The PhotorefractiveEffect
A laserbeampassingthrough a crystal can suddenlyburst into a
spray of light. Thisphotorefractiveeffect may be the key to
developingcomputersthat exploit light insteadof electricity
by David M. Pepper,jack Feinberg and Nicolai V. Kukhtarev
W
hen Arthur
Ashkin
and his
colleagues at Bell Laboratories
first noticed
the photorefrac-
tive effect some 25 years ago, they considered the phenomenon a curiosity at
best and a complete nuisance at worst.
Today photorefractive materials are being shaped into components for a new
generation of computers that exploit
light instead of electridty.
Ashldn was experimenting with a
crystal of lithium niobate (UNbO 3) that
he hoped would convert one color of
intense laser light to another (a process technically called second-harmonic generation). As part of his tests, he
directed a laser beam through the
crystal. At first, the crystal performed
quite admirably,allowing light to pass
through undisturbed. But after a few
minutes, the crystal began to distort
the beam, scattering light around the
laboratory. Somehow the laser light
had altered the optical properties of
the crystal itself. 1bis photorefractive
effect would persist in the crystal for
days. If the workers bathed the crystal
in a uniform beam of light, however,
I
DAVID M. PEPI'B., JACK FEINBERG
and NICOLAI V. KUKHTAREV wrote
part of this article on the beaches of
HawaJi and simultaneously participated
in an experiment on the photocbromIc elfect: they developed suntans. Pepper Is a senior staft' physicist in the
optical physics department at Hughes
Research Laboratories and adjunct pr0fessor at Pepperd1neUniversity. In 1980
he received his Ph.D. in applied physics
from the California Institute of Technology. This is his second article for SaENTmC AMERICAN.Feinberg is associate
professor of physics and electrical engineering at the University of Southern
California. In 1977 he earned his PhD.
in physics from the University of Callfomia, Berkeley. Kukhtarev is a senior
sdentiat at the Institute of Physics in
Kiev, UkraIne, U.S.S.R. In 1983 he received a doctorate degree for his studies
of the theory of dynamic holography.
cO)
C"",,,rrrnr
A Ul'lJlr
AN
the crystal would once again transmit
an undistorted beam.
During the past 2S years investigators have discovered a wide variety
of photorefractlve materials, including
insulators, semiconductors and organic compounds. Photorefractive materials, like film emulsions, change rapidly when exposed to bright light, respond slowly when subjected to dim
light and capture sharp detail when
struck by some intricate pattern of
light. Unlike film, photorefracttve materials are erasable: images can be stored
or obliterated at whim or by design.
By virtue of their sensitivity, robustness, and unique optical properties,
pbotorefractive materials have the p0tential to be fashioned into data-processing elements for optical co.-oputers. In tlteory, these devices would
allow optical computers to process information at much faster rates than
their electronic counterparts. Employing photorefractive materials, workers
have already developed the optical analogue to the transistor: if tWo laser
beams interact within a pbotorefractlve
material, one beam can conttol, switch
or amplify the second beam. Photoretractive materials also lie at the heart
of devicesthat tracethe edgesof bnages,
that connect netWOrks of lasers and
that store three-dimensional images.
B
ecause
photorefracttve
the
optical
properties
matertals
of
can
be
modified by the very Ught that
passes through them, they are categorized as nonlinear optical media.
In linear optical me<tia-such as lenses, prisms and polarizing filters-light
beams merely pass through one anoth.
er, without changing the properties of
the material.
The photorefractive effect is closely
related to another nonlinear phenomeDOnknown as the pbotochromic effect.
The ltght that strikes a pbotochromic
material can change the amount of
Ught that the medium absorbs. Photochromic matertals, which are incorpo-
nrtnIrn- 1qqn
rated in certain brands of sunglasses,
darken in bright sunlight and lighten In
dark rooms.
In photorefractive materials, the light
that bombards the material affects how
fast I1ght travels through It. More specifically, the photorefractive effect is a
process in which light alters the refractive index of a material. (The refractive
index is the ratio of the speed of light
in a vacuum to that in the material.)
Most transparent materials will
change their refractive index if bombarded by light of sufficient intensity. Ught is a traveling electromagnetic wave whose electric field strength
is proportional to the square root of
the intensity of light. For instance. an
optical beam whose intensity is 100
million watts per square centimeter is
equivalent to an electric field strength
of about 100,000 volts per centimeter.
