A holographic recording medium comprising a conductive substrate, a photoconductive layer and an electrically alterable layer of a linear, low molecular weight hydrocarbon polymer has improved fatigue resistance. An acrylic barrier layer can be interposed between the photoconductive and electrically alterable layers

Gange, Robert Allen

NASA Technical Reports Server

United States Patent [ I9]  
Gange 
4,O 12,253 
[45] Mar. 15, 1977 
~~ 
HOLOGRAPHIC RECORDING MEDIUM 
Inventor: Robert Allen Gange, Belle Mead, 
N.J. 
Assignee: RCA Corporation, New York, N.Y. 
Filed: Oct. 16, 1974 
Appl. No.: 515,369 
Related U.S. Application Data 
Division of Ser. No. 309,754, Nov. 27, 1972, 
abandoned. 
U.S. CI. .................................. 96/1.5; 96/27 H; 
96/1.1 
Int. CI.' ..................... G03G 5/00; G03G 5/10; 
G03C 5/04 
Field of Search ............ 96/1.1, 1.5, 1 PS, 1 M, 
96/27 H; 346/74 TP, 77 E 35013.5; 17816.61 
P 
1561 References Cited 
UNITED STATES PATENTS 
3,243,293 3/1966 Stockdale 
3,266,045 8/1966 Schattery . 
3,565,612 2/I971 Clark .... 
3,598,581 8/1971 Reinis ... 
3,640,708 2/1972 Humphri 
3,723.1 12 3/1973 Luebbe 
3,741.759 6/1973 Cicarelli 
Primary Examiner-David Klein 
Assistant Examiner-John L. Goodrow 
Attorney, Agent, or Firm-G. H. Bruestle; B. E. Morris; 
J. T. Harcarik 
[571 ABSTRACT 
A holographic recording medium comprising a conduc- 
tive substrate, a photoconductive layer and an electri- 
cally alterable layer of a linear, low molecular weight 
hydrocarbon polymer has improved fatigue resistance. 
An acrylic barrier layer can be interposed between the 
photoconductive and electrically alterable layers. 
3 Claims, 2 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=20080012220 2019-08-30T03:58:45+00:00Z
U.S. Patent Mar. 15, 1977 
I I 
7 4  I 
4,012,253 
1, 
HOLOGRAPHIC RECORDING MEDIUM 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provision of Section 3'05 of the National 
Aeronautics and Space Act of 1958, Public Law 
85-568 (72 Stat. 435; 42 U.S.C. 2457). 
This is a division of application Ser. No. 309,754, 
filed Nov. 27, 1972, now abandoned. 
BACKGROUND OF THE INVENTION 
This invention relates to an improved electrically 
alterable recording medium. More particularly, this 
invention relates to an electrically alterable holo- 
graphic recording medium which does not fatigue after 
numerous write-erase cycles. 
Phase holograms can be formed on a heat softened 
thermoplastic surface which selectively deforms during 
exposure to an applied charge pattern, as has been 
disclosed by Urbach in US. Pat. No. 3,560,205. Ac- 
cording to this system, a corona discharge device con- 
nected to a suitable recording medium ionizes the air 
near the surface of the thermoplastic whereupon posi- 
tive ions are deposited uniformly on the surface of the 
thermoplastic. This surface is now exposed to an image 
by means of coherent light split into an object beam 
and reference beam in known manner. The light inter- 
acts with a photoconductor which causes a redistribu- 
tion of the charge in the areas where it impinges on the 
thermoplastic. When the thermoplastic is charged 
again, the electric field increases in the previously illu- 
minated areas. The thermoplastic is then exposed to a 
temperature sufficient to soften its surface which de- 
forms according to the electric field, becoming thinner 
or forming valleys in the areas of high field intensity. 
When cooled to room temperature, a hologram is re- 
corded as a thickness variation or pattern in the ther- 
moplastic. Such holograms can be erased in the ab- 
sence of exposure and corona discharge by heating the 
thermoplastic above its softening point to a tempera- 
ture sufficient to allow the surface tension of the ther- 
moplastic to revert to its undeformed state. 
