  We propose an improved structure of an optical biosensor based on a photonic
crystal (PC) with a defect layer (DL), which can detect the concentration of
SARS-CoV-2 in water by defect mode (DM) shift. We investigated 4 types of
defective PCs with different arrangements of layers inside the perfect PCs and
their impact on the performance of the sensor. The sensitivity and amplitude of
DM were examined as a function of DL thickness. Also, the peculiarities of edge
modes in the presence of DL were investigated. Finally, we obtained a
characteristic equation to determine the wavelengths of DMs for an arbitrary 1D
PC with an isotropic defect inside.
