3
Electrically accessible Lippmann hologram memory T. Yasuhira, Y. Mitsuhashi, T. Morikawa, J. Shimada and T. Kamijo Using Fe-doped LiNbO 3 crystals we have constructed Lippmann-type holograms where access both in writ- ing and in reading is performed electrically. Discussions are given for the interlayer cross talks in a stacked structure. Device operations are demonstrated with a double-layered memory. Irradiation of some kind of ferroelectric crystals with light beams has long been known to cause local changes in the refractive index along the c axis of the crystals.' Because of this property, these crystals have been considered as promising media for hologram storage. 2 Based furthermore on the fact that this photoinduced effect can be controlled by an electric field externally applied along the c axis, 3 a layered structure was suggested and demonstrated for an elec- trically accessible holographic memory. 4 In the sug- gested structure, however, a configuration of ordinary hologram recording was adopted, where the c axis lies in the hologram plane, and consequently the external field is to be applied along a direction lying in this plane. Therefore a comparatively high voltage must be sup- plied for a satisfactory device operation. Our opinion-is that higher electric field can be ob- tained along the c axis with a comparatively low voltage when a thin c-cut wafer is used instead, and the voltage is applied between the c faces. In this configuration, the grating vector of the recorded hologram must be parallel, or nearly parallel, to the c axis, and therefore the most suitable hologram recording scheme is such that the object beam incident on the c face is made to interfere with the reference beam traveling in the op- posite or conjugate direction, resulting in Lippmann type holograms.' In order to implement this idea, we constructed an electrically accessible Lippmann hologram (EALH) memory using an Fe-doped LiNbO 3 crystal. In this paper, we report the basic properties of our EALH's and analyze the interlayer crosstalks for the case when they are stacked to a layered structure. We will also dem- onstrate a double-layered EALH memory. T. Kamijo is with Hosei University, School of Engineering, Koganei, Tokyo, 184 Japan; the other authors are with Electrotechnical Lab- oratory, Tanashi, Tokyo, 188 Japan. Received 15 March 1977. The poled LiNbO 3 crystal used in our experiment is doped with Fe by 0.1 mol%. The crystal was sliced in a c-cut wafer of 7 X 7 mm 2 and polished to a thickness of 100 /im. n 2 0 3 was sputtered, 2 mm in radius, on the central part of each side of the wafer. In the wafer, Lippmann holograms were recorded at a spatial frequency of 3000 lines/mm. The direction of the grating vector was tilted off the c axis by an angle of 50 so as to make the diffraction beam distinguishable from the specularly reflected beam. The argon ion laser beams of X = 488 nm were used in the hologram recording at a power density of 0.5 W/cm 2 . This recording power was chosen to make the recording sensitivity vary linearly with the applied electric voltage in the range between +1.0 kV and -1.5 kV. Beyond +1.0 kV a symptom of electrical break- down begins to appear. This is not the case for the re- verse voltage so far as -1.5 kV is not reached. In the hologram reconstruction the beam intensity was re- duced to 10 mW/cm 2 to avoid erasure of the holo- gram. In Fig. 1 the effects of applied voltages on the holo- gram diffraction efficiencies are shown for a single ele- ment of EALH. Compared with the case without ap- plied voltage an enhancement in the diffraction effi- ciency by a factor of 1.6 was observed when the holo- gram was recorded and reconstructed with an applied voltage of +1.0 kV, while a suppression by a factor of 2.6 was observed for an applied voltage of-1.5 kV. 5 The measured angular half-power width of the resultant Bragg diffraction grating is about 0.6°. When the voltage is switched from +1.0 kV to -1.5 kV, the Bragg angle shifts 6 by about 0.40. As a result of these char- acteristics a decrease in diffraction efficiency by a factor of 2.5 is obtained by such a voltage switching in holo- gram reconstruction. When one stacks such memory layers, there will naturally arise a problem of interlayer cross talks. Let us consider the case where both for recording a pattern selectively in one of the stacked layers and for recon- structing the pattern stored in this layer, we apply +1.0 kV on it and -1.5 kV on the other'layers. Without a loss of generality, we will analyze here the case of a tri- 2532 APPLIED OPTICS/ Vol. 16, No. 9 / September 1977

