Spatial light modulation (22) in a III-V single crystal (12), e.g., gallium arsenide, is achieved using the photorefractive effect. Polarization rotation created by beam coupling is utilized in one embodiment. In particular, information (16)on a control beam (14) incident on the crystal is transferred to an input beam (10), also incident on the crystal. An output beam (18) modulated in intensity is obtained by passing the polarization-modulated input beam through a polarizer (20)

Cheng, Li-Jen

Gheen, Gregory O.

Partovi, Afshin

NASA Technical Reports Server

United States Patent 1191 pi] Patent Number: 4,898,436 
Cheng et al. [45] Date of Patent: Feb. 6, 1990 
[54] SPATIAL LIGHT MODULATION IN 
COMPOUND SEMICONDUCI'OR 
MATERIALS 
[75] Inventors: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed 
Li-Jen Cheng, LaCrescenta; Gregory 
0. Gheen, Pasadena; Afshin Partovi, 
Los Angeles, all of Calif. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
172,007 
Mar. 23,1988 
[51] Int. CI.4 ......................... GO3H 1/13 G02F 1/35; 
G02B 27/28 
[52] U.S. CI. .................................. 350/3.64; 350/3.85; 
350/389; 350/390 
[58] Field of Search .................... 350/3.62, 3.64, 3.85, 
350/389, 390 
[561 References Cited 
U.S. PATENT DOCUMENTS 
4,761,059 8/1988 Yeh et al. ....................... 350/3.64 X 
4,767,195 8/1988 Pepper ........................... 350/3.62 X 
Primary Examiner-Bruce Y. Arnold 
Assistant Examiner-David J. Edmondson 
Attorney, Agent, or Firm-Thomas H. Jones; John R. 
Manning; Charles E. B. Glenn 
Spatial light modulation (22) in a 111-V single crystal 
(12), e.g., gallium arsenide, is achieved using the 
photorefractive effect. Polarization rotation created by 
beam coupling is utilized in one embodiment. In particu- 
lar, information (16)on a control beam (14) incident on 
the crystal is transferred to an input beam (lo), also 
incident on the crystal. An output beam (18) modulated 
in intensity is obtained by passing the polarization- 
modulated input beam through a polarizer (20). 
PI ABSTRACI' 
10 Claims, 1 Drawing Sheet 
12 
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U.S. Patent Feb. 6,1990 
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BEST MODES FOR CARRYING OUT THE 
INVENTION SPATIAL LIGHT MODULATION IN COMPOUND SEMICONDUCTOR MATERIALS 
Gallium arsenide (GaAs) and other 111-V compound 
The invention described herein was made in the per- 5 semiconductors, Such as InP, and 1I-W compound 
semiconductors, such as CdTe, are potential materials 
for spatial light modulation. In particular, semi-insulat- 
ing GaAs is known to be photorefractive. Photorefrac- 
tive crystals have potential optical processing applica- 
10 tions, such as spatial light modulation, holography, 
beam-steering, convolution and correlation, incoherent- 
been demonstrated using conventional oxide materials, 
15 (Bi12Si020), and lithium niobate (LiNbO3). However, 
the use of GaAS for optical devices has many advan- 
tages over those of conventional oxide materials. They 
include hi& sensitivity, low power consumption, infra- 
modulators red operation, fast response, tunable information stor- 
Processing and Optical comPuth3. An ideal Spatial fight 20 age time, availability of high quality and large-size crys- 
tals, and finally, the most important one, compatibility 
with semiconductor lasers, sensors, and VLSI technolo- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of public L~~ 96-517 (35 USC 
202) in which the antractor has elected to title. 
TECHNICAL FIELD 
The Present invention to 'Patid light modula- toGoherent light conversion, and so on. These have more to transferring an image 
wried On One light beam to =Other light beam where such as titanate (BaTiOs), bismuth silicon oxide the light beams intersect in a GaAs crystal. 
BACKGROUNDART 
The availability of high performance spatial fight 
very dtical  for the Progress in image 
modulator should have many advantageous characteris- 
tics of performance, such as high sensitivity, fast re- 
sponse, high uniformity, low power consumption, sim- 
ple fabrication, low cost, and compact size. In addition, 
it should be compatible to technologies of the light 
source (i.e. lasers), sensor and VLSI. However, such 
spatial light modulators do not exist at this moment. 
DISCLOSURE OF LNVENTION 
In accordance with the invention, spatial light modu- 
lation in GaAs is disclosed, namely, an image carried by 
one light beam can be transferred to another beam when 
they intercept each other in a GaAs crystal. The tech- 
nique used is a cross polarization beam coupling pro- 
cess, in which two coherent beams with their polariza- 
tion bisecting the major electro-optic axes intercept 
each other in the crystal and exchange energy via self- 
induced index grating created due to the photorefrac- 
tive effect. During this process, there is no net energy 