When such intense light is directed at
a transparent material, It disrupts the
positions of the atoms, changing the
refractive index by a few parts in one
million. As a result of the change, the
material
can
act like a prism or a lens
to deflect
light.'
.
The term "photorefractive" Is usually
reserved, however, for materials whose
refractive index changes in response to
light of low intensity. In photorefractive materials, I1ght beams as weak as
one thousandth of a watt per square
centimeter can alter the arrangement
of atoms in a crystal, changing the re-
fractiveindex as muchas a fewparts
in 10,000. And unlike most transparent
materials, the change in photorefractive crystals is semipermanent: if an
altered crystal is isolated from all s0urces of I1gbt, the change in the refractive
index can last from milliseconds to
years, depending on the material. In
this manner, one can store Infonnation
in the fonn of images in a crystal.
LASER.BEAM strildng a 5-mm crystal of
barium titanate scatters into a fan of
Hght owing to the photorefracdve effect.
r
§es,
oiD
ight
row
;peisa
raclive
ight
)
will
om,nsinet19th
t of
"
an
100
~r is
18th
~ter.
d at
the
the
one
, the
lens
Jally
!lose
.e to
frac.k as
uare
nent
ere>arts
U'eDt
fracif an
.OUI'cttve
Is to
II. In
ition
tal of
m of
jfect.
HOW LIGHT ALTERS THE OPTICAL PROPERTIES
OF A PHOTOREFRACTIVE CRYSTAL
~
In
Z
W
1. Two laser beams interfere in a
photorefractiveCl'yStai to forma
~
~
pattern of bright and dark regions.
x
<:J
:J
u.(/)
OW
~~
So
2. Mobile electrons migrate away
from bright regions of the crystal.
!!!~
~iD
(/)0
0:1
1/1
~W
0"
za:
oel:
3. MobHe electrons accumulate In
the dark regions. leaving regions
of positive charge.
g5
IOU
ii:ii:
t;t
c~
w
9
w
it
(J
~
t
~
w
T
~..~..~..~..~..~..
~~~..~..~~~..~..
4. An electric field forms between
regions of positive and negative
charge.
w
§
5. The electric field distorts the
crystal lattice.
~
(.)
6. Th.e 1I1S!OttfolJcauses light to
travel slower through some
regions and faster through others.
More specifically, the refractive
index Is altered periodically. The
refractive-index grating is shifted
one quarter of a period in space
from the light pattern.
How can a weakbeamof light cause
such a strong changein the refractive
index of a crystal? In the late 1960s
F. S. Chen of Bell Laboratories advancedthe basicmodel of the photorefractive effect. just as a single ant can
move a large mound of sandone grain
at a time, a weak beam of light can
gradually build up a strong electric
field by moving electric charges one
by one. In photorefractlve crystals,
chargesdiffuse away from bright regions and pile up in dark regions. As
more and more chargesare displaced,
the electric field inside the crystal in64
order of parts per million, can cause
the photorefracttve effect.
Each crystal defect can be the source
of an extra charge, which can be either
electrons (particles of negative charge)
or holes (regions of positive charge),
depending on the particular crystal. In
the dark, these charges are trapped; in
the presence of light, they are free to
roam within the crystal until they eventually become caught again. If light illuminates charges in one region of the
crystal, they will diffuse away from
that region and accumulate in the dark,
in the way cockroaches scurry underneath furniture to avoid light.
Each charge that moves inside the
crystal leaves behind an immobile
charge of the opposite sign. In the region between these positive and negative charges, the electric field is strongest, and the crystal lattice will distort
the most. A beam of light that passes
through this region of the crystal will
experience a different refractive index
from that of the unaffected regions.
creases,attaining a strength as high as
10,000volts per centimeter.The electric field w1ll distort the crystal lattice
slightly (about .01 percent), thereby
modifying the refractiveindex.
