  We write the equation of geodesic deviations in the spacetime of $pp$-waves
in terms of the Newman-Penrose scalars and apply it to study gravitational
waves in quadratic curvature gravity. We show that quadratic curvature gravity
$pp$-waves can have a transverse helicity-0 polarization mode and two
transverse helicity-2 general relativity-like wave polarizations. A concrete
example is given in which we analyze the wave polarizations of an exact
impulsive gravitational wave solution to quadratic curvature gravity.
