try second order elements

DiffractionGratings/grating2D.geo

@@ -250,7 +250,10 @@ Physical Surface(PMLTOP)   = {48}; // pmltop
 
 Physical Point(PRINT_POINT) = {1};
 
+If (Flag_o2_geom==1)
+  Mesh.ElementOrder = 2;
+EndIf
+
 Solver.AutoMesh=1;
 Geometry.Points = 0;
-Mesh.SurfaceEdges = 0;
-// Hide "*";
+Mesh.SurfaceEdges = 0;
\ No newline at end of file