Intersubband electro-optic modulators for near and mid infrared applications

Optical transitions between conduction subbands of asymmetrical quantum-well structures can produce large second-order optical nonlinearities. In particular, it is possible to engineer asymmetrical quantum-well structures to have very large electro-optic (EO) coefficients. In this paper we study three alternative approaches to design and fabricate intersubband EO modulators, i.e. the Stark modulator, the quantum-interference (QI) modulator, and the carrier-density (CD) modulator. We show that near a given resonance the Stark modulator has the largest EO coefficient. However, the linear intersubband absorption limits the usability of this modulator to a very short device length. Far from resonance we found that the CD and the QI modulators are more efficient. Furthermore, we found that these modulators can be utilized for near-infrared modulation in the 1.3–1.5 μm spectral range. These modulators are almost unaffected by the linear intersubband absorption so that their efficiency can be very high. We present preliminary experimental results that demonstrate the operation of the QI modulator.