transfer, but the resultant polarization of each beam is 
rotated due to the fact that energy transfer is polariza- 
tion-dependent. By using a polarizer, the diffracted 
beam bearing the image can be separated from the origi- 
nal one. Consequently, the image transfer can be 
achieved. 
Spatial light modulation in GaAs has not previously 
been demonstrated, and is accordingly a siBnifcant step 
toward the realization of using GaAs crystals for real- 
time spatial light modulation, one of the critical pro- 
cesses for optical information processing. Because they 
are based on the same semiconductor materials technol- 
ogy, this provides a potential to integrate optical pro- 
cessing devices with advanced technologies of VLSI 
and opto-electronics (lasers and sensors) for the devel- 
opment of future versatile, compact, and sophisticated 
intelligent systems to find use in many technological 
areas. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. la is an illustration of the experimental setup 
demonstrating the essential features of spatial light mod- 
ulation using cross polarization beam coupling configu- 
ration in accordance with the invention; and 
FIG. l b  is a schematic diagram depicting the electro- 
optic properties of GaAs with space-charge field per- 
pendicular to the (101) plane. 
gies. The successful demonstration of spatial light mod- 
ulation in GaAs disclosed herein is a significant step 
25 toward the alization of using GaAs for real-time spatial 
light modulation, beamsteering, and data transfer. 
Gallium arsenide is the most technologically impor- 
tant compound semiconductor for the development of 
high speed electronics and opto-electronics. In addition, 
30 the GaAs crystal is electro-optic. The semi-insulating 
GaAs crystals are photorefractive and can be used in 
optical information processes. 
This invention concern the first demonstration of 
spatial light modulation in GaAs crystals using a cross 
35 polarization beam coupling technique. At the present 
time, the quality of the transferred image is less than 
optimum, due to equipment limitation, poor crystal 
quality and other factors. Nevertheless, spatial light 
modulation using a GaAS crystal has been demon- 
40 strated. This establishes the potential of GaAs as an 
optical processing medium. 
The ordinary beam coupling process includes: (1) 
formation of intensity interference pattern due to the 
interception of two coherent light beams; (2) creation of 
45 a periodic space charge distribution due to net carrier 
transfer from light regions to the dark regions; (3) gen- 
eration of a refractive index grating via electro-optic 
effects due to the electric field associated to the space 
charge, which is about 90" spatially phase shifted with 
50 respect to the interference pattern; (4) diffraction of the 
beams due to the existence of the grating in which one 
diffracted beam constructively interferes with the in- 
coming beam and the other is destructively interfered 
because of the phase shift. This results in a net energy 
55 transfer from one beam to the other. The transfer direc- 
tion depends on the crystal orientation with respect to 
incident beam and light polarization directions. 
The cross polarization beam coupling process of the 
invention is a special case of the beam coupling process. 
60 In this process, the polarizatiion of the incoming beams 
bisect the two major electro-optical axes and the elec- 
tric field vector of light can be decomposed into two 
equal components with each being parallel to one elec- 
tro-optical axis. Because the refractive index change 
65 along the two electro-optic axes are of equal magnitude 
but opposite sign, the diffracted beam from one compo- 
nent experiences a constructive interference with the 
incoming beam and that from the other experiences a 
4,898,436 
3 4 
destructive interference. Consequently, the electric ness of the crystal, the transferred information is 
field vector of the resultant diffracted beam is rotated smeared in the direction parallel to the plane of intersec- 
by 90" toward the axes which experiences constructive tion. The smear length is given by the projection of the 
interference. Then, the diffracted beam can be sepa- coupling length of the control beam onto the output 
rated from the original beam by using a polarizer. It 5 beam's wave front. Assuming near normal incidence 
should be noted that in the cross polarization beam onto the crystal for both beams, the smear length is 
coupling process, there is no net energy transfer from approximately equal to D sin 8, where D is the thick- 
one to the other. ness of the crystal and 8 is the angle of intersection in 
FIG. la illustrates a cross Polarization beam coupling the crystal. Since the crystal must have substantial 
configuration used in the demonstration of spatial light 10 thickness to ensure adequate coupling, 8 must be small 
