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 possessing an oxide layer having a first surface; - a set of semiconductor regions consecutively extending over the substrate in the propagation direction (X'-X) of the electromagnetic wave, the semiconductor regions 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 ; the set of semiconductor regions and the set of dielectric layers forming a grating of elementary structures 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 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 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 is single-crystal, and possesses an interface with the first surface of the oxide layer . - a substrate possessing an oxide layer having a first surface; - a set of semiconductor regions consecutively extending over the substrate in the propagation direction (X'-X) of the electromagnetic wave, the semiconductor regions 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 ; the set of semiconductor regions and the set of dielectric layers forming a grating of elementary structures 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 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 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 is single-crystal, and possesses an interface with the first surface of the oxide layer . <EOS>