This holography system would be highly useful in 
optical memory devices due to its ability to write and 
erase a series of holograms in situ. However, the ther- 
moplastics employed heretofore gradually degrade or 
show fatigue after repeated write-erase cycles, when 
sharp, clear holograms can no longer be formed. In 
some cases, fatigue is discernible after several write- 
erase cycles. 
Electrically alterable recording media described as 
suitable for recording deformation holograms comprise 
conductive substrates, a photoconductive insulating 
layer over the substrate and a deformable insulating 
thermoplastic layer over the photoconductor. The ther- 
moplastics suggested heretofore for this system include 
natural resins, such as glycerol and pentaerythritol 
esters of partially hydrogenated rosin, and synthetic 
polymers such as p-alpha-methylstyrene, terpolymers 
of styrene, indene and isoprene, polyterpene resins 
from ppinene, olefin-diene resins, styrene-butadiene 
resins, polystyrene, coumaroneindene resins, chlori- 
nated polyphenyl resins, styrene-acrylate copolymers, 
vinyltolueneacrylate copolymers, alkyd resins, mix- 
tures of styrene and silicone resins and unreactive phe- 
nol-formaldehyde resins. 
These materials show fatigue after repeated write- 
erase storage cycles; in many cases the onset of fatigue 
4,012,253 
is noted after a few hundred cycles. In order to be able 
to be incorporated into a practical optical memory 
system, the recording medium must be immune to fa- 
tigue over numerous write-erase cycles. Thus the 
5 search for an improved electrically alterable or deform- 
able recording medium for holographic storage has 
continued. 
SUMMARY OF THE INVENTION 
10 I have discovered that linear, low molecular weight 
hydrocarbon polymers can be employed as the electri- 
cally alterable layer of a holographic recording me- 
dium. The resultant recording medium can withstand 
many write-erase cycles with no discernible change in 
15 its physical or chemical properties nor molecular struc- 
ture, and it can continue to form and erase sharp, clear 
, holograms. 
BRIEF DESCRIPTION OF THE DRAWINGS 
2o FIG. 1 is a cross-sectional view of one embodiment of 
a recording medium of the invention and FIG. 2 is a 
cross-sectional view of another embodiment of a re- 
cording medium of the invention. 
25 DETAILED DESCRIPTION OF THE INVENTION 
The present holographic recording medium com- 
prises a conductive substrate, a photoconductive insu- 
lating layer on the substrate and a linear low molecular 
weight hydrocarbon polymer as the electrically alter- 
The conductive substrate is conventional, and can be 
flexible or rigid. It can be made of a conductive metal, 
such as aluminum, brass, copper and the like; or of a 
non-conductive substrate coated with a thin conductive 
35 layer. Suitable non-conductive substrates include glass, 
quartz, polymers and the like, which can be coated with 
a conductor such as tin oxide, copper iodide, indium 
oxide and the like. Preferably, the substrate is transpar- 
4o ent. Such substrates are well known, and include glass 
coated with a thin, transparent tin oxide or indium 
oxide film, or like coated polymeric films of polyethy- 
lene terephthalate, polycarbonates, polyurethanes, 
acrylics and the like. 
The insulating photoconductor is also conventional 
and can be inorganic, such as a layer of amorphous 
seleneium, or pigments such as cadmium sulfide, cad- 
mium selenide, zinc sulfide, zinc selenide, zinc oxide, 
lead oxide, lead sulfide, mercuric sulfide, antimony 
50 sulfide, mercuric oxide, indium trisulfide, titanium di- 
oxide, arsenic sulfide, gallium selenide, lead iodide, 
lead selenide, lead telluride, gallium telluride, mercuric 
selenide, and the like. Alternatively, the photoconduc- 
tor can be organic, such as anthracene-3-benzylidene- 
55 aminocarbazole, poly-N-vinylcarbazole, 2,5-bis(p- 
aminophenyl- I )- 1,3 ,4-oxadiazole, 1 ,Cdicyanoaphtha- 
lene, 2,4-diphenylquinazolin, 1 -methyl-2-( 3',4'-dihy- 
droxymethylenepheno1)benzimidazole and the like. 
