By shining white light on a glass slide stippled
with millions of tiny titanium dioxide pillars,
researchers at the National Institute of Standards
and Technology (NIST) and their collaborators
have reproduced with astonishing fidelity
the luminous hues and subtle shadings of “Girl
With a Pearl Earring,” Dutch artist Johannes
Vermeer’s masterpiece.
The approach has potential applications in
improving optical communications and making
currency harder to counterfeit.
For example, by adding or dropping a particular
color, or wavelength, of light traveling in
an optical fiber, scientists can control the
amount of information carried by the fiber.
By altering the intensity, researchers can
maintain the brightness of the light signal
as it travels long distances in the fiber.
The approach might also be used to “paint”
paper money with small but intricate color
details that a counterfeiter would have great
difficulty forging.
Other scientists have previously used tiny
pillars, or nanopillars, of varying sizes
to trap and emit specific colors when illuminated
with white light.
The width of the nanopillars, which are about
600 nanometers in height, or less than one-hundredth
the diameter of a human hair, determines the
specific color of light that a pillar traps
and emits.
For a demanding test of such a technique,
researchers examined how well the nanopillars
reproduced the colors of a familiar painting,
such as the Vermeer.
Although several teams of researchers had
successfully arranged millions of nanopillars
whose sizes were tailored to transmit red,
green or blue light to create a specific palette
of output colors, the scientists had no way
to control the intensity of those colors.
The intensity, or brightness, of colors determines
an image’s light and shadow, and enhances
the ability to convey impressions of perspective
and depth, a signature feature of Vermeer’s
work.
Now, by fabricating nanopillars that not only
trap and emit specific colors of light but
also change its polarization by varying degrees,
the NIST researchers and their collaborators
from Nanjing University in China have for
the first time demonstrated a way to control
both color and intensity.
The researchers describe their findings in
the journal Optica.
In their new work, the NIST team fabricated
on a glass slide nanopillars of titanium dioxide
that had an elliptical cross section rather
than a circular one.
Circular objects have a single uniform diameter,
but elliptical objects have a long axis and
a short axis.
The researchers designed the nanopillars so
that at different locations their long axis
was more aligned or less aligned with the
polarization of the incoming white light.
(Polarized light is light whose electric field
vibrates in a particular direction as it journeys
across space.)
If the nanopillar’s long axis was exactly
aligned with the direction of polarization
of the incoming light, the polarization of
the transmitted light was unaffected.
But if the long axis was rotated by some angle
— for instance 20 degrees — relative to
the direction of polarization of the incoming
light, the nanopillar rotated the polarization
of the incident light by twice that angle
— in this case, 40 degrees.
At each location on the glass slide, the orientation
of a nanopillar rotated the polarization of
the red, green or blue light it transmitted
by a specific amount.
By itself, the rotation imparted by each nanopillar
would not in any way alter the intensity of
the transmitted light.
But in tandem with a special polarizing filter
placed on the back of the glass slide, the
team achieved that goal.
The filter was oriented so that it prevented
any light that had retained its original polarization
from passing through.
(Sunglasses work in much the same way: The
lenses act as vertically polarized filters,
reducing the intensity of horizontally polarized
glare.)
That would be the case for any place on the
glass slide where a nanopillar had left unaltered
the polarization of the incident light.
Such a region would project as a dark spot
on a distant screen.
In places where a nanopillar had rotated the
polarization of the incident white light,
the filter permitted a certain amount of the
red, green or blue light to pass.
The amount depended on the rotation angle;
the greater the angle, the greater the intensity
of the transmitted light.
In this way, the team, for the first time,
controlled both color and brightness.
Once the NIST researchers had demonstrated
the basic design, they created a digital copy
of a miniature version of the Vermeer painting,
about 1 millimeter long.
They then used the digital information to
guide the fabrication of a matrix of millions
of nanopillars.
The researchers represented the color and
intensity of each picture element, or pixel,
of the Vermeer by a group of five nanopillars
— one red, two green and two blue — oriented
at specific angles to the incoming light.
Examining the millimeter-size image that the
team had created by shining white light through
the nanopillars, the researchers found that
they reproduced “Girl With the Pearl Earring”
with extreme clarity, even capturing the texture
of oil paint on canvas.
To construct the nanopillars, the research
team first deposited a layer of an ultrathin
polymer on glass, just a few hundred nanometers
thick.
Using an electron beam like a miniature drill,
they then excavated an array of millions of
tiny holes of varying dimensions and orientations
in the polymer.
Then, using a technique known as atomic layer
deposition, they backfilled these holes with
titanium dioxide.
Finally, the team etched away all of the polymer
surrounding the holes, leaving behind millions
of tiny pillars of titanium dioxide.
The dimension and orientation of each nanopillar
represented, respectively, the hue and brightness
of the final millimeter-size image.
The nanopillar technique can easily be adapted
to transmit specific colors of light, with
particular intensities, to communicate information
through an optical fiber, or to imprint a
valuable item with a miniature, multihued
identification mark that would be hard to
replicate.
