U.S. patent application number 15/935132 was filed with the patent office on 2019-11-07 for methods and apparatus for transparent display using scattering nanoparticles.
The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Chia Wei Hsu, Wenjun Qiu, Ofer Shapira, Marin Soljacic, Bo Zhen.
Application Number | 20190339522 15/935132 |
Document ID | / |
Family ID | 51016998 |
Filed Date | 2019-11-07 |
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United States Patent
Application |
20190339522 |
Kind Code |
A1 |
Hsu; Chia Wei ; et
al. |
November 7, 2019 |
METHODS AND APPARATUS FOR TRANSPARENT DISPLAY USING SCATTERING
NANOPARTICLES
Abstract
Transparent displays enable many useful applications, including
heads-up displays for cars and aircraft as well as displays on
eyeglasses and glass windows. Unfortunately, transparent displays
made of organic light-emitting diodes are typically expensive and
opaque. Heads-up displays often require fixed light sources and
have limited viewing angles. And transparent displays that use
frequency conversion are typically energy inefficient. Conversely,
the present transparent displays operate by scattering visible
light from resonant nanoparticles with narrowband scattering cross
sections and small absorption cross sections. More specifically,
projecting an image onto a transparent screen doped with
nanoparticles that selectively scatter light at the image
wavelength(s) yields an image on the screen visible to an observer.
Because the nanoparticles scatter light at only certain
wavelengths, the screen is practically transparent under ambient
light. Exemplary transparent scattering displays can be simple,
inexpensive, scalable to large sizes, viewable over wide angular
ranges, energy efficient, and transparent simultaneously.
Inventors: |
Hsu; Chia Wei; (Cambridge,
MA) ; Qiu; Wenjun; (Chicago, IL) ; Zhen;
Bo; (Cambridge, MA) ; Shapira; Ofer;
(Cambridge, MA) ; Soljacic; Marin; (Belmont,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Family ID: |
51016998 |
Appl. No.: |
15/935132 |
Filed: |
March 26, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15237800 |
Aug 16, 2016 |
9927616 |
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15935132 |
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15090348 |
Apr 4, 2016 |
9677741 |
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15237800 |
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14067471 |
Oct 30, 2013 |
9335027 |
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15090348 |
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14143558 |
Dec 30, 2013 |
9458989 |
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15237800 |
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61748228 |
Jan 2, 2013 |
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61748259 |
Jan 2, 2013 |
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61866468 |
Aug 15, 2013 |
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61748228 |
Jan 2, 2013 |
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61748259 |
Jan 2, 2013 |
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61866468 |
Aug 15, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10S 977/773 20130101;
G02B 5/0278 20130101; G02B 2027/0112 20130101; G02B 2027/0196
20130101; F21V 9/12 20130101; G02B 2027/0145 20130101; B82Y 20/00
20130101; F21V 9/08 20130101; G09F 13/00 20130101; G02B 27/0172
20130101; G02B 5/0242 20130101; G02F 1/0126 20130101; G02F 1/19
20130101; H04N 9/3129 20130101; G03B 21/62 20130101; G02B 2027/0147
20130101 |
International
Class: |
G02B 27/01 20060101
G02B027/01; G02F 1/01 20060101 G02F001/01; F21V 9/08 20060101
F21V009/08; G09F 13/00 20060101 G09F013/00; F21V 9/12 20060101
F21V009/12; G02F 1/19 20060101 G02F001/19; G03B 21/62 20060101
G03B021/62; G02B 5/02 20060101 G02B005/02 |
Goverment Interests
GOVERNMENT SUPPORT
[0004] This invention was made with government support under Grant
No. DMR0819762 awarded by the National Science Foundation and under
Contract No. W911NF-07-D-0004 awarded by the Army Research Office
and under Grant Nos. DE-SC0001299 and DE-FG02-09ER46577 awarded by
the Department of Energy. The government has certain rights in the
invention.
Claims
1. A transparent display comprising: a transparent substrate; at
least one light source, in optical communication with the
transparent substrate, to illuminate the transparent substrate with
light comprising a first spectral component at a first wavelength
and a second spectral component at a second wavelength; and at
least one nanoparticle, disposed on and/or within the transparent
substrate, to scatter the first spectral component and the second
spectral component and to transmit ambient light at other
wavelengths in the visible spectrum.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 15/237,800, which was filed on Aug. 16, 2016, as a
continuation-in-part of U.S. application Ser. No. 15/090,348, now
U.S. Pat. No. 9,677,741, which was filed on Apr. 4, 2016, as a
continuation of U.S. application Ser. No. 14/067,471, now U.S. Pat.
No. 9,335,027, which was filed on Oct. 30, 2013.
[0002] U.S. application Ser. No. 15/237,800 is also a continuation
of U.S. application Ser. No. 14/143,558, which was filed on Dec.
30, 2013.
[0003] U.S. application Ser. No. 14/067,471 and U.S. application
Ser. No. 14/143,558 each claim the benefit, under 35 U.S.C. .sctn.
119(e), of: U.S. Application No. 61/748,228, filed on Jan. 2, 2013,
and entitled "Methods and Apparatus for Transparent Display Using
Scattering Particles"; U.S. Application No. 61/748,259, filed on
Jan. 2, 2013, and entitled "Methods and Apparatus for Transparent
Display Using Up-Converting Particles"; and U.S. Application No.
61/866,468, filed on Aug. 15, 2013, and entitled "Methods and
Apparatus for Transparent Display Using Up-Converting
Particles."
[0005] Each of the aforementioned applications is hereby
incorporated by reference in its entirety.
BACKGROUND
[0006] Transparent displays are long sought-after by scientists and
engineers. Two-dimensional (2D) transparent displays can create
images that appear floating in the air, in contrast to traditional
displays where images appear on a visible screen. Aside from
creating special visual impressions, such displays can have a wide
variety of applications. A glass window can be turned into the
screen of a home theater. Eyeglasses can become a mini computer
screen. The windshield of a vehicle can show information such as
maps without blocking the driver's view. The display window of a
store can show not only products but also their information.
[0007] A number of transparent display technologies exist, but none
have gained a widespread usage. Liquid crystal displays (LCD) can
be made transparent by eliminating the backlight, but they are not
very transparent (the typical transmittance may be less than 15%).
Organic light-emitting diodes (OLEDs) can also be made transparent,
but the production remains costly and the transmittance is also
limited (typically less than 40%). OLED displays can be made
flexible and foldable, so transparent flexible displays are also
possible. Electroluminescent displays have also been made
transparent, but have so far been limited to single colors.
Recently, fluorescent films have been combined with ultraviolet
(UV) lights to make multi-colored displays that are transparent;
however, an intense UV light source, such as an ultra high
performance (UHP) lamp, is required due to the small emission cross
sections of the fluorescent particles.
SUMMARY
[0008] In view of the foregoing, various inventive embodiments
disclosed herein relate generally to achieving clear displays with
potentially low production costs, and may be the enabling
technology to bring transparent displays into the consumer
market.
[0009] Exemplary embodiments include a display with a transparent
substrate, at least one nanoparticle disposed on the transparent
substrate, and at least one light source in optical communication
with the nanoparticle. Illuminating the nanoparticle with a
monochromatic beam from the light source causes the nanoparticle to
scatter at least a portion of the monochromatic beam in the
direction of a viewer.
[0010] Other exemplary embodiments include methods of making
displays with one or more nanoparticles that scatter monochromatic
light. Such a display may be constructed by depositing at least one
nanoparticle that has a scattering cross section with a full-width
half maximum of about 1 nm to about 70 nm and a center wavelength
from about 390 nm to about 760 nm on a substrate with a
transmittance of about 60% to about 100% (e.g., 65%, 70%, 75%, 80%,
85%, 90%, or 95%) from about 390 nm to about 760 nm.
