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170
/
CLE0'97
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TUESDAY AFTERNOON
-4
-3
~2
-1
0
I
2
3
4
Magnetic
Field
(T)
CTuUl
Fig.
1
Total phase-conjugate signal
and its time-reversed component as a function of
magnetic field for the diffraction grating oriented
parallel to the
(100)
crystallographic direction.
The time-reversed component varies with the
magnetic field, while the non-time-reversed com-
ponent constitutes a constant offset.
-4-3
2-1
n
I
2
3
4
Magnetic
Field
(T)
CTuUl
Fig.
2
Phase conjugate intensity as a
function
of
magnetic field when the grating vec-
tor
is
oriented in the
(1
10) direction, for which the
phase conjugate beam has only a time-reversed
component. The circles are experimental points
and the curve is the theoretical phase conjugate
efficiency.
on the emergence of circular birefringence
(Fig.
2).
In conclusion, we have shown that in dis-
tinct experimental situations a magnetic field
will quench only the time-reversed phase con-
jugate beam while leaving the non-time-
reversed component unaffected. The relation-
ship between this phenomenon and the
removal of time reversal symmetry and Kram-
ers degeneracy under an applied magnetic field
is the subject of ongoing work.
*Brimrose Corporation, Baltimore, Maryland
21236
1.
R.
S.
Rana etal.,Opt.Lett. 20,1238 (1995).
CTuU2
2:45
pm
Oscillatory mode coupling from strobed
gratings in photorefractive quantum
well diodes
1. Lahiri,
D.
D.
Nolte,
M.
R.
Melloch,*
M.
B.
Klein,**
Department ofphysics and
the MRSECfor Technology-Enabling
Heterostructure Materials Purdue University,
1396 Physics Building, West Lafayette,
Indiana 47907.1396; E-mail: lahiri@physics.
purdue.edu
The longitudinal geometry photorefractive di-
odes respond transiently to changes in electric
fields and exhibit a rich variety of spatiotem-
poral dynamics. By use of short electrical
-
0.40
+-
v
a
0.20
=
0.00
a
'i!
.-
d
y
-0.20
0
-0.40
-0.40
-0.20
0.00
0.20
0.40
Time
(ms)
CTuU2
Fig.
1
Oscillation in the transmitted
intensity (mode coupling) resulting from moving
gratings for the
two
laser beams (1 and
2)
at a
detuning,
R
=
19.6
kHz.
Oscillations continue to
persist after the electric field has been turned
off.
pulses (-100
ps)
at
lcHz
repetition rates it is
possible to set up
a
quasi-static grating created
by two interfering laser beams. By moving the
intensity pattern at a ked velocity, mode cou-
pling between the
two
laser beams can be
achieved because of the refractive index grat-
ing created in the device. We present the first
experimental evidence for osciuatory photore-
fractive gain in these devices
as
a result
of
mode
coupling between
two
laser beams, which is the
result of the beating of the moving interference
pattern against a quasi-static strobed grating.
The p-i-n photorefractive diodes were
grown by molecular beam epitaxy on n+ GaAs
substrates. The
i
region consisted of a low-
temperature-growth (-320°C) multiple quan-
tum well layer consisting of lz0.5 periods
of 100-A
GaAs
wells and 35-A Al,.,Ga,,l.
As
barriers, sandwiched between 5000-A
Al,,,Ga,.,As buffer layers. We performed
degenerate two-wave mixing using
a
cw Ti:
sapphire laser tuned to 850 nm and a
fringelgrating spacing of
A
=
30
km. Two
acousto-optic modulators controlled the fre-
quency difference,
0,
between the two writing
beams. The electric field was strobed at a rep-
etition rate
of
2
kHz
by use of a 5 V/p.m single-
sided 100
ps
square pulse. This resulted in a
quasi-static grating in the photorefractive
quantum well diode.
The temporal response
of
the mode cou-
pling, AI (change in transmitted intensity),
that is due to the moving grating is shown in
Fig.
1 for a detuning,
a=
19.6
kHz.
Nonrecip-
rocal energy transfer is demonstrated where
one beam is amplified at the expense of the
other. Oscillations continue to persist after the
electric field has been turned off, which is a
consequence of the beating of the moving in-
terference fringes against the quasi-static
strobed grating created in the device.
The frequency
of
the oscillatory gain is
shown in Fig.
2
as a function of the frequency
difference,
CL,
between the two laser beams. We
find that the oscillatory frequency
is
equal to
the frequency difference between the two laser
beams.
The asymmetry in the transmitted signal
divided by the transmitted intensity and the
active crystal length defines the photorefrac-
tive gain,
r.
The peak photorefractive gain as a
function of the electric field is shown in Fig. 3
for three different detunings. Photorefractive
gains approaching 1000 cm-' have been dem-
onstrated.
,100
c
E
10
e
L
m,
E
=
5
VIpm
A=30pm
h
=
850
nm
1
10
100
Frequency Difference,
R
(kHz)
CTuU2
Fig.
