Guiding and electrooptic modulation of light at 1;15μ from a HeNe laser has been achieved in a thin film semiconductor waveguide.The guide was composed of a thin (~ 10μ) epitaxial film of GaAs sandwiched between a GaAs substrate and an evaporated aluminum coating.The slightly higher refractive index of the guide relative to the substrate allowed the propagation of one single optical TM mode and one single optical TM mode. Large electric fields generated in the epitaxial film by applying a voltage to the aluminum coating, induced an electrooptic change in the refractive index and a consequent modulation of the guided light.
An important new effect, optical mode propagation cut-off, was discovered. Calculations showed that no guided modes propagated below a threshold value of the refractive index difference between guide film and substrate; above· that value guiding occurred. This was observed when samples were switched from a non-guiding "off" state to a guiding "on" state by applying a modulation voltage that increased the refractive index of the guide, making it go through cut-off.