[0011] It should be appreciated that all combinations of the
foregoing concepts and additional concepts discussed in greater
detail below (provided such concepts are not mutually inconsistent)
are contemplated as being part of the inventive subject matter
disclosed herein. In particular, all combinations of claimed
subject matter appearing at the end of this disclosure are
contemplated as being part of the inventive subject matter
disclosed herein. It should also be appreciated that terminology
explicitly employed herein that also may appear in any disclosure
incorporated by reference should be accorded a meaning most
consistent with the particular concepts disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0013] The skilled artisan will understand that the drawings
primarily are for illustrative purposes and are not intended to
limit the scope of the inventive subject matter described herein.
The drawings are not necessarily to scale; in some instances,
various aspects of the inventive subject matter disclosed herein
may be shown exaggerated or enlarged in the drawings to facilitate
an understanding of different features. In the drawings, like
reference characters generally refer to like features (e.g.,
functionally similar and/or structurally similar elements).
[0014] FIG. 1A is a schematic drawing of a transparent scattering
display.
[0015] FIG. 1B is a plot of the scattering cross section (solid
line) and absorption cross section (dashed line) for a blue
scattering particle suitable for use in the transparent scattering
display of FIG. 1A.
[0016] FIG. 1C is a close-up of the transparent scattering display
shown in FIG. 1A.
[0017] FIG. 1D illustrates the screen of the transparent scattering
display shown in FIG. 1A used for viewing three-dimensional
images.
[0018] FIG. 2 is a schematic drawing of a multi-layer transparent
scattering display with predetermined patterns of scattering
nanoparticles.
[0019] FIG. 3 is a plot indicative of wavelength-selective
scattering performance derived from the dielectric functions of
various materials suitable for making scattering particles.
[0020] FIG. 4A is a plot of the scattering cross section (solid
line) and the absorption cross section (dashed line) versus
wavelength for a silver-coated silica nanoparticle (inset) that
strongly scatters blue light and is suitable for use in a
transparent scattering display.
[0021] FIG. 4B is a plot of the scattering cross section (solid
line) and the absorption cross section (dashed line) versus
wavelength for a silver-coated silica nanoparticle (inset) that
strongly scatters green light and is suitable for use in a
transparent scattering display.
[0022] FIG. 4C is a plot of the scattering cross section (solid
line) and the absorption cross section (dashed line) versus
wavelength for a silver-coated silica nanoparticle (inset) that
strongly scatters red light and is suitable for use in a
transparent scattering display.
[0023] FIG. 5A is a plot of the scattering cross section (solid
line) and the absorption cross section (dashed line) versus
wavelength for a titanium dioxide nanoshell (inset) that strongly
scatters blue light and is suitable for use in a transparent
scattering display.
[0024] FIG. 5B is a plot of the scattering cross section (solid
line) and the absorption cross section (dashed line) versus
wavelength for a silicon nanoshell (inset) that strongly scatters
green light and is suitable for use in a transparent scattering
display.
[0025] FIG. 6 is a plot of the scattering cross section versus
wavelength for a solid silicon nanosphere with a radius of r=115
nm.
[0026] FIG. 7A illustrates a first process for making a transparent
scattering display.
[0027] FIG. 7B illustrates a second process for making a
transparent scattering display.
[0028] FIG. 7C illustrates a third process for making a transparent
scattering display.
[0029] FIG. 8A is a plot of the measured extinction cross section
(solid line) and the theoretical extinction cross-section (dashed
line) versus wavelength for a solution of spherical silver
nanoparticles in PVA with a concentration of about 5 .mu.g/mL.
[0030] FIG. 8B is a plot of the measured extinction cross section
(solid line) and the theoretical extinction cross-section (dashed
line) versus wavelength for a solution of spherical silver
nanoparticles in polyvinyl alcohol (PVA) with a concentration of
about 10 .mu.g/mL.
[0031] FIG. 8C is a plot of the theoretical extinction
cross-section versus wavelength for two silver nanoparticles that
stick together (e.g., as shown in the inset of FIG. 9A) when
illuminated with light polarized along the axis of alignment.
[0032] FIG. 9A is a plot of the measured transmittance spectrum
(solid line) and predicted transmittance spectrum (dashed line) for
silver nanoparticles with a diameter of 62 nm.+-.4 nm embedded in a
PVA film (inset) as in a transparent scattering display.
[0033] FIG. 9B is a plot the measured extinction ratio (solid
line), predicted extinction ratio (dashed line), and predicted
contributions from scattering (dot-dashed line) and absorption
(dot-dot-dashed line) for the silver nanoparticles of FIG. 9A.
[0034] FIG. 10A is a color photograph the Massachusetts Institute
of Technology (MIT) logo projected onto the upper portion of a
transparent scattering display placed in front of three coffee mugs
with the MIT logo.
[0035] FIG. 10B is a color photograph of the MIT logo projected
onto a regular piece of glass in front of three coffee mugs with
the MIT logo.
[0036] FIG. 11A is a color photograph of the MIT logo projected
onto a transparent scattering display with a black backing.
[0037] FIG. 11B is a color photograph of the MIT logo projected
onto a piece of white paper.
[0038] FIG. 12 shows a metallic nanoparticle whose shape supports a
narrowband magnetic resonance.
DETAILED DESCRIPTION
[0039] In a traditional display system based on light
projection--for example, projectors used for presentations and in
movie theaters--the screens are opaque to maximize scattering of
the projected light so that viewers can see images on the screen.
In such cases, the screen scatters light efficiently, but cannot be
transparent. Existing transparent 2D and 3D laser display
technologies convert ultraviolet (UV) or infrared (IR) light into
visible light with a screen that is transparent in the visible
portion of the electromagnetic spectrum but not in the UV or IR
regions of the electromagnetic spectrum. Fluorescent or nonlinear
materials on the screen convert the UV or IR light into visible
light that is displayed to the viewer. Fluorescent or nonlinear
conversion enables screen transparency, but most fluorescent and
nonlinear materials do not convert the incident light efficiently
(in other words, they suffer from low frequency-conversion
efficiency). As a result, fluorescent and nonlinear displays
typically employ high-power light sources the UV/IR light
sources.
[0040] As explained herein, however, it is possible to create a
passive screen that is both substantially transparent at visible
wavelengths and uses visible illumination to display images
efficiently. This sounds contradictory, but can be explained as
follows. Sunlight and light from typical indoor/outdoor lighting
fixtures (e.g., incandescent light bulbs, fluorescent lamps, etc.)
include broad ranges of wavelengths. However, a mixture of
monochromatic light at three or more specified wavelengths--for
example red, green, and blue (RGB)--is enough to produce almost all
colors perceived by humans. This makes it possible to create a
screen (display) that is transparent in the visible spectrum except
at narrowband regions near the specified wavelengths.
[0041] An exemplary transparent scattering display uses
nanoparticles that scatter strongly at one or more specific visible
wavelengths and transmit at all other visible wavelengths to
produce color images that can be perceived easily by most humans.
For instance, a transparent scattering display might scatter about
30-70% (e.g., 35%, 40%, 45%, 50%, 55%, 60%, or 65%) at one or more
predetermined narrowband wavelength ranges and transmit 20% or more
(e.g., 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100%) of incident
visible light outside those wavelength ranges. The nanoparticles
can be coated onto or embedded in a transparent substrate or
sandwiched between a pair of transparent substrates to form a
transparent scattering screen. If the nanoparticles' scattering
efficiency at the specified wavelength(s) is high enough,
illuminating the transparent scattering screen with a relatively
low-power beam from a laser, light-emitting diode, or other
spectrally coherent source at one or more of the specified
wavelengths produces an image visible to an observer in front of or
behind the screen.
[0042] Depending on the nanoparticles' scattering wavelengths and
the light source, the transparent scattering display may be
monochromatic or multi-color. For instance, if the nanoparticles
scatter strongly only at wavelengths at or near about 450 nm, then
the display will be blue. (Absorbing nanoparticles for black
displays are also possible.) Similarly, a transparent scattering
display with nanoparticles that scatter strongly at multiple
wavelengths (e.g., 450 nm and 650 nm) can be used to show
multi-color images. And a transparent scattering display with
nanoparticles that scatter strongly at three or more specified
wavelengths (e.g., 450 nm, 550 nm, and 650 nm) can be used as a
full-color display.
