Abstract: Mid-infrared transmittance of subwavelength silver slit structures was numerically studied with the finite difference time domain method. The TM (transverse magnetic) wave transmission behavior at normal incidence was also further investigated by looking into electromagnetic fields and Poynting vectors of selected slit geometries. The results showed that for the slit structures without square features attachment， the transmittance increases monotonically with the wavelength， but is not proportional to the slit width. Given the slit structures with square features at the inlet， the effect of square features attachment mode and side length on the transmittance are different for the various slit widths. Based on the analysis of electromagnetic field and Poynting vectors， such transmittance change can be attributed to the cavity resonance effect inside the slit arrays.

Key words: finite difference time domain method, transmittance, metallic slit structure, cavity resonance effect, Poynting vectors