A pn junction layer is formed in a semiconductor body having a forbidden band the width of which is greater than the photon energy of an irradiated light beam. The pn junction is forwardly biased through transparent electrodes provided on the opposite principal surfaces to inject carriers at a concentration of the order of 1018 to 1022 cm 3. The refractive index of the semiconductor body is varied according to the carrier concentration to arbitrarily deflect the incident light beam.