[0043] A full-color transparent display may be based on an additive
color scheme in which combining appropriately weighted amounts of
red, green, and blue light yields the desired color. For example,
adding red and blue yields magenta, adding blue and green yields
cyan, and adding green and red yields yellow. Adding colors on a
transparent screen that hosts red-, green-, and blue-scattering
particles (or another suitable set of scattering particles) is
simply a matter of illuminating a single spot on the screen with
beams of red, green, and blue light with appropriately weighted
intensities. If the beams' intensities are modulated as the beams
are scanned across the screen at rates faster than the eye's
integration period, the beams produce what appears to an observer
as a full-color image on the screen. Changing the intensity
modulation and/or scan pattern produces animated images.
[0044] Because the nanoparticles scatter or absorb light only
within the narrowband regions near the specified wavelengths, the
transparent scattering display may appear transparent under normal
lighting conditions if the substrates are relatively transparent as
well. In fact, an exemplary transparent scattering display can
exhibit a transparency in the visible portion of the
electromagnetic spectrum of 90% or higher, while scattering 90% or
more of the incident laser light at the scattering wavelength(s).
(Higher and lower scattering percentages are also possible,
depending on the nanoparticles' characteristics.) Such high
scattering efficiency makes it possible to illuminate the screen
with a relatively low-intensity beam (e.g., an intensity of about 1
mW or higher), which in turn makes it possible to operate the
display's light source at a relatively low level of power
consumption.
[0045] Exemplary transparent scattering displays can be used for
displaying moving images (e.g., video data), static images, etc.
They may be suitable for use in heads-up displays;
eyeglass/spectacle displays (e.g., Google Glass); contact lens
displays; goggle-based displays; large-area displays; and so on.
The usable display size depends on the substrate size and the light
source's scanning/illumination range, and the refresh rate depends
on the light source's scanning rate. Depending on the exact
implementation, a transparent scattering display can be used for
showing movies, television programs, video games, computer
displays, billboards, outdoor and indoor advertising displays, and
so on.
[0046] A transparent scattering display can also be used for
illumination purposes. The screen, instead of being used to display
images, can be used to produce diffuse white light and possibly
other colors as well, for instance, by selecting an ensemble of
scattering particles whose scattering wavelengths span the visible
spectrum. The exact color can be customized depending on the
nanoparticles' scattering wavelength(s) and the wavelengths and
relative intensities of the beams emitted by the light source.
Illumination using transparent scattering display offers the energy
efficiency of LED illumination coupled with the potential for
large-area illumination, e.g., by turning windows in houses,
offices, and other buildings into sources of illumination,
replacing or supplementing light bulbs on the ceilings. This may
create lighting that better imitates natural sunlight.
[0047] If desired, a transparent scattering display can be doped or
used to host particles that scatter IR or UV light. Illuminating
these IR- or UV-scattering particles produces IR or UV images that
can be detected with appropriately configured cameras.
[0048] Transparent Scattering Displays with Front or Back Light
Illumination
[0049] FIG. 1A is a diagram of a transparent scattering display 100
suitable for displaying full-color images using illumination from
either the front or the rear. The display 100 includes a passive
screen 110 made of a transparent medium (substrates 112a and 112b)
that hosts nanoparticles 114 with low absorption loss and strong
scattering only for light at particular visible wavelengths. When
illuminated by a broadband ambient light source (such as the sun,
an incandescent light bulb, or a fluorescent lamp), this screen 110
is practically transparent since only a small fraction of the
impinging light is absorbed or scattered. However, under
monochromatic illumination from a spectrally coherent light source
120 at one or more of the specified visible wavelengths, the screen
110 scatters the incoming light strongly into all directions. Thus,
one can efficiently project color images at the scattering
wavelength(s) onto this seemingly transparent screen 110. Similar
to a regular projector screen, the image on this screen 110 appears
through scattering rather than through specular reflection, so the
viewing angle is relatively unlimited. Furthermore, the scattering
is highly efficient because it does not involve non-linear
processes, so the light source 120 can be a low-power source, such
as a personal-use laser projector.
[0050] The screen 110 is formed of a layer 116 of scattering
nanoparticles 114 sandwiched between a pair of substantially
transparent (e.g., 90%, 95%, or 99% transparent) substrates 112a
and 112b. The substrates 112a and 112b may be formed of any
suitable material, including but not limited to glass, plastic,
acrylic, and polymers. The substrates can be rigid or flexible; for
instance, they may be as rigid as plate glass, e.g., when used in a
window display, or as flexible as a sheet of plastic, e.g., when
used as a flexible display. For instance, the substrates 112a and
112b can be thin sheets of plastics that are flexible enough to be
rolled into a cylinder with a radius of less than one inch.
Similarly, the substrates can be thick or thin, e.g., 0.1 mm, 0.25
mm, 0.5 mm, 0.75 mm, 1.0 mm, 2.5 mm, or thicker, depending on the
application and the nanoparticle concentration. If desired, the
substrates 112a and 112b may be tinted, textured, or otherwise
patterned to achieve a particular effect, such as an area of
opacity or diffuse transmission.
[0051] The layer 116 of scattering nanoparticles 114 sandwiched
between the substrates 112a and 112b may be formed by coating,
printing, painting, spraying, or otherwise depositing a
nanoparticle solution onto one or both of the substrates 112a and
112b as described in greater detail below. In some cases, the
substrates 112a and 112b may be pressed together to remove air
bubbles and to ensure that the nanoparticle layer's thickness and
surface are relatively uniform. The nanoparticle layer 116 may be
as thin as the diameter of the largest nanoparticle 114; it can
also be much thicker, e.g., several times the largest
nanoparticle's diameter, depending on whether or not the
nanoparticles 114 clump together and whether the nanoparticle layer
116 includes a matrix (e.g., a polymer matrix) that separates the
substrates 112a and 112b. Alternatively, or in addition, spacers
(not shown) may separate the substrates 112a and 112b to form a
cavity that to holds the nanoparticles 114.
[0052] The scattering nanoparticles 114 in the nanoparticle layer
116 may be distributed in a periodic, aperiodic, or random fashion
when viewed along the display's optical axis (i.e., the axis normal
to the surfaces of the substrates 112a and 112b). In some cases,
the nanoparticles 114 are distributed uniformly (if randomly)
within the nanoparticle layer 116 with an areal density of about
10.sup.8 cm.sup.-2 to about 10.sup.11 cm.sup.-2 (e.g.,
5.times.10.sup.8 cm.sup.-2, 10.sup.9 cm.sup.-2, 5.times.10.sup.9
cm.sup.-2, 10.sup.10 cm.sup.-2, or 5.times.10.sup.10 cm.sup.-2).
The volumetric density may be about 10.sup.10 cm.sup.-2 to about
10.sup.13 cm.sup.-2 (e.g., 5.times.10.sup.10 cm.sup.-2, 10.sup.11
cm.sup.-2, 5.times.10.sup.11 cm.sup.-2, 10.sup.12 cm.sup.-2, or
5.times.10.sup.12 cm.sup.-2). Other areal and volumetric densities
may also be possible. In other cases, the nanoparticles 114 may be
distributed more or less densely in certain areas, e.g., to form
opaque areas or transparent areas on the screen 110.
[0053] Given the cross-section(s) and the concentration of the
nanoparticles, the thickness of the screen 110 should be chosen to
balance the extinction, which is the product of the nanoparticle
scattering cross section(s), nanoparticle density, and screen
thickness, at the desired wavelength .lamda..sub.0 (which should be
higher for higher scattering efficiency) and the extinction away
from resonance (which should be lower for higher transparency).
Generally, the screen 110 should be thick enough that more than
half of the light at .lamda..sub.0 is scattered, but thin enough
that more than half of the light away from resonance is
transmitted.