2
Frequency of oscillatory gain is
found to be equal to the difference frequency,
a,
between the two laser beams during two-wave
mixing
-r
1000
"
1
I
=
25
mW/cmz
A=30pm
h
=
852
nm
n
0246810
Electric Field
(V/pn)
CTuU2
Fig.
3
Peak photorefractive gain
as
a
function of the electric field
for
several detunings,
R.
Photorefractive gains approaching 1000 cm-'
have been achieved.
In
conclusion we present the first experi-
mental demonstration
of
oscillatory mode
coupling in Stark-geometry photorefractive
p-i-n quantum well diodes. The simple corre-
lation between the mode oscillation frequency
and the detuning between the two laser beams
could make these devices candidates for opti-
cal applications such as optical vibration anal-
ysis
and laser velocimetry.
D.
D.
Nolte acknowledges support by the
NSF under grant #ECS-9414800.
*School of Electrical and Computer Engineering
and the MRSEC for Technology-Enabling Het-
erostructure Materials, Purdue Universtty, 1285
Electrical Engineering Building, West Lafayette,
Indiana 47907-1285
**Lasson Technologies,
1331
Avenida de Cortez,
Pacific Palisades, California 90272
CTuU3
3:OO
pm
Fast transient phase conjugation with
use of photorefractive surface waves
Erik Raita, Alexei
A.
Kamshilin,
Timo Jaaskelainen,
Vaisala Lnboratory,
Department
of
Physzcs, Untverszty
of
Joensuu,
P.O. Box
111,
FIN-80101, Joensuu, Fznland
Mutually pumped phase conjugation (MPPC)
is a nonlinear-optical process in which two
mutually incoherent beams pump each other
generating phase conjugate wavefront. The
diffusion-type photorefractive nonlinearity
was usually exploited to create the steady-state
MPPC.
Nevertheless, it
is
known that the
strong transient energy-exchange occurs be-
TUESDAY
AFTERNOON
/
CLE0'97
/
171
0
30
35
40 45
50
55
60
65
applied electric
field,
kV/cm
CTuU3 Fig.
1
Response time (a) and conver-
sion efficiency (b)
of
transient MPPC as a func-
tion
of
the applied electric field.
tween interacting beams at the initial stage of
their coupling in the case ofthe drift-type
non-
linearity.' We have experimentally observed
that such a transient coupling is enough to
create the mutually pumped phase-conjugate
mirror in a Bi,,TiO,, crystal.
In
this work the
nonstationary conditions were implemented
by applying a rising front ofthe dc electric field
to the sample while both pump beams were
derived from cw He-Ne lasers at
A
=
633
nm.
Transient fanning effect of the pulse form was
observed under these circumstances at smaller
(few degrees) angles of scattering. When we
adjust the pump-beams incidence
so
that for-
mation of both fanning gratings coincides
both in space and in time, two transient phase-
conjugate waves of the pulse form were gener-
ated from the opposite crystal's end faces.
Both the response time and the conversion
efficiency of transient MPPC depend on the
applied electric field as shown in Fig.
1.
The
curve (a) shows that the higher electric field is
applied to the crystal the faster MPPC response
time is observed. This contradicts usual behav-
ior of photorefractive media, where the re-
sponse time grows up with the increasing elec-
tric field. However, we have reported recently
the anomalous response-time behavior for the
fanning effect in a thin Bi,,TiO,, crystal.' It is
attributed with the generation of photorefrac-
tive surface waves, which appear owing to the
internal reflections of fanned beams on the
crystal surface and from the photorefractive
gratings recorded near this s~rface.~,~ As a re-
sult, the light energy is self-confined near the
crystal surface without any prefabricated
waveguide. As fast response time as 4.5 ms was
obtained for the external electric field of 61
kV/cm and the pump beam power
10
mW. To
the best of our knowledge, this is the fastest
MPPC observed in photorefractive crystals
under the cw pumping. Note that the conver-
sion efficiency of the transient phase conjugate
mirror
also
grows up while the electric field is
increased, as shown in Fig. 1b. Maximal con-
version efficiency is measured to be 25%,
which corresponds to 39% after the correction
to Fresnel reflections.
1.
J.
M.
Heaton, L. Solymar, IEEE.
J.
Quan-
tum Electron 24,558 (1988).
2.
A. A. Kamshilin,
E.
Raita,
V.
V. Prokofiev,
T.
Jaaskelainen, Appl. Phys. Lett. 67,3242
(1995).
A.
A.
Kamshilin,
E.
Raita, A.
V.
Khomenko,
J.
Opt. Soc. Am.
B
13,2536 (1996).
M.
Cronin-Golomb, Opt. Lett.
20,
2075
(1995).
3.
4.
