Phase and amplitude interferometric characterization of infrared nanostructured gratings

Sub-wavelength gratings allow to code complex transmittance functions that introduce both amplitude and phase variations in the propagation of a given wavefront. These micro-structures are a promising technique to miniaturize optical functions such as light polarizing, light confinement, spectral filtering... Realizations in the visible and the infrared domain have been fulfilled: for example micro-lenses, anti-reflection coatings or sinusoidal-transmittance can easily be coded. This technique is all the more advantageous in the mid-wavelength infrared (MWIR) or long-wavelength infrared (LWIR) spectral range since there are only a few materials available in this spectral range. However the characterization of these structures is problematical, since it involves phase and amplitude measurements. It is even more complicated in the far infrared domain (8 - 14 μm), as will be detailed. Besides, the finite size of the gratings introduces phase steps, which is well-known to be a problematic issue. We describe here a dedicated bench to characterize sub-wavelength gratings in the LWIR spectral range. The core of the bench is a quadri-wave lateral shearing interferometer based on a diffraction grating, which allows a complete two-dimensional characterization of both phase and amplitude in a single measurement. We present here theoretical and experimental results of a characterization of such a sub-wavelength grating.