Capacitive electro-optical modulator comprising a capacitive-electro-optical-modulator waveguide, the waveguide being intended to be coupled to at least one light source that emits an electromagnetic wave in a propagation direction (X'-X), the electromagnetic wave having a wavelength denoted λ, the waveguide comprising: - a substrate (2, 20) possessing an oxide layer having a first surface; - a set of semiconductor regions (3a, 3b) consecutively extending over the substrate (2, 20) in the propagation direction (X'-X) of the electromagnetic wave, the semiconductor regions (3a, 3b) being electrically doped a first conductivity type and a second conductivity type alternately in the propagation direction (X'-X); - a set of dielectric layers, each dielectric layer being formed between two consecutive semiconductor regions (3a, 3b); the set of semiconductor regions (3a, 3b) and the set of dielectric layers forming a grating of elementary structures (5a, 5b) consecutively comprising a first elementary structure formed by a semiconductor region of the first conductivity type and an adjacent dielectric layer, and a second elementary structure formed by a semiconductor region of the second conductivity type and an adjacent dielectric layer, the first and second elementary structures (5a, 5b) having first and second dimensions L A, L B in the propagation direction (X'-X), respectively; - a medium surrounding the grating, the medium having a real part n env of an effective index; the first dimensions L A and the second dimensions L B of the elementary structures (5a, 5b) being matched to λ so that the grating permits optical modes propagating in the propagation direction (X'-X), and so that the optical modes permitted by the grating have a wavenumber k respecting k 0 k < 1 n env, where k 0 is the wavenumber of the electromagnetic wave in free space; characterized in that the set of semiconductor regions (3a, 3b) is single-crystal, and possesses an interface with the first surface of the oxide layer. - a substrate (2, 20) possessing an oxide layer having a first surface; - a set of semiconductor regions (3a, 3b) consecutively extending over the substrate (2, 20) in the propagation direction (X'-X) of the electromagnetic wave, the semiconductor regions (3a, 3b) being electrically doped a first conductivity type and a second conductivity type alternately in the propagation direction (X'-X); - a set of dielectric layers, each dielectric layer being formed between two consecutive semiconductor regions (3a, 3b); the set of semiconductor regions (3a, 3b) and the set of dielectric layers forming a grating of elementary structures (5a, 5b) consecutively comprising a first elementary structure formed by a semiconductor region of the first conductivity type and an adjacent dielectric layer, and a second elementary structure formed by a semiconductor region of the second conductivity type and an adjacent dielectric layer, the first and second elementary structures (5a, 5b) having first and second dimensions L A, L B in the propagation direction (X'-X), respectively; - a medium surrounding the grating, the medium having a real part n env of an effective index; the first dimensions L A and the second dimensions L B of the elementary structures (5a, 5b) being matched to λ so that the grating permits optical modes propagating in the propagation direction (X'-X), and so that the optical modes permitted by the grating have a wavenumber k respecting k 0 k < 1 n env, where k 0 is the wavenumber of the electromagnetic wave in free space; characterized in that the set of semiconductor regions (3a, 3b) is single-crystal, and possesses an interface with the first surface of the oxide layer.