In multiple quantum wells system, a large dipole strength and a narrow band width were observed. This suggests that the intersubband optical transitions in a quantum well may have very large optical nonlinearities. The intersubband transition have potential for device applications in far-infrared laser amplifiers, photodetectors and high-speed electrooptical modulators. In device design, the region of input optical density for linear response is a very important parameter. And the optical saturation is related with tunnelling effects and the intersubband relaxations. So the intersubband nonlinear absorption is very valuble for a fundamental and a practical point of views. Recently, there are some works on this region [1], [2].

1.
D.
Ahn
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S-L
Chuang
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IEEE J. Quantum Electron
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23
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2.
S-H
Pan
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S-M
Feng
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D-F
Cui
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G-Z
Yang
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Acta Physica Sinica
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40
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1990
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3.
K.
Nakamura
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A.
Shimizu
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M.
Koshiba
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K.
Hayata
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IEEE J. Quantum Electron
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27
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2035
(
1991
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