In this work we report on synchronized plasma wave resonances in ultrathin-membrane GaN heterostructures. In contrast to commonly employed grating-gate configurations, the analyzed structure contains periodically-patterned ohmic contacts to the two-dimensional electron gas (2DEG), which are laid-out parallel to the gate fingers. Our work demonstrates that the proposed approach allows: more efficient excitation of high order plasmon modes, and superior overall coupling, even in configurations having less number of devices per unit area.