[0054] The scattering nanoparticles 114 may be of any suitable
shape, including but not limited to spheres, ellipsoids, oblate
spheroids, and prolate spheroids. They may be solid particles made
of a single material, hollow particles, or solid particles coated
with outer layers. The nanoparticle may also be made as a metallic
nanoparticle 1214 with a shape that supports formation of a current
loop as shown in FIG. 12. Such nanoparticles can support magnetic
resonances that may have narrower bandwidths than solid
nanoparticles.
[0055] The nanoparticles 114 may include dielectric materials,
including silica, silicon, and titanium dioxide; metals, including
silver, gold, and copper; and combinations of dielectric materials
and metals. Their outer diameters may range from about 5 nm to
about 250 nm (e.g., 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40
nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm,
90 nm, or 95 nm, 100 nm, 125 nm, 150 nm, 175 nm, 200 nm, or 225
nm). The nanoparticles' exact shape, size, and composition may
depend on the desired scattering properties.
[0056] As explained in greater detail below, the nanoparticles 114
scatter incident light at one or more specified wavelengths
depending on their composition and size. If desired, the
nanoparticles 114 may include different types of nanoparticles,
each of which scatters light at only a narrowband region centered
about one or more wavelengths--for example, at about 460 nm, about
530 nm, and about 650 nm. (The nanoparticles 114 scatter and absorb
negligible amounts of light at the other wavelengths in the visible
spectrum.) For example, the nanoparticles 114 may include
red-scattering nanoparticles 114a, green-scattering nanoparticles
114b, and blue-scattering nanoparticles 114c. These red-scattering
nanoparticles 114a, green-scattering nanoparticles 114b, and
blue-scattering nanoparticles 114c may be mixed in equal
proportions, in proportions weighted based on the human eye's
sensitivity to different colors, or in any other desired
proportion. Alternatively, or in addition, some or all of the
nanoparticles 114 may be configured to scatter light at multiple
wavelengths (e.g., at wavelengths corresponding to red, green, and
blue). Other mixtures of scattering nanoparticles 114 are also
possible (e.g., monochromatic, two-color, etc.).
[0057] FIG. 1B is a plot of the scattering cross section (solid
line) and the absorption cross section (dashed line) versus
wavelength for a blue-scattering nanoparticle 114b. The plot shows
that the blue-scattering nanoparticle 114b scatters light strongly
at a wavelength of about 460 nm, but does not scatter light
significantly at any other wavelength in the visible spectrum. It
also shows that the blue-scattering nanoparticle 114b absorbs
hardly any light in the visible spectrum. As a result, the
blue-scattering nanoparticle 114b is effectively transparent at
every wavelength in the visible spectrum except for those in a
narrowband region centered at about 460 nm.
[0058] In operation, the light source 120 illuminates the
nanoparticles 114 at one or more of the nanoparticles' scattering
wavelengths. Depending on the implementation, the light source 120
may include one or more lasers, light-emitting diodes (LEDs), or
other spectrally coherent sources configured to generate light at
wavelengths scattered by one or more of the nanoparticles 114. For
instance, the light source 120 may include a first laser diode 122a
that emits a first beam 123a at a wavelength of about 460 nm, a
second laser diode 122b that emits a second beam 123b at a
wavelength of about 530 nm, and a third laser diode 122c that emits
a third beam 123c at a wavelength of about 650 nm (collectively,
laser diodes 122 and monochromatic laser beams 123). Lens 124a,
124b, and 124c (collectively, lenses 124) and other beam-shaping
optics, such as pinholes, prisms, and diffractive elements,
collimate or loosely focus the laser beams 123 to prevent
divergence. Beam-shaping optics 126, shown in FIG. 1A as a mirror
126a and dichroic beam combiners 126b and 126c, combine the laser
beams 123 to form a polychromatic beam 121. Additional beam-shaping
optics (not shown) may focus the polychromatic beam 121 to a spot
in the plane of the nanoparticle layer 114.
[0059] Alternatively, the light source 120 may include a broadband
light source (e.g., an ultra high performance (UHP) lamp or
household projector). One or more dichroic filters or bandpass
filters selects the desired wavelength(s). Because a broadband
light source is not monochromatic, its scattering efficiency may
not be as high as that of a laser or monochromatic source. However,
a broadband light source may provide higher power than a laser
diode at a relatively low cost.
[0060] To produce an image on the screen 110, a beam-steering
element 130, which may include a galvo-scanning mirror or
acousto-optic deflector, directs the polychromatic beam 121 to
different areas on the screen. In some cases, the beam-steering
element 130 may a micro-electromechanical systems (MEMS) device
integrated into the light source 120 (e.g., as in the MicroVision
SHOWW+ Laser Pico Projector). For example, the beam-steering
element 130 may scan the polychromatic beam 121 along a raster
pattern (or any other suitable scan pattern) while a controller 140
modulates the intensity of the monochromatic laser beams 123
emitted by the laser diodes 122 to produce the desired image(s) on
the screen. The controller 140, which may include a processor,
memory, communications interface, user interface, and any other
suitable components, may control the beam-steering element 130 and
the light source 120 in response to user input, input from a video
or image data source, or both to form a particular image or series
of images on the screen 110.
[0061] FIG. 1C is a close-up of a section of the screen 110
illuminated by the polychromatic beam 121. The polychromatic beam
121 forms a spot 125 that illuminates several nanoparticles 114,
including at least one red-scattering nanoparticle 114a and at
least one blue-scattering nanoparticle 114b. Red light 123a
scatters off the red-scattering nanoparticle 114a to produce
scattered red light 111a, and blue light 123b scatters off the
blue-scattering nanoparticle 114b to produce scattered blue light
111b. Observers 1 and 3 looking at the screen 110 may perceive the
scattered red light 111a and scattered blue light 111b as a purple
spot on the screen 110.
[0062] Together, the size of the focused spot and the nanoparticle
density set the display's pixel size, or resolution: as long as the
focused spot is large enough to encompass at least one nanoparticle
114, it should produce scattered light visible to an observer.
Unlike conventional displays, however, the pixel size of the
transparent scattering display 100 can be adjusted on the fly by
simply increasing or decreasing the focused spot size, e.g., using
a zoom lens. In addition, the focused spot size can be very small,
e.g., on the order of the illumination wavelength, for resolution
much finer than the finest resolution achievable with conventional
pixelated displays. (In some cases, however, the beam-steering
element's scanning capability may limit the number of resolvable
spots on the display if the number of distinct scan
angles/positions is smaller than the display's area divided by the
focused spot size.)
[0063] FIG. 1D shows how the transparent screen 110 can be used to
achieve a three-dimensional (3D) viewing effect using polarization
multiplexing. A first polarized light source 152a projects a first
polarized image 151a onto the screen 110 with a first polarized
beam 153a (e.g., a right-handed circularly polarized beam as shown
in FIG. 1D). A second polarized light source 152b projects a second
polarized image 151b onto the screen 110 with a second polarized
beam 153b (e.g., a left-handed circularly polarized beam as shown
in FIG. 1D). An observer 5 views the superimposed image with
eyeglasses 160 that filter right-handed circular polarization for
one eye and left-handed circular polarization for the other eye
with filters 162a and 162b. Each eye perceives a different image,
so a 3D effect can be achieved. This works on the transparent
screen 110 because most of the scattered light comes from a
single-scattering event rather than multiple scattering, and so the
scattered light retains the polarization state of the incident
light.
[0064] Those of skill in the art will readily understand that the
display 100 can use other architectures instead of those shown in
FIG. 1A. For instance, the light source 120 may include LEDs that
emit red, green, and blue light instead of laser diodes. The red,
green, and blue beams may be steered independently, e.g., by using
a separate beam-steering element for each beam. The display can
also use a single beam-steering element to steer pulsed
monochromatic beams in a repetitive sequence, e.g., the red beam
121a, then the green beam 121b, and then the blue beam 121c. The
beam-shaping optics 124 and the beam-combining optics 126 can also
be selected depending on the desired spot size (display
resolution), working distance (distance from the light source 120
to the screen 110), etc.
