Electro-optical modulator

An electro-optical modulator comprising:

• a first semiconductor layer;
• a second semiconductor layer;
• and
• a dielectric film,
• wherein
• the first semiconductor layer and the second semiconductor layer are being doped so that a conductivity type of the first semiconductor layer is different from that of the second semiconductor layer,
• a portion of the first semiconductor layer is a first highly-doped portion with a high doping concentration compared with the other portion of the first semiconductor layer,
• a portion of the second semiconductor layer is a second highly-doped portion with a high doping concentration compared with the other portion of the second semiconductor layer,
• the first highly-doped portion and the second highly-doped portion each is connectable to an external terminal directly or via other member,
• a rib waveguide is formed by stacking the second semiconductor layer on an upper side of the first semiconductor layer via the dielectric film,
• in a region in the vicinity of contact surfaces of the first semiconductor layer and the second semiconductor layer with the dielectric film, a free carrier is accumulated, removed, or inverted by an electrical signal from the external terminal, and whereby a concentration of the free carrier in an electric field region of an optical signal is modulated, so that a phase of the optical signal can be modulated,
• at least one of the first semiconductor layer and the second semiconductor layer is wider than a stacked portion including the first semiconductor layer, the dielectric film, and the second semiconductor layer,
• at least one of the first highly-doped portion and the second highly-doped portion is formed outside the stacked portion,
• a portion of the first semiconductor layer on which the dielectric film and the second semiconductor layer are stacked has a concavo-convex shape,
• the dielectric film and the second semiconductor layer are stacked on the portion so as to be fitted in the concavo-convex shape,
• the concavo-convex shape of the first semiconductor layer is a shape having a curvature, and
• with respect to a thickness W (nm) of a region of the second semiconductor layer in which a free carrier is accumulated, removed, or inverted, at least one of a maximum width of a convex portion in the concavo-convex shape of the first semiconductor layer and a maximum width of a buried portion of the second semiconductor layer buried in a concave portion of the first semiconductor layer is 2 W (nm) or less.