Electrically accessible Lippmann hologram memory

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Page 1: Electrically accessible Lippmann hologram memory

Electrically accessible Lippmann hologram memory

T. Yasuhira, Y. Mitsuhashi, T. Morikawa, J. Shimada and T. Kamijo

Using Fe-doped LiNbO 3 crystals we have constructed Lippmann-type holograms where access both in writ-ing and in reading is performed electrically. Discussions are given for the interlayer cross talks in a stackedstructure. Device operations are demonstrated with a double-layered memory.

Irradiation of some kind of ferroelectric crystalswith light beams has long been known to cause localchanges in the refractive index along the c axis of thecrystals.' Because of this property, these crystals havebeen considered as promising media for hologramstorage.2 Based furthermore on the fact that thisphotoinduced effect can be controlled by an electricfield externally applied along the c axis,3 a layeredstructure was suggested and demonstrated for an elec-trically accessible holographic memory.4 In the sug-gested structure, however, a configuration of ordinaryhologram recording was adopted, where the c axis liesin the hologram plane, and consequently the externalfield is to be applied along a direction lying in this plane.Therefore a comparatively high voltage must be sup-plied for a satisfactory device operation.

Our opinion-is that higher electric field can be ob-tained along the c axis with a comparatively low voltagewhen a thin c-cut wafer is used instead, and the voltageis applied between the c faces. In this configuration,the grating vector of the recorded hologram must beparallel, or nearly parallel, to the c axis, and thereforethe most suitable hologram recording scheme is suchthat the object beam incident on the c face is made tointerfere with the reference beam traveling in the op-posite or conjugate direction, resulting in Lippmanntype holograms.'

In order to implement this idea, we constructed anelectrically accessible Lippmann hologram (EALH)memory using an Fe-doped LiNbO3 crystal. In thispaper, we report the basic properties of our EALH's andanalyze the interlayer cross talks for the case when theyare stacked to a layered structure. We will also dem-onstrate a double-layered EALH memory.

T. Kamijo is with Hosei University, School of Engineering, Koganei,Tokyo, 184 Japan; the other authors are with Electrotechnical Lab-oratory, Tanashi, Tokyo, 188 Japan.

Received 15 March 1977.

The poled LiNbO3 crystal used in our experiment isdoped with Fe by 0.1 mol%. The crystal was sliced ina c-cut wafer of 7 X 7 mm 2 and polished to a thicknessof 100 /im. n2 03 was sputtered, 2 mm in radius, on thecentral part of each side of the wafer.

In the wafer, Lippmann holograms were recorded ata spatial frequency of 3000 lines/mm. The direction ofthe grating vector was tilted off the c axis by an angleof 50 so as to make the diffraction beam distinguishablefrom the specularly reflected beam.

The argon ion laser beams of X = 488 nm were usedin the hologram recording at a power density of 0.5W/cm2 . This recording power was chosen to make therecording sensitivity vary linearly with the appliedelectric voltage in the range between +1.0 kV and -1.5kV. Beyond +1.0 kV a symptom of electrical break-down begins to appear. This is not the case for the re-verse voltage so far as -1.5 kV is not reached. In thehologram reconstruction the beam intensity was re-duced to 10 mW/cm2 to avoid erasure of the holo-gram.

