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Posted: Aug 13, 2015

A nanoscale color filter

(Nanowerk Spotlight) For the past decade, researchers have searched for robust, inorganic color filters that can replace traditional organic dye-based filters for better stability, lifetimes, performance, and amenability to miniaturization.
Some popular solutions have come in the form of metallic gratings, where the filtering capability is based on the excitation and interference of surface plasmon polaritons (see for instance: "Biomimetic photodetector 'sees' in color"). Due to the nature of their operational principle, these types of color filters provide the best spectral-selectivity when the grating includes a large number of grating elements spaced regularly apart from one another.
Since more gratings means larger size, achieving truly nanoscale color filters with reliable spectral-selectivity can be a challenging task with plasmonic grating-based filters.
There has been tremendous interest in trying to scale down micron-sized photonic components to the nanoscale, such that they can become compatible with nanoscale electronic components. One of the key components to photonics is the optical filter.
New work by a research team in Korea, realizing an inorganic filter that can operate with a single element, represents an important step toward nanoscale color filters. The team devised a a simple design in which light can be filtered and tuned over wavelengths through the use of a single nanoscale element in the form of a ZnO nanorod integrated with a silver cavity.
The findings, published in the August 3, 2015 online edition of Nano Letters ("Nanoscale Single-Element Color Filters"), describe the smallest color filter to date. This work also ushers in possibilities of realizing filters at even smaller sizes.
"Our work takes advantage of a recent development in the field of nanophotonics, namely metal-shell induced scattering cancellation aka metal-shell induced transparency," Jerome K. Hyun, an assistant professor in the Department of Chemistry and Nano Science at Ewha Womans University, and the paper's first author, tells Nanowerk.
Hyun and colleagues from Prof. Gyu-chul Yis team at Seoul National University, fabricated a device that consists of a ZnO nanowire sandwiched between two silver films.
single-element optical filter consisting of a ZnO nanorod positioned between two Ag layers
(a) Schematic of the single-element optical filter consisting of a ZnO nanorod positioned between two Ag layers whose top and bottom thicknesses are 30 and 15 ± 5 nm, respectively. (b) SEM image of a representative hybrid nanorod filter. Scale bar is 1 µm. (c) Setup for measuring the far-field transmission through the hybrid nanorod. (d) Numerically simulated transmission as a function of diameter and wavelength by modeling the measurement setup in 3D. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
"At the filter wavelength, the metal effectively acts as an imperfect cloak to the nanowire, and makes the nanowire appear more transparent than its surrounding," explains Hyun. "This allows the light at this particular wavelength to transmit through the nanowire more than it would through the neighboring silver film. We can choose the color with excellent spectral selectivity by simply changing the diameter of the nanowire."
He points out that this work is an application of the theory developed by professors Andrea Alù at UT Austin and Nader Engheta at the University of Pennsylvania ("Achieving transparency with plasmonic and metamaterial coatings").
The inspiration for the team's work came from the realization that metal coatings offer intriguing ways of manipulating the scattering and absorption properties of small nanostructures.
Small nanostructures already provide interesting optical characteristics not seen in the bulk material. The metal component adds another degree of freedom to modifying their properties.
"We had also been interested in the filtering properties of gratings and had been investigating new type of strategies for manipulating their transmission characteristics," recounts Hyun. "These two themes helped drive the project forward toward realization of a new model of optical filters."
According to the researchers, a remaining challenge is to increase the absolute transmission efficiency. Because this nanoscale device is several times smaller than the smallest probe size achievable with optical lenses, it is difficult to achieve transmission efficiencies greater than 60% with the current measurement setup.
"However, we found that by adding more of our filters, we can increase the transmission efficiency while maintaining the excellent spectral-selectivity," notes Hyun. "We now are working in this direction."
Future work would also address challenges in the construction of these nanofilters over a large scale, aimed at display applications, and increasing the absolute transmission efficiency using different core materials.
These nanoscale filters could serve as optical bandpass filters in nanoscale communication, for instance to clean up the signal sent from a local emitter such as a single molecule.
Another specific application is ultra-high-resolution color filters. By arranging these nanowires into three types of arrays, each consisting of nanowires of a different diameter, it is possible to build a RGB color filter but at sizes that are substantially smaller than traditional color pixels and plasmonic grating based pixels.
"I believe our work is noteworthy because it provides new possibilities for color filters with practical implications in ultra high-resolution displays and nanoscale communication, nanoscale bio-imaging etc.,"Hyun concludes.
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