A waveguide for guiding an electro-magnetic wave comprises: a first waveguide part; and a second waveguide part; wherein the first waveguide part has a first width in a first direction (Y) perpendicular to the direction of propagation of the electro-magnetic wave and the second waveguide part has a second width in the first direction (Y), wherein the second width is larger than the first width; and wherein the first and the second waveguide parts are spaced apart by a gap in a second direction (Z) perpendicular to the first and second planes in which the waveguide parts are formed, wherein the gap has a size which is sufficiently small such that the first and second waveguide parts unitely form a single waveguide for guiding the electro-magnetic wave. The waveguide may be used in numerous applications, such as in a photonic integrated circuit, in a sensor or in an actuator.