Research and design of a metasurface with an extended depth of focus in the near field

A metasurface with an extended depth of focus has broad application prospects in security detection. However, in the near field, the simulation results obtained by using traditional methods to achieve an extended depth of focus have a significant deviation from the preset value. This paper discusses the relationship between the depth of focus and focusing position, and the reason why the simulation results deviate from the preset focus position in the radial modulation method. The angle modulation method is found by a simulation. A more accurate method for an extended depth of focus was proposed by combining the radial modulation method with the quasi-optical path principle. Finally, a polarization-insensitive reflective metasurface element was designed, and elements were arranged to form a polarization-insensitive focus between 150 and 400 mm based on the focusing effect settings. The simulation results indicate that the metasurface achieves the same focusing effect between 175 and 425 mm when different linear-polarization waves are incident. This focus is greater and more accurate than the radial modulation method under the same conditions, which indicates that the method is superior to the radial modulation method in the near-field region. The simulation verifies the accuracy of the method and shows potential application prospects in fields such as microwave imaging.