Intersubband /spl chi//sup 3/ in coupled InGaAs-AlGaAs multiple quantum wells

The third-order nonlinearity, /spl chi//sup 3/(/spl omega/,/spl omega/,-/spl omega/), is measured for a mid-infrared intersubband transition in strained InGaAs-AlGaAs multiple quantum wells (MQW's). The high conduction band offset of this system allows an intersubband transition at 3.1 /spl mu/m. The level structure of the quantum well is designed to include a meta-stable trapping level, resulting in a peak saturation intensity of 6 MW/cm/sup 2/ at Brewster's angle, approximately 20 times lower than would be found in a square quantum well with similar linewidth. A near-resonant n/sub 2/ of 8.4/spl times/10/sup -7/ cm/sup 2//W at 3.1 /spl mu/m is calculated. The off-resonant n/sub 2/ is also calculated and shown to be attractive at wavelengths as short as 1.55 /spl mu/m.