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Intersubband Transitions and Absorptions in Nano–Scale Quantum Well structures

W. Liu, D.H. Zhang, X.H. Tang, W.J. Fan and S.F. Yoon
Nanyang technological University, SG

Keywords:
nanometer scale, quantum well, intersubband transition, absorption

Abstract:
Intersubband transitions and absorptions in conduction-band quantum wells (QWs) are investigated theoretically using 8 band k?p model and envelope-function Fourier expansion method, and they are experimentally examined. Compared to conventional method, we don’t make any simplification to the Hamiltonian but directly work out the subbands and intersubband transitions by numerical calculations. It’s found that the intersubband absorptions in the QW wells with narrow wells don’t follow the conventional selection rules. In the case of moderate or wide wells, the absorption of normal-to-plane polarized (TM) light mainly comes from the transitions between the ground state (e1) and the even-index excited states (e2, e4 …), while the absorption of in-plane polarized (TE) light mainly comes from the transitions between the ground state (e1) and the odd-index excited states (e3, e5 …). However, In the case of well width being narrower than a critical value (only a few nanometers), the TM absorption mainly comes from the transitions between the ground state (e1) and the odd-index excited states (e3, e5 …), while the TE absorption mainly comes from the transitions between the ground state (e1) and the even-index excited states (e2, e4 …). GaAs/AlGaAs n-type multiple QWs with different well widths were grown and characterized, and the experimental observations were found to be consistent with the theoretical results. This work provides new information for design of quantum well infrared photodetectors (QWIPs) and quantum cascade lasers (QCLs).

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