An optically-active multi-layer dielectric structure comprises an optically active zone between two mirrors forming a Fabry-Perot microcavity. The optically active zone comprises an optically active material of wavelength lambda centred in a layer of high refractory index medium of optical thickness less than lambda/2 surrounded by two layers of low refractory index medium each of optical thickness less than &lambda/4,the combined optical thickness of said three layers making up the optically active zone being less than or equal to 3lambda/4. This structure behaves like a lambda/2 high index cavity except that there is a maximum of the optical field in the centre of the cavity instead of the usual node. This phase-shifted structure is useful for planar light emitting devices, vertical cavity lasers, and photo-detectors.