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 . <EOS>