In Fig. 1 the effects of applied voltages on the holo-gram diffraction efficiencies are shown for a single ele-ment of EALH. Compared with the case without ap-plied voltage an enhancement in the diffraction effi-ciency by a factor of 1.6 was observed when the holo-gram was recorded and reconstructed with an appliedvoltage of +1.0 kV, while a suppression by a factor of 2.6was observed for an applied voltage of-1.5 kV.5 Themeasured angular half-power width of the resultantBragg diffraction grating is about 0.6°. When thevoltage is switched from +1.0 kV to -1.5 kV, the Braggangle shifts6 by about 0.40. As a result of these char-acteristics a decrease in diffraction efficiency by a factorof 2.5 is obtained by such a voltage switching in holo-gram reconstruction.

When one stacks such memory layers, there willnaturally arise a problem of interlayer cross talks. Letus consider the case where both for recording a patternselectively in one of the stacked layers and for recon-structing the pattern stored in this layer, we apply +1.0kV on it and -1.5 kV on the other'layers. Without aloss of generality, we will analyze here the case of a tri-

2532 APPLIED OPTICS / Vol. 16, No. 9 / September 1977

Page 2: Electrically accessible Lippmann hologram memory

Write-in %1tage Vw+1.0 kV+1.0-1.5-1.5

0

2.0

1.0

9

eyt~

ua

.. Shift of Bi

_ .-'- -

----- -… 0

*0_ ~ *0___ .

-Q6 -0.4 -0.2

Angle Deviation of Rf

Fig. 1. Effects of applied voltages ciency in a single element of EALH.ages in writein and readout is indicVarious values of il are defined her

structing beam has n

ple-layered memory. When tthe intensity of the readout Tsented, with the notation ad(lowing equation:

P(1) = lEEPI + 77SEP2 + 77Si

+ 77SSP1 + 77ESP2 +

+ 7SSP1 + qsSP2 +

+ 6

Read-Out Voltoge VR where Pi (i = 1, 2, 3) is the intensity distribution of the+1.0 kV-1.5 pattern stored in the ith layer, and 6 is the noise com-+1.0 ponents due to the interference effects among the re-

°gle 0 constructed patterns. In this expression the termsGr!199 Angle T bearing Pi will contribute to the desired output Pi while

E _ the others are regarded as the cross talks from the pat-tern P2 and P3 intentionally stored in the second andthird layer, respectively. Using this expression and thedata given in Fig. 1, one should be able to evaluate thecross talks.

One should note, however, that in the case of dou-_ r -s . . -. ' ble-layered memory the cross talk originates solely from

w . -° -ok X~~7ES and 7SE- In this case, if the shift of the Bragg angle0 0.2 0.4 0.6 is large enough or reconstructing angular selectivity is.constructing Beam (deg) good enough to make l7ES or liSE negligible for 7hEE, one

will not suffer from the cross talk problem.n the hologram diffraction effi- The memory device which we fabricated to demon-The condition of applied volt- strate our multilayer concept comprises two EALH el-

ated in the Inset for each curve.at the point where the recon- ements stacked with a spacing of 1 mm. The pattern

o angle deviation. stored in each layer and the result of electrical access ofthe two layers are shown in Fig. 2. The operationalconditions are the same as for the single layer describedearlier. The effect of one layer on the other with regard

;he first layer is addressed, to the absorption of the light beam and the erasure ofpattern P(1) can be repre- the stored pattern in recording were estimated to be less)pted in Fig. 1, by the fol- than a few percent. The first and the second photo-

graphs show the images formed by the recording lightthat passed through both layers. The third and fourth

EP3 (from the 1st layer) photographs are for the selective readout of the first7SSP3 (from the 2nd layer) layer and the second layer, respectively. The fifth

photograph was obtained when both layers were ac-'ESP3 (from the 3rd layer) cessed simultaneously.

Previous workers have described holographic image

(1) Writing

on

1 st Layer

(2) Writing

on

2nd Layer

(3) Reading

from

1 st Layer

(4) Reading

from

2 nd Layer

(5) Reading

from

Both Layers

For 1st Layer

Vw = +1.0

For 2nd Layer

Vw= -1.5

Vw = -1.5

Vw=+1.0

VR =+1.0 VR = -1.5

VR -1.5 VR +1.0

Fig. 2. Input and output images of the experimental double-layered EALH memory shown in sequence for selective writing and selectivereading.

