Optical Modulation by Conducting Interfaces
We analyze the interaction of a propagating guided electromagnetic wave with a quantum well embedded in a dielectric slab waveguide. First, we design a quantum well based on InAlGaAs compounds with the transition energy of 0.8 eV corresponding to a wavelength of 1.55 μm. By exploiting the envelope function approximation, we derive the eigenstates of electrons and holes and the transition dipole moments. Next, we calculate the electrical susceptibility of a three-level quantum system (as a model for the 2-D electron gas trapped in the waveguide), by using phenomenological optical Bloch equations. We show that the 2-D electron gas behaves as a conducting interface, whose conductivity can be modified by controlling the populations of electron and hole energy levels. Finally, we design a slab waveguide in which a guided wave with the wavelength of 1.55 μm experiences a strong coupling to the conducting interface. We calculate the propagation constant of the wave in the waveguide subject to the conducting interface, by exploiting the modified transfer matrix method, and establishing linear dependence on the interface conductivity. By presenting a method for controlling the populations of electrons and holes, we design a compact optical modulator with an overall length of around 60 μm.