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This invention relates to light polarizers and more particularly to a
method for producing light polarizing material and the resulting product
thereof.
Light polarizers are conveniently classified as either prism, reflectance
or sheet polarizers. Although prism and reflectance polarizers are
extremely effective, their use is restricted by the fact that they are
bulky and often have small areas and limited angular aperture. The
advantages of sheet polarizers have been well established with the
introduction of Polarioid sheet film polarizers. The Polaroid process
consists of orienting microscopic dichroic crystals in a film and is
described in the Journal of the Optical Society of America, Vol. 41, page
957, 1951. This type polarizer has good performance in the visual spectral
region and a type HR Polaroid can be used for the near infrared. The
disadvantage of this approach in general is that it requires a film and
cannot be directly applied to the surface of an optical element. Further,
since it is a chemical polarizer, the temperature stability is determined
by the dichroic chemical and film stability. In particular, it has been
determined that the HR Polaroid deteriorates rapidly at temperatures above
55.degree.C. Because of the transmission of the material, the useful
wavelength of the HR Polaroid is from 0.8 to 2.2 microns and no Polaroid
is available for longer wavelengths.
A second approach for producing sheet polarizers is the wire-grid approach
such as is disclosed in Journal of the Optical Society of America, Vol.
50, page 72, 1960. The principle first used by Hertz to polarize radio
waves is based on the use of wires with length greater and diameter and
spacing smaller than the wavelength of the radiation to be polarized. When
the electric vector of light is parallel to the wire direction, it is
reflected and when it is perpendicular it is transmitted. U.S. Pat. No.
3,046,839 to Bird et al describes a transparent diffraction grating with a
sawtooth cross section with deposited gold on the tips of the sawteeth to
form regular, continuous parallel strips of gold. These wire grids are
effective polarizers in the infrared.
A number of problems are associated with prior art wire grid approaches.
First the infrared region where the grid spacing is close enough to
provide good polarization requires substrates which are transparent and
can be impressed with a grid pattern prior to metal grid deposition. These
materials are soft plastics which have unwanted absorption bands or other
materials which cold flow and deteriorate with temperature. As the
wavelengths approach the visible spectrum, the required grid spacing is so
small that it is impractical to deposit metal at the required spacing as
evidenced by degraded performance at shorter wavelengths.
An alternate approach is to actually scribe the grid pattern onto an IR
transmitting surface by means of a ruling engine such as is described by
the article in Applied Optics by J. B. Young et al., Vol. 4, page 1023,
1965. However, the tedious step of ruling each polarizing filter obviously
precludes production of large quantities of polarizers.
A coating approach to sheet polarizer production is to apply a polarizing
solution coating to an oriented surface and allow it to dry such as is
disclosed in the Journal of the Optical Society of America, Vol. 37, page
983, 1947. These coatings can be applied to rubbed surfaces of glass or
plastic; however, these devices can only operate from the ultraviolet
region of the spectrum out to 7000 Angstroms.
Accordingly it is an object of the present invention to provide a polarizer
which has excellent degree of polarization over a wide range of
wavelengths from the visible to the infrared regions.
Another object of the present invention is to provide a light polarizing
material which can be formed by appling a thin film coating composed of
whiskers which may be metallic to any smooth, transparent surface.
Another object of the present invention is to provide a polarizing coating
which polarizes both the light reflected as well as the light transmitted
by said coating.
Another object of the present invention is to provide a polarizer which can
be fabricated in sheet form or applied as a film to optical elements.
A further object of the present invention is to provide a polarizer which
has greater temperature stability and wider spectral band as well as
comparable dichroic characteristics than conventional polarizers.
Another object of the present invention is to provide a method of
fabrication polarizing material which has a uniform coating density
thereon.
A still further object of the present invention is to provide a method
which allows production of polarizers, which method can be performed
relatively rapidly and inexpensively and which can be controlled for
consistent quality thereby lending itself to arge production quantities.
Other objects and features of the invention will become more readily
understood from the following detailed description and appended claims
when read in conjunction with the accompanying drawings; in which:
FIG. 1 is a schematic side view, with elements partly in cross section,
illustrating the step of directing a vaporized metal upon a surface
forming a part of the process of the present invention.
