A biosensor is provided. The biosensor includes a plurality of sensor units. Each of the sensor units includes one or more photodiodes, a first aperture feature disposed above the photodiodes, an interlayer disposed on the first aperture feature, a second aperture feature disposed on the interlayer, and a waveguide disposed above the second aperture feature. The second aperture feature includes an upper grating element and the first aperture feature includes one or more lower grating elements, and a grating period of the upper grating element is less than or equal to a grating period of the one or more lower grating elements. A difference of the absolute values between a first polarizing angle of the upper and lower grating elements in one of the sensor units and a second polarizing angle of the upper and lower grating elements in adjacent one of the sensor units is 90°.