We investigate polarizations of gravitational waves in generalized theories of gravity whose action contains a nonminimal coupling (NMC) between an exponential function of f(R) gravity and the matter field. Specifically, we consider two classes of such models. In the first case, we assume an explicit NMC between the matter Lagrangian density and curvature while in the second case, we consider the exponential f(R) gravity coupled to the trace of the stress-energy tensor of a scalar field. Using the Newman–Penrose quantities, we show that these models can be considered as suitable choices in order to produce new polarization modes.
