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Normal incidence infrared modulator using direct–indirect transitions in GaSb quantum wells

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2 Author(s)
Xie, H. ; Department of Electrical Engineering, Columbia University, New York, New York 10027 ; Wang, W.I.

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We propose a novel normal incidence infrared modulator using the direct–indirect transitions induced by an applied electric field in GaSb quantum wells (QWs). The device is based on the principles that the quantum‐confined Stark shift is proportional to the effective mass, and that the interconduction subband absorption at normal incidence is forbidden in direct‐gap QWs but allowed in indirect‐gap QWs. Since the effective mass of the L valleys is larger than that of the Γ valley, the corresponding Stark shift is also larger. As a result, the ground state of a QW associated with the Γ point at zero voltage becomes L state under bias (direct–indirect transition). Consequently the device switches from being transparent to normal incidence light to strongly absorbing it. Based on our calculations for a GaSb/Ga0.5Al0.5Sb QW with a well width of 85 Å, changes in absorption coefficients up to 104 cm-1 in the modulation wavelength range of 14–18 μm could be achieved under an electric field in the range of 200 kV/cm. This is the most effective mechanism ever reported for normal incidence infrared modulators.  

Published in:

Applied Physics Letters  (Volume:63 ,  Issue: 6 )