Plane-fronted gravitational wave
= Plane-fronted gravitational wave
For
$$
ds^2=2\,du\,dv+dx^2+dy^2+H(u,x,y)\,du^2,
$$
the only potentially nonzero Ricci component is
$$
R_{uu}=-\frac12(H_{xx}+H_{yy}).
$$
The vacuum equation is therefore the transverse Laplace equation.