Application of the arbitrary decomposition to finite spot size Mueller matrix measurements

Finite spot size Mueller matrix polarimetric measurements whereby the light spot impinges on two different areas of the sample, e.g., a grating and a substrate, are relatively frequently met in practice. It has been shown that if the Mueller matrix of one of the areas (the substrate) is known from an additional measurement then the Mueller matrix of the remaining medium (the grating) can be obtained from the (substrate-grating) overall response by the polarimetric subtraction method. We show that, provided a specific condition is fulfilled, the individual polarimetric responses of the two areas can be retrieved from the finite spot size measurement by using a special form of the arbitrary decomposition even if none of the individual responses is known a priori. The decomposition method is illustrated on a microelectronics grating structure and its accuracy, as well as limits of applicability, is discussed.