FIG. 2 is a magnified view of the resulting metal whiskers formed on the
surface of the material resulting from the process illustrated in FIG. 1.
FIG. 3 illustrates the step of applying a protective coating to the
resulting polarized material and the degree of polarization of light
through the resulting polarizing material.
FIG. 4 is an electron microscope picture of a surface coated according to
the present invention.
FIG. 5 illustrates an alternative schematic side view, with elements partly
in cross section, illustrating a method of obtaining more uniform coatings
on a surface according to the present invention.
FIG. 6 illustrates a magnified view of the resulting polarizing material
showing the relative orientation of the metal whiskers.
FIG. 7 is a graph illustrating the whisker density versus the surface
distance resulting from the process shown in FIG. 5.
Referring now to FIG. 1, one method of forming a light polarizing material
according to the present invention is illustrated. A substrate 12 having a
microscopically smooth surface 10 is mounted on substrate holder 14 in
bell jar 16. The smooth surface is required to eliminate shadowing effects
which cause gaps in the surface of the deposited whiskers. Prior to
mounting the substrate 12 in bell jar 16, the surface 10 of substrate 12
is cleaned and/or coated. The substrate 12 may be made of, for example, a
glass or plastic which is transparent to visible light or an IR
transmitting material, such as germanium or mercury cadmium telluride
compound which is transparent to IR light and may be an optical element
such as a lens, mirror or beam splitter. The substrate 12 is mounted to
substrate holder 14 by any suitable means, such as screws or adhesive.
Substrate holder 14 is attached to support 18 through pin 20 which allows
the substrate holder 14 and substrate 12 to pivot or rotate relative to
support 18. Heater element 22 has a receptacle means 24 for retaining the
material, or in the preferred embodiment, the metal 26 to be vaporized.
The heater element 22 is connected to heater electrodes 28 and 30 which
are in turn connected to a source of power (not shown). Between the metal
source 26 and the surface 10 is a collimating apparatus 32 having a
collimating slit or aperture 34.
In operation, the process according to the preferred embodiment of the
present invention includes first cleaning a 1 inch by 1 inch square
surface 10 of, for example, a glass substrate 12 and mounting it at room
temperature (24.degree.C) on substrate holder 14 positioned about 50 cms
from a metal source. A suitable high-vacuum of about 2 .times.
10.sup.-.sup.5 Torr is provided within the enclosure of bell jar 16
utilizing known techniques. Next the metal source 26 which may be, for
example, 0.065 troy ounces of silver, gold, copper, or aluminum is heated
to its evaporating temperature by passing a current through heater
electrodes 28 and 30 thereby producing a vaporized metal stream which is
permitted to pass through collimating slit 34 to the surface 10 of
substrate 12 for about ten minutes. Collimating slit 34 forms parallel
streams of vaporized metal which impinge directly on surface 10 at an
angle of incidence .theta. with respect to the axis 38 normal to the
surface 10. This angle of incidence, .theta., is normally between
80.degree. and 90.degree. with best results at about 88.degree. with
respect to axis 38 in order for the vaporized metal 36 to adequately cover
surface 10. It will be understood by those skilled in the art that the
source 26 may be any other metal or material which is reflective at the
wavelength to be polarized.
As the metal source 26 evaporates in the vacuum enclosure 16, the vaporized
metal 36 impinges on surface 10 thereby initially producing metal atoms
attached to nucleation sites on the surface 10. For further information of
nucleation site see the article by Henry Levinsten, "The Growth and
Structure of Thin Metal Films," Journal of Applied Physics, Vol. 20, April
1949, p. 311, and Chopra, Kasturi L., "Thin Film Phenomena" pp 177-178,
McGraw-Hill Book Company, Inc., New York, 1969. As the vaporized metal 36
continues to impinge upon surface 10, a plurality of metal whiskers 40 (as
shown in the magnified view illustrated in FIG. 2) are grown on the metal
atoms at the nucleation sites of the smooth, transparent surface 10 to
form a discontinuous film thereof. The term "whisker" as employed herein
denotes an elongated projection attached or anchored at one end to the
surface and unatached on the other end. As deposition continues, the metal
whiskers 40 are grown on the initial sites in the direction of the
incident vaporized metal 36 until a coating consisting of a uniform
distribution of these metal whiskers 40 covers the exposed surface 10 of
substrate 12. During the whisker growth process, metal whiskers 40 are
grown on the surface with their long axis essentially parallel and in line
with the vaporized metal direction 36 and the projection 44 of each long
axis 42 of each metal whisker 40 onto surface 10 is parallel to one
another. An alternative, it will be recognized that whiskers 40 can be
formed on surface 10 from a dielectric, such as silicon monoxide, and
subsequently rendered reflective at the wavelength of light to be
polarized by chemical or evaporative techniques. In the preferred
embodiment, the whiskers 40 are grown until its length is at least as long
or longer than the wavelength of the light to be polarized and the
diameter and separation from each other is small compared to the
wavelength of the light to be polarized; with these characteristics, the
metal coating acts as a linear polarizer for both reflected and
transmitted light.
