Fast progress in modern information technologies requires the creation of new optical devices and diffractive elements that determine emergent activities in the related research fields. In this study, we propose and implement a novel approach for the design of photo- and electro-controllable diffraction gratings. This approach is based on polymerization-induced fixation of periodic dissipative structures in nematic liquid crystals created by application of a low-frequency AC electric field (50 Hz). The electrohydrodynamic (EHD) instability and convection flow lead to the formation of 2D diffraction gratings, which are then preserved through the polymerization of mono- and diacrylates incorporated into the nematic mixture. After dissolving and removing the non-polymerized low-molar-mass components, the resulting porous nematic networks with fixed EHD patterns are refilled with a nematic photochromic azobenzene-containing mixture. UV-irradiation induces E–Z isomerization of azobenzene-containing molecules, causing a decrease in birefringence, and, consequently, reducing the diffraction efficiency of the grating. This effect can be attributed to the formation of bent-shaped Z-isomers disrupting the liquid crystalline order. Visible light action recovers a high concentration of E-isomers and significantly increases the diffraction efficiency. A similar phenomenon is observed when a high-frequency AC electric field (1 kHz) is applied, inducing the alignment of mesogenic molecules along the field direction. The created photo- and electro-controllable diffraction gratings represent a promising class of advanced materials for applications in photonics and optoelectronics.