  The emission from open cavities with non-integrable features remains a
challenging problem of practical as well as fundamental relevance.
Square-shaped dielectric microcavities provide a favorable case study with
generic implications for other polygonal resonators. We report on a joint
experimental and theoretical study of square-shaped organic microlasers
exhibiting a far-field emission that is strongly concentrated in the directions
parallel to the side walls of the cavity. A semiclassical model for the
far-field distributions is developed that is in agreement with even fine
features of the experimental findings. Comparison of the model calculations
with the experimental data allows the precise identification of the lasing
modes and their emission mechanisms, providing strong support for a physically
intuitive ray-dynamical interpretation. Special attention is paid to the role
of diffraction and the finite side length.