FIG. 3 illustrates such a polarizer constructed according to the present
invention. A transparent optical coating 46, such as, for example, silicon
monoxide (SiO), silicon dioxide, glass, plastics or magnesium flouride
(MgF.sub.2) may be applied to the metalized surface 10 for enhanced
optical properties, thermal stabilization and physical protection of the
surface 10. In use as a light polarizer, unpolarized light 48 incident on
this polarizer from a near normal direction will have its reflected
component parallel to the long axis of the metal whiskers 40 and the
transmitted component 52 is perpendicular to the long axis of the whiskers
40. If substrate 12 is a light absorbing material, such as, for example,
black glass, porcelain, and pigmented plastics, then a reflective light
polarizer is produced in that the light impinging thereon is absorbed by
the substrate 12 leaving only reflective component 50 present.
Light transmitted through a polarizer constructed according to the present
invention was linearly polarized with a good degree of polarization. In an
actual test,0.065 troy ounce of gold was positioned 50 cms from the
substrate and evaporated at a pressure of 2 .times. 10.sup.-.sup.5 Torr
for ten minutes through the collimated slit onto a polished transmparent 1
inch by 1 inch square optical glass blank with 96 percent transmission at
a wavelength of 1.08 microns. A transmission electron microscope picture
of the coating formed in this test according to the present invention is
shown in FIG. 4 with the vaporized metal direction indicated.
Magnification is 14,200.times.. Unpolarized light can be resolved into two
perpendicular components with transmittances k.sub.1 (transmitted) and
k.sub.2 (extinguished). For unpolarized light k.sub.1 = k.sub.2 = 1.00.
Typical transmittance values for this polarizer evaluated at four
wavelengths ranging from the visible to the near infrared are:
Wavelength (microns, .mu.m)
k.sub.1 k.sub.2
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.5500 .37 .14
1.0830 .77 .04
1.3000 .79 .02
1.8900 .73 .02
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Polarizers constructed according to the process described hereinabove have
good surface coverage and uniform density. FIG. 5 illustrates a modified
procedure for forming a polarized coating of the metals mentioned above on
smooth, transparent surface 10 with a further improvement in uniform
density across the face of the surface 10. The substrate 12 may be
constructed of the above-mentioned substrate materials. The apparatus
illustrated in FIG. 5 is similar to that illustrated in FIG. 1 except that
there is a mirror image vapor metallizatin system included in bell jar 16.
This enclosure 16 includes heater elements 22a-b connected to heater
electrodes 28a-b and 30a-b. Metal sources 26a-b are included within heater
elements 22a-b. Collimating apparatus 32a-b is intermediate the heater
elements 22a-b and the surface 10 to to coated. Bell jar 16 is evacuated
using standard techniques and heater electrodes 28a-b and 30a-b have
current passed therethrough in order to heat metal sources 26a and 26b to
produce vaporized metal streams 36a and 36b, respectively. Collimating
slits 34a and 34b are so positioned such that the vaporized metal streams
36a and 36b impinge upon the surface 10 at substantially the same angle of
incidence with respect to the axis 38 normal to surface 10; in other
words, angle .theta..sub.a is substantially equal to angle .theta..sub.b.
As was the case with the embodiment shown in FIG. 1, .theta..sub.a and
.theta..sub.b will be in the range, in the preferred embodiment, of
80.degree. to 90.degree..
