For most commonly used GaAs/AlGaAs n-type quantum well infrared photodetectors (QWIPs), the normal incident
absorption is not possible because of the transition rule. The optical grating is required to achieve high absorption
quantum efficiencies. When some gratings are patterned on the metal plate, the polarization direction can be changed
greatly because of the diffraction effect. Finite difference time domain (FDTD) method has been used to investigate the
effect of a reflection metal grating on the couple efficiency previously. However, the authors only take one metal grating
and apply periodic boundary condition along the grating direction due to the computation limit. For a real QWIP system,
such simulation is crude. In this work we consider a real GaAs/AlGaAs QWIP with a wavelength response around 15um
and use FDTD method to investigate the effect of a reflection metal grating on the electric field pattern and the couple
efficiency. The simulating results show that the electric field pattern is not periodic for every metal grating in a real
QWIP system. We have also studied the influence of the substrate thickness and the grating period on the electric field
pattern and the couple efficiency. These results offer a guideline for the design of QWIP.
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