modulation in GAS. One Of the beams 10 b p h g h g  in order to produce good resolution. 
upon the GaAs Crystal 12 is a uniform collimated beam An experimental demonstration was performed using 
called the input beam- The Other beam 14 Passes the arrangement shown in FIG. la A semi-insulating, 
through a transparency 16 c o n t a k g  the letters JpL liquid-encapsulated Czochrhki-grown G A S  crystal 
that is imaged onto the crystal by a lens (not shown). 15 with dimensions 5 mmx8 mmx 10 along the 
This beam is referred to as the beam. Both < l O i > ,  <101>, and <010> crystalline directions, 
beams are parallel to the x-z plane and polarized in the respectively, was used for the photorefractive material. 
y direction as defined in FIG. lb. Both (1Oi) surfaces were polished with the technique 
The formation of the interference pattern along the commonly used for preparing G~~ integrated-circuit < 101 > direction results in a modulated space-charge 20 wafers. 
field along the same direction. This in turn results in a The input beam and the control beam had intensities 
modulated refractive index as determined by the elec- of and 30 mW/cmz, respectively, and were tro-optic tensor. The principal axes of this modulated from a 1.5 ~m He-Ne The polarization of the refractive index can be shown to lie along the x' and y' incident beams was to the <olo> direction, 
25 bisecting the principal electro-optic axes created by the axes, as illustrated in FIG. lb. Because the polarization of the light incident upon light-induced space-charge field. 
A transparency containing the opaque letters JPL the crystal 12 bisects the two principal axes, the light can be decomposed into two equal components with was placed in the control beam and imaged onto the polarizations along the x' and y' directions. For x'-pola- GaAs crystal. The angle of intersection between the rized light, energy is coupled from the control beam 14 30 
coupled from the input beam 10 to the control beam 14. this the measured gain coefficient Of the beam 
energy transfer; however, the polarizations of the two A G1an-nompson polarizer 
beams are rotated in opposite directions, since the two 35 was placed behind the crystal* and the Output was pro- 
along different directions jected onto a screen and viewed using an infrared televi- 
of polarization. Thus, the input beam 10 becomes modu- sion camera and monitor. 
lated in p o l h t i o n  across its wave front. k e a s  cone- While both positive and negative images were ob- 
spending to the dark letters JPL have the Same served, non-uniformities in the results were considered 
tion as the incident light, no coupling has oc- 40 to arise from non-ideal bulk and surface qualities of the 
curred. Areas corresponding to the light background GAS sample and other miscellaneous factors affecting 
wiln be rotated in polarization, since they haare partici- the experiment. However, better-quality images are 
pated in the beam-coupling process. expected as crystal quality is improved. In addition, 
To obtain an output beam 18 that is moddated in improvement in image resolution Can be achieved by 
intensity, the polarization-modulated input beam 10 is 45 reducing the angle of intersection between the beams, as 
passed through a polarizer 20. The combination of the dk~ussed above. The resulting decrease in the beam- 
single crystal 12 and the polarizer 20 forms the spatial Coupling coefficient Can be offset by applying an elec- 
light modulator 22 of the invention. The use of a cross tric field to the Crystal. 
INDUSTRIAL APPLICABILITY polarizer to convert a two-dimensional polarization- modulated beam into a corresponding intensity- 50 
modulated beam is well-known. When the orientation The spatid light modulator of the invention is useful 
of the polarizer 20 is set perpendicular to the polariza- in a variety of applications, including image PrOCesSing 
tion of the incoming beam 10, the incoming beam is and Optical computing. 
completely blocked; and only those areas that were Thus, there has been disclosed a spatial light modula- 
rotated in polarization pass. The result is a positive 55 tor, using the photorefractive effect in a 111-V material 
two-dimensional intensity-modulated beam. and employing beam coupling. Varios changes and 
When the polarization is set perpendicular to the light modifications of an obvious nature will be readily ap- 
rotated in polarization, the unrotated light passes, and a parent to one Of ordinary skill in this art, and all such 
negative two-dimensional intensity-modulated beam is changes and modifications are considered to fall within 
observed. In practice, the actual setting of the polarizer 60 the scope of the invention, as defined by the appended 
for the best performance may depend on a number of claims. 
to the input beam 10. F~~ yt-polaked fight, energy is 
The net result ofthese two coupling processes is zero 
input and Control beams Was 13" outside the Crystal. For 
