over
some range of angles that covered the Bragg-matched
position with only the reference beam on. A typical
recording and erasure curve for one of the Mn-doped
crystals is shown in Fig. 1. From single-hologram
recording and erasure, we obtain a sensitivity of
0.5 cmJ and M# of 6.5mm. Here the sensitivity
is defined as S  dphdtjt0IL, and M# is
defined as M#  dphdtjt0te, where h is the
diffraction eff iciency calculated as IdiffractedIincident, L
is the thickness of the crystal, I is the total intensity
of the recording beams, and te is the erasure-time
constant.
To verify that such a high M# is achievable for
practical applications, we multiplexed 100 holograms
(extraordinary light polarization; light wavelength,
Fig. 1. Typical recording and erasure curve for
1-mm-thick LiNbO3:0.2-at.% Mn.© 2002 Optical Society of America
February 1, 2002 / Vol. 27, No. 3 / OPTICS LETTERS 159458 nm). Figure 2 shows the comb function of the 100
multiplexed holograms. We used a precalculated ex-
posure schedule to equalize the diffraction eff iciency.
The angle between two neighboring holograms is 0.4±.
The M# obtained from this multiplexing, which was
calculated with M# 
P100
n1
p
hn, is 5.0mm, where
hn is the diffraction eff iciency of the nth hologram.
The loss of some M# is due to the nonideal exposure
schedule.6 The larger diffraction eff iciencies around
the center of the comb function are due to some back-
ref lection. The M#’s obtained from single-hologram
recording and multiplexing agree well.
One can change the oxidation state of LiNbO3 crys-
tals by annealing at an elevated temperature in the
appropriate atmosphere, typically oxygen for oxidation
and argon for reduction. Typically, M# and sensi-
tivity are strong functions of the oxidation state. We
measured the M# and the sensitivity of the Mn-doped
crystal with different oxidation states. Table 1 con-
tains information about the thermal treatment for each
oxidation state. Since the absorption band of the Mn
center is very wide, it is hard to determine the ratio of
CMn21CMn31 quantitatively. The measured M# and
sensitivities for different oxidation states are shown in
Fig. 3. The sensitivity is 0.5 cmJ and is almost inde-
pendent of the oxidation state, whereas the M# drops
by a factor of 15 from the highly oxidized to the highly
reduced state. This independence of sensitivity from
the oxidation state the LiNbO3:Mn is in strong contrast
with LiNbO3:Fe and is good for holographic applica-
tions. The highest M# 12mm was obtained for the
highly oxidized state.
We measured the dark decay by placing the crys-
tals on a heatable plate whose temperature was
controlled to within 0.1 ±C accuracy. An argon-ion
laser beam with wavelength 514 nm was used to
record holograms. We performed the dark decay
experiments with the Mn-doped crystal described
above and also with an Fe-doped crystal (0.25-wt. %
Fe2O3) for comparison. The laser beam was split into
two equal-intensity extraordinarily polarized beams
that were expanded to cover the whole crystal during
recording. Recorded holograms had a grating period
of 1.3 mm and were written with the grating vector
oriented along the c axis. Recording was performed
at room temperature. Afterward, the crystals were
heated to a certain temperature in the dark, and a
weak laser beam of 514 nm was used to monitor the
holographic diffraction eff iciency. The weak readout
light illuminated the crystal only from time to time,
and the intervals between two measurements were
long enough to keep the erasure of the holograms by
the probing beam negligible. After each experiment
the crystal was heated to 230 ±C and kept at this
temperature under uniform illumination for 45 min
to erase the gratings completely. The results are
shown in Fig. 4. One can see that the dark decay
time constants in both samples, the Mn- and the
Fe-doped crystals, obey an Arrhenius-type depen-
dence on the absolute temperature, T , but with two
different activation energies, 1.0 and 0.28 eV. The
different activation energies explicitly indicate two
distinct dominant dark decay mechanisms, whichhave been identified as proton compensation7,8 and
electron tunneling,9 – 11 respectively. The projected
lifetime of holograms in the Mn-doped crystal at room
temperature is approximately three years.