[0065] Multi-Layer Transparent Scattering Displays
[0066] FIG. 2 illustrates a multi-layer transparent scattering
display 200 that uses selectively scattering nanoparticles 214 to
produce images visible to observers in front of and behind the
screen. Like the display 100 shown in FIG. 1A, the multi-layer
transparent scattering display 200 includes a screen 210 and light
sources 222a and 222b (collectively, light sources 222). In this
case, however, the screen 210 includes three substantially
transparent substrates: a first substrate 212a, a second substrate
212b, and third substrate 212c (collectively, substrates 212).
These substrates 212 may be made of glass, plastic, acrylic, or any
other suitable material. Their thickness and rigidity/flexibility
may be selected based on the application as explained above.
[0067] The screen 210 also includes two nanoparticle layers: a
first nanoparticle layer 216a, which includes nanoparticles 214a
that scatter light at a first wavelength (e.g., 460 nm), and a
second nanoparticle layer 216b, which includes nanoparticles 214b
that scatter light at a second wavelength (e.g., 650 nm)
(collectively, nanoparticles 214 and nanoparticle layers 216). A
given nanoparticle layer 216 may include more than one type of
nanoparticle 214 (e.g., as in the display 100 of FIG. 1A). If
desired, the nanoparticles 214 may be deposited or arranged between
the substrates 216 in predetermined patterns. For example, the
nanoparticles 214 may be arranged to form logos, letters, numbers
(e.g., the numbers "1" and "2" as shown in FIG. 2), or other
patterns that can be displayed by illuminating the screen 210 with
light at the scattering wavelength(s).
[0068] FIG. 2 also shows a pair of light sources 222a and 222b
disposed to illuminate the screen 210 with diverging beams 221a and
221b (collectively, diverging beams 221) of monochromatic light. In
this example, the first light source 222a emits a first diverging
beam 221a at wavelength scattered by the nanoparticles 214a in the
first nanoparticle layer 216a, and the second light source 222b
emits a second diverging beam 221b at wavelength scattered by the
nanoparticles 214b in the second nanoparticle layer 216b.
Illuminating the entire screen 210 with the diverging beams 221
causes images to appear on the screen 210 in the shapes of the
patterns formed by the nanoparticles. The diverging beams 221 can
be turned on and off (modulated) to produce images that appear to
flicker, move, change color, etc. as desired.
[0069] Scattering Nanoparticles
[0070] The nanoparticles in the transparent scattering displays
shown in FIGS. 1A and 2 have scattering cross sections that are
sharply peaked at particular wavelengths and substantially zero
throughout the rest of the visible spectrum. They have also have
absorption cross sections that are substantially zero throughout
the visible spectrum. This combination of selective scattering and
low absorption, combined with the substrates' substantial
transparency throughout the visible spectrum, means that the
display itself can be substantially transparent.
[0071] There are several ways to achieve wavelength-selective
narrowband scattering (resonant scattering) in nanoparticles,
including surface plasmonic resonances in metal-coated
nanoparticles, resonant features (e.g., cavities), and Fano
resonances, which are resonances that exhibit asymmetric profiles
due to interference between the resonant and background scattering
probabilities. Other particles, such as high-index dielectric
nanoparticles, and other types of resonances, such as higher-order
resonances, may also exhibit suitable wavelength-selective
scattering. Moreover, the size, shape, and composition of the
nanoparticles can be selected to achieve particular scattering
wavelengths, bandwidths, and bandshapes.
[0072] As understood by those of skill in the art, when a particle
is much smaller than the wavelength of incident light (e.g., a
nanoparticle), the particle experiences a local electromagnetic
field that is substantially constant in space. As a result, the
optical response of this small particle can be determined from the
corresponding electrostatic problem. This is called the
quasi-static approximation (also known as the electrostatic
approximation or dipole approximation).
[0073] The quasi-static approximation can be used to estimate the
sharpness of the scattering cross-section in the localized surface
plasmon resonances of small metallic particles. This derivation is
general and applies to arbitrary particle shapes, including but not
limited to spheres, ellipsoids, oblate spheroids, and prolate
spheroids. In the quasi-static approximation, the scattering
cross-section .sigma..sub.sca averaged over angle and polarization
of the incoming light can be written as:
.sigma. sca = k 4 18 .pi. j = 1 , 2 , 3 a j ( ) 2 , ( 1 )
##EQU00001##
where the angle brackets denote an average over angle and
polarization, k=2.pi. .epsilon..sub.m/.lamda. is the wavenumber in
the surrounding medium (whose dielectric constant .epsilon..sub.m
is purely real and positive), and .alpha..sub.1, .alpha..sub.2, and
.alpha..sub.3 are the particle's static polarizabilities in three
orthogonal directions. The particle can be a uniform material
(e.g., a solid sphere) or a composite of multiple materials (e.g.,
a core-shell structure).
[0074] One way to achieve wavelength-selective scattering is to use
the localized surface plasmon resonances in metallic nanoparticles.
Without being bound by any particular theory, those skilled in the
art understand that a metallic nanoparticle can support a surface
plasmon because its dielectric function will have a negative real
part at some wavelength range. In particular, a metallic
nanoparticle supports a localized surface plasmon resonance that
occurs approximately at the wavelength .lamda..sub.0 for which
1 .alpha. j ( Re ( ( .lamda. 0 ) ) ) = 0. ( 2 ) ##EQU00002##
(For a sphere, this condition can be simplified to
Re(.epsilon.(.lamda..sub.0))=-2.epsilon..sub.m.) Near the
resonance, the static polarizability .alpha..sub.j provides the
dominant contribution to the averaged scattering cross-section, so
the ratio between the on-resonance (at .lamda..sub.0) and
off-resonance (at .lamda..sub.0+.DELTA..lamda., for a small
.DELTA..lamda. of interest) scattering cross-sections is
approximately
.sigma. sca ( .lamda. 0 ) .sigma. sca ( .lamda. 0 + .DELTA..lamda.
) .apprxeq. .alpha. j ( ( .lamda. 0 ) ) .alpha. j ( ( .lamda. 0 +
.DELTA..lamda. ) ) 2 . ( 3 ) ##EQU00003##
[0075] This expression can be simplified by writing the
polarizability as a rational function and taking the changes in the
real and imaginary components of the permittivity with
.DELTA..lamda. to be small relative to their on-resonance values.
Analytical expressions for the polarizability of spheres, coated
spheres, ellipsoids, and coated ellipsoids appear in C. F Bohren
and D. R. Huffman, Absorption and Scattering of Light by Small
Particles (Wiley, New York, 1998), which is incorporated herein by
reference in its entirety. All of these analytical expressions may
take the form of rational functions. For more arbitrary geometries,
the polarizability can often be locally approximated as a rational
function near the resonance.
[0076] Applying these simplifications to Equation (3) yields the
following expression for the ratio between the on-resonance (at
.lamda..sub.0) scattering cross-section and the off-resonance (at
.lamda..sub.0+.DELTA..lamda., for a small .DELTA..times., of
interest) for a small particle (e.g., diameter<<wavelength)
characterized by a dielectric function .epsilon.:
.sigma. sca ( .lamda. 0 ) .sigma. sca ( .lamda. 0 + .DELTA..lamda.
) .apprxeq. 1 + Re [ ( .lamda. 0 + .DELTA..lamda. ) - ( .lamda. 0 )
] Im [ ( .lamda. 0 ) ] 2 . ( 4 ) ##EQU00004##
The derivation leading to Equation (4) applies to arbitrary
particle shapes, and assumes only that the particle is much smaller
than the wavelength .lamda..sub.0 and that .DELTA..lamda. is small
enough that the permittivity does not change much. Equation (4)
applies to localized surface plasmon resonances, but not
necessarily to other types of resonances.
[0077] For strong wavelength-selective scattering, the on-resonance
scattering cross section, .sigma..sub.sca(.lamda..sub.0), should be
much larger than the off-resonance scattering cross section,
.sigma..sub.sca(.lamda..sub.0+.DELTA..lamda.), which that the ratio
given by Equation (4) should be (much) greater than 1. To achieve a
large ratio (e.g., much greater than 1), Equation (4) suggests that
the nanoparticle material should be characterized by a dielectric
function with a small imaginary component, Im(.epsilon.), and a
fast-changing real component, Re(.epsilon.), near the resonance
wavelength, .lamda..sub.0. Materials whose dielectric functions
have small imaginary components and fast-changing real components
near a particular resonance wavelength include Drude metals with
negligible loss.
