We demonstrate selective fixing of volume holograms in photorefractive media. Each holographic page may be fixed individually and overwritten without destroying the other fixed pages. We present experimental results describing this process in Cr-doped Sr0.75Ba0.5Nb2O6 at room temperature, with hologram lifetimes exceeding 100 days during continuous readout with an intense beam (1 W/cm^2)

Kewitsch, Anthony S.

Neurgaonkar, Ratnakar

Segev, Mordechai

Yariv, Amnon

English

Caltech Authors - Main

1262 OPTICS LETTERS / Vol. 18, No. 15 / August 1, 1993
Selective page-addressable fixing of volume holograms in
Sr0 .75Bao.25Nb2 0 6 crystals
Anthony Kewitsch, Mordechai Segev, and Amnon Yariv
Department of Applied Physics 128-95, California Institute of Technology, Pasadena, California 91125
Ratnakar Neurgaonkar
Rockwell International Science Center, Thousand Oaks, California 91360
Received February 24, 1993
We demonstrate selective fixing of volume holograms in photorefractive media. Each holographic page may be
fixed individually and overwritten without destroying the other fixed pages. We present experimental results
describing this process in Cr-doped SrO.7 5 Bao. 5 Nb2 0 6 at room temperature, with hologram lifetimes exceeding 100
days during continuous readout with an intense beam (1 W/cm2).
Permanent storage of volume holograms in photore-
fractive media, or fixing, was first demonstrated in
LiNbO3 (Ref. 1) by use of a thermal fixing process.
The physical mechanism responsible for the fixed
grating is believed to be ionic compensation of the
electronic space-charge grating during a thermal de-
velopment cycle2 or space-charge-induced local fer-
roelectric domain reversal.3 Permanent fixing was
soon thereafter reported in Sr0.75Ba0.25Nb2O6 (SBN:75)
by applying an electrical fixing pulse to bias the holo-
gram about the coercive field' or photoinduced ferro-
electric domain nucleation on cooling from the para-
electric phase.' Recent research on electrical fixing6
and fixing based on screening7 has been reported
for SBN. The electrical fixing technique was also
applied to BaTiO3.5 In addition, fixing has been re-
ported in Bi12 TiO2 0, 9 BiU2 SiO 20,' 0 and KTa1_,NbO 3 ."1
The common characteristic of these earlier fix-
ing studies is a single development stage during
which time all the space-charge holograms are si-
multaneously fixed. After the development process,
the holographic memory is unable to be updated
selectively; that is, to update a single holographic
page of data, one must rewrite the entire memory.
For applications such as a random access memory,
the ability to update existing fixed holograms selec-
tively is highly desirable. While selective erasure
and overwriting of dynamic holograms have been
reported in the literature, we believe that this is the
first demonstration of selective overwriting of fixed
holograms. Recently13 we presented experimental
results of electric-field multiplexing, including two
holograms individually fixed in SBN:75. In this Let-
ter we describe this selective page-addressable fix-
ing technique. We present data on the holographic
diffraction efficiency during the reading and writing
process and demonstrate the selective overwriting of
a single fixed hologram that does not destroy adjacent
fixed holograms that share the same volume.
The crystals used in this experiment are SBN:75
single crystals, 6 mm X 6 mm X 6 mm, grown at
the Rockwell International Science Center. Sample
A has facets cut perpendicularly to the principal
axes, whereas the axes of sample B are rotated 450
about the y axis in the x-c plane. The crystals are
poled by application of a dc electric field of 5 kV/cm
while they are uniformly heated to 80'C in a high-
dielectric-strength oil bath. With the field on, the
samples are cooled at a rate of 0.5OC/min to 25°C.
The poling field is then removed. Note that the 450
cut of sample B makes the task of poling difficult;
hence the diffraction efficiencies for this sample are
very small.
The experimental setup is depicted in Fig. 1. Dur-
ing the writing process, A, and A2 are the signal
and reference beams, respectively. During the co-
herent reconstruction with A2, B, and B2 are the
reconstructed signal and remnant reference beams,
respectively. Since the holograms are sufficiently
weak, we have defined the diffraction efficiency as
Bl/B 2, to include reflection and absorption losses(absorption coefficient 1.9 cm-'). We also adopt a
