U.S. patent application number 13/234444 was filed with the patent office on 2012-01-12 for multilayer wire-grid polarizer with off-set wire-grid dielectric grid.
Invention is credited to Cheng-Yuan Cheng, Eric W. Gardner, Douglas P. Hansen, Raymond T. Perkins.
Application Number | 20120008205 13/234444 |
Document ID | / |
Family ID | 36573849 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120008205 |
Kind Code |
A1 |
Perkins; Raymond T. ; et
al. |
January 12, 2012 |
MULTILAYER WIRE-GRID POLARIZER WITH OFF-SET WIRE-GRID DIELECTRIC
GRID
Abstract
A wire-grid polarizer includes a stack of thin film layers
disposed over a substrate including a wire grid layer and a
plurality of thin film layers disposed between the wire grid layer
and the substrate. The wire-grid layer includes an array of
elongated metal elements, comprising lengths longer than a
wavelength of visible light, a period less than 200 nanometers, a
height less than 400 nanometers; and a material selected from the
group consisting of aluminum, silver, gold, copper, or combinations
thereof. The plurality of thin film layers disposed between the
wire grid layer and the substrate include at least one dielectric
grid layer and at least one thin film layer comprising silicon. The
dielectric grid layer and the wire-grid are substantially parallel
with one another, have substantially equal periods, and have
substantially equal widths.
Inventors: |
Perkins; Raymond T.; (Orem,
UT) ; Cheng; Cheng-Yuan; (Chandler, AZ) ;
Hansen; Douglas P.; (Spanish Fork, UT) ; Gardner;
Eric W.; (Eagle Mountain, UT) |
Family ID: |
36573849 |
Appl. No.: |
13/234444 |
Filed: |
September 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12879315 |
Sep 10, 2010 |
8027087 |
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13234444 |
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12400100 |
Mar 9, 2009 |
7813039 |
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12879315 |
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11005927 |
Dec 6, 2004 |
7570424 |
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12400100 |
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Current U.S.
Class: |
359/485.05 |
Current CPC
Class: |
G02B 5/3058
20130101 |
Class at
Publication: |
359/485.05 |
International
Class: |
G02B 5/30 20060101
G02B005/30 |
Claims
1. A wire-grid polarizer device for polarizing light, comprising:
a) a stack of thin film layers disposed over a substrate including:
b) a wire grid layer and a plurality of thin film layers disposed
between the wire grid layer and the substrate; c) the wire-grid
layer including an array of elongated metal elements, wherein the
elements comprise: i. lengths longer than a wavelength of visible
light; ii. a period less than 200 nanometers; iii. a height less
than 400 nanometers; and iv. a material selected from the group
consisting of aluminum, silver, gold, copper, or combinations
thereof; d) the plurality of thin film layers disposed between the
wire grid layer and the substrate including: i. at least one
dielectric grid layer; and ii. at least one thin film layer
comprising silicon; and e) the dielectric grid layer and the
wire-grid are substantially parallel with one another, have
substantially equal periods, and have substantially equal
widths.
2. The device of claim 1, wherein at least one of the plurality of
thin film layers disposed between the wire grid layer and the
substrate has a thickness and material selected for decreasing
reflection of p polarized light.
3. The device of claim 1, wherein the substrate comprises glass or
quartz.
4. The device of claim 1, wherein the dielectric grid comprises at
least two contiguous dielectric grids and at least one of the
dielectric grids comprises a material selected from the group
consisting of aluminum oxide; antimony trioxide; antimony sulphide;
beryllium oxide; bismuth oxide; bismuth triflouride; cadmium
sulphide; cadmium telluride; calcium fluoride; ceric oxide;
chiolite; cryolite; germanium; hafnium dioxide; lanthanum fluoride;
lanthanum oxide; lead chloride; lead fluoride; lead telluride;
lithium fluoride; magnesium fluoride; magnesium oxide; neogymium
fluoride; neodymium oxide; praseodymium oxide; scandium oxide;
silicon; silicon oxide; disilicon trioxide; silicon dioxide; sodium
fluoride; tantalum pentoxide; tellurium; titanium dioxide; thallous
chloride; yttrium oxide; zinc selenide; zinc sulphide; zirconium
dioxide; and combinations thereof.
5. The device of claim 1, wherein the metal elements have a height
less than 200 nanometers.
6. The device of claim 1, further comprising at least one thin film
disposed on top of the wire grid.
7. The device of claim 1, further comprising at least two thin
films disposed on top of the wire grid and wherein at least one of
the at least two thin films disposed on top of the wire grid has a
thickness and material selected for decreasing reflection of p
polarized light.
