<p>In this paper, a GaAs/AlGaAs quantum well infrared photodetector (QWIP) using a rectangular plasmonic microcavity structure is proposed. Based on the fact that different side lengths of the rectangular patch responding to different TM waves during surface plasmon resonance, the device has three modes of operation. In the case of TM₁₀ and TM₀₁ waves being incident alone, the response peaks of the device are located at 6.48&#xa0;μm and 7.38&#xa0;μm, which correspond to the two single-peak modes of Ex and Ey polarisation, respectively. The two response wavelengths generated by the two polarisation modes are independently regulated within a certain range and are not affected by each other. In the case of simultaneous incidence of TM₁₀ and TM₀₁ waves, the full width at half maximum (FWHM) of the device’s responsivity spectrum reaches 1.56&#xa0;μm, which is broadband response mode. The simulation study reveals that the average electric field enhancement |<i>E</i>_Z|/|<i>E</i>₀| in all three modes is more than 5 times, while the single-peak and broadband response mode responsivities reach 3 A/W and 2 A/W, respectively. Our findings indicate that the average energy inside the microcavity is positively correlated with the microcavity volume within a certain range. Moreover, the coupling structure exhibits favourable incident angle compatibility, and the device performance in each mode is insensitive to the incident angle in the range of ~ 45°.</p>