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Optically controllable photonic structures with zero absorption

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