These and other organic photoconductors can be com- 
60 plexed with Lewis acids, such as 2,4,7-trinitrofluore- 
none, as is known. High sensitivity photoconductors 
such as poly-N-vinylcarbazole containing 2,4,7-trini- 
trofluorenone are preferred. 
Nonfatiguable linear hydrocarbons useful as the elec- 
65 trically alterable layer have low molecular weights, i.e., 
about 300 to about 2000, which preferably are solid at 
room temperature but which have low softening points. 
Suitable materials include microcrystalline natural 
30 able layer on the insulating layer. 
45 
2 J 
waxes or low molecular weight, linear polymers of 
ethylene. 
These hydrocarbons are straight chains of -CH2- 
units, substantially without branched chains, unsatura- 
tion, or active end groups. They are further character- 
ized by a narrow molecular weight distribution, that is, 
most of the molecules of the resin have similar chain 
lengths. Suitable commercial products include Boreco 
Polywax, a linear polyethylene available in molecular 
weights in the range about 1000 to about 2000 and "Be 
Square" 190/195 Amber or No. I white microcrystal- 
line waxes having about 60 carbon atoms per molecule, 
available from the Boreco Division of Petrolite Corpo- 
ration. These materials have melting points between 
about 40" C. and about 125" C. 
Although the reason for the efficacy of the present 
materials is not completely understood, it is believed 
the straight chain configuration is responsible for the 
long life of the recording medium described herein, 
2,253 
4 
source, reaction between the thermoplastic and the 
photoconductor may be encountered. In that case it 
may be desirable to apply a barrier layer between the 
photoconductor layer and the recording layer. A thin 
5 layer of highly polar, transparent material such as an 
acrylic resin commericially available as Elvacite 20 13 
resin from du Pont de Nemours and Company can be 
employed for this purpose. This resin is a low molecular 
weight methyl/n-butylmethacrylate copolymer having 
I O  an inherent viscosity of 0.2, determined from a solution 
containing 0.25 gram of polymer in 50 ml of chloro- 
form at 25" C. 
A defect-free, thin film of the barrier polymer can be 
applied by dipping the layer to be coated into a solution 
15 of the acrylic resin or by spin coating in known manner. 
The thickness of the barrier layer, although not critical, 
should be regulated so that an excessive amount of the 
applied electric field will not be lost across this layer. 
Suitably the barrier layer can be up to about 0.25 mi- 
The invention will be further illustrated by the fol- 
lowing examples, but it is to be understood that the 
invention is not meant to be limited to the details de- 
scribed therein. In the examples, parts and percentages 
EXAMPLE .I 
since little cross-linking or reaction between end 20 cron in thickness. 
groups of these straight chain molecules occurs in the 
presence of high electric fields throughout numerous 
cycles of softening and hardening or heating and cool- 
ing. 
convenient manner, but is preferably applied from 
solution, as by brushing, dip coating, spraying and the 
like. Electrostatic deposition can also be employed. A glass substrate coated with a thin, transparent in- 
The electrically alterable layer must be thick enough so dium Oxide layer was into a Solution of Poly-N- 
that well defined hills and valleys may be formed in it 30 vinylcarbazole: trinitrofluorenenone ( 10: 1 ) in 1: 1 p- 
upon exposure, but the exact thickness is not critical dioxane-methylene chloride So as to apply a layer about 
and can be up to 5 microns or more, Thicker layers 1-2 microns thick. Another Solution Of 0.35 parts of 
have deeper peaks and valleys in the holographic pat- Amber wax, a microcrystalline hydrocarbon wax hav- 
tern and thus may give a clearer, better defined bolo- ing about 60 carbon atoms per molecule in straight 
gram; but the available holographic density, Le., the 35 Bareco Company and having a 
number of bits that can be stored in a given hologram melting Point of 88" c . ,  softening point (Ring and ball) 
area, will be decreased due to reduced spatial band- of 42" c . ,  and a Saybolt viscosity at 99" C. of 75 in 40 
width. The optimum thickness for a given application parts by Of n-hexane was Prepared at about 
can be readily determined by a series of test runs. Pref- 60"-65" c. The coated Substrate was in the 
erably, the thickness of the electrically alterable layer 40 Per Second 
will be from about 0.3 to about 0.7 microns thick. so as to deposit a layer about 0.6 micron thick when the 
Referring now to the drawings, in FIG. 1 there is 
shown a cross-sectional view of a recording medium of Holograms were formed in the recording medium 
the invention. The recording medium comprises a con- prepared as above with a helium-neon laser using a 
ductive layer 1 having a photoconductive layer 2 45 Continuous corona discharge, writing with heat pukes 
thereon and an electrically deformable layer 3 over the Of 2-5 in duration and erasing by applying 
layer 2. heat above the softening point of the wax in the ab- 
FIG. 2 is a cross-sectional view of another embodi- SenCe Of light, Using heat pulses Of similar duration. 
