Switchable diffraction gratings consisting of a dual-frequency-addressable nematic liquid crystal embedded in a polymer matrix were prepared using a holographic technique. The photopolymerization of a polymer-precursor, initiated by an optical intensity grating leads to the grating formation within a few seconds. The diffraction efficiency of the resulting grating structure can be modulated by an external electric field due to a variation of the frequency at constant voltage. The corresponding response time is a few ms, i. e. much faster than the relaxation time of the liquid crystal at zero field. The dispersion of white light and the surface modulation of a PDLC sample with one free surface are demonstrated as possible applications for PDLC gratings.