Angle resolved Mueller polarimetry with a high numerical aperture and characterization of transparent biaxial samples

We present a polarimetric instrument suitable for the simultaneous measurement of angle resolved normalized Mueller matrices for polar angles ranging from 0 degrees to 60 degrees and all azimuths. The polarimetric modulation and analysis are performed by means of an optimized polarization state generator and analyzer based on nematic liquid crystals. A high numerical aperture (0.95) microscope objective is used in double pass to illuminate the sample, with its rear focal plane imaged on a low noise CCD. This polarimeter can be used either in reflection, with the sample set in the objective front focal plane, or in transmission, for thin transparent samples. This latter configuration, which involves an additional spherical mirror with its center of curvature at the objective front focus, is described in detail. This instrument was used to accurately determine the directions of the optic axes and the angular dependence of the retardation of a biaxial polyethylene terephthalate (PET) plastic substrate in spite of the strong depolarization essentially due to the source 10 nm spectral width or the limitation in angular resolution due to the pixels distribution of the CCD combined with the sample large retardation.