merit of a recording medium. This recording medium After 7000 Write-erase Cycles, the recording medium 
has a conductive layer 4, a photoconductive layer 5 50 showed no sign Of fatigue. 
thereon, a thin barrier layer 6 on the layer 5 and an EXAMPLE 2 
electrically deformable layer 7 over the barrier layer 6. was followed except 
Holograms can be written and erased in the abovede- that a barrier layer was applied onto the photoconduc- scribed recording medium many times without sign of tive layer as follows. First, o.3 part of an acrylic resin 
source, such as pulsed ohmic heating via the transpar- de Nemours and Company) was dissolved in 4o parts 
stant thermal bias using a suitable constant direct cur- resin solution was spun onto the photoconductive layer 
heat gradient over short time intervals. This thermal 60 barrier layer about looo A thick had been deposited. 
bias can be above room temperature, suitably just 
below the softening point of the linear hydrocarbon EXAMPLE 3 
employed, which minimizes and controls the tempera- The procedure of Example 1 is followed except that 
ture excursions on pulsing, and results in further im- a barrier layer is applied onto the photoconductive 
provement in the life of the recording medium. 65 layer as follows. A solution containing 0.2 part of Elva- 
In the event that the thermoplastic is exposed to cite 2013 in 5 parts by volume of acetone warmed to 
sustained elevated temperatures for long periods of 60" C. is prepared to which is added dropwise 50 parts 
time, such as when using direct current as the bias heat by volume of ethanol also warmed to about 60" C. A 
The electrically alterable layer can be applied in any 25 are by weight unless otherwise noted. 
available 
and withdrawn at a rate Of * 
was 
The procedure of Example 
'an be written using a pulsed heat 5' (available commercially as Elvacite 20 13 from dupont 
'onductor layer, Or, preferably, exposed to a 'On- by volume of acetone at room temperature. The acrylic 
rent and current pulses which produce a at about 1000 rpm. After evaporating the solvent, a 
4,O 12,253 
5 6 
glass plate having a photoconductive layer as in Exam- 
ple 1 is immersed in the above solution and withdrawn 
at a rate of 2 inches per second. A uniform barrier film 
about 1000 A thick is deposited onto the photoconduc- 
tive layer. 
I claim: micron in thickness. 
1. A medium for recording and erasing phase holo- 
grams in the form of a surface relief pattern consisting 
essentially of in sequence an electrically conductive 
substrate, a photoconductive insulating layer, a trans- I O  barrier layer is up to about 0.25 micron in thickness. 
storage layer consisting essentially of a linear, micro- 
crystalline hydrocarbon polymer having a molecular 
weight in the range from about 300 to about 2000. 
2. A medium according to claim 1 wherein the bar- 
5 rier layer is a layer of an acrylic resin up to about 0.25 
3. A medium according to claim 1 wherein the bar- 
rier layer is a layer of a methylh-butyl-methacrylate 
copolymer having an inherent viscosity of 0.2 and the 
parent, polar, barrier layer, and an electrically alterable * * * * *  
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Holographic recording medium

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