8. A wire-grid polarizer device for polarizing light, comprising:
a) a stack of thin film layers disposed over a substrate including
a wire grid layer and a dielectric thin film layer; b) the
wire-grid layer including an array of elongated metal elements,
wherein the elements comprise: i. lengths longer than a wavelength
of visible light; ii. a period less than 200 nanometers; iii. a
height less than 400 nanometers; and iv. a material selected from
the group consisting of aluminum, silver, gold, copper, or
combinations thereof; c) the dielectric thin film layer disposed
between the wire grid layer and the substrate; and d) the
dielectric thin film layer comprising: i. a material having a
refractive index greater than a refractive index of the substrate;
and ii, a material selected from the group consisting of aluminum
oxide; antimony trioxide; antimony sulphide; beryllium oxide;
bismuth oxide; bismuth triflouride; cadmium sulphide; cadmium
telluride; calcium fluoride; ceric oxide; chiolite; cryolite;
germanium; hafnium dioxide; lanthanum fluoride; lanthanum oxide;
lead chloride; lead fluoride; lead telluride; lithium fluoride;
magnesium fluoride; magnesium oxide; neogymium fluoride; neodymium
oxide; praseodymium oxide; scandium oxide; silicon; silicon oxide;
disilicon trioxide; silicon dioxide; sodium fluoride; tantalum
pentoxide; tellurium; titanium dioxide; thallous chloride; yttrium
oxide; zinc selenide; zinc sulphide; zirconium dioxide; and
combinations thereof.
9. The device of claim 8, further comprising at least one thin film
disposed on top of the wire grid.
10. The device of claim 8, further comprising at least two thin
films disposed on top of the wire grid and wherein at least one of
the at least two thin films disposed on top of the wire grid has a
thickness and material selected for decreasing reflection of p
polarized light.
11. The device of claim 8, wherein: a) the dielectric thin film
layer comprises at least two contiguous dielectric grids; b) at
least one of the at least two contiguous dielectric grids comprises
silicon; and c) the dielectric grids and the wire-grid are
substantially parallel with one another and have substantially
equal periods.
12. The device of claim 11, wherein the dielectric grids and the
wire-grid have substantially equal widths.
13. The device of claim 8, wherein the dielectric thin film layer
has a thickness and material selected for decreasing reflection of
p polarized light.
14. The device of claim 8, wherein the metal elements have a height
less than 200 nanometers.
15. A wire-grid polarizer device for polarizing light, comprising:
a) a stack of thin film layers disposed over a substrate including
a wire grid layer and a dielectric thin film layer; b) the
wire-grid layer including an array of elongated metal elements,
wherein the elements comprise: i. lengths longer than a wavelength
of visible light; ii. a period less than 200 nanometers; iii. a
height less than 400 nanometers; and iv. a material selected from
the group consisting of aluminum, silver, gold, copper, and
combinations thereof; and c) the dielectric thin film layer having
a refractive index greater than a refractive index of the
substrate.
16. The device of claim 15, wherein the dielectric thin film layer
has a thickness and material selected for decreasing reflection of
p polarized light.
17. The device of claim 15, wherein the dielectric thin film layer
comprises silicon.
18. The device of claim 15, wherein the dielectric thin film layer
comprises at least two contiguous dielectric thin film layers and
wherein at least one of the dielectric thin film layers is a
dielectric grid and at least one of the dielectric thin film layers
is continuous.
19. The device of claim 15, wherein the metal elements have a
height less than 200 nanometers.
20. The device of claim 15, wherein the substrate comprises glass
or quartz.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY
[0001] This is a continuation of U.S. patent application Ser. No.
12/879,315, filed on Sep. 10, 2010; which is a continuation of U.S.
patent application Ser. No. 12/400,100, filed on Mar. 9, 2009, now
U.S. Pat. No. 7,813,039; which is a divisional of U.S. patent
application Ser. No. 11/005,927, filed on Dec. 6, 2004, now U.S.
Pat. No. 7,570,424; which are herein incorporated by reference.
BACKGROUND
[0002] The present invention relates generally to wire-grid
polarizers for the visible and near visible spectrum.
[0003] A wire grid polarizer (WGP) is an array of parallel wires
disposed on the surface of a substrate, such as glass. Usually
wire-grid polarizers are a single, periodic array of wires on the
substrate. The grid acts as a diffraction grating when the period
of the wires is greater than about half of the wavelength of light.
The grid acts as a polarizer when the period of the wires is less
than about half the wavelength of light.
[0004] While it is desirable for a WGP to transmit all of the light
of one polarization and reflect all of the other polarization, no
polarizer is perfect. Real WGPs will transmit some of the light of
both polarizations and will reflect some of the light of both
polarizations. When light is incident on the surface of a
transparent material, such as a sheet of glass, a small amount of
the light is reflected. For example, at normal incidence, about 4%
of the incident light is reflected from each surface of the
glass.
[0005] It has been suggested to dispose a film under a WGP, or
between the wires and the substrate, to move the first diffraction
order to shorter wavelengths in order to improve performance in
part of the visible spectrum, such as blue light. See U.S. Pat. No.
6,122,103. The film has an index of refraction less than that of
the substrate. It has also been suggested to etch into either the
substrate or underlying layer to further reduce the effective
refractive index under the wire grid. See U.S. Pat. No. 6,122,103.