The source of these electric chargesapparentlylies in defectsin the crystal lattice of the material. The defects
can be mechanicalflaws in the lattice
structure (missing atoms at certain
lattice sites),substitutional dopants(a
foreign atom at some lattice site) or
interstitial dopants (a foreign atom
wedged between native atoms). Very
small amountsof thesedefects,on the
Scn:NTm:c AMERICAN October 1990
he time it takes for light to rearrange chargesin a crystal depends
on the intensity of the light and
alsoon how fast chargesmigratein the
crystal. Weak light takes longer than
strong light to build up the sameelectric field. For low-intensitylight (about
.01 watt per squarecentimeter).it can
take minutes for the chargesto reach
their equilibrium pattern. For high-intensitylight (abouta billion wattsper
squarecentimeter),the responsetime
canbe lessthan a nanosecond.A photorefractive crystal, like photographic
film, requiresa certainamountof light
to completeits "exposure."
The changein refractiveindex is linearly proportional to the strength of
the electric field if the crystal lattice
lacksa certainproperty calledan inversion symmetry.The electric field will
remain in the crystal long after the
light is removed,just as the mound of
sand remains in its new location long
after theantshaveleft.
One of the most useful consequences of the photorefractiveeffect is the
exchangeof energybetweentwo laser
beams,which is also known as twobeam coupling. If two laser beamsof
the samefrequencyintersect,they will
interfere and producea stationarypattern of bright and dark regions-or
more specifically a pattern whose intensity varies sinusoidally with position in the crystal. If this sinusoidal
pattern of light forms within a photorefractive crystal, electric chargeswill
move to generate an electric field
whose strength also varies sinusoidally. The resultant field will distort the
crystal lattice in a similar periodic manner, producing changes in the refractive index. Ultimately a "refractive-index grating" (also called a refractive-index volume hologram) will be fonned
within the crystal.
The electric field and the refractive-
index grating will have the sameperiodicity as the Ught pattern, but they
will be shifted by one quarter of a period in space from the incident light.
lbis displacement-called a 90-degree
phaseshift-is the optimal configuration for the exchangeof energy betweenthe original two laserbeams.
Oncethe refractive-indexgrating has
beenestablishedin the crystal,someof
the light of one beamwill be deflected,
or diffracted, by the grating in the directionof the other beam(andviceversa). Hence, the two deflected beams
will interfere with the two original
beams-constructively in one caseand
destructively in the other. In the case
of constructiveinterference,the peaks
of the light wavesin one of the deflected beamscombine with the peaks of
one of the original beams, and the
beamstherefore reinforce each other.
In the caseof destructiveinterference,
the peaksof the wavesfrom the other
deflectedbeam combine with the valleys from the other original beam,and
the light waves dtmintsh each other.
Thebeamformed from constructiveinterferencewill emergefrom the crystal
strongerthan whenit entered,whereas
the beam fonned from destructiveinterferencewill emergeweaker.Hence,
one of the beamswill havegainedenergy from the other. Which beam gains
and which beam loses is determined
by the orientation of the crystal and
whether the chargecarriers are holes
or electrons.
Photorefractivematerialsexhibittwobeamcouplingbecausethe optical pattern and the refractive-indexgrating
are shifted in space.Two-beamcoupling is not found in most nonlinear
materials, however, becausethey respond "locally" to optical beams(for
example,atomic orbitals are deformed
by the intense electric fields of the laser beams).In most nonlinear materials. therefore. the optical pattern and
grating preciselyoverlap.The light de.
fleeted by the grating interferes with
eachof the undeflectedbeamsin exactly the sameway. Thus, the two beams
exchangean equal amount of energy,
so neithergrowsin intensity.
T
o
enhance
effect,
the
investigators
photorefractive
have
learned
to control the flow of charge
within a material.Thetwo mechanisms
that control the flow of charge in a
crystalare diffusion and drtft.1bey are
analogousto the diffusion and drift of
smoke from a burning ember.Left on
its own, smoke w1ll diffuse to regions
of low smokedensity.If a slight breeze
is blowing, however, the smoke w1ll
crystal such as bartmn titanate (Ball03~
Initially the beam passes through the
crystal unaltered. After a second or so
(the time depends on the intensity of
the light), the beam begins to spread
COHERENTOYI1CAL EXOSOR scatters intense light from a laser but transmits out In the crystal, cwving to one side.