By first exposing the surface 10 to the vaporized metal stream 36a, metal
whiskers 40a (see FIG. 6) will form on surface 10 in a manner as described
with respect to FIG. 1. This process will continue until a whisker density
defined by Curve A results on surface 10. It will be noted from FIG. 7 and
Curve A in particular that the side of surface 10 which is closest to
metal source 26a will have a higher density (for example 55 percent ) than
the side which is furthest away from metal source 26a (45 percent ); as
illustrated, there is approximately a 10 percent differential in whisker
density across the surface 10 created by the metallic whisker coating
formed by vaporized metal stream 36a.
By subjecting the surface 10 to a second vaporized metal stream 36b, which
may or may not be the same metal as the first vaporized metal, at
substantially the same angle of incidence as the vaporized metal stream
36a, second metal whiskers 40b (see FIG. 6) will form on surface 10 with
their long axes 42b essentially parallel to the vaporized metal stream 36b
and the projections 44a and 44b of the long axes 40a and 40b,
respectively, onto surface 10 are essentially parallel to each other.
Referring now to FIG. 7 again, it will be seen that the vaporized metal
stream 36b produces a mirror image Curve B of the whisker density produced
by the metal stream 36a. The total whisker density across surface 10 will
be the summation of Curve A and Curve B which is represented by Curve C
which is substantially uniform across the width of surface 10.
A modified procedure for obtaining the same results of a very uniform
whisker density across surface 10 can be implemented by the apparatus
illustrated in FIG. 1. Referring again to FIG. 1, after subjecting surface
10 to vaporized metal 36, such as, for example, silver, to produce the
metallic whiskers 40 illustrated in FIG. 2, surface 10 can then be rotated
180.degree. about the normal axis 38, and the surface 10 once again
subjected to the vaporized metal stream 36 of silver or a different metal,
such as gold, may be used to form the metal stream. This will produce a
second group of metal whiskers identical to that illustrated in FIG. 6 and
having the same whisker density characteristics shown in the Curves of
FIG. 7.
Although the surface 10 has been illustrated as having finite width for
purposes of convenience of illustration, it is understood that surface 10
can be in a continuous flexible form and successive portions of it can be
moved into and out of the vaporized metal stream to form sheet-type
polarizers. Of course, one of the primary advantages of the polarizing
material produced according to the present invention is that it is no
longer tied to a sheet or substrate whose optical properties limit the
useful spectral range of the device. As previously mentioned, some prior
art polarizers are limited by the spectral response of the plastic base
and chemicals which form the matrix of dichroic molecules. In the case of
wire grid polarizers, they are tied to materials which can be impressed
with a grid. In the near infrared (8,000 to 35,000 Angstroms) the
principal sheet polarizers are wire grid polarizers. Wire grid polarizers
at present have degraded performance below 12,000 Angstroms. Over the near
infrared, the polarizers produced according to the present invention have
better performance than available wire grid polarizers and have, for
example, both military and police applications in the near infrared night
vision equipment.
Over the past ten years, a significant effort has been made to produce
sheet infrared (3.5 to 20 microns) polarizers. Wire grid polarizers for
this spectral range have been used but the substrate materials have in
general been limited to transparent plastics and materials which cold
flow; as a result, these polarizers have poor temperature characteristics
and/or dead bands in their intended spectral regions. The polarizer
constructed according to the present invention permits forming coatings on
large area transparent materials such as Irtran (a pressed zinc sulfide),
silicon, germanium or any smooth infrared transmitting material and
provides a simple and inexpensive method when compared with the technique
of scribing Irtran with grids.
In the visible spectral region, a thin coating polarizer can be produced
using the method according to the present invention by utilizing a metal
which reflects in the visible spectral region which would also make
possible a single element which works from the near ultraviolet to the
infrared provided a suitable transmitting substrate is utilized. In the
visible spectral region the polarizer described herein can be used in
sunglasses (and has the additional capability of allowing the coating to
be directly applied to corrective eyeglass lenses), window glass (to
reject heat, such as infrared radiation, and eliminate reflective glare in
the view), in the automative industry (by polarizing the headlight beam
and providing a polarizing filter for the drivers), and in liquid crystal
displays which employ transmitting and/or reflective polarizing filters.
Although the present invention has been shown and illustrated in terms of a
specific method and apparatus, it will be apparent that changes or
modifications can be made without departing from the spirit and scope of
the invention as defined by the appended claims.
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