It has been further suggested to form each wire as a composite with
alternating metal and dielectric layers. See U.S. Pat. No. U.S.
Pat. No. 6,532,111.
SUMMARY
[0006] It has been recognized that it would be advantageous to
develop a wire-grid polarizer with improved performance, or a
wire-grid polarizer with increased transmission of a desired
polarization state, such as p, and decreased transmission (or
increased reflection) of another polarization state, such as s. In
addition, it has been recognized that a wire-grid polarizer can act
as a metal for reflecting one polarization state and act as a thin
film of lossy dielectric for the other polarization state. Thus, it
has been recognized that form birefringence and effective index of
refraction can be applied to a wire-grid polarizer. In addition, it
has been recognized that a wire-grid polarizer can be treated as a
thin film layer, and incorporated into an optical stack.
[0007] Briefly, and in general terms, the invention is directed to
multilayer wire-grid polarizers for polarizing light. In accordance
with one aspect of the invention, the polarizer includes a stack of
thin film layers disposed over a substrate including: a wire grid
layer and a plurality of thin film layers disposed between the wire
grid layer and the substrate. The wire-grid layer includes an array
of elongated metal elements, wherein the elements comprise: lengths
longer than a wavelength of visible light; a period less than 200
nanometers; a height less than 400 nanometers; a material selected
from the group consisting of aluminum, silver, gold, copper, or
combinations thereof. The plurality of thin film layers disposed
between the wire grid layer and the substrate include: at least one
dielectric grid layer; and at least one thin film layer comprising
silicon. The dielectric grid layer and the wire-grid are
substantially parallel with one another, have substantially equal
periods, and have substantially equal widths.
[0008] In accordance with another aspect of the present invention,
the polarizer includes a stack of thin film layers disposed over a
substrate including a wire grid layer and a dielectric thin film
layer. The wire-grid layer includes an array of elongated metal
elements, wherein the elements comprise: lengths longer than a
wavelength of visible light; a period less than 200 nanometers; a
height less than 400 nanometers; and a material selected from the
group consisting of aluminum, silver, gold, copper, or combinations
thereof. The dielectric thin film layer is disposed between the
wire grid layer and the substrate. The dielectric thin film layer
comprises: a material having a refractive index greater than a
refractive index of the substrate; and a material selected from the
group consisting of aluminum oxide; antimony trioxide; antimony
sulphide; beryllium oxide; bismuth oxide; bismuth triflouride;
cadmium sulphide; cadmium telluride; calcium fluoride; ceric oxide;
chiolite; cryolite; germanium; hafnium dioxide; lanthanum fluoride;
lanthanum oxide; lead chloride; lead fluoride; lead telluride;
lithium fluoride; magnesium fluoride; magnesium oxide; neogymium
fluoride;
[0009] neodymium oxide; praseodymium oxide; scandium oxide;
silicon; silicon oxide; disilicon trioxide; silicon dioxide; sodium
fluoride; tantalum pentoxide; tellurium; titanium dioxide; thallous
chloride; yttrium oxide; zinc selenide; zinc sulphide; zirconium
dioxide; and combinations thereof.
[0010] In accordance with another aspect of the present invention,
the polarizer includes a stack of thin film layers disposed over a
substrate including a wire grid layer and a dielectric thin film
layer. The wire-grid layer includes an array of elongated metal
elements, wherein the elements comprise: lengths longer than a
wavelength of visible light; a period less than 200 nanometers; a
height less than 400 nanometers; and a material selected from the
group consisting of aluminum, silver, gold, copper, and
combinations thereof. The dielectric thin film layer has a
refractive index greater than a refractive index of the
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Additional features and advantages of the invention will be
apparent from the detailed description which follows, taken in
conjunction with the accompanying drawings, which together
illustrate, by way of example, features of the invention; and,
wherein:
[0012] FIGS. 1 and 2 are cross-sectional side schematic views of
multilayer wire grid polarizers in accordance with embodiments of
the present invention (the figures are not to scale and features
are shown greatly exaggerated for clarity);
[0013] FIG. 3 is a cross-sectional side schematic view of a
multilayer wire grid polarizer in accordance with an exemplary
embodiment of the present invention (the figure is not to scale and
features are shown greatly exaggerated for clarity);
[0014] FIG. 4a is a graph of p-polarization reflection versus
wavelength for the multilayer wire grid polarizer of FIG. 3
compared to other polarizers;
[0015] FIG. 4b is a graph of s-polarization transmittance versus
wavelength for the multilayer wire grid polarizer of FIG. 3
compared to other polarizers;
[0016] FIG. 4c is a graph of p-polarization transmittance versus
wavelength for the multilayer wire grid polarizer of FIG. 3
compared to other polarizers;
[0017] FIG. 5 is a cross-sectional side schematic view of a
multilayer wire grid polarizer in accordance with an exemplary
embodiment of the present invention (the figure is not to scale and
