ELECTRIC-FIELD ENHANCEMENT AND EXTINGUISHMENT OF OPTICAL 2ND-HARMONICGENERATION IN ASYMMETRIC COUPLED QUANTUM-WELLS

The second-harmonic susceptibilities of asymmetric coupled quantum wel ls (ACQW) and compositionally asymmetric coupled quantum wells (CACQW) under the influence of the applied electric field are investigated th eoretically. Analytic forms of the second-harmonic susceptibility are derived by using the density matrix formalism. Coupled one-dimensional Schrodinger equation and Poisson's equation ae solved self-consistent ly to find the subband eigenenergies and the envelope wavefunctions fo r the ACQW and CACQW structures. Dipole moment matrix elements for the ACQW and CACQW are evaluated from the resulting envelope wavefunction s and large nonlinear optical effects are predicted for these two stru ctures. Based on the theoretical calculations, the second-harmonic sus ceptibilities of 50 nm/V and 35 nm/V can be achieved for the ACQW and the CACQW, respectively. This is more than two orders of magnitude enh ancement as compared to that of the bulk GaAs. By a suitable choice of the composition of CACQW, a novel structure which the second-order no nlinear optical effect can be turned on or off by the applied electric field is proposed based on the results of the theoretical calculation . This phenomenon is attributed to the symmetry restoration of envelop e wavefunctions of the CACQW under the quenching electric field F-off. A simple physical model to estimate the F-off has also been developed successfully.