Propagation properties of dielectric waveguides with optically induced plasma layers

Propagation of millimeter waves in a semiconductor dielectric waveguide is controlled by illuminating the waveguide by light with a photon energy larger than the forbidden gap because of the generation of plasma. A theoretical and experimental study is made of this and the effects of waveguide size and plasma layer thickness on attenuation and phase shift are analyzed for various illumination intensities. Further, the modulation characteristics are examined in relation to the response of the plasma density. A shorter plasma recombination time is found to be favorable for high speed control, but this requires intense illumination to maintain the required plasma density. A test made with a Si waveguide at 50 GHz substantiates the usefulness of this technique and the validity of the theory.