  We propose a new optical biosensor based on a photonic crystal with a defect
layer, which can determine the SARS-CoV-2 concentration in water by the defect
mode shift. Two models of the dependence of the refractive index of the defect
layer on the concentration of the pathogen in water were considered. The
optimal parameters for the photonic crystal in our device and the optimal
thickness of the defect layer were determined. It was also demonstrated that in
the presence of absorption in the investigated structure it is much more
advantageous to work in the reflection mode compared to the transmission mode.
Finally, the wavelength dependence of the defect mode on the SARS-CoV-2
concentration was obtained and the sensitivity of the sensor was determined.