weak light from a lamp. Here the exdsor protects a camera from the damaging In the process the cwved beam divides
laser rays. The laser beam, which consists of a single frequency of light, scatters
off defects in the photorefractive crystal and then interferes with itself. The inter- into many rays that appear to spray
ference pattern creates a refracdve-index grating. As a result, most of the laser out into a broad fan of light-hence the
light is scattered to one side of the crystaJ, a process called beam fanning. Because term -beam fanning: Depending on
the lamp light consists of many frequencies, neither an interference pattern nor a the choice of photorefractive crystal,
grating is created, and so most of the lamp light is transmitted through the crystal. the emerging light In cross section can
66
SClENTmc AMERICAN October 1990
~
Becauseof this property, phase-coJijugate mirrors have myriad applications
in the fields of optical communications, high-power lasers and optical
computing. As an example, they can
be incorporated into a system to correct undesirable aberrations that laser
beams sometimes acquire during propagation through mstorting media or
powerful laser amplifiers (see "Applications of Optical PhaseCorijugation," by
David M. Pepper; SCII:NTIFICAMER!CAN,January, 1986).
In 1977 Robert W. Hellwarth of the
University of Southern California suggested a basic configuration for phasecorijugate mirrors, and two years later
Sergei G. Odulov and one of us (Kukhtarev) and independently Huignard and
his co-workers produced such a mirror that incorporated photorefractive
materials. In 1982 one of us (Feinberg)
PHASE-CONJUGATE
MIRROR made from a photorefractive crystal allows com- serendipitously mscovered a class of
munication through the atmosphere. The receiving station emits a beam of light. phase-corijugate mirrors that many inAs the beam travels through the atmosphere, it spreads out and distorts. At the vestigators use today. Feinberg had fotransmitting station a photorefractive crystal either reflects or transmits the light
cused three laser beams on a crystal of
that bits it, depending on the voltage applied through the electrodes. Becausethe
crystal acts as a pbase-conjugate mirror, the reflected light is time-reversed. A barium titanate. One beam contained
messagecan be encoded by alternately reflecting and transmitting the light. As the the light waves whose time-reversed
time-reversed message beam interacts with the atmosphere, all distortions are replica was sought; the two additional
removed from the beam, and the messagecan be decoded at the receiving station. "pump" beams were needed to form
the phase-corijugate mirror (or so Feinberg and his colleagues thought at the
time). To check the experiment, Feinassumethe shapeof a beautiful array novelty filters and edge enhancement berg blocked the pump beams to enof elliptical patterns and can havedif- of images.
sure that the presumed time-reversed
The coherent optical exctsor-flrst
fering polarizationcomponents.
beamdid not arise merelyfrom a simdescribed
in
1985
by
Mark
CroninBeamfanningresults from an energy
exchangebetweenthe inddent beamof Golomb and Amnon Yariv of the Cali- ple reflection from a crystal face. At
first, the time-reversed beam obedientlight and light randomly scatteredby fornia Institute of Technology-can po- ly vanished. But after a short time, the
crystal defects.tight scatteredby a de- tentially filter out light that scatters
beam surprisingly reapfect interferes with the unscattered from hlgh-intensity laser beams.The ex- time-reversed
peared. Feinberg had found a phasecisor
has
the
potential
to
protect
senbeam, forming a refractive-indexgratconjugate mirror that required only a
ing becauseof the photorefractiveef- sitive optical detectors, which may be
fect. Oncethis occurs,the unscattered necessary to monitor high-power lasers single beam. Feinberg's elegant device
light can transfer additional energyto during industrial processes such as an- is an example of a more general class
the scatteredbeam. It turns out that nealing and welding. The excisor can of self-pumped, phase-cOIyugate mirthe energy-exchange
mechanismis not also prevent damage to video cameras ror, which was "pioneered by Jeffrey
O. White'; Mark Crcinin-Golomb, Baruch
isotropic, and so the scatteredbeams [see illustration on page 66].
Fischer and Aumon Yariv of Cal Tech.
