Optical wavefront sensor based on sub-wavelength metallic structures

Lateral shearing interferometers (LSIs) are efficient tools for optical analysis. They allow classical optical wavefront aberrations measurements as well as the precise evaluation of abrupt steps. The basic element of an LSI is the transmittance grating, which diffracts a number of orders (two in the case of a mono-dimensional LSI, ideally three or four non coplanar orders in the case of bi-dimensional LSI). This brings the need for specifically designed transmittance gratings. For instance, a mono-dimensional LSI needs a sinusoidal-shaped transmittance, since its Fourier transform carries exactly 2 orders. Such transmittances are however either impossible or at least extremely costly to design using classical macroscopic techniques, mainly because the usual thin film deposition techniques require several technological steps, in order to get the desired light filtering effect. Given these constraints, we made use of sub-wavelength structures in order to build a new class of LSI. They are made of sub-wavelength lamellar metallic gratings specifically designed for the mid-infrared, and allow the precise coding of the desired transmission shape all over the LSI grating.