How to introduce a obliquely