Photonic crystals

If a medium is periodic with the periodicity on the order of the wavelengths of visible light it can acquire extremely interesting optical properties. Light with the same wavelength as the structural periodicity may be blocked from transmission, being strongly reflected instead, whereas wavelengths a bit longer or shorter will generally be admitted. This property is described as an optical band gap of the material, which is often referred to as a photonic crystal. Chiral liquid crystals of nematic or smectic-C type generally exhibit a helical superstructure, in which the director is modulated helicoidally with a period that can often be in the 300-400nm range, i.e. the same period as the wavelengths of visible light within the liquid crystal. The result is that they constitute a very peculiar type of self-assembled photonic crystal, reflecting light within a narrow wavelength range around the helix pitch, if it has a circular polarization of the same handedness as that of the director helix. The opposite handedness is transmitted. In case of unpolarized or linearly polarized light, it is divided in two circular polarized components, one fully reflected and one fully transmitted. Since the liquid crystal pitch is generally highly temperature sensitive the band gap of this photonic crystal can be tuned by changing the temperature.

In our research we currently use the photonic crystal properties of chiral thermotropic nematics primarily in electrospun fibers filled with the chiral nematic. This gives the fibers an intense color that can change or be removed upon changing the temperature.

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