features are shown greatly exaggerated for clarity);
[0018] FIG. 6a is a graph of s-polarization reflection versus
wavelength for the multilayer wire grid polarizer of FIG. 5
compared to another polarizer;
[0019] FIG. 6b is a graph of p-polarization transmittance versus
wavelength for the multilayer wire grid polarizer of FIG. 5
compared to another polarizer;
[0020] FIG. 7 is a cross-sectional side schematic view of a
multilayer wire grid polarizer in accordance with an exemplary
embodiment of the present invention (the figure is not to scale and
features are shown greatly exaggerated for clarity);
[0021] FIG. 8 is a cross-sectional side schematic view of a
multilayer wire grid polarizer in accordance with an exemplary
embodiment of the present invention (the figure is not to scale and
features are shown greatly exaggerated for clarity);
[0022] FIG. 9 is a graph of p-polarization reflection versus
wavelength for the multilayer wire grid polarizers of FIGS. 7 and 8
compared to another polarizer;
[0023] FIGS. 10a and b are cross-sectional side schematic views of
multilayer wire grid polarizers in accordance with exemplary
embodiments of the present invention (the figures are not to scale
and features are shown greatly exaggerated for clarity);
[0024] FIG. 11 is a graph of s-polarization reflection versus
wavelength for the multilayer wire grid polarizers of FIGS. 10a and
b compared to another polarizer;
[0025] FIG. 12 is a cross-sectional side schematic view of another
multilayer wire grid polarizer in accordance with exemplary
embodiments of the present invention (the figure is not to scale
and features are shown greatly exaggerated for clarity);
[0026] FIG. 13 is a graph of s-polarization transmittance versus
wavelength for the multilayer wire grid polarizer of FIG. 112
compared to another polarizer;
[0027] FIG. 14 is a side cross-sectional view of a wire grid layer
with a dielectric material in spaces between metal elements of the
wire grid layer in accordance with an exemplary embodiment of the
present invention; and
[0028] FIG. 15 is a side cross-sectional view of a dielectric grid
layer with two dielectric grids with elements of two different
materials in accordance with an exemplary embodiment of the present
invention.
[0029] Reference will now be made to the exemplary embodiments
illustrated, and specific language will be used herein to describe
the same. It will nevertheless be understood that no limitation of
the scope of the invention is thereby intended.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT(S)
[0030] It has been recognized that, for one polarization of light,
a wire-grid polarizer substantially acts as a metal that reflects
the light (or one polarization thereof), while for the other
polarization of the light, the wire-grid polarizer substantially
acts as a thin film of lossy dielectric that transmits the light
(or another polarization thereof). Thus, it has been recognized
that two concepts, namely form birefringence and effective index of
refraction, can be applied to improve the performance of the
polarizer.
[0031] A wire-grid polarizer is not typically considered an example
of form birefringence. Generally, birefringence means that a
material has a different index of refraction for different
polarizations. Birefringence is very common in crystalline
materials, such as quartz, and in stretched polymers. Form
birefringence refers to birefringence caused by the shape of a
material.
[0032] When a material has variations in material properties, such
as density, with the scale of the variation being smaller than the
wavelength of light, the index of refraction is different from the
index of uniform bulk material. There is an effective refractive
index, which is the index that a uniform thin film would have that
causes the same affect on light. The theoretical treatment of this
effect is called effective medium theory. This phenomenon is used
with dielectric materials to make such things as moth-eye
antireflection coatings.
[0033] In addition, a wire-grid polarizer is not typically
considered a thin film. In optics, both form birefringence and
effective index are typically considered only for dielectric
materials. It has been recognized, however, that treating a
wire-grid polarizer as an equivalent birefringent thin film with
effective indices of refraction allows one to consider it as an
element in a thin film stack, and to use thin film design
techniques with particular performance goals.
[0034] The present invention utilizes thin films in combination
with a metallic wire grid polarizer to improve performance of the
polarizer. Generally this may include films under and on top of the
wire grid. Any one of these films may be uniform or a dielectric
grid. The wire grid may be a composite grid, or have composite
wires. Combining the wire grid with multiple layers of different
material, and thus different refractive indices, can reduce
reflection of the polarization that is desired to be transmitted.
For example, a wire grid can be configured to reflect s polarized
light, and transmit p polarized light. As discussed above, while it
is desirable to transmit all the p polarized light and reflect all
the s polarized light, a typical wire grid will transmit some of
both polarizations and reflect some of both polarizations. It has
been found, however, that treating the wire grid as a birefringent
thin film, and combining the wire grid with multiple thin films,
reduces reflection of p polarized light.
