Sub-wavelength periodic metamaterial structures are enabling the design of silicon photonic devices with unprecedented performance in the near infrared band. However, for applications in the promising mid-infrared band it is expected that they acquire even more prominence because for longer wavelengths it is far easier to fabricate structures with a sub- wavelength pitch. Here we report our recent progress in the electromagnetic modeling of sub-wavelength structures, and we will review some of our latest advances in the development of sub-wavelength based devices operating both at near and mid- infrared wavelengths.