Optically controllable photonic structures with zero absorption

We show the possibility to periodically modulate the refractive index in a homogeneous resonant atomic medium in space or/and time while simultaneously keeping vanishing absorption or gain. Such modulation is based on periodic resonant enhancement of the refractive index, controlled by an external optical field, and opens the way to produce coherently controllable photonic structures. We suggest the possible implementation of the proposed scheme in rare-earth doped crystals with excited state absorption.