A holographic recording medium with a substrate which has a diffraction grating composed of a number of spaced line ridges on the surface is discussed. The first layer consists of a metal with a plasma wavelength shorter than that of the selected light source. A second layer, or coating, on top of the first layer consists of a thin film of photosensitive emulsion. A diagram of the device is provided. The principles of operation are explained

Cowan, J. J.

English

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REPLY TO
ATTN OF: GP
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 20546
JUN 2 1 1974
TO: KSI/Scientific & Technical Information Division
Attn: Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP
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U.S. Patent No.
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NASA Patent Case No.
NOTE - If this patent covers an invention made by a corporate
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an invention of ..."
Bonnie L. Woerner
Enclosure
https://ntrs.nasa.gov/search.jsp?R=19740018833 2020-03-17T15:00:23+00:00Z
United States Patent<t • ^
Fletcher et al.
[in 3,815,969
[45] June 11, 1974
[54] HOLOGRAPHY UTILIZING SURFACE
PLASMON RESONANCES
[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration, in respect to an
invention by; James J. Cowan,
Huntsville, Ala.
[22] Filed: May 31, 1973
[21] Appl. No.: 365,644
[52] U.S. Cl 350/3.5, 96/38.3, 96/79
[51 ] Int. Cl G02b 27/00, G03c 1/72
[58] Field of Search 350/3.5, 162 SF; 96/38.3,
96/79, 81
[56] References Cited
UNITED STATES PATENTS
3,623,798 11/1971 Sheridon 350/3.5
3,743,507 7/1973 Ih et al... 350/3.5
Primary Examiner—Ronald J. Stern
Attorney, Agent, or Firm—L. D. Wofford, Jr.; G. J.
Porter; J. R. Manning
[57] ABSTRACT
A holographic recording medium employing a sub-
strate having a diffraction grating composed of a plu-
rality of spaced line ridges on a surface thereof to-
gether with a first layer consisting of a metal having a
plasma wavelength snorter than that of a selected light
source and a second layer, or coating, on top of the
first layer, consisting of a thin film of photo-sensitive
emulsion.
5 Claims, 4 Drawing Figures
ftNASA-case-MFS-22040^T) HOLOGRAPHY
UTILIZING SURFACE PLASMON
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HOLOGRAPHY UTILIZING SURFACE PLASMON directed nearly normal to the sensitized surface and the
RESONANCES reference beam directed at a smaller, selected, angle to
ORIGIN OF THE INVENTION the ,SurfaCe' The "^ is Plane-polarized light from a
continuous source of coherent light. There results a
The invention described herein was made in the per- 5 second reference beam or surface wave which passes
formance of work under a NASA contract and is sub- across the grating in the photographic emulsion just
ject to the provisions of Sec. 305 of the National Aero- under the surface, and as a result of the configuration
nautics and Space Act of 1958, Public Law 85-568 (72 described, plasmon resonances appear. The optimum
STAT. 435;42 U.S.C. 2457). angle of incidence of the reference beam is achieved
BACKGROUND OF THE INVENTION 10 whe" °™™e images become dark only in certain
patches, those corresponding to the dielectric thick-
1. Field of the Invention ness, at which the plasmon resonance has its maximum
This invention relates to the process of production intensity.
and reconstruction of holograms and particularly to an
improved medium for the production of holograms uti- 15 BRIEF DESCRIPTION OF THE DRAWINGS
lizing the propagation of a surface wave in a layer of FIG. 1 is an edge view of a portion of a holographic
photographic emulsion on a diffraction grating. recording structure constructed in accordance with the
2. General Description of the Prior Art invention.
Holography, a lens-less form of photography, is based FIG. 2 is a schematic, plan oriented, illustration of
upon the recording on a photographic film or plate of 20 one possible holographic system constructed in accor-
a diffraction pattern created by the interference be- dance with the invention.
tween light scattered from an object or scene that has FIG. 3 is a schematic illustration showing the light
been illuminated with coherent light and light directly patterns involved in the construction of a holographic
impingent upon the film from the same light source. image.
