Focusing of electromagnetic waves into a biaxial crystal, experimental results

We present experimental results for focusing of an electromagnetic wave from air into a biaxial crystal. These results are compared with numerical results, and good agreement is found. The experimental results show distinct tangential and sagittal foci, whose positions agree with numerical predictions. The theory is outlined both for the exact solution, which includes extra-paraxial geometries and double refraction, and for the paraxial solution, in which double refraction is ignored. We also present a simple ray-optical interpretation of the results, which allows us to predict the positions of the sagittal and tangential foci.