[0078] FIG. 3 is a plot of the square of the ratio of the real part
of the derivative of the dielectric function with respect to
wavelength to the imaginary part of the dielectric function,
.eta.=|Re(d.epsilon./d.lamda.)/Im(.epsilon.)|.sup.2, for different
metals. In other words, .eta. provides an estimate of a material's
performance in a scattering nanoparticle, with higher values
indicating higher scattering and/or lower absorption. FIG. 3 shows
that realistic metals tend to be lossy and to deviate significantly
from the Drude model in the visible spectrum. Within most of the
visible spectrum, silver has the highest value of
.eta.=|Re(d.epsilon./d.lamda.)/Im(.epsilon.)|.sup.2 among the
common metals, and is thus suitable for use in a scattering
nanoparticle. Other materials suitable for making scattering
nanoparticles include but are not limited to gold and copper.
[0079] As mentioned above, a nanoparticle suitable for use in a
transparent scattering display should have a uniformly low
absorption cross-section gabs across the visible spectrum, and a
high scattering cross section .sigma..sub.sca at the resonance
wavelength .lamda..sub.0 with low scattering cross section
.sigma..sub.sca elsewhere in the visible spectrum. These parameters
can be used to define the following figure of merit (FOM):
FOM = .sigma. sca ( .lamda. 0 ) 2 .sigma. sca _ + max { .sigma. abs
} , ( 5 ) ##EQU00005##
where the overline and the symbol max{ . . . } denote the mean and
the maximum, respectively, in the visible spectrum (from 390 nm to
750 nm). The FOM is defined as a ratio, instead of in terms of the
absolute values of the particle's scattering and absorption cross
sections, because the screen's total scattering and absorption can
be set by picking an appropriate areal density of the nanoparticles
on the screen. The number 2 is an empirically determined weight
that provides a good balance between optimizing for sharp
scattering and for low absorption. (Other empirically determined
weights (e.g., a number in the range of 1.5-2.5) may work as well.)
And using the maximum absorption cross section gives an FOM for a
flat absorption spectrum for a colorless transparent screen instead
of a peaked absorption cross section for a tinted transparent
screen. If desired, the FOM can be adjusted to include a
wavelength-dependent weight on the scattering and absorption cross
sections to account for the human eye's spectral sensitivity.
[0080] If desired, the FOM defined in Equation (5) can be used to
determine a nanoparticle's size and composition for a given
scattering wavelength, scattering cross section, and absorption
cross section. More specifically, a nanoparticle can be designed
numerically by combining Mie-theory calculation of the scattering
and absorption cross sections with a nonlinear optimization engine
that uses an appropriate figure of merit, such as the FOM given in
Equation (5). Other suitable figures of merit may be defined as the
average scattering cross section over a specified narrowband region
divided by the average extinction cross section across the whole
visible spectrum or by minimizing the full width at half maximum
(FWHM) of the scattering cross section times the average extinction
cross section across the whole visible spectrum. Nonlinear
optimization based on a figure of merit can be applied to any
material, any geometry, and other types of resonances. This
FOM-based optimization can also be used to design nanoparticles
with multiple scattering cross sections in the visible spectrum
and/or scattering cross sections with different spectral
widths.
[0081] FIGS. 4A-4C are plots of scattering cross sections (solid
lines) and absorption cross sections (dashed lines) versus
wavelength for spherical nanoparticles with shells made of silver
and cores made of silica generated using the nonlinear optimization
process described above. The particles, which are shown in the
insets of FIGS. 4A-4C, can be synthesized, for example, using the
Stober process, are assumed to be embedded in a transparent medium
with a refractive index n=1.44, which is typical of a polymer
matrix. The scattering and absorption cross-sections shown in FIGS.
4A-4C were calculated with the transfer matrix method, using n=1.45
for silica and experimental values for the wavelength-dependent
complex permittivity of silver. The particle size distribution is
assumed to follow a Gaussian distribution with a standard deviation
equal to about 10% of the mean. Using the FOM from Equation (5)
gives a core radius and shell thickness by performing a global
optimization via a multi-level, single-linkage algorithm
implemented, e.g., within the nonlinear optimization package NLopt
(available at http://ab-initio.mit.edu/wiki/index.php/NLopt).
[0082] FIG. 4A shows a silica nanosphere, with a radius of about
1.3 nm, that is coated with a 30.8 nm thick silver shell and
scatters blue laser light (.lamda..sub.0=458 nm). FIG. 4B shows a
silica nanosphere, with a radius of about 22.2 nm, that is coated
with a 15.8 nm thick silver shell and scatters green laser light
(.lamda..sub.0=532 nm). And FIG. 4C shows a silica nanosphere, with
a radius of about 34.3 nm, that is coated with a 11.0 nm thick
silver shell and scatters red laser light (.lamda..sub.0=640 nm).
The FWHM of the peaks in FIGS. 4A, 4B, and 4C are about 66 nm,
about 62 nm, and about 69 nm, respectively, and the FOMs are 1.01,
0.91, and 0.81, respectively. Even at these FWHM, a transparent
substrate that hosts any of the nanoparticles shown in FIGS. 4A-4C
is substantially transparent except at the resonance
wavelength(s).
[0083] FIGS. 5A and 5B are plots of the calculated scattering and
absorption cross-sections of a titanium dioxide nanoshell that
scatters blue laser light at .lamda..sub.0=458 nm and a silicon
nanoshell that scatters green laser light at .lamda..sub.0=532 nm,
respectively. The calculations take into account a .+-.10% random
distribution in the shell thickness. (Without being bound by any
particular theory, the quasi-static approximation may not describe
these nanoshell resonances accurately because the nanoshells are
relatively large.)
[0084] The titanium dioxide nanoshell in FIG. 5A has an inner
radius of 25.5 nm, an outer radius of 70.1 nm, and a FOM=1.76. The
silicon nanoshell in FIG. 5B has an inner radius of 43.8 nm, an
outer radius of 68.2 nm, and a FOM=1.14. The nanoshells' resonances
exhibit relatively low absorption loss, but the index contrast
should be high enough to provide sufficient confinement for the
resonances. In FIGS. 5A and 5B, for example, the nanoshells' cores
and surrounding media are assumed to have refractive index n=1. In
practice, the cores and surrounding media may include low-index
materials, such as transparent aerogels with transmission lengths
of up to 60 mm.
[0085] FIG. 6 is a plot of the scattering cross section for a
silicon nanoparticle with a radius of about 115 nm. The scattering
cross section has peaks at wavelengths corresponding roughly to
blue, green, and red. The silicon nanoparticle also scatters light
in the near infrared portion of the electromagnetic spectrum, but
because humans cannot see infrared light, the infrared scattering
is unlikely to affect the display's performance.
[0086] Making a Transparent Scattering Display
[0087] FIGS. 7A-7C illustrate different processes for making
screens with scattering nanoparticles for use in transparent
scattering displays, e.g., as shown in FIGS. 1A and 2. The
nanoparticles 714 may have shapes, sizes, and/or compositions
selected to provide scattering at particular wavelengths (e.g.,
wavelengths corresponding to red, blue, and green). The
nanoparticles 714 can also be synthesized with existing techniques
including but not limited to wet chemistry, physical vapor
deposition, ion implantation, and fiber drawing. If desired, the
nanoparticles' outer surfaces can be treated (e.g., with
polyvinylpyrrolidone (PVP)) to prevent undesirable aggregation
(clustering, described below), chemical reactions with the host
(substrate) material, or both. The nanoparticle solution may be
diluted to a reduce the nanoparticle concentration, to improve the
coating's deposition characteristics, or both.
