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Research at CMMPE — Electrical/optical phenomena — Photonic band gapThe
research group is interested in photonic band structures that are
formed either naturally or artificially from liquid crystalline phases:
examples of naturally forming band structures include the chiral nematic
phase, the chiral smectic C phase, and the blue phases. In each of
these cases the molecules self-organise to form a periodic structure
on a length scale that is of the order of the wavelength of light.
The photonic band gap describes a range of wavelengths which are forbidden
to propagate through the structure. For the chiral nematic and chiral
smectic phases this results in a one-dimensional photonic band gap
whereas for blue phase I and II a band gap exists in three dimensions.
Examples of the photonic band gap observed for a chiral nematic, a
chiral smectic C and a blue phase are shown in the figure. Through
the chemical composition it is possible to control the properties
of the band gap such as the central wavelength and the width of the
gap.
Figure
1. Photonic band gaps occur naturally in the carapace of
certain beetles.
Our research is interested in all aspects of these structures although
there is a particular focus on controlling the photonic band gap using
a range of external stimuli such as temperature and electric fields.
We continue to explore and develop new ways with which to tune the
wavelength of the band gap using electric fields that do not degrade
the quality of the structure. A recent example is the reversible wavelength
tuning of the band gap of a chiral nematic liquid crystal using electrically
commanded surfaces (link to page in the applications section). Alternatively,
we have also been exploring wavelength tuning of the photonic band
gap using blue phase liquid crystals. This combined research is of
interest for reflective displays, tuneable filters, and liquid
crystal lasers.
Figure 2. Photonic band gaps of liquid crystals. (a) chiral nematic, (b) chiral smectic C, and (c) blue phase I. In addition to the naturally forming photonic band structures, we are also interested in creating photonic band gaps from soft-matter that have been engineered to have a periodic structure. Examples include polymer dispersed liquid crystals and more recently the hybrid systems consisting of periodic carbon nanotube arrays and a nematic liquid crystal. Other examples include using polymer structures to form a periodic template and then combining this with an achiral nematic liquid crystal so as to alter the refractive index using an electric field and thus tune the wavelength of the band gap. |