Generally the interference occurs in free space. In cer- 25 FIG. 4 is a schematic illustration showing the light
tain special cases bound or surface waves within some patterns involved in the reconstruction of a holo-
medium other than free space have been used as either graphic image,
the reference or object beam. Heretofore, however, the
use of bound or surface waves within a layer of photo- DESCRIPTION OF THE PREFERRED
graphic emulsion has not been satisfactory. One, this 30 EMBODIMENT
has required that the emulsion be coated with a trans- Referring to FIG. 1, holographic recording device 10
parent material of higher index of refraction than that utilizes a construction in which a layer 12 of a selected
of the emulsion itself, and two, the object or reference
 metal is formed to provide a grating surface 14, the se-
light must pass through this coating. The result is that |ected metal having a plasma wavelength which lies at
either the object light or reference light must be mci- 35
 a snorter wave iengtn than the spectral region of the
dent at excessively large incident angles in order to |ight which is to be employed in making holograms. If
stimulate surface waves. Further, such methods do not
 the meta| |ayer is thin, j.e ; a few hundred angstroms,
achieve surface plasmon resonances as contemplated jt may be supported by a flat glass, quartz, or similar
by this invention nor do they enable the intensity of sur-
 type of substrate !6. The grating surface 14 shouid
face waves to be sufficiently controlled. -: 40 have sharp, rather than-rounded ridges so that efficient
SUMMARY OF THE INVENTION stimulation of the surface waves can occur.
As examples of metals for the grating surface, in the
Accordingly, it is the object of this invention to pro-
 visibie spectrum region, aluminum or silver may be em-
vide an apparatus and arrangement for making holo- ployed. To the metal grating is applied a thin layer of
grams wherein an improved used is made of surface 45
 high reso,ution recording emulsion 18 with a suffi-
plasma waves. ciently low absorption coefficient so that incident light
In accordance with the invention, a reflection type
 will not be appreciably absorbed before being reflected
diffraction grating is constructed in which the grating
 by metal grating surface 14 The ,ayer of emuision has
surface consists of a metal having a plasma wave length
 a thickness on the order of one micron and preferably
which lies at a shorter wave length than the spectral re- 50
 on|y a fcw hundred angstrom units. A photosensitive
gion of the light which is to be used with the system. emulsion such as AgfaGevaert 1OE56 has been satisfac-
The plasma frequency, o>p , is defined as torily empioyed as the recording medium. The emul-
top = V(4?jWym~, sion may be applied by heating it until it is in a liquid
where e = electron charge, n = conduction electron
 55 form and in turn heating the grating and thereafter ap-
density, and m*= effective electron mass of the metal. P'yin8 the emulsion to grating surface 14. A thin layer
The plasma wavelength, \p, is defined in terms of the 1S achieved by simply turning substrate on its side and
speed of light, c, and o>p by ' all°wing a portion of the emulsion to run off. There is
_ ' . left a thin layer of emulsion which adheres to the sur-
XP ~ 27TC/ft>p • 60 face and hardens-
These definitions apply to an idealized free-electron- An optical system for making holograms is illustrated
like metal, but give very good values for a metal like in FIG. 2. Typically, a hologram would be made by first
aluminum. Modifications to the formula are necessary masking all but a top edge region 20 of grating surface
for non-ideal metals, such as gold or silver. Over the 14 (FIG. 1). This top edge is then exposed to laser light,
metal surface is applied a photographic emulsion. A
 6j e.g., plane-polarized 5145 angstrom light from argon
hologram is recorded by directing conventional image laser 22. In most cases the polarization plane of a laser
and reference beams on. the surface of the photo- beam is vertical, or S-polarized, but this plane may he
graphic emulsion, with the object beam being typically rotated by 90 degrees to the horizontal plane by passing
3,815,969
the laser beam through a half-wave plate, plate 23 as
shown, and a horizontal, or P-polarized plane beam 24,
obtained. In the present case, this horizontal plane is
the plane corresponding.to the plane in which the angle
of incidence on grating surface 14 is measured. 5
Beam 24 is directed onto beam splitter 25. From it
there is formed two beams, beams 26 and 27. Beam 26
is expanded to sufficient working size and spatially fil-
tered by beam expander 28 to cover object 29 upon
which it is directed. The resulting reflected beam, beam 10
S from object 29, passes on to recording device 10, ar-
riving essentially normal to grating surface 14. The
other beam, referenced beam R, passes through beam
splitter 25, then through beam expander 30 where this
beam is also spatially filtered, and then finally onto is
grating surface 14 at an angle a. This produces a sur-
face reference wave in emulsion 18 which, stimulated
by rotating recording device 10, varying angle a; until
the surface wave corresponds to the —1 diffracted or-
der, (FIG. 3), and passes off, as shown, parallel to sur- 20
face 14. A slight further rotation, stimulates the surface
wave which is indicated when the off blaze orders +1
and +2 become quite dark. When the critical angular
position has been thus determined, the principal sur-
face of grating surface 12 may be unmasked and holo- 25
grams made in the usual way.
