RESONANT 3RD-ORDER OPTICAL SUSCEPTIBILITY CHI((3))(3-OMEGA) IN ASYMMETRIC COUPLED SI1-XGEX SI QUANTUM-WELLS/

We have calculated the enhancement of the third-order nonlinear optica l susceptibility owing to the multi-photon process associated with the intersubband transition in the valence band in asymmetric coupled Si1 -xGex/Si quantum wells. The hole states are described by the 6 x 6 Lut tinger-Kohn Hamiltonian which incorporates the heavy-hole, light-here and spin-orbit split-off bands and the effect of strain of lattice con stants in the multi-quantum-well structure has been taken into account . In order to optimize the resonant third-order susceptibility the lay er thicknesses of wells and barriers are selected in such a way that t he energy spacings between the bound states in the valence band are ne arly equal and the product of the dipole matrix elements of the releva nt intersubband transitions is maximized. The step structure in the ba rrier height has been also considered. We found that the susceptibilit y chi((3))(3 omega) is one or two orders of magnitude larger than that of the bulk Si.