[0035] As illustrated in FIGS. 1 and 2, multilayer wire-grid
polarizer devices, indicated generally at 10a and 10b,
respectively, are shown as exemplary implementations in accordance
with the invention for polarizing light 12, or substantially
separating one polarization state from an orthogonal polarization
state, and doing so in an improved manner, with less reflection
and/or transmission of unwanted polarizations. Such devices are
believed to have substantial utility in visible light applications,
or for use with visible light in the range of approximately 400-700
nm (nanometers), or 0.4-0.7 .mu.m (micrometers or microns). Such
visible light applications can include projection display devices
such as projectors. The multilayer wire-grid polarizer devices
described herein can be utilized in various different capacities,
such as polarizers, beam splitters, analyzers, etc. It is also
believed that the devices herein have utility in near-visible
applications, such as ultraviolet and/or infrared applications, or
for use with light in the range of approximately 250-400 nm or
700-10,000 nm. Thus, the term "light" is used broadly herein to
refer to visible light, ultraviolet light and infrared light, or
electromagnetic waves in the range of 250-10,000 nm.
[0036] The polarizers 10a and 10b include a substrate 14 carrying
or supporting a plurality or stack of thin film layers 18,
including a wire grid or a wire grid layer 22. The substrate 14 can
be transparent to the light being treated. For example, the
substrate can be glass (Bk7). Other substrates can be quartz or
plastic. In addition, the substrate 14 can have a substantial
thickness t.sub.s with respect to the remaining thin film layers.
Furthermore, the substrate can have a refractive index (or index of
refraction) n.sub.s. For example, a glass substrate (Bk7) has a
refractive index n.sub.s of 1.52 (at 550 nm). (It will be
appreciated that the refractive index varies slightly with
wavelength.)
[0037] The wire grid or wire grid layer 22 includes a wire-grid
array of elongated metal elements 26. The elements 26 have lengths
longer than a wavelength of the light, and are located in a
generally parallel arrangement with a period P less than half the
wavelength of the light. Thus, for use with visible light, the
elements 26 have a length larger than the wavelength of visible
light, or greater than 700 nm (0.7 .mu.m). The length, however, can
be much longer. The elements 26 can have a center-to-center
spacing, pitch or period P less than half the wavelength of visible
light, or less than 200 nm (0.2 .mu.m). The elements 26 can also
have a width w in the range of 10 to 90% of the pitch or period.
The elements 26 can also have a thickness or a height t less than
the wavelength of the light, or less than 400 nm (0.4 82 m) for
visible light applications. In one aspect, the thickness can be
less than 0.2 .mu.m for visible light applications.
[0038] The elements 26, or the array, generally interact with the
visible light to generally 1) transmit a transmitted beam 30 having
a substantially uniform and constant linear polarization state
(such as p polarization), and 2) reflect a reflected beam 34 also
have a substantially uniform and constant linear polarization state
(such as s polarization). The elements generally transmit light
with a first polarization state (p polarization), oriented locally
orthogonal or transverse to the elements, and reflect light with a
second polarization state (s polarization), oriented parallel to
the elements. It will be appreciated that the wire-grid polarizer
will separate the polarization states of the light with a certain
degree of efficiency, or some of both polarization states may be
transmitted and/or reflected. It will also be appreciated that a
portion of the elements can be configured to transmit or reflect a
different polarization state.
[0039] The elements 26 or array can be formed on or over the
substrate by photo-lithography. The elements 26 can be conductive,
and can be formed of aluminum, silver, gold or copper.
[0040] The plurality of thin film layers 18 can include layers
under and/or over the wire grid layer 22. Thus, one or more layers
18a-c can be disposed between the substrate 14 and the wire grid
layer 22. In addition, one or more layers can be disposed over the
wire grid layer 22. The layers 18 can be formed of different
materials, or materials different than the substrate 14, and even
from each other. Thus, the layers 18 can have refractive indices n
different than the refractive index n.sub.s of the substrate 14.
Furthermore, it has been found that at least one of the layers
18a-c having a refractive index n.sub.1-3 greater than the
refractive index n.sub.s of the substrate 14 decreases reflection
of the p polarized light. Thus, in accordance with one aspect of
the invention, the polarizer 10a or 10b has at least one thin film
layer 18a disposed between the substrate 14 and the wire grid layer
22, and the thin film layer 18a has a refractive index n.sub.1
greater than the refractive index n.sub.s of the substrate 14. In
accordance with another aspect of the invention, the polarizer 10a
or 10b can have at least two thin film layers 18a and b, or at
least three thin film layers 18a-c.
[0041] The thin film layers 18a-c can extend continuously across
the substrate 14, and can be consistent or constant layers,
indicated by 18a and 18c. The layers 18a-c can be formed of
dielectric material. For example, the layers can be formed of:
aluminum oxide; antimony trioxide; antimony sulphide; beryllium
oxide; bismuth oxide; bismuth triflouride; cadmium sulphide;
cadmium telluride; calcium fluoride; ceric oxide; chiolite;
cryolite; germanium; hafnium dioxide; lanthanum fluoride; lanthanum
oxide; lead chloride; lead fluoride; lead telluride; lithium
fluoride; magnesium fluoride; magnesium oxide; neogymium fluoride;
neodymium oxide; praseodymium oxide; scandium oxide; silicon;
silicon oxide; disilicon trioxide; silicon dioxide; sodium
fluoride; tantalum pentoxide; tellurium; titanium dioxide; thallous
chloride; yttrium oxide; zinc selenide; zinc sulphide; and
zirconium dioxide. The film layers can be deposited on the
substrate. In the case of metal oxides, they can be deposited by
starting with an oxide evaporant material (with additional oxygen
backfill as needed). The material, however, can also be deposited
by evaporating a base metal, then oxidizing the deposited material
with O2 in the background.
