Abstract
One of the extensions of Einstein's theory is the Quadratic Gravity, where the action contains additional terms that are quadratic combinations of Riemann tensor and its contractions. A metric that admits a non-expanding, non-twisting and shear-free null geodesic congruence is called the Kundt metric.
Its denition is purely geometrical and it does not depend on the field equations; inserting the Kundt metric into Quadratic Gravity field equations then gives us restrictions on the metric. We focus on the vacuum solutions with a cosmological constant.
We study specic subcases of the Kundt metric in Quadratic Gravity, such as the pp-waves and VSI spacetimes, and look for their geometrical and physical interpretation.