Quantum interference versus stark intersubband electro-optical modulation in asymmetrical quantum wells

Intersubband optical transitions in asymmetrical quantum well structures can generate very large second order optical nonlinearities. In particular it is possible to develop phase and amplitude electro-optical modulators that operate in the mid-infrared spectral range. In this paper we show that there are two alternative approaches how to develop efficient intersubband electro-optical modulators. The first approach is based on the linear Stark effect in quantum well structures for which the transition energy between the subbands is linearly shifted under the application of a dc-electric field across the structure. The second approach is based on envelope states mixing (or quantum interference) due to the application of a dc-electric field. In both cases either the change of the transition energies or the exchange dipole matrix elements (or both) lead to the generation of large electro-optical coefficients in the structures. We have investigated the characteristics of these two types of modulators and found that the quantum interference modulator is more suitable for phase modulation. Finally, we have demonstrated the operation of quantum interference modulator in asymmetrical coupled quantum wells structure.