Metal nanoparticles shine with customizable color (w/ video)

Feb 23, 2012
The color output of a new type of optical filter created at Harvard depends on the polarization of the incoming light. Credit: Image courtesy of Tal Ellenbogen.

(PhysOrg.com) -- Engineers at Harvard have demonstrated a new kind of tunable color filter that uses optical nanoantennas to obtain precise control of color output.

Whereas a conventional can only produce one fixed , a single active filter under exposure to different types of light can produce a range of colors.

The advance has the potential for application in televisions and , and could even be used to create invisible security tags to mark . The findings appear in the February issue of .

Kenneth Crozier, Associate Professor of at the Harvard School of Engineering and Applied Sciences (SEAS), and colleagues have engineered the size and shape of metal nanoparticles so that the color they appear strongly depends on the of the light illuminating them. The nanoparticles can be regarded as antennas—similar to antennas used for wireless communications—but much smaller in scale and operating at visible frequencies.

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The optical nanoantenna technology can create a pixel with a uniform color or complex patterns with colors varying as a function of position.

"With the advances in nanotechnology, we can precisely control the shape of the optical nanoantennas, so we can tune them to react differently with light of different colors and different polarizations," said co-author Tal Ellenbogen, a postdoctoral fellow at SEAS. "By doing so, we designed a new sort of controllable color filter."

Conventional RGB filters used to create color in today's televisions and monitors have one fixed output color (red, green, or blue) and create a broader palette of hues through blending. By contrast, each pixel of the nanoantenna-based filters is dynamic and able to produce different colors when the polarization is changed.

To demonstrate their work, researchers at Harvard created a plate of chromatic plasmonic polarizers that spells out the acronym "LSP." Under light of different polarizations, the letters and the background change color. The image at far right shows the antennas themselves, as viewed through a scanning electron microscope. Credit: Photos courtesy of Tal Ellenbogen.

The researchers dubbed these filters "chromatic plasmonic polarizers" as they can create a pixel with a uniform color or complex patterns with colors varying as a function of position.

To demonstrate the technology's capabilities, the acronym LSP (short for localized surface plasmon) was created. With unpolarized light or with light which is polarized at 45 degrees, the letters are invisible (gray on gray). In polarized light at 90 degrees, the letters appear vibrant yellow with a blue background, and at 0 degrees the color scheme is reversed. By rotating the polarization of the incident light, the letters then change color, moving from yellow to blue.

"What is somewhat unusual about this work is that we have a color filter with a response that depends on polarization," says Crozier.

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The acronym LSP, short for localized surface plasmon, is displayed. With unpolarized light or with light which is polarized at 45 degrees the letters are invisible (gray on gray). In polarized light at 90 degrees the letters appear vibrant yellow with a blue background and at 0 degrees the color scheme is reversed. By rotating the polarization of the incident light the letters then change color, moving from yellow to blue.

The researchers envision several kinds of applications: using the color functionality to present different colors in a display or camera, showing polarization effects in tissue for biomedical imaging, and integrating the technology into labels or paper to generate security tags that could mark money and other objects.

Seeing the color effects from current fabricated samples requires magnification, but large-scale nanoprinting techniques could be used to generate samples big enough to be seen with the naked eye. To build a television, for example, using the nanoantennas would require a great deal of advanced engineering, but Crozier and Ellenbogen say it is absolutely feasible.

Crozier credits the latest advance, in part, to taking a biological approach to the problem of color generation. Ellenbogen, who is, ironically, colorblind, had previously studied computational models of the visual cortex and brought such knowledge to the lab.

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Each pixel of the nanoantenna-based filters is dynamic and able to produce different colors when the polarization is changed.

"The chromatic plasmonic polarizers combine two structures, each with a different spectral response, and the human eye can see the mixing of these two spectral responses as color," said Crozier.

"We would normally ask what is the response in terms of the spectrum, rather than what is the response in terms of the eye," added Ellenbogen.

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Expiorer
not rated yet Feb 24, 2012
So instead of 3 elements per pixel we will have only 2?
It is great news.
And what will be the advantages (television)?
Better contrast, cleaner colors or better power consumption?
antialias_physorg
not rated yet Feb 24, 2012
And what will be the advantages (television)?

My guess would be:
- no more blending (error diffusion artefacts).
- no more rainbow effects.
- better antialiasing.

The metamaterial 'filter' is applied on an entire pixel (whereas in LCD TVs up to three quarters of the light is blocked and the remaining quarter is then filtered). So we could get either brighter TVs or lower power consumption at the same brightness.

This might be offset by reducing the brightness from the lightsourc by having to pass it through a polarisation filter, but I think there are methods for polarizing light without losses.

However, it would be tough to integrate this with 3D TV sets/projectors that use polarized light.

antialias_physorg
5 / 5 (1) Feb 24, 2012
Additional bonuses:

- with the same size of patterning structures as in LCDs you could get double the resolution
- no more need for transistors in the filter layer which block part of the light (so better resulting brightness at same energy input)
- no more dead pixels

It remains to be seen whether it is posible to create a nanomaterial layer with good quality in a roll-on process. anything less would probably be to expnsive.
Eikka
not rated yet Feb 24, 2012

This might be offset by reducing the brightness from the lightsourc by having to pass it through a polarisation filter, but I think there are methods for polarizing light without losses.


LCDs use polarization filters anyways, because they work by bending the polarization of light between two polarizing filters. The output of the LCD panel is already polarized, so you can just make a monochrome panel and slap a filter mask on top.

But this is a filter - it doesn't change the color of the light, it simply removes from it. If the selectivity is good, then the filter can be thin and pass more light through without passing the uwanted wavelenghts. A better option is to tune the backlight to only include those waveleghts that are wanted, so you can use thinner less selective filters that are cheaper.
Eikka
not rated yet Feb 24, 2012
It would be technically possible to make a double-layer LCD panel where you have a monochrome LCD panel to act as the shutter, and on top of that you'd have another plane of liquid crystals that twist the polarization through the nanoparticle filter to alter the color of the output.

That'd create a truly infinitely variable color display, but then you need a backlight with a continous spectrum as well, which means you can't use LEDs or fluorescent tubes.
holoman
not rated yet Feb 24, 2012
This concept may also come into play.

http://colossalst...D_tv.htm
geo999
not rated yet Feb 27, 2012
20 plus years ago I did something very similar by coupling a polarising filter, a UV filter and an IR filter - by rotating the polariser you could dial up either warm ( pink) colors or cool (blue) colours