[0088] The process 700 shown in FIG. 7A involves dissolving
nanoparticles 714 in an appropriate solvent 715, such as polyvinyl
alcohol (PVA), which dissolves in water, to form a nanoparticle
solution (step 702). If desired, the nanoparticle solution can be
mixed with a polymer powder to form a polymer/nanoparticle
solution. Alternatively, the nanoparticles can also be dissolved
into a liquid polymer matrix to form a polymer/nanoparticle
solution that is deposited onto one surface of a transparent
substrate 712 and allowed to dry slowly.
[0089] The nanoparticles can be coated onto the transparent
substrate 712, which can made of glass, plastic, acrylic, or any
other suitable transparent material, by depositing the nanoparticle
solution onto the substrate's surface (step 704). Making the
coating uniform, both in thickness and surface appearance, may
reduce or eliminate undesired scattering. To achieve a uniform
thickness, the nanoparticle solution may be spin-coated, painted,
or otherwise deposited onto the substrate's surface to form the
coating. For example, the nanoparticle solution may be painted onto
the substrate in the form of liquid, foam, or spray. Such paint can
be sprayed onto a transparent surface, without the need for any
special facility, and so can be done easily at almost any occasion.
If desired, another transparent substrate (not shown) may be placed
or pressed onto the coated surface of the first substrate 712 to
remove bubbles in the coating, reduce the coating's surface
roughness, or both. The coating is allowed to dry (step 706)
through gentle evaporation (e.g., via dessication in a vacuum
chamber) to form a uniform surface. If desired, additional coats
(e.g., containing different types of nanoparticles) may be
deposited onto the dried coating or onto other substrate
surfaces.
[0090] In some cases, the nanoparticle solution is printed onto the
substrate surface to produce a uniform layer of nanoparticles.
Suitable printing techniques include but are not limited to screen
printing, inkjet printing, rotogravure printing, and flexography
printing. Printing is particularly useful for depositing
nanoparticles onto flexible substrates.
[0091] If desired, the nanoparticle solution may be printed or
other otherwise deposited onto predetermined portions of the
substrate surface, e.g., to form patterns like those shown in FIG.
2, using a mask or other appropriate technique. For instance, if
the nanoparticle solution is an aqueous solution, the substrate
surface may be selectively coated with hydophilic and/or
hydrophobic coatings to attract and repel, respectively, the
nanoparticle solution.
[0092] FIG. 7B illustrates another process 730 for making a screen
suitable for use in a transparent display. In this process 730, the
substrate is softened to form a softened substrate or melted to
form a liquid substrate (step 732). The nanoparticles are pressed
into the softened substrate or dissolved into the liquid substrate
(step 734), then allowed to harden into a screen that includes
nanoparticles embedded within the substrate (step 736). For
instance, the nanoparticles can be dissolved directly into molten
glass, molten/liquid plastic, or molten/liquid acrylic to form a
doped substrate material. The doped substrate material can then be
extruded, molded, or otherwise formed into a transparent board
doped with nanoparticles using standard glass (or plastic) making
techniques.
[0093] FIG. 7C illustrates yet another process 760 for making a
screen suitable for use in a transparent display. In this process
760, one or more nanoparticles are suspended in a liquid polymer
matrix or mixed with a dry polymer and a solvent to form a
nanoparticle suspension (step 762). In step 764, the nanoparticle
suspension is injected or wicked into a cavity formed between a
pair of substrates or within a single substrate, e.g., using
capillary action. The cavity is sealed in step 766 to prevent the
suspension from escaping, and the nanoparticle suspension is cured,
e.g., using ultraviolet light, to prevent the nanoparticles from
settling.
[0094] Nanoparticle Clustering
[0095] In some cases, the nanoparticles in solution can cluster
together to produce undesired shifts in the scattering wavelength.
Without being bound by any particular theory, it appears that
osmatic pressure, capillary forces, and van der Waals forces could
each play a role in nanoparticle clustering. Osmatic pressure
yields short-range (sticking) attraction, but may not apply in
solvents with relatively low nanoparticle concentrations (e.g.,
volume fractions of about 2.times.10.sup.-5 or less). Capillary
forces can be reduced or eliminated by lowering the interfacial
energy between the nanoparticle surface and the solvent (e.g.,
PVA), for example, by coating the particles with a polar coating
like PVP.
[0096] Van der Waals forces may be more likely to cause clustering
than osmatic pressure or capillary forces. The potential energy
associated with the van der Waals force for a pair of spheres with
radius R and separated by a distance D is
W(D)=-AR/(12D),
where A is a constant. For silver nanospheres in water,
A=4.times.10.sup.-19, which, at a separation distance D=200 nm,
corresponds to an attractive energy roughly equal to kT
(Boltzmann's constant times temperature, or the thermal excitation
energy). Decreasing the nanosphere density reduces the probability
that a given pair of nanospheres will be separated by 200 nm or
less, which in turn reduces the probability that van der Waals
attraction will lead to clustering:
TABLE-US-00001 Nanosphere Probability of <200 nm Density Nearest
Neighbor 2.0 .mu.g/L 1.7% 1.0 .mu.g/L 0.8% 0.5 .mu.g/L 0.4%
[0097] FIGS. 8A-8C illustrate the effects of nanoparticle
clustering. FIGS. 8A and 8B show the experimental extinction cross
sections (.sigma..sub.ext=.sigma..sub.sca+.sigma..sub.abs; solid
lines) and theoretical extinction cross sections (dashed lines)
versus wavelength for spherical silver nanoparticles in PVA. The
widths of the experimental curves indicate plus/minus one standard
deviation. The nanoparticle concentration is 5 .mu.g/mL in FIG. 8A
and 10 .mu.g/mL in FIG. 8B, which shows a larger extinction cross
sections near 620 nm (as indicated by the arrow). FIG. 8C is a plot
of the theoretical extinction cross-section spectrum of two silver
nanoparticles that stick together (e.g., as shown in the TEM image
in the inset of FIG. 9A) calculated for polarization along the axis
of alignment. It shows that the clustered nanoparticles scatter
light strongly at 620 nm, which suggests that the extra peak in
FIG. 8B is due to nanoparticle clustering at the higher
nanoparticle concentration.
EXEMPLIFICATION
[0098] The following example is intended to illustrate aspects of
the present technology without limitation of the claims.
[0099] In one example, a transparent display capable of displaying
blue images was made and tested as follows. FIG. 4A shows that a
nanoparticle that scatters blue laser light has a negligible silica
core, so the transparent display was made using solid spherical
silver nanoparticles (e.g., from nanoComposix) for simplicity. The
nanoparticles were hosted in a transparent polymer matrix by adding
PVA (e.g., from Sigma-Aldrich) into an aqueous solution of silver
nanoparticles (nanoparticle density 0.01 mg/mL). The liquid was
mixed thoroughly, poured onto one surface of a piece of glass that
measured 25 cm by 25 cm, desiccated in a vacuum chamber, and
allowed to dry at room temperature for 40 hours. This yielded a
transparent screen with a film of nanoparticle-doped PVA that was
0.46 mm thick, with almost no air bubbles inside, on a transparent
substrate.
[0100] FIGS. 9A and 9B are plots of the transmittance and
extinction, respectively, versus wavelength for the film of
nanoparticle-doped PVA. (The inset of FIG. 9A shows a transmission
electron microscope (TEM) image of the measured nanoparticles,
which have a diameter of 62 nm.+-.4 nm.) Solid lines indicate
measured values and dashed lines indicate calculated values. The
width of the solid lines indicates plus/minus one standard
deviation. FIG. 9B also shows theoretical values for extinction due
to scattering (dot-dash) and absorption (dot-dot-dash). The
transmittance falls to about 20% at the resonance wavelength and is
close to 100% elsewhere in the visible sprectrum. Both plots
indicate very good agreement between the experiment and the theory,
with a slight discrepancy near 620 nm that can be explained by the
occasional clustering of the nanoparticles (see FIGS. 8B and 8C).
FIG. 9B also shows that the theory predicts that the majority of
the extinction at the resonance wavelength comes from scattering
rather than from absorption.
