Terahertz Electromagnetic Pulse Generation and Four-Wave Mixing in Coupled Quantum Wells

Recent experiments by Roskos et al. in ultrashort optical pulse excitation of biased coupled quantum wells have shown remarkable generation of terahertz electromagnetic radiation. Two different effects were identified: (i) an oscillatory field of frequency corresponding to the separation between the coupled levels in the quantum-well pair (~ 1.5 THz); and (ii) an “instantaneous” electromagnetic field pulse that appears to follow the time evolution of the exciting laser pulse. The oscillatory field is understood to be caused by the coherent oscillation of optically created electrons from one well to the other.[1] The “instantaneous” part we believe is caused by a new mechanism proposed in Ref. 2. The essence of this “instantaneous” mechanism is that electron-hole pairs are created in a biased semiconductor with a net polarization. The density of these pairs follows the light field, as does the induced polarization, hence, generating an electromagnetic source that follows the light pulse. Such mechanisms have been discussed before for photon energies below the optical bandgap energy,[3] although they have so far been difficult to observe. In our work, we have generalized and extended such effects to photon energies above the handgap energy. In this paper we will discuss both of these mechanisms as well as the fourwave-mixing (FWM) signal,[4] and show how they can be understood within one unified formalism. We will also show that the calculations agree well with the experimental results.