An optical modulator, comprising: a substrate; a metal layer on the substrate, the metal layer having a grooved channel; a dielectric layer on the metal layer and in the grooved channel; a transparent conducting material on the dielectric layer and in the grooved channel, such that the grooved channel forms a plasmonic waveguide; and a tunable voltage applied across the transparent conducting material and the metal layer; characterised by: the grooved channel comprising a first end of the grooved channel at a first edge of the metal layer and a second end of the grooved channel at a second edge of the metal layer; the transparent conducting material comprising a transparent conducting oxide, TCO, filling the grooved channel; and in that: the tunable voltage applies an electrical bias voltage across the TCO and the metal layer to form an accumulation layer at the metal /dielectric interface that subsequently changes the properties of the propagation mode in the grooved channel; and an optical input signal is configured to be applied to the grooved channel at the first end, and a modulated optical signal is configured to be outputted at the second end. a substrate; a metal layer on the substrate, the metal layer having a grooved channel; a dielectric layer on the metal layer and in the grooved channel; a transparent conducting material on the dielectric layer and in the grooved channel, such that the grooved channel forms a plasmonic waveguide; and a tunable voltage applied across the transparent conducting material and the metal layer; characterised by: the grooved channel comprising a first end of the grooved channel at a first edge of the metal layer and a second end of the grooved channel at a second edge of the metal layer; the transparent conducting material comprising a transparent conducting oxide, TCO, filling the grooved channel; and in that: the tunable voltage applies an electrical bias voltage across the TCO and the metal layer to form an accumulation layer at the metal /dielectric interface that subsequently changes the properties of the propagation mode in the grooved channel; and an optical input signal is configured to be applied to the grooved channel at the first end, and a modulated optical signal is configured to be outputted at the second end. the grooved channel comprising a first end of the grooved channel at a first edge of the metal layer and a second end of the grooved channel at a second edge of the metal layer; the transparent conducting material comprising a transparent conducting oxide, TCO, filling the grooved channel; and in that: the tunable voltage applies an electrical bias voltage across the TCO and the metal layer to form an accumulation layer at the metal /dielectric interface that subsequently changes the properties of the propagation mode in the grooved channel; and an optical input signal is configured to be applied to the grooved channel at the first end, and a modulated optical signal is configured to be outputted at the second end. the tunable voltage applies an electrical bias voltage across the TCO and the metal layer to form an accumulation layer at the metal /dielectric interface that subsequently changes the properties of the propagation mode in the grooved channel; and an optical input signal is configured to be applied to the grooved channel at the first end, and a modulated optical signal is configured to be outputted at the second end.