**7.1 Operating principle**

When a mixture of LC, monomer and photoinitiator (PhI) is exposed under the standing wave, formed from the interference of two or multiple coherent laser beams, it generates periodic dark and bright fringes. These periodic fringes regulate polymerization and hence phase separation process. High polymerization rate in the bright region (diffusion of monomer from dark to bright region) and low polymerization rate in the dark region (diffusion of LC from bright to dark region) create Bragg grating with alternate polymer-rich and LC-rich regions. Similar to the PDLC, the polymer and LC material are chosen such that the RI of the polymer should match with the ordinary RI of LC. Upon application of electric field, HPDLC film becomes optically transparent/homogenous, and the grating is in its OFF state. When the electric field is removed, LC molecules return to their original random state, and grating is in its ON state. This HPDLC grating reflects light of a particular wavelength and transmits light of all other wavelengths [163]. Morphology and diffraction properties of grating depends upon writing set-up, materials, diffusion rate, curing conditions and phase separation process [164, 165]. The particular wavelength that is reflected is a function of the refractive index difference and the width of the layers in the grating. When a voltage is applied, the liquid crystals align with the field, and their new refractive index matches that of the polymer, causing the grating to become transparent.
