Infrared Transmission Spectrum Broadening in Metallic Gratings with Gradual Thickness

Transmission properties of transverse magnetic mid-infrared through sub-wavelength metallic gratings with gradual thickness are studied by finite-difference-time-domain method. The result shows that the infrared transmission spectrum is broadened, but also the maximum transmission is weakened. This is attributed to the positive coupling between the incident wavelength and the slits. Any incident wavelength has a match with the gratings where corresponding thickness stands. On the other hand, unlike the case in conventional gratings, optimal coupling cannot be gained in every slit at the same time, resulting in weakened maximum transmission. Such transmission broadening effect may be applied in these fields where wide spectrum response is required.