coupling was 0.15/cm, and the dculated 
about 3*4 
factors, such as the intensity ratio of two incoming 
beams and the polarization changes along the beam 
paths in the system. 
A fundamental l i t a t ion  on the quality of the output 
image 24 is determined by the angle of intersection 
between the two beams. Because the beams are not 
parallel and information is transferred over the thick- 
What is claimed is: 
1. Apparatus for transferring information encoded on 
one beam of electromagnetic radiation to another beam 
(a) a spatial light modulator comprising (1) a semi- 
insulating single crystal of a 111-V or 11-VI mate- 
rial, said single crystal cleaved to form a slab hav- 
65 of electromagnetic radiation comprising: 
4,89 8,43 6 
5 6 
ing a front-major surface and a rear major surface 6. The method of claim 5 wherein said single crystal 
and (2) a polarizer adjacent said rear surface; comprises GaAs. 
(b) means for forming a coherent input beam polar- 7. A method for cross-coupling an input beam and a 
ked in a given direction; control beam carrying information, both beams of co- 
(c) means for directing said input beam onto said 5 herent electromagnetic radiation and polarized in a 
similar direction and incident on the surface of a single 
crystal Cpmprising a 1II-V Or II-VI semiconductor ma- 
(a) fo-g an intensity interference Pattern along a 
front major surface of said single crystal to cause 
diffraction of said beam, said beam passing through 
(d) means for f o h g  a coherent control beam which 
terial, comprising: said polarizer to form an output beam; 
is coherent with said input beam and 
similar direction as said input beam; 
mation; and 
onto said front major surface of said single crystal 
to interact with said input beam within said single 
crystal to cause said input beam to be modulated in 
polarization across its wave front, whereby said 
ferred to said input beam and whereby said output 
beam from said polarizer is modulated in intensity. 
2. The modulator of claim 1 wherein said output 
3. The modulator of claim 2 wherein said single crys- 25 
in a 10 given direction in said crystal due to the intercep- 
tion of said input and control beams; 
given direction, which results in a modulated re- 
fractive index; 
(c) generating two components of polarization such 
that energy from one beam is coupled to the other 
beam along one direction of polarization and en- 
ergy from said other beam is coupled to said one 
beam along another direction of polarization to 
result in a net zero energy transfer and a rotation of 
the polarization of said two beams in opposite di- 
rections such that said input beam is modulated in 
polarization across its wave front; beam is coherent and vertically polarized. (d) passing said polarization-modulated input beam 
though a pol-er upon its emergence from said 
tal comprises GaAs and said slab is oriented parallel to single crystal to obtain an output beam that is mod- 
its (1 10) plane. ulated in intensity; and 
wherein said vertical (e) rotating said polarizer to select rotated light from 
polarization is along the <010> direction of said single unrotated light. 
crystal. 8. The method of claim 7 wherein said single crystal 
5. A method for transferring information from a con- comprises G ~ A ~ .  
trol beam of polarized, coherent electromagnetic radia- 9. The method of claim 7 wherein the orientation of 
tion incident on a single Crystal material to an input said polarizer is set perpendicular to the polarization of 
beam Of polarized, coherent electromagnetic radiation said incoming beam to block said incoming beam and 
polarized in a similar direction as said control beam and 35 permit only those areas rotated by polarization to pass, 
coherent therewith and incident on said single crystal to produce a positive two-dimensional intensity- 
comprising placing said single crystal such that both modulated beam. 
said beams interact within said single crystal to diffract 10. The method of claim 7 wherein the orientation of 
said input beam and modulate said beam in polarization, said polarizer is set perpendicular to the light rotated in 
said single crystal comprising a 111-V or 11-VI semicon- 40 polarization to block said rotated light and permit un- 
ductor material, said method further comprising placing rotated light to pass, to produce a negative two-dimen- 
a polarizer in the path of said polarization-modulated sional intensity-modulated beam. 
beam to obtain an output beam modulated in intensity. 
(e) means for encoding said control beam with infor- (b) creating a modulated space-charge field along said 
(' means for directing said encoded beam 15 
encoded On said beam is trans- 20 
4* The modu1ator of claim 
30 
* * * * *  
45 
50 
55 
60 
65 


Spatial light modulation in compound semiconductor materials

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