Schematic of the tunable color filter. The combination of a gold film
with ring-shaped holes and the use of liquid crystals (red and green) enables
pixels of a defined color that can be turned on and off.
A thin liquid crystal film on gold sheets makes an ultra-compact color
filter
Flat panel displays, mobile
phones and many digital devices require thin, efficient and low-cost
light-emitters for applications. The pixels that make up the different colors on
the display are typically wired to complex electronic circuits that control
their operation. Jing Hua Teng at the A*STAR Institute of Materials Research
and Engineering and co-workers have now developed a display technology that
requires a much simpler architecture for operation. They demonstrated that
combining a thin perforated gold film with a liquid crystal layer is all that
it takes to make an efficient color filter1.
“Our color filters are a lot
thinner and more compact than conventional thin-film-based color filters,” says
Teng. “The colors of these filters can be tuned with ease so they are very
versatile in applications.”
The color selection of the
devices comes from the patterned gold film. The collective motions of the
electrons on the film surface — the so-called surface plasmons — absorb light
at wavelengths that depend on the details of these patterns. In the present
case, the patterns are narrow, nanometer-sized rings cut out of the films (see
image). As the diameter of the rings changes, so does the color of the metal
film. Pixels of a different color can be realized simply by patterning rings of
different sizes across the same gold film.
To realize a full display,
however, each of these pixels needs to be turned on and off individually. This
is where liquid crystals come in.
Liquid crystals are molecules
that can be switched between two different states by external stimuli, such as
ultraviolet light. In their normal state the crystals let visible light pass
through so that the pixel is turned on. But when ultraviolet is also present,
the structure of the liquid crystal molecules will change so that it absorbs
visible light (i.e. the pixel is turned off). This process can be repeated over
many cycles without degrading the device itself.
Although the device works in
principle, it remains a concept on the drawing board for now. This is because
there are still many issues that need to be overcome, for example, the
optimization of the switching speed and the contrast between ‘on’ and ‘off’ states.
In future work, the researchers will need to extend their ideas so that their
device can serve a larger area and produce the fundamental colors red, green
and blue.
Teng and his team are quite
optimistic that they will achieve this soon.
The A*STAR-affiliated researchers
contributing to this research are from the Institute of Materials Research and
Engineering
References
- Liu, Y. J., Si, G. Y., Leong, E. S. P., Xiang,
N., Danner, A. J. & Teng, J. H. Light-driven plasmonic color filters
by overlaying photoresponsive liquid crystals on gold annular aperture
arrays. Advanced Materials 24, OP131–OP135 (2012).
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