  For the oblique helicoidal structure of the chiral twist-bend nematic-forming
mixture of CB7CB/CB6OCB/5CB doped by light-sensitive chiral compound based on
azo-fragment, two consequent states of Bragg reflection of the light in the
visible spectral range from 400 to 750 nm were experimentally observed in the
course of decreasing the applied electric field. These states were assigned to
different orders of Bragg's diffraction. While the second and third orders of
Bragg's diffraction can be easily observed in conditions of standard
electrooptic cell, the first order could be revealed only at very high electric
field near the "red" edge of the visible spectral range. For the second and
third orders of diffraction there is a narrow range of electric field where
both diffraction orders are observed (so-called hybrid state), with
transmittance spectra of the $Ch_{OH}$ structure showing two peaks at the
short-wavelength and the long-wavelength edges of the visible spectral range.
Model calculations of Bragg's reflection of light were carried out using
literature values of elasticity constants and dielectric permittivity, and the
calculated plots of the selective reflection maximum vs. voltage were in good
agreement with experimental data.
