Metal nanoparticles shine with customisable colour

Engineers at Harvard have demonstrated a new kind of tunable colour filter that uses optical nanoantennas to obtain precise control of colour output.
 
Whereas a conventional colour filter can only produce one fixed colour, a single active filter under exposure to different types of light can produce a range of colours.
 
The advance has the potential for application in televisions and biological imaging, and could even be used to create invisible security tags to mark currency. The findings appear in the February issue of Nano Letters.
 
Kenneth Crozier, Associate Professor of Electrical Engineering 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 polarization 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.
 
''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 colours and different polarisations,'' 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 colour (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 colours when the polarisation is changed.
 
The researchers dubbed these filters ''chromatic plasmonic polarisers'' as they can create a pixel with a uniform color or complex patterns with colours varying as a function of position.
 
To demonstrate the technology's capabilities, the acronym LSP (short for localised surface plasmon) was created. With unpolarised light or with light which is polarised at 45 degrees, the letters are invisible (gray on gray).
 
In polarised light at 90 degrees, the letters appear vibrant yellow with a blue background, and at 0 degrees the colour scheme is reversed. By rotating the polarisation 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 colour filter with a response that depends on polarisation,'' says Crozier.
 
The researchers envision several kinds of applications: using the color functionality to present different colours in a display or camera, showing polarisation 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 colour 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.
 
''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 colour,'' 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.
 
The researchers have filed a provisional patent for their work.
 
Kwanyong Seo, a postdoctoral fellow in electrical engineering at SEAS, also contributed to the research. The work was supported by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences; and Zena Technologies. In addition, the research team acknowledges the Center for Nanoscale Systems at Harvard for fabrication work.