[0101] FIG. 10A shows a color photograph of the transparent screen
used as a transparent scattering display, which sits in front of
three colored coffee mugs. The photograph shows a blue MIT logo
projected onto this screen using a low-power laser projector (e.g.,
MicroVision SHOWWX+, maximum output power 1 mW), with a GaN laser
diode that emits light at a wavelength of 458 nm.+-.2 nm. The
projected logo was visible from all directions because it was
formed by light scattering. As a comparison, FIG. 10B is a color
photograph that shows the same logo projected onto a piece of
regular glass in front of the same coffee mugs shown in FIG. 10A.
No image appears in FIG. 10B because the glass is transparent at
the 458 nm wavelength of the projected blue laser light. FIGS. 10A
and 10B also show that the screen is nearly as transparent as the
glass: the colored coffee mugs appear properly behind both the
screen and the glass.
[0102] FIGS. 11A and 11B are color photographs of the MIT logo
projected onto the transparent screen and a piece of white paper,
respectively. Both photographs were captured with the same lighting
condition and exposure and converted to JPEG files directly from
the raw image files without any editing. FIGS. 11A and 11B show
that the image projected image onto the screen is slightly dimmer
than the image on the paper, but the screen achieves better image
contrast than the paper because it scatters less ambient light than
the paper. (Unlike the paper, the screen does produce any diffuse
scattering.) The black backing behind the transparent screen in
FIG. 11A also improves the contrast ratio.
CONCLUSION
[0103] While various inventive embodiments have been described and
illustrated herein, those of ordinary skill in the art will readily
envision a variety of other means and/or structures for performing
the function and/or obtaining the results and/or one or more of the
advantages described herein, and each of such variations and/or
modifications is deemed to be within the scope of the inventive
embodiments described herein. More generally, those skilled in the
art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the inventive teachings is/are used. Those
skilled in the art will recognize, or be able to ascertain using no
more than routine experimentation, many equivalents to the specific
inventive embodiments described herein. It is, therefore, to be
understood that the foregoing embodiments are presented by way of
example only and that, within the scope of the appended claims and
equivalents thereto, inventive embodiments may be practiced
otherwise than as specifically described and claimed. Inventive
embodiments of the present disclosure are directed to each
individual feature, system, article, material, kit, and/or method
described herein. In addition, any combination of two or more such
features, systems, articles, materials, kits, and/or methods, if
such features, systems, articles, materials, kits, and/or methods
are not mutually inconsistent, is included within the inventive
scope of the present disclosure.
[0104] The above-described embodiments can be implemented in any of
numerous ways. For example, the embodiments (e.g., of designing
and/or operating transparent displays) may be implemented using
hardware, software, or a combination thereof. When implemented in
software, the software code can be executed on any suitable
processor or collection of processors, whether provided in a single
computer or distributed among multiple computers.
[0105] Further, it should be appreciated that the present displays
and methods of making and operating displays may be used in
conjunction with a computer, which may be embodied in any of a
number of forms, such as a rack-mounted computer, a desktop
computer, a laptop computer, or a tablet computer. For instance,
the controller 140 shown in FIG. 1A may be implemented as a
computer, smart phone, or other processor-based device.
Additionally, a computer may be embedded in a device not generally
regarded as a computer but with suitable processing capabilities,
including a Personal Digital Assistant (PDA), a smart phone or any
other suitable portable or fixed electronic device.
[0106] Also, a computer may have one or more input and output
devices, including one or more displays as disclosed herein. These
devices can be used, among other things, to present a user
interface. Examples of output devices that can be used to provide a
user interface include printers or display screens for visual
presentation of output and speakers or other sound generating
devices for audible presentation of output. Examples of input
devices that can be used for a user interface include keyboards,
and pointing devices, such as mice, touch pads, and digitizing
tablets. As another example, a computer may receive input
information through speech recognition or in other audible
format.
[0107] Such computers may be interconnected by one or more networks
in any suitable form, including a local area network or a wide area
network, such as an enterprise network, and intelligent network
(IN) or the Internet. Such networks may be based on any suitable
technology and may operate according to any suitable protocol and
may include wireless networks, wired networks or fiber optic
networks.
[0108] The various methods or processes outlined herein may be
coded as software that is executable on one or more processors that
employ any one of a variety of operating systems or platforms.
Additionally, such software may be written using any of a number of
suitable programming languages and/or programming or scripting
tools, and also may be compiled as executable machine language code
or intermediate code that is executed on a framework or virtual
machine.
[0109] In this respect, various inventive concepts may be embodied
as a computer readable storage medium (or multiple computer
readable storage media) (e.g., a computer memory, one or more
floppy discs, compact discs, optical discs, magnetic tapes, flash
memories, circuit configurations in Field Programmable Gate Arrays
or other semiconductor devices, or other non-transitory medium or
tangible computer storage medium) encoded with one or more programs
that, when executed on one or more computers or other processors,
perform methods that implement the various embodiments of the
invention discussed above. The computer readable medium or media
can be transportable, such that the program or programs stored
thereon can be loaded onto one or more different computers or other
processors to implement various aspects of the present invention as
discussed above.
[0110] The terms "program" or "software" are used herein in a
generic sense to refer to any type of computer code or set of
computer-executable instructions that can be employed to program a
computer or other processor to implement various aspects of
embodiments as discussed above. Additionally, it should be
appreciated that according to one aspect, one or more computer
programs that when executed perform methods of the present
invention need not reside on a single computer or processor, but
may be distributed in a modular fashion amongst a number of
different computers or processors to implement various aspects of
the present invention.
[0111] Computer-executable instructions may be in many forms, such
as program modules, executed by one or more computers or other
devices. Generally, program modules include routines, programs,
objects, components, data structures, etc. that perform particular
tasks or implement particular abstract data types. Typically the
functionality of the program modules may be combined or distributed
as desired in various embodiments.
[0112] Also, data structures may be stored in computer-readable
media in any suitable form. For simplicity of illustration, data
structures may be shown to have fields that are related through
location in the data structure. Such relationships may likewise be
achieved by assigning storage for the fields with locations in a
computer-readable medium that convey relationship between the
fields. However, any suitable mechanism may be used to establish a
relationship between information in fields of a data structure,
including through the use of pointers, tags or other mechanisms
that establish relationship between data elements.
[0113] Also, various inventive concepts may be embodied as one or
more methods, of which an example has been provided. The acts
performed as part of the method may be ordered in any suitable way.
Accordingly, embodiments may be constructed in which acts are
performed in an order different than illustrated, which may include
performing some acts simultaneously, even though shown as
sequential acts in illustrative embodiments.
[0114] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0115] A flow diagram is used herein. The use of flow diagrams is
not meant to be limiting with respect to the order of operations
performed. The herein described subject matter sometimes
illustrates different components contained within, or connected
with, different other components. It is to be understood that such
depicted architectures are merely exemplary, and that in fact many
other architectures can be implemented which achieve the same
functionality. In a conceptual sense, any arrangement of components
to achieve the same functionality is effectively "associated" such
that the desired functionality is achieved. Hence, any two
components herein combined to achieve a particular functionality
can be seen as "associated with" each other such that the desired
functionality is achieved, irrespective of architectures or
intermedial components. Likewise, any two components so associated
can also be viewed as being "operably connected," or "operably
coupled," to each other to achieve the desired functionality, and
any two components capable of being so associated can also be
viewed as being "operably couplable," to each other to achieve the
desired functionality. Specific examples of operably couplable
include but are not limited to physically mateable and/or
physically interacting components and/or wirelessly interactable
and/or wirelessly interacting components and/or logically
interacting and/or logically interactable components.
[0116] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0117] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Multiple elements listed with "and/or" should be construed in the
same fashion, i.e., "one or more" of the elements so conjoined.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified. Thus, as a
non-limiting example, a reference to "A and/or B", when used in
conjunction with open-ended language such as "comprising" can
refer, in one embodiment, to A only (optionally including elements
other than B); in another embodiment, to B only (optionally
including elements other than A); in yet another embodiment, to
both A and B (optionally including other elements); etc.
[0118] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of" "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0119] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0120] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
* * * * *
References