[0042] The thicknesses t.sub.1-3 and materials (or refractive
indices n.sub.1-3) of the thin film layers 18a-c can be manipulated
to reduce reflection of p polarized light, as described in greater
detail below.
[0043] One or more of the thin film layers 18a-c can include a
dielectric grid including an array of non-metal elements 38. The
non-metal and metal elements 38 and 26 of the arrays can be
oriented substantially parallel with one another. In addition, the
arrays can have substantially equal periods and/or widths. In one
aspect, the non-metal elements 38 of the dielectric grid and the
metal elements 26 are aligned, or the non-metal elements 38 are
aligned with the metal elements 26 of the wire grid layer, as shown
in FIG. 1. In another aspect, the non-metal elements 38 of the
dielectric grid and the metal elements 26 are off-set, or the
non-metal elements 38 are off-set with respect to the metal
elements 26 of the wire grid layer, as shown in FIG. 2.
[0044] As stated above, the plurality of thin film layers 18 can
include one or more other thin film layers disposed over the
wire-grid layer 22. The other thin film layer can include a
dielectric material, and can be continuous or constant. In
addition, the other thin film layer 42 can include a dielectric
grid including an array of non-metal elements 46. The non-metal and
metal elements 46 and 26 of the arrays can be oriented
substantially parallel with one another, and can have substantially
equal periods. In one aspect, the non-metal elements 46 and metal
elements 26 are aligned, or the non-metal elements 46 of the
dielectric grid are aligned above or over the metal elements 26 of
the wire grid layer 22, as shown in FIG. 1. In another aspect, the
non-metal elements 46 and metal elements 26 are off-set, or the
non-metal elements 46 of the dielectric grid are off-set above the
metal elements 26 of the wire grid layer 22.
[0045] As discussed above, the number, thicknesses t, and materials
(or refractive indices) of the thin film layers 18 can be varied to
reduce reflection of p polarized light (increase transmission of p
polarized light) and/or reduce transmission of s polarized light
(increase reflection of s polarized light). Some of the layers 18a
and c can be uniform in structure and material, while other layers
can include grids, such as metal elements 26 of the wire grid layer
22 or non-metal elements 38 and 46 of a dielectric grid. Examples
of specific configurations are discussed below.
[0046] Referring to FIG. 3, an example of a multilayer wire-grid
polarizer 10c is shown. The polarizer includes three uniform thin
film layers 50a-c on a glass (Bk7) substrate 14 and between the
substrate and the wire grid or wire grid layer 22. The substrate 14
has a refractive index n.sub.s of 1.52. The first thin film layer
50a is a uniform material of magnesium oxide (MgO) having a
thickness t.sub.1 of 65 nm. Thus, the first layer 50a has a
refractive index n.sub.1 of 1.74 (for a wavelength of 550 nm)
greater than the refractive index n.sub.s of the substrate 14. The
second thin film layer 50b is a uniform material of ZrO.sub.2
having a thickness t.sub.2 of 130 nm, and a refractive index of
2.0. Thus, the second layer 50b also has a refractive index n.sub.2
greater than the refractive index n.sub.s of the substrate 14. The
third thin film layer 50c is a uniform material of magnesium
fluoride (MgF2) having a thickness t.sub.3 of 70 nm. Thus, the
third layer 50c has a refractive index n.sub.3 of 1.38 (for a
wavelength of 550 nm).
[0047] The wire grid layer 22 or wire grid is disposed on top of
the third layer 50c. The wire grid includes elements made of
aluminum. The elements can have a period P of 144 nm, a width w of
39.5% of the period, or 57 nm, and a thickness t.sub.wg or height
of 155 nm.
[0048] Referring to FIGS. 4a-c, the performance of the polarizer
10c of FIG. 3 is compared to a similar polarizer with no thin film
layers between the wire grid and substrate, and a similar polarizer
with a 30 nm layer of magnesium fluoride (MgF.sub.2) between the
wire grid and substrate (and thus has a thin film layer with a
lower refractive index than the substrate). Light 12 is incident on
the polarizer 10c at an incidence angle of 45 deg. In this case,
the p polarization 30 is primarily transmitted, and the s
polarization 34 is primarily reflected. Referring to FIG. 4a, the
transmittance of the p polarization through the polarizer 10c is
greater than the other two polarizers (or the reflectance of p
polarization from the polarizer is less), as shown by curve at 54.