The
key
component
of
the
coherent
of light becomepreferentially intensiAlthough phase-conjugate mirrors
optical
exctsor
is
a
photorefractive
crysfied only over a finite range of angles.
can be made from many classes of
tal,
which
is
placed
in
front
of
the
delbis intensified,scatteredbeamis then
opticat materials, photorandomlyscatteredby other crystalde- tector or the camera's lens. The intense nonlinear
refractive elements have several disfects, and the processis repeated.As beam can be diverted away from the tinct advantages: first, the mirrors rea result. a large number of scattered detector because the beam-fanning efbeamsfan out from the original beam. feet essentiallyguidesthe coherentla- quire only one input beam-the very
Depending on the orientation of the serlight to the sideasit travelsthrough beam whose phase-conjugate replica
crystal and the point at which the ind- the crystal.The detectoror cameracan is sought-thus forming the so-called
dent laserbeamenters the crystal. the view the object in the presenceof in- self-pumped phase conjugator; and
second, very low laser powers and infanned beam can be made to curve tenselight, however,becausethe backtensities can initiate the process leadground
room
light
scattered
from
the
through shallow angles (barely curving through the crystal) or over rather rest of the object passesthrough the ing to the time-reversed beam.
Why does nature love the phase-conlarge angles(strildng an extreme cor- crystal essentiallyunaltered.
jugate beam? A partial answer to this
Beam
fanning
also
plays
an
imporner of the crystal).
tant role in phase-conjugatemirrors question posed by Hellwartb can be advanced at least in the case of barium tiearnfanning and other photore- made from photorefractive materials. tanate, as postulated by Kenneth MacThese
mirrors
possess
the
peculiar
fractiveeft'ectshavebeenexploitDonald, then at the University of Southed in such applications as coher- property that an optical beam "reflectern California, and one of us (Feinberg).
ed"
from
them
w1ll
travel
exactly
backent beam excisors, optical interconnecAfter a short time, beam fanning causward
in
space
as
if
time
were
reversed.
tion elements, phase-coQjugatemirrors,
B
70
SCIENTIFICAMERICAN
October1990
ever the intensities of the two beams
match, the optical interferencepattern
at that particular region in the crystal
will have the largest modulation from
bright to dark to bright and so on. The
strong interference pattern will then
generatea strong refractive-indexgrating. Whenthe readout beam interacts
with the strong grating, it will be most
efficiently deflected. or diffracted, at
that location,and so the reconstructed
imagewill emergewith all of its edges
highlighted.
Becausephotorefractivecrystalscan
act as both energycouplersand phaseconjugatemirrors, they areparticularly
useful for reconftgurableoptical interconnectsand frequency-lockingof lasers.Photorefractivecrystalscanrelay
information from one optical element
(say. an optical fiber) to another (a
data-processingelement).free of complicated optical elementsor electronic interconnects.1bis optical relaying
schemecan also force two (or more)
separatelasersto "lock" onto eachother so that the two lasers oscillate at
preciselythe sameoptical wavelength;
whirh I~ rlirPrtM Into a -omtorefractive
crystal along with a "reference" beam. in this way, the two separatelasersesThe two beams interfere inside the sentiallybehaveas one largerlaser.
How do two or more mutually incocrystal and produce a hologram of the
ortginal image. The image can be recov- herentbeamsof light becomeconnected in a photorefractivecrystal? When
ered by a third "readout" beam. which
is aimed in a direction opposite to the the different beamsilluminate a photorefractive crystal, eachwill produce
reference beam. If the object beam is
its own armadaof scatteredbeamsand
relatively weak, then the reconstructrandom refractive-indexgratings (or
ed image will be a faithful replica of
the ortglnal picture. If the object beam holograms)within the crystal. If one
hologramfrom onebeamexactlymatchis more intense than the other two
beams, however, then the edges of the esoneof the hologramsfrom the other
~
other
application
that takes
adbeam, then that hologram will grow
reconstructed image will be enhanced.
vantage
of the energy-excbange
The intensity of the object beam faster in strength than the pthers.The
mechanismis a devicecalled a
novelty filter, whichhighllghts what- varies locally in the crystal because the result Is that the two beams will be
ever is changingin a highly complex beam contains an image. Hence, its in- connectedto eachother.1bis deviceIs
scene.Suchdevicescan pick out mov- tensity will match that of the reference called a mutually pumped. or doubly
mirror.
ing airplanesagainsta backgroundof beam at every edgein tht>~imag«:,_be- pumped,phase-conjugate
The
device-first
demonstrated
in
cause
an
edge
contains
a
full
range
of
buildings,a sportswomandiving into a
1987
by
Fischer,
now
at
the
Techn1onintensities,
from
dark
to
bright.