FIG. 3 illustrates the process of construction of a ho-
lographic image. Reference beam R is shown as being
incident on grating surface 14 and diffracted into the
propagating orders, 0, +1, and +2 and surface wave —1. 30
A hologram is formed when surface wave —1 interferes
coherently with object beam or light S.
After an appropriate exposure, the whole grating is
immersed in a photographic developer, stop bath, and
fixer, in that order, and recording device 10 is then 35
dried. With the photographic process thus completed
and the hologram thus recorded, the image may there-
after be reconstructed when desired.
In order to reconstruct the recorded image, the grat-
ing is placed in the same position in the optical system 40
as when the hologram was recorded. Light from the ob-
ject beam is blocked off and reconstruction is made
with reference beam R alone. In case the emulsion
layer has shrunk or its thicknesses changed slightly, the
incident angle of the reconstruction beam may be 45
slightly different from the angle of construction. This
may be determined experimentally by looking at the
off-blaze images and rotating recording device 10
about its center until these images become dark; that
is until the same small areas of equal thickness that got 50
dark in construction are made dark in reconstruction.
Properly oriented, the result is that there may be ob-
served a bright reproduced image of the photographed
object when viewed at a critical angle.
Upon reconstruction as illustrated in FIG. 4, the 1(R) 55
surface wave can be regarded as a new reference beam,
which in turn is diffracted into its own propagating or-
ders (+1), (+2), and (+3). Dashed lines 32 denote real
holographic images and dashed lines 34 represent con-
jugate or imaginary (virtual) images. 60
While the process thus far described is for an absorp-
tion type hologram, it is also possible to make a phase-
type hologram simply by bleaching the emulsion using
standard bleaching techniques. The invention has a
particular advantage over previous methods and de-
vices for surface wave holography in that the reference
or object light can be made to successfully pass di-
rectly from air to the emulsion layer and be incident at
small as well as large angles of incidence. Further, con-
struction can be made at one angle using certain order
diffracted light, for example, as the reference beam,
and reconstruction be made at a different angle, using
a different order diffracted light. In addition, the inven-
tion enables holograms to be made using surface
plasma waves of narrow resonances and wherein the
intensity distribution can be controlled by using grat-
ings of different line spacing and blaze angles and
wherein surface waves can be stimulated at large or
small angles in very thin recording layers by light that
passes directly from air into the recording medium.
What is claimed is:
1. A holographic device for recording holographic
images comprising:
a substrate having a surface thereof formed of a plu-
rality of spaced lined ridge disposed to provide a
predetermined diffraction grating on said surface;
a metal comprising said surface; and
a coating of photosensitive emulsion on said surface.
2. A holographic device as set forth in claim 1
wherein said metal comprises said substrate.
3. A holographic device for recording holographic
images as set forth in claim 1 further comprising:
means for generating a beam of coherent light;
positioning means for positioning an object to be
photographed in a selected position with respect to
said surface of said substrate;
means for splitting said beam of coherent light and
directing one portion of said beam onto a said ob-
ject and then onto said surface of said substrate and
directing a second portion of said beam, being a
reference portion, onto said surface at a selected
angle of incidence;
whereby a further and second reference beam is gen-
erated in said coating which propagates across the
surface of said diffraction grating creating a plas-
mon resonance in said coating.
4. A holographic device as set forth in claim 3
wherein said device includes means for polarizing said
reference portion of said beam in a plane, which plane
is oriented with respect to said surface at an angle cor-
responding to the plane in which said selected angle of
incidence is measured.
5. A holographic device as set forth in claim 4
wherein said metal has a shorter plasma wave length
characteristic than coherent light generated by said
means for generating a beam of coherent light.
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Holography utilizing surface plasmon resonances

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