A low-loss waveguide that can be curved aggressively, that is, curved with a radius of curvature that is substantially zero, in the plane of propagation, without radiating, is formed by a slab of dielectric material having four metal plates, two on each opposite surface of the slab and mutually spaced to define in the dielectric slab between the four metal plates a confinement zone. In use, electromagnetic radiation injected in one end of the zone by suitable input means will propagate throughout the zone to an extraction means. Lower loss and better confinement of the radiation may be obtained by providing plugs of dielectric material adjacent the inwardly-facing edge of each of the metal plates. Embodiments of the invention can be used to implement integrated optical devices and circuits for routing or processing light signals.