Research at CMMPEElectrical/optical phenomena — Photonic band gap

The 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.
The liquid crystalline microstructure of their shells gives rise to selective reflection of a specific
wavelength range within the visible spectrum, which gives them their metallic green lustre.

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.

For more information about how photonic band gaps are ustilised in CMMPE research into LC lasers, please click here.

For more information about CMMPE research into devices made with tunable photonic band-gaps, please click here.

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