We have prepared cross-grating gate electrodes on top of a two-dimensional electron system in AlxGa1-xAs/GaAs heterostructures, which act as grating couplers. In addition to the well-known two-dimensional plasmon modes at ωp(q1) and ωp(2q1), with wave vector q1=2π/a, where a is the grating period, we observe an excitation at ωp(√2q1). This plasma excitation oscillates in the diagonal direction with respect to the cross-grating structure, and is formed by a superposition of the plasmons in the orthogonal lattice directions with wave vectors qx and qy.