Magnetically tunable selective reflection of light by heliconical cholesterics

We present studies of chiral nematic liquid crystals composed of flexible dimer molecules subject to large dc magnetic fields between 0 and 31 T. We observe that these fields lead to selective reflection of light depending on temperature and magnetic field. The band of reflected wavelengths can be tuned from ultraviolet to beyond the IR-$C$ band. A similar effect induced by electric fields has been presented previously, and was explained by a field-induced oblique-heliconical director deformation in accordance with early theoretical predictions. The use of magnetic field here instead of electric field allows precise measurements of some material constants and holds promise for wireless tuning of selective reflection.

Figure 1

(a) Molecular structures of the components and their composition used in the studied mixtures M1 and M2. (b) Schematic of the experimental setup for measurement of optical reflection spectra in an applied magnetic field. (c) Illustration of the director profile of the liquid crystal at zero magnetic field (left), at high magnetic field that aligns the director uniformly along the magnetic field (middle), and at magnetic field below a threshold that creates heliconical structure.

Figure 2

Reflection spectra recorded from the M1 mixture for different values of the magnetic fields at temperatures of ${32}^{\circ }\mathrm{C}\left(\mathrm{bottom}\right),35{}^{\circ }\mathrm{C}$ (middle), and ${38}^{\circ }\mathrm{C}\left(\mathrm{top}\right)$; all are within the $N^{*}$ range.

Figure 3

Reflection spectra recorded from the M2 mixture for various magnetic fields at temperature of ${34}^{\circ }\mathrm{C},{38}^{\circ }\mathrm{C}$, and ${40}^{\circ }\mathrm{C}$.

Figure 4

Magnetic field dependence of the wavelength and full width at half maximum (FWHM) of the selective reflection peak observed at different temperatures. (a) ${\lambda }_p\left(B\right)$[inset $1/{\lambda }_p\left(B\right)$] for M1, (b) $\mathrm{\Delta }\lambda \left(B\right)$ [inset $1/\mathrm{\Delta }\lambda \left(B^2\right)$] for M1, (c) ${\lambda }_p\left(B\right)$ [inset $1/{\lambda }_p\left(B\right)$] for M2, and (d) $\mathrm{\Delta }\lambda \left(B\right)$ [inset $1/\mathrm{\Delta }\lambda \left(B^2\right)$] for M2.