Nonlinear optical technique for accurate retardation measurements

Summary form only given. We present the first nonlinear optical technique for retardation measurements. The technique is based on second-harmonic generation (SHG) from poled achiral thin films with in-plane isotropy using two input beams at the fundamental frequency. We demonstrated our technique by determining the retardation of a zero-order half-wave plate. The technique relies on fundamental symmetry properties of nonlinear interactions and does therefore not require a sophisticated experimental arrangement nor extreme care in the alignment to achieve a high precision.