In recent years our research has focused on finding new ways to tune the photonic band gap of chiral nematic liquid crystals using electric fields. Methodologies include using polymer stabilization to form a structure whereby the helix axis lies in the plane of the device, which is formed by cooling the presence of a low frequency electric field. After polymerization, an electric field applied across the cell (normal to the substrates) results in a redshift in the reflected wavelengths as the helical structure is unwound. An example of the change in the reflection of a liquid crystal cell using this technique is shown in Figure 1 and the corresponding optical texture of the polymerized structure is shown in Figure 2.
Figure 2. The optical texture viewed between crossed polarizers on a microscope .
Doping the ferroelectric liquid crystal into the bulk of the chiral nematic liquid crystal has also been shown to result in wavelength tuning of the photonic band gap and is currently a subject that is under investigation at CMMPE. Examples of tuning using this approach is shown in Figures 5 and 6.
5. Broadband wavelength tuning of the photonic band gap.
Figure 6. The change in the reflected color from the cell as the electric field strength was increased, at a frequency of 1 kHz.
Electrically tuneable liquid crystal photonic bandgaps
The switching properties of chiral nematic liquid crystals
using electrically commanded surfaces
tuning the photonic band gap in chiral nematic liquid crystals using
electrically commanded surfaces
color switching from blue to red in a polymer stabilized chiral nematic
Figure 1. The change in reflected colour of a polymer stabilized chiral nematic liquid crystal .
Another approach that has been explored is to use electrically commanded surfaces to tune the wavelength of the photonic band gap of a chiral nematic liquid crystal indirectly. A 100 nm layer of ferroelectric liquid crystal is coated onto each substrate before cell assembly. After cell fabrication, the application of an electric field results in the director in the FLC layer rotating in the plane of the device. This rotation creates a torque on the neighbouring chiral nematic liquid crystal molecules resulting in a macroscopic contraction of the pitch of the helix. A simple schematic of the basic principle of the tuning mechanism is shown in Figure 3 whilst an example of the results of the photonic band gap recorded on a spectrometer for different electric field strengths are presented in Figure 4.
Figure 3. Schematic of the chiral nematic liquid crystal/ ferroelectric liquid crystal command surface cell configuration and proposed wavelength tuning mechanism .
Figure 4. Wavelength tuning of the photonic band gap using dual electrically commanded FLC surfaces. (a) Shift of the PBG with amplitude of electric field at a constant frequency of 1 kHz, and (b) PBG shift with frequency at constant amplitude of 20.8 V/micron. All data was recorded at a temperature of 25° C .