A method to generate terahertz radiation comprising the steps of providing a plane-parallel optical element formed of medium with non-linear optical properties having a plane entry surface and a rear boundary surface parallel to the entry surface, coupling a pump beam into the optical element through the entry surface, perpendicularly to the entry surface, directing said pump beam along a first propagation direction to one of a periodic relief structure formed in the rear boundary surface of the optical element, and a periodic relief structure formed in a surface of an additional element, said surface facing the rear boundary surface, extending parallel to the rear boundary surface and being optically coupled to the rear boundary surface of the optical element, subjecting the pump beam to at least one of reflection and diffraction, splitting thereby the pump beam into partial beams, said partial beams having a common planar envelope, and generating terahertz radiation by the partial beams, as pump beams, in the nonlinear optical medium through nonlinear optical interaction, and decoupling the terahertz radiation from the optical element through the entry surface, wherein said periodic relief structure (53; 153) comprises at least first zones perpendicular to the first propagation direction and parallel to each other, each of said first zones having a symmetrical V-shape in a plane section along the first propagation direction, a magnitude of angles formed by legs of said V-shape and the rear boundary surface being half an angle (γ) of pulse-front-tilting required to satisfy the velocity matching condition of v p, cs cos γ = v THz, ƒ within the optical medium, in relation v p;cs being a group velocity of the pump beam, v THz;f being a phase velocity of the terahertz radiation and y being an angle formed between pulse and phase fronts of said pump beam, i.e. the angle of pulse-front-tilting itself, and wherein, after said reflection and/or diffraction, the partial beams propagate at an angle γ to the first propagation direction, and the planar envelope travels at a speed v THz;f towards the entry surface of the optical element. coupling a pump beam into the optical element through the entry surface, perpendicularly to the entry surface, directing said pump beam along a first propagation direction to one of a periodic relief structure formed in the rear boundary surface of the optical element, and a periodic relief structure formed in a surface of an additional element, said surface facing the rear boundary surface, extending parallel to the rear boundary surface and being optically coupled to the rear boundary surface of the optical element, a periodic relief structure formed in the rear boundary surface of the optical element, and a periodic relief structure formed in a surface of an additional element, said surface facing the rear boundary surface, extending parallel to the rear boundary surface and being optically coupled to the rear boundary surface of the optical element, subjecting the pump beam to at least one of reflection and diffraction, splitting thereby the pump beam into partial beams, said partial beams having a common planar envelope, and generating terahertz radiation by the partial beams, as pump beams, in the nonlinear optical medium through nonlinear optical interaction, and decoupling the terahertz radiation from the optical element through the entry surface, wherein said periodic relief structure (53; 153) comprises at least first zones perpendicular to the first propagation direction and parallel to each other, each of said first zones having a symmetrical V-shape in a plane section along the first propagation direction, a magnitude of angles formed by legs of said V-shape and the rear boundary surface being half an angle (γ) of pulse-front-tilting required to satisfy the velocity matching condition of v p, cs cos γ = v THz, ƒ within the optical medium, in relation v p;cs being a group velocity of the pump beam, v THz;f being a phase velocity of the terahertz radiation and y being an angle formed between pulse and phase fronts of said pump beam, i.e. the angle of pulse-front-tilting itself, and