September 1977 / Vol. 16, No. 9 / APPLIED OPTICS 2533

VR = +1.0

VR =+1.0

.. . v ..

Page 3: Electrically accessible Lippmann hologram memory

(001) W

<1 ) 5O_-m

(1) Y-Cut Plate

We have demonstrated that the EALH layeredmemory can be implemented with Fe-doped LiNbO3which shows the linear electrooptic effects. It can alsobe implemented with SBN having nonlinear elec-trooptic effects,4 and in this case, the resultant devicesare expected to work with much lower voltages.

We thank K. Sakurai and S. Ishihara for valuablediscussions. The assistance of S. Asakawa, M. Harada,and H. Fujino is gratefully acknowledged.

001)

(010> 5t

(2) X-Cut Plate

Fig. 3. Deformational damages caused in X- or Y-cut Fe-dopedLiNbO:3 plates by the irradiation with Ar laser beams having an in-

tensity level on the order of 2 W/cm2 .

deteriorations due to laser scattering in Fe-dopedLiNbO3's.7 The laser scattering has been found to becaused also by the bubblelike patterns originating fromimperfect poling of the crystals as well as by the gener-ation of such deformational damage 8 as shown in Fig.3. The deformational damages are .observed only in X-or Y-cut plates irradiated by argon laser beams andnever in c-cut plates. Thus, in our case of the EALHin the c-cut plate, such holographic image deteriorationsas those caused by the generation of deformationaldamages can be avoided to its advantage.

References1. A. Ashkin, G. D. Boyd, J. M. Dziedzic, R. G. Smith, A. A. Ballman,

J. J. Levinstein, and K. Nassau, Appl. Phys. Lett. 9, 72 (1966).2. F. S. Chen, J. T. LaMacchia, and D. B. Fraser, Appl. Phys. Lett.

13, 223 (1968).3. D. L. Staebler and J. J. Amodei, J. Appl. Phys. 43, 1042 (1972).4. J. B. Thaxter and M. Kestigian, Appl. Opt. 13, 913 (1974).5. W. D. Cornish, M. G. Moharam, and L. Young, J. Appl. Phys. 47,

1479 (1976).6. R. P. Kenan, C. M. Verber, and Van E. Wood, Appl. Phys. Lett.

24, 428 (1974).7. W. Phillips, J. J. Amodei, and D. L. Staebler, RCA Rev. 33, 97

(1972).8. T. Yasuhira, T. Morikawa, J. Shimada, and K. Sakurai, J. Appl.

Phys. 47, 1229 (1976).

From Piv icistscontznue to laughrMIR PublishingHouse, Moscow1968. rranslatedfrom he Russianby MNrs LorraineT Kapitanoff.

Schools of physics

When Niels Bohr visited the Physics Institute of the Academy ofSciences of the USSR, to the question of how he had succeededin creating a first-rate school of physicists he replied: 'Presumablybecause I was never embarrassed to confess to my students thatI amafool . . .'.

On a later occasion, when E M Lifschitz read out this sentencefrom a translation of the speech it emerged in the following form:'Presumably because I was never embarrassed to declare to mystudents that they are fools . . .'.

This sentence caused an animated reaction in the auditorium,then Lifschitz, looking at the text again, corrected himself ndapologized for his accidental slip of the tongue. However, P IKapitsa who had been sitting in the hall very thoughtfully notedthat this was not an accidental slip of the tongue. It accuratelyexpressed the principal difference between the schools of Bohrand of Landau to which E M Lifschitz belonged.

Reproduced from A RANDOM WALK IN SCIENCE, an anthologycompiled by R. L. Weber and published by the Instituteof Physics 1973.

2534 APPLIED OPTICS / Vol. 16, No. 9 / September 1977