Design principle of Au grating couplers for quantum-well infrared photodetectors.

Based on analytic formulas and numerical simulations, we investigate the enhancement effect of Au gratings with spoof plasmon resonances on quantum-well infrared photodetectors (QWIPs) operating between 2 and 30 μm. It is found that a simple analytic formula can well estimate the resonant wavelengths of Au gratings. Using optimal grating parameters, the absorption in the QW active region can be enhanced by 4-5 times compared with that in the reference structure (without gratings and with an isotropic active region). For s-polarized light, a high enhancement (>1.4) can occur in a broad range of incident angle (|θ|<40°).