conventional definition of the term fixed; that is, the
Al
0 X
B2
A2
0 /
Kg
C
B1
Fig. 1. Typical experimental setup.
0146-9592/93/151262-03$6.00/0 © 1993 Optical Society of America
August 1, 1993 / Vol. 18, No. 15 / OPTICS LETTERS 1263
n
e1
I
;.0 0.5 1.0 1.5 2.0
Write Time (Hours)
Fig. 2. Diffraction efficiency of the fixed hologram dur-
ing an extended writing process, illustrating the mono-
tonic increase in diffraction efficiency (sample A).
lifetime of the fixed hologram is several orders of
magnitude longer than that of the dynamic hologram.
The transmission holograms are written in the image
plane at 488 nm, with equal angles of incidence
(0 = 16°). The following experiments have all been
performed at or near room temperature (25-30'C)
in the ferroelectric phase.
The selective page-addressable fixing process
occurs simultaneously with the dynamic hologram
writing process; no thermal development cycle
or electrical fixing pulse is required. However,
increases in incident intensity, in temperature (while
well below T, = 56°0), or in write time all tend
to enhance the diffraction efficiency of the fixed
hologram. The hologram diffraction comprises a
dynamic component, with a characteristically fast
response time, and a fixed component, with a much
slower response and a lifetime that is several orders
of magnitude larger than that of the dynamic grating.
This dependence of relaxation rate on exposure level
was first reported by Thaxter and Kestigian,14 who
measured a fixed component with a lifetime of several
minutes. We obtain a significant fixed component by
simply writing holograms with an incident intensity
greater than 1 W/cm2 for a duration of 2 min or
more, or an exposure level of 100 J/cm 2 . For a
total intensity of the signal and reference beams of
100 mW/cm 2 , only a significant dynamic grating is
present. As the intensity is increased to 4 W/cm 2,
the hologram diffraction efficiency monotonically
increases with an extremely slow response time
(several orders of magnitude larger than the dynamic
grating). In fact, the diffraction efficiency grows by
a factor of 25 from the initial value of 0.1% during
a writing process of 2 h, surpassing the diffraction
efficiency of the dynamic hologram alone (Fig. 2).
While it is exceedingly difficult to maintain stability
of the interference pattern during these long write
times, the diffraction efficiency was found to increase
for approximately 20 h until saturation occurred at a
maximum diffraction efficiency of 3%.
Thus the hologram written at a higher exposure
level is automatically fixed during the writing
stage. Figure 3 (sample B) shows that the dynamic
hologram disappears completely after only 7 s. The
diffraction efficiency of the fixed hologram decays for
40 s and then settles into steady state. This behav-
ior is contrary to early fixing techniques, in which the
diffraction efficiency initially grows during readout,
since the reconstruction with uniform illumination
depletes the dynamic grating and leaves behind
only the uncompensated ionic grating. In contrast,
the development stage in this selective fixing tech-
nique occurs simultaneously with the writing or
exposure stage.
The inset of Fig. 3 (sample A) illustrates the long-
term decay of a fixed hologram written at room
temperature with total intensity of 4 W/cm2 for 1 h.
The diffraction efficiency decreases by a factor of 3
F
0
.2
.8
._
1.2e.3
1.Oe-3
8.0e-4
6.0e-4
4.0e-4
2.0c-4 1
0 20 40 60 80
Readout Time (sec)
Fig. 3. Comparison of the time response of the dynamic
(written and read out with a low-intensity beam of
100 mW/cm2) and fixed (written and read out with
a high-intensity beam of 4 W/cm2 ) readout processes
(sample B). The inset shows the diffraction efficiency of
the fixed hologram during an extended readout process
under intense illumination (sample A).
(a) (d)
(b) (e)
(c) (D
Fig. 4. Selective overwriting of a single angle-
multiplexed volume hologram: Hologram (c) is overwrit-
ten with (f), leaving (a) and (b) slightly degraded as (d)
and (e) (sample A).
3 . .
Incident Intensity = 4 W/cm
2
2 -
. .. 
1.0. . .
0.0
0 2 4 6 8(days)
.....................................................
llxedhologam "' 4W/m 2
* dynamc hologam 1 mW0 m2
1264 OPTICS LETTERS / Vol. 18, No. 15 / August 1, 1993
during the initial 4 days of readout and then reaches
a steady-state value. The hologram was continu-
ously reconstructed with an intense beam of 1 W/cm2