CTuU4
3:15
pm
~
Time-resolved visualization of localized
photorefraction in LiNbO, crystal
Akira Shiratori, Ryuji Aida, Ryohei Hagari,
Minoru Obara,
Department
of
Electrical
Engineering, Keio University, 3-14-1 Hiyoshi,
Kohoku-ku, Yokohama 223, Japan; E-mail:
akira@obara.elec. keio. ac.jp
For the practical use of photorefractive ferro-
electric crystals such as LiNbO, and BaTiO, in
versatile applications, an important feature is
the stability of the wave mixing process, in-
cluding phase of the light, output power, and
efficiency. However, temporal instability or
chaotic behavior has been often observed in
some photorefractive crystals.
','
To
overcome
these issues, it is necessary to know the dy-
namic mechanism of the photorefraction. Al-
though theoretical studies on the mechanism
of the photorefraction have been reported by
many researchers
so
far,3 experimental under-
standing of the photorefraction is still difficult,
in fact, because the photoinduced index
change is invisible. In this paper, we have de-
veloped a novel three-dimensional imaging
technique for visualization of the photorefrac-
tion in the LiNbO, crystal with a Mach-
Zehnder interferometry. Time-resolved imag-
ing of the localized photorefraction is also
demonstrated.
Figure
I
shows the experimental setup. A
photorefractive LiNbO, crystal is placed in the
Mach-Zehnder interferometer, where
a
He-Ne laser (632.8 nm) is used as a light
source. A cw Ar+ laser (514.5 nm) irradiates
the crystal as shown in the insets of Fig.
1,
which induces photoinduced index change in
the crystal. The dynamic index change affects
the phase of the He-Ne laser, which is moni-
tored as the variation of the interferograms by
two-dimensional CCD array.
We produced three-dimensional images of
the photorefraction in the LiNbO, crystal from
the interferograms. Figure 2 shows an example
of the images, which clearly visualizes the lo-
calized photorefraction induced by 10 minutes
Arf laser irradiation (intensity:
4
W/cmZ). We
also produced
10
time-resolved images (irra-
diation duration
1
-
10
minutes), and success-
fully visualized the dynamics of the photore-
fraction in the LiNbO, crystal, which will be
presented. The effect of the absorption and
diffraction of the laser beam
on
the dynamic
photorefraction is visually investigated.
This technique is applicable to the visualiza-
tion of the photorefractive grating induced by
CTuU4
Fig.
1
Experimental setup
of
the
pho-
torefraction imaging
with
Mach-Zehnder inter-
ferometry. The insets show the geometry
of
the
Art
laser irradiation.
CTuU4 Fig.
2
Three-dimensional image of
the localized photorefraction in the
LiNbO,
crys-
tal.
two-wave mixing process and phase conjuga-
tion. Dynamics of the grating formation, de-
cay, and competition will be visualized, which
leads to the explication of the mechanism of
the photorefractive devices including the self-
pumped or mutually pumped phase conjuga-
tors.
1.
D.
Wang,
Z.
Zhang,
X.
Wu, P. Ye,
J.
Opt.
Soc. Am.
B
7,2289 (1990).
2. G.
R.
Gray, D. Huang, G. P. Agrawal,
Phys. Rev. A. 49,2096 (1994).
3. C. Gu,
J.
Hong, H. Li,
D.
Psaltis, P. Yeh,
7.
Appl. Phys. 69,
1167
(1991).
CTuU5
3:30
pm
~ ~
_____
~~
~_____
Holographic grating oscillations in
photorefractive materials
M.
P.
Petrov,
V.
M. Petrov, I. Zouboulis,*
A.
Gerwens,**
E.
Kratzig,**
A.
F.
Iofe
Physical Technical Institute, Russian Academy
of
Sciences, 194021,
St.
Petersburg, Russia;
E-mail: mpetr@shuv.pti.spb.su
When a photorefractive crystal is exposed to
two
beams, one of which is phase modulated,
the recorded holographic grating oscillates
near its equilibrium position. The analysis of
the grating oscillations is of particular interest
for the study of relaxation processes in pho-
torefractive materials and kinetic parameters
of charge carriers (mobility, lifetime, drift
length), as well as for optimization ofthe adap-
tive holographic interferometric technique
aiming at detection of phase-modulated opti-
cal signals. One of the most powerful tech-
niques for studying the grating dynamics is the
technique of two-wave mixing. However, it is
efficient enough only for phase modulation
frequencies
R
<
T-',
where
T
is the character-
istic relaxation time of holographic recording,
because at
R
1
7-l
the grating oscillations
affect the output signal much less than a direct
phase modulation of one of the incident
beams.
In
this
report
we
show
for
the
first
time
that
in case of induced three-wave mixing (ITWM)
that was discovered earlier' the grating oscilla-
tions can be detected for any
R
(R
<
T-'
or
R
>
7-
')
because an alternating output signal
(U3w)
originating from ITWM is determined
directly by the grating oscillations rather than
by phase modulation of one of the incident
beams.
So
measurements of
U,,
is
a
new tech-
nique for direct investigations of the holo-
graphic grating dynamics.
