A waveguide structure includes core 1 formed of a semiconductor such as Si, two external regions 2 which are not optically connected to the core but arranged at a certain distance from the core and bridges 3 which electrically connect the external regions to the core. Light propagating in the waveguide core is strongly confined in the waveguide core and optically disconnected from (i.e. not coupled with) the external regions, so that light can propagate in the waveguide without being affected by the existence of the external regions. Furthermore, the waveguide core is electrically connected to the external regions through the bridges, so that a voltage can be applied and a current can be caused to flow to the core from the external regions.