with a beam diameter of 2 mm. Extrapolating this
curve, we estimate that the hologram will survive at
least several hundred days during continuous read-
out. The lifetime is further enhanced for readout
with a plane wave, as this will prevent accumulation
of space-charge fields that is due to the intensity
gradient in the radial direction for a focused reference
beam. A fixed page written for 15 min has been
monitored intermittently for over 1 month without
any degradation in image quality.
A significant advantage of this fixing technique is
the ability to address an individual page and selec-
tively overwrite the hologram only at that location.
Figure 4 shows the selective overwriting of one of
three fixed angle-multiplexed holograms, each holo-
gram separated from the next by AO = 0.01°. Note
that because the Bragg peaks of these holograms are
thermally broadened (no thermal stabilization during
writing), the minimum angular separation between
adjacent holograms is a factor of 10 larger than
the theoretical limit of 0.001° in this configuration.
Initially, the three holograms, labeled (a)-(c), were
written with 4 W/cm2 for 30 min each. Hologram
(c) was then overwritten by hologram (f) for 20 min,
during which time the diffraction efficiency of the
remaining two fixed holograms, (d) and (e), decayed
by approximately a factor of 2. The three holograms
remained fixed following the overwriting procedure.
Fundamentally, for a writing process in which mul-
tiple volume holograms share the same volume, each
time a hologram is overwritten the diffraction ef-
ficiencies of the remaining holograms sharing the
same volume are diminished. Thus even though this
fixing technique is selective in the sense that fixed
holograms at a particular address (wavelength, field,
and angle) may be overwritten completely, holograms
at other addresses are slightly degraded. We expect
that proper scheduling during the selective overwrit-
ing process will minimize the degradation of adjacent
fixed holograms.
We present the following evidence that demon-
strates that the fixing mechanism is local ferroelectric
domain reversal: (1) In an electro-optic amplitude-
modulation measurement, the half-wave voltage V,
increased by a factor of 10 following the hologram-
writing process, because the fixed hologram periodi-
cally polarizes the crystal. By applying a field equal
to the coercive field (-1.2-1.6 kV/cm) for 1 s, the
crystal is partially repoled as the half-wave voltage
decreases by a factor of 3. (2) In some configurations
the fixed grating displays a large second spatial har-
monic because the index modulation is proportional
to the square of the dielectric polarization. (3) The
motion of domains under applied dc electric fields is
apparent during coherent reconstruction of the holo-
gram, and the hologram can be erased by uniformly
illuminating and applying a field larger than the coer-
cive field. (4) Photorefractive response is diminished
locally in regions where many holograms have been
written, because the effective electro-optic coefficient
is reduced. (5) Domain gratings are revealed by a
microphotometric method'5 and erased by applying
large fields.
In summary, fixing and writing occur simultane-
ously during exposure levels of at least 100 J/cm2 .
This technique is identical to the method of writ-
ing dynamic space-charge gratings, except that the
fixed hologram exhibits an extremely large read/write
time asymmetry. This method has three primary
advantages: (1) holograms are individually written
and fixed, (2) the process is automatic, and (3) fixing
occurs at room temperature. As a consequence, this
technique is well suited for applications requiring an
updatable ensemble of gratings, such as a holographic
random access memory.
This study was supported by the Defense Advanced
Research Projects Agency, the Army Research Office,
and the U.S. Air Force Office of Scientific Research.
A. Kewitsch gratefully acknowledges the support of
a U.S. Office of Naval Research graduate fellowship.
The authors thank R. Hofmeister and A. Agranat for
their scientific assistance.
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Selective page-addressable fixing of volume holograms in Sr0.75Ba0.25Nb2O6 crystals

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Selective page-addressable fixing of volume holograms in Sr0.75Ba0.25Nb2O6 crystals

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