  Spontaneous formation of polarization gratings by liquid crystals made of
bent dimeric molecules is reported. The grating is formed within the
temperature range of the twist bend modulated nematic phase, NTB, without the
necessity to pattern the cell surfaces, therefore the modulated nematic phase
is a promising candidate for low-cost modulators and beam steering devices, the
polarization properties of which can be tuned by temperature. In addition, the
study of the diffracted light properties turns out to be a sensitive measuring
technique for determination of the 3D spatial variation of the optic axis in
the cell.