For holographic recording in congruently melting
LiNbO3 by use of transmission geometry, the bulk
photovoltaic current is dominant. The saturation
Fig. 2. Comb function of multiplexing 100 holograms in
LiNbO3:0.2-at.% Mn. The M# calculated from this comb
function is 5.
Table 1. Summary of Oxidation States of the
Mn-Doped LiNbO3
Oxidation
State Thermal Treatment
1 Highly oxidized
Starting with state 6, in oxygen at
930±C for 24 h
2 Starting with state 1, in argon at 780 ±C for 1 h
3 Starting with state 2, in argon at 780 ±C for 3 h
4 Starting with state 3, in argon at 780 ±C for 4 h
5 Starting with state 4, in argon at
780 ±C for 11 h
6 Highly reduced in vacuum at 1000 ±C for 14 h,
then in oxygen at 925 ±C for 4 h
Fig. 3. Measured sensitivity and M# versus oxidation
state in 1-mm-thick LiNbO3:0.2 at.% Mn.
160 OPTICS LETTERS / Vol. 27, No. 3 / February 1, 2002Fig. 4. Arrhenius plot of the dark decay time constants of
nonfixed holograms stored in Mn-doped LiNbO3 (0.2-at. %
Mn), and Fe-doped LiNbO3 (0.25-wt.% Fe2O3).
refractive-index change, Dns, is proportional to g
and CX31 , where g is the recombination coefficient
and CX31 is the concentration of ionized donors. For
LiNbO3:Mn crystals, the recombination coefficient
gMn is much larger than that of LiNbO3:Fe because the
Mn center is deeper (gMn  2.4 3 10213 m3s versus
gFe  1.65 3 10214 m3s).12 Furthermore, the doping
level (0.2 at. %) is high. The large recombination
coeff icient, gMn, coupled with the high doping level
in the Mn-doped crystal makes the large M# easily
achievable.
One possible way to account for the lack of a trade-off
between M# and sensitivity in the LiNbO3:Mn crys-
tal is by assuming that a large majority of the Mn
traps are occupied by electrons. When we make this
assumption, the sensitivity, which is proportional to
the filled trap density, does not decrease when the
LiNbO3:Mn crystal is oxidized and a small percentage
change in the Mn21 concentration takes place. How-
ever, the percentage change of Mn31 is large, leading
to an increase in M# as the crystal is oxidized. Since
we assume that most of the Mn traps are Mn21, consid-
ering the doping level (0.2 at. %), the sensitivity could
be high.
Generally, both proton compensation and electron
tunneling contribute to the dark decay in LiNbO3 crys-
tals. But in LiNbO3 crystals with low doping levels,
electron tunneling is very weak and the dark decay is
dominated by proton compensation. In crystals withhigh doping levels, the dominant dark decay is elec-
tron tunneling.11 There are two possible reasons that
the dark decay in the Mn-doped crystal is dominated
by proton compensation. One possibility is that the
actual doping level in the Mn-doped crystal is much
lower than the nominal one. In this case, the large
M# could be just the result of the large recombination
coefficient, gMn, and the dark decay mechanism, which
is dominated by proton compensation, could just be
typical in LiNbO3 with low doping levels. The other
possibility is that the actual doping is 0.2 at. %, and,
since the Mn center is deeper than the Fe center, the
electron-tunneling effect is still weak in the Mn-doped
LiNbO3, and therefore the dark decay is dominated by
proton compensation.
This effort was sponsored by the National Science
Foundation (NSF), the Engineering Research Center
for Neuromorphic Systems Engineering, and the
Defense Advanced Research Projects Agency. The
authors thank the NSF and the Deutscher Akademis-
cher Austauschdienst for sponsoring the U.S.–German
collaboration. We thank Ingo Nee for fruitful dis-
cussions. Y.-P. Yang’s e-mail address is yunping@
sunoptics.caltech.edu.
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Photorefractive recording in LiNbO3:Mn

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Photorefractive recording in LiNbO3:Mn

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