While it can be seen that the polarizer with a thin layer of lower
refractive index performs better than the plain polarizer, the
polarizer 10c with the three thin film layers 50a-c performs even
better. Referring to FIG. 4b, transmittance (leakage) of s
polarization light is less with the polarizer 10c than with either
of the other polarizers (or the transmittance of s polarization
through the polarizer is less), as shown by curve 56. Referring to
FIG. 4c, the reflection of the p polarization is generally less
with the polarizer 10c than with the other polarizers (or the
transmittance of p polarization is greater), as shown by curve 58.
The net result is that there is more transmitted p polarization,
and improved contrast in both transmission and reflection, which
means the purity of the transmitted and reflected polarizations is
greater with the multiplayer polarizer 10c.
[0049] Referring to FIG. 5, another example of a multilayer
wire-grid polarizer 10d is shown. The polarizer 10d includes two
dielectric layers or two dielectric grids 60a and 60b disposed
directly on top of a wire grid layer 22 or wire grid with elements
of aluminum. The wire grid or wire grid layer 22 is disposed on a
glass (Bk7) substrate 14. The thickness or height t.sub.wg of the
elements 26 of the wire grid is 160 nm. The first dielectric grid
60a is disposed on the wire grid and has a thickness t.sub.1 is 100
nm, and formed of silicon oxide (SiO2), with an index of refraction
n.sub.1 of 1.45. The second dielectric grid 60b also has a
thickness t.sub.2 of 100 nm, and is formed of a material with an
index of refraction n.sub.2 of 2.5. The period P of the grids is
144 nm. The width of the elements is 45% of the period P, or 57 nm.
Light 12 is incident at 45 degrees.
[0050] Referring to FIGS. 6a and b, the performance of the
polarizer 10d of FIG. 5 is compared to a similar polarizer without
dielectric grids on top. Because the period P of the grids is less
than the wavelength of visible light, they all essentially behave
as thin films. In FIG. 6a it is seen that the reflected s
polarization is substantially greater with the polarizer 10d, as
shown by curve at 62. In FIG. 6b it is seen that the transmitted p
polarization is also greater with the polarizer 10d, as shown by
curve at 64.
[0051] Referring to FIG. 7, another example of a multilayer
wire-grid polarizer 10e is shown. The polarizer 10e includes three
uniform thin film layers 70a-c between a wire grid or wire grid
layer 22 and a glass (Bk7) substrate 14. The first layer 70a is
disposed on the substrate 14, has a thickness t.sub.1 of 33 nm
thick, and has a refractive index n.sub.1 of 1.8. The second layer
70b is a material of magnesium fluoride (MgF.sub.2) with a
refractive index n.sub.2 of 1.38, and a thickness t.sub.2 of 30 nm.
The third layer 70c has a thickness t.sub.3 of 20 nm, and has a
refractive index n.sub.3 of 1.8. Thus, the first and third layers
70a and c have refractive indices n.sub.1 and n.sub.3 greater than
the refractive index n.sub.s of the substrate 14. The wire grid or
wire grid layer 22 includes elements of aluminum with a period P of
144 nm. The element height t.sub.wg is 160 nm, and the element
width w is 45% of the period, or 57 nm. Light 12 is normally
incident (0 deg.).
[0052] Referring to FIG. 8, another example of a multilayer
wire-grid polarizer 10f is shown. The polarizer 10e includes three
thin film layers 80a-c, similar to those described above for FIG.
7, except that the first layer 80a has a thickness t.sub.1 of 28
nm; the second layer 80b has a thickness t.sub.2 of 25 nm; and the
third layer 80c has a thickness t.sub.3 of 17 nm. In addition, the
polarizer 10f includes a thin film layer 84 above the wire grid
layer 22. The thin film layer 84 includes a dielectric grid with
non-metal elements disposed on the metal elements of the wire grid.
The wire grid or wire grid layer 22 is similar to the wire grid
described above for FIG. 7. The elements of the dielectric layer 84
have a thicknesses t.sub.4 of 100 nm. The elements of the
dielectric layer 84 are formed of silicon dioxide (SiO.sub.2).
[0053] Referring to FIG. 9, the performance of the polarizers 10e
and f is compared with a similar wire grid polarizer without the
thin film layers. Both polarizers 10e and f reflect less p
polarization (pass more p polarization), as shown by curves at 86
and 88. The polarizer 10f with thin film layers under the wire grid
layer and dielectric grids above the wire grid shows significant
improvement, as shown by curve at 88.
