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

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.