Wberstill pond or bacteriaswlmmlngagainst
Israellnstitute of Technologyin HaIfa.
and his co-workers.by RobertW.Eason
and A.M.c. Smoutof the Universityof
Essexin GreatBritain and subsequently by Mark D. Ewbankof the Rockwell
International ScienceCenter in 1bousandOaks,Calif.-can connectany twO
beamsoriginating from any direction.
If the two beamscontain images,then
eachimagewill be convertedinto the
other as the beamstraversethe crystal (seeillustration on page74). If one
beam Is pulsed rapidly, the temporal
information is relayedback toward the
other beam.
If two beamscomefrom different lasers that oscillate at slightly different
PARAMEcruMis hidden among algaeand debris (left), but when the sceneis viewed frequendes,then the beam from one
through a novelty filter, only the swimming microorganism appears (right). Novlaser will be diverted into the approxelty ftlters can highlight any object that moves against a stationary background.
"Ibis mter was designed by R. M. Pierce. R. Cudney, G. D. Bacher and j. Feinberg. imate coJijugate,or time-reversed,dt-
es the incident laser light to be swept
preferentially to one side of the crystal. If the incidentbeamand crystal are
positioned so that the fanned beam
is swept into a far comer of the crystal. the fannedbeamundergoestwo internal reflections, essentially folding
back onto itself. This reflected beam
fans again back along the direction
of the incident beam. Out of all the
scatteredbeamsinside the crystal, the
time-reversedbeam-by virtue of its
backwardtrajectory-gains more energy than the other scatteredbeams.This
beam-reversalprocesscan be very emdent: 60 percentof the powerin the incident beamcan emergeas the phasecorijugatebeam.
Oneof us (Pepper)hasaddedan additional twist to this novelphase-corijugatemirror. Pepperattachedelectrodes
to a photorefractivecrystal to apply a
time-varying electric field across the
crystal. Whena laserbeamstrikes this
crystal,it not only is time-reversedbut
also can be pulsed in time Weea shutteredmirror. In this manner,pulsedinfonnation canbe relayedfrom the conjugate mirror back to the laser source;
the time-reversednature of the beam
guaranteesthat the two communication points remain locked onto each
other. lbis schemecan be used to es.
tablish a communicationschannelbetween two satell1tesor to relay information from a remotesensorplacedat
one end of an optical fiber link (seeIllustrationon page70).
72
a background of motionless algae.
One type of novelty filter demonstrated by Cronin-Golomb (now at Tufts
University) involves two beams that illuminate a photorefractive crystal. An
image is encoded spatially onto the
first laser beam. The crystal is oriented so that this image-bearing beam
transfers most of its energy to the
second beam. After the image-bearing
beam passes through the crystal. it becomes almost completely dark. Whenever something in the image-bearing
beam changes, however, the energy-exchange mechanism is disturbed momentarily. and the part of the image
beam that has changed will pass
throughthe crystal.This part of the
beam can then be viewed on a video
monitor. Once the motion has ceased.
the image-bearing beam will become
dark once again after passing through
the crystal.
Photorefract1ve crystals can also be
employed in other applications to enhance the edges of an image. An image is encoded onto an "object" beam.
SCIENTIFIC
AMERICANOctober 1990
-
--
As workers increasethe concentration of the defects in a photorefractive ~ate~l, t~ey'ha~efound that the
numoer 01 avauaole cnarges Increases
thereby enhancing the strength of th~
internal electric field and the refractiveindex gratings. On the other hand, defects scatter and absorb light from the
incident beams. Defect concentrations
of from one to 100 parts per million
appear to be optimal in terms of providing a reasonable number of charges
without appreciably attenuating the input light.
Perhaps the best photorefractive materials will be those painstakingly fashioned out of semiconductor materials.
Stephen E. Ralph, David D. Nolte and
Alastair M. Glass of AT&T Bell laboratories have recently shown that layers
of gallium alwninum arsenide alloys
only a few atoms thick can be assemPHOTOREFRACTIVECRYSTAL couples two laser beams, a process that may be bled into structures-called superlatexploited In communications systems, optical computers or other laser networks. tices and quantum wells-that exhibit a
measurable photorefractlve effect. Such
materials are currently being studied
rection of the beamfrom the other la- crystals in the different materialsvary for their novel electrical properties and
ser, and vice versa. If two such lasers greatly as wen: gallium arsenideand high speed of response. These crystals
are optically connectedby a crystal, other semiconductorsaretypicallypho- may also lead to a new dass of intein thenearinfraredpart of grated processors based on optics and
they can, under the proper conditions, tosensitive
be made to oscillatecoherently,there- the optical spectnUn,whereasmost in- electronics.