[0054] Referring to FIGS. 10a and b, examples of multilayer
wire-grid polarizers 10g and h are shown. Both polarizers 10g and h
include a wire grid or wire grid layer 22 disposed on a substrate
14. The wire grid can include elements of aluminum and the
substrate can be glass (Bk7). The period P of the wire grid is 144
nm, and the elements have a thickness t.sub.wg of 150 nm. The width
w of the elements is 45% of the period, or 65 nm. In addition, the
elements 26 define spaces 92 therebetween that include a material
with a refractive index n.sub.1 of 1.17. A second uniform layer 96
is disposed on top of the elements 26 and spaces 92, or the wire
grid layer 22, that has a thickness t.sub.2 of 100 nm and a
refractive index n.sub.2 of 1.17. A third thin film layer 100 is
disposed over the second layer 96. The third layer 100 has uniform
layer of silicon dioxide (SiO.sub.2) and a thickness t.sub.3 of 60
nm. Thus, the third layer 100 has an index of refraction n.sub.3 of
1.45. A fourth layer 104 is disposed on the third layer 100, and
includes a dielectric grid with non-metal elements. The elements of
the dielectric grid have a thickness t.sub.4 of 50 nm. The elements
of the dielectric grid are formed of silicon dioxide (SiO2) and
have a refractive index n.sub.4 of 2.0. The width w of the elements
of the dielectric layer is 50% of the period. The elements of the
dielectric layer are disposed substantially directly above the
elements of the wire grid, as shown in FIG. 10a. Alternatively, the
elements of the dielectric layer can be off-set with respect to the
elements of the wire grid, or are shifted one half period so that
they are substantially above the spaces between the elements of the
wire grid, as shown in FIG. 10b. The light 12 is incident at 45
degrees.
[0055] Referring to FIG. 11, the performance of the polarizers 10g
and h are compared with a similar polarizer with only a wire grid
on a glass substrate. The polarizers 10g and h have improved
reflectance of s polarization, as shown by curves at 104 (which
overlap each other). In addition, it appears that the alignment of
the dielectric grid to the wire grid is not relevant when the
conditions for effective medium theory apply. These examples also
show that uniform layers and dielectric layers may be combined and
used to advantage. In addition, these examples demonstrate the
principle of the effective medium theory.
[0056] Referring to FIG. 12, another example of a multilayer
wire-grid polarizer 10i is shown. The polarizer 10i is similar to
the polarizer 10c of FIG. 3, but includes a wire grid or wire grid
layer 112 with composite elements. The composite elements can
include alternating layers of metal and non-metal layers. Examples
of such composite elements are found in U.S. Pat. No. 6,532,111,
which is herein incorporated by reference. For example, each
element can include of alternating layers of aluminum and magnesium
fluoride.
[0057] Referring to FIG. 13, the performance of the polarizer 10i
is compared to a similar polarizer with composite elements, but
without the thin film layers between the substrate and the wire
grid layer. The polarizer 10i has less leakage or transmittance of
s polarization, as shown by curve at 116.
[0058] Referring to FIG. 14, a wire grid layer 22 similar to those
described above but with a dielectric material 120 in spaces
between metal elements of the wire grid layer. Such a wire grid or
wire grid layer can be substituted for any of those described
above.
[0059] Referring to FIG. 15 a dielectric grid layer is shown with
two dielectric grids 124 and 128 with elements of two different
materials having two different indices of refraction n.sub.1 and
n.sub.2 respectively. Thus, the dielectric layer or grid has
alternating elements of different material, or elements of one grid
disposed in the spaces of another grid. Such a dielectric grid or
layer can be substituted for any of those described above.
[0060] The examples presented here are but a few of the many
possibilities that may be realized from this invention. In general,
a combination for uniform layers and dielectric grids may be
combined for specific applications such as optimizing transmittance
or reflectance over a given range of angles of incident of a given
band of light. Optimization may be made for transmittance or
reflectance or for both together. Optimization may be made for
incidence from the air side on the polarizer or from the substrate
side or both.
[0061] Various aspects of wire-grid polarizers, optical trains
and/or projection/display systems are shown in U.S. Pat. Nos.
5,986,730; 6,081,376; 6,122,103; 6,208,463; 6,243,199; 6,288,840;
6,348,995; 6,108,131; 6,452,724; 6,710,921; 6,234,634; 6,447,120;
and 6,666,556, which are herein incorporated by reference.
[0062] Although the wire-grid polarizers have been illustrated as
facing the light source, or with the elongated elements facing
towards the light source, it is understood that this is for
illustrational purposes only. Those skilled in the art will
appreciate that the wire-grid polarizers can be oriented to face
towards imaging bearing beams, such as from a liquid crystal array,
for the simple purpose of avoiding passing the image bearing beam
through the substrate, and thus avoiding ghost images or multiple
reflections associated with light passing through mediums, such as
the substrate. Such configurations may result in the wire-grid
polarizer facing away from the light source.
[0063] While the forgoing examples are illustrative of the
principles of the present invention in one or more particular
applications, it will be apparent to those of ordinary skill in the
art that numerous modifications in form, usage and details of
implementation can be made without the exercise of inventive
faculty, and without departing from the principles and concepts of
the invention. Accordingly, it is not intended that the invention
be limited, except as by the claims set forth below.
* * * * *