The periodic scattering of the surface plasmon modes employed in the waveguide of terahertz quantum cascade lasers is shown to be an efficient method to control the properties of the laser emission. The scatterers are realized as thin slits in the metal and top contact layer carrying the surface plasmon. This technique provides larger coupling strengths than previously reported and can be used in various device implementations. Here the method is applied to realize a distributed feedback resonator without back-facet reflection, to achieve vertical emission of the radiation with second-order gratings, and to increase the facet reflectivity by fabricating passive distributed Bragg reflectors.