  Unique electro-optical properties of the oblique helicoidal cholesteric
(ChOH) stem from its heliconical director structure. An applied electric field
preserves the single harmonic modulation of the director while tuning the ChOH
period and the corresponding Bragg peak wavelength within a broad spectral
range. We use the response of ChOH to the electric field to measure the elastic
constants of twist K22 and bend K33 directly in the cholesteric phase. The
temperature dependencies of K22 and K33 allow us to determine the range of the
electric tunability of the ChOH pitch and the heliconical angle. The data are
important for understanding how molecular composition and chirality influence
macroscopic elastic properties of the chiral liquid crystals and for the
development of ChOH-based optical devices.
