Irreducible weight modules over Witt algebras with infinite dimensional weight spaces

Let $d>1$ be an integer. In 1986, Shen defined a class of weight modules $F^\alpha_b(V)$ over the Witt algebra $\mathcal{W}_d$ for $\a\in\C^d$, $b\in\C$, and an irreducible module $V$ over the special linear Lie algebra $\sl_d$. In 1996, Eswara Rao determined the necessary and sufficient conditions for these modules to be irreducible when $V$ is finite dimensional. In this note, we will determine the necessary and sufficient conditions for all these modules $F^\alpha_b(V)$ to be irreducible where $V$ is not necessarily finite dimensional. Therefore we obtain a lot of irreducible $\mathcal{W}_d$-modules with infinite dimensional weight spaces.