What makes photorefractive materiby locking the frequencyof the two la- sulators and organic compounds are
als so promising for optical computing
sers.With this scheme,workers hope sensitivein the visible spectrum.
that thousands of semiconductor laA flurry of experimentscurrently in is their high sensitivity to light, couserscan be emdently combinedInto a progressaim to identify and possibly pled with their ability to connect light
high-powersourceof coherentlight.
control the defectsresponsiblefor the beams from different lasers and to
photorefractlve effect in various crys- change one information pattern into
T
o construct
better optical
comtals. As an example,the sourceof the' another. The future of photorefractive
ponents
and devices.
investigaphotorefractlve effect in barium tita- materials will depend on whether their
tors are searching
among
the
nate is an open question: many re- optical properties can be as meticumany different kinds of photorefracsearchersattribute the effect to the lously tailored as semiconductors are
tive materials to find the most effident
presenceof various ionization states today. Ideally, photorefractlve devices
and reliable crystals. Photorefractlve
of transition metal impurities such as would be integrated with semiconducmaterials vary greatly in their optical.
iron, cobaltand manganese;
oxygenva- tor lasers and detectors to form a sinelectrical and structural propertiescanciesin the lattice may also contrib- gle, compact device capable of processfor example, the insulator barium ti- ute to the-Charge:camers.In gallium ing m1ll1onsof bits of data simultanetanate. the semiconductor gallium ar- arsenide,on the other hand, the pho- ously in each microsecond, yielding a
senide and the organic compound 2- torefractive effect is attributed to an total data-processing rate of trillions of
cyclooctylamino-S-nitropyridine doped intrinsic lattice defect thought to be bits per second.
with 7,7,8.8.-tetracyanoquinodbnethane. formed by a combinationof an arsenic
These seemingly different materials atom replacing a gallium atom in the
nonetheless exhibit similar photorelattice and an additional arsenicatom
RJR1HER READING
tractive effects. They all have a crys- placedin the samelattice cell, forming
0P11CAI. PHAsE C ONJUGATJON IN ~
tallattlce that is relatively easy to dis- the so-calledEL2 center.
UFRAC11VE MATERJALS. jack Feinberl
tort, and they all contain defects, which
in Optfcall"ht.ue CDrIJugatIon. Edited by
Regardlessof the character of the
R. A. Ftsber. Academic Press. 1983.
act as a source of charge earliers and crystalline defects, the properties of
PHo'roREFRAC11VE NONUNEAR 0Pncs.
charge traps. Under the influence of an most photorefractlve crystals can be
jack Feinberg In Physics Today, Vol. 41.
electric field, however, charges move altered by doping them with impuriNo. 10, pages 46-52; October. 1988.
about 10,000 times faster in gallium
ties or by drawingatomsout of the latPHOTORE.fRAC11VE MA TERJALS AND
arsenide than they do in barium titice. For example,one of us (Feinberg)
'J'HEm APPuCA11ONS I " D: TOPICS IN
tanate. For the same inddent light in- and StephenDucharmeof U.S.c.found
APPUm PHYSICS. Volumes 61 and 62.
tensity, therefore. the photorefractive
that when a crystal of barium titanate
Edited by P. GOnter and j.-P. HWgDard.
SprinpT-Ver!ag.
1988.
effect evolves much more rapidly in is heated in an oxygen-freeenvironNONUNEAll OPTICAL PHAsE CONJUGAgallium arsenide than it does in barium
ment to removesomeoxygenfrom the
11ON. Special Issue. Guest edited by
titanate. The resulting refractive-index
crystal lattice,its photorefractlvepropDavid M. Pepper. IEEE journal of QIIaIIgrating Is not as strong in gallium ar- ertiesare alteredmarkedlybecausethe
film EI«tronIcs, Vol 25. No.3. pages
senide, however, as it is in barium tI- dominant chargecarriers are changed
312-647; March, 1989.
tanate. The optical properties of the from holesto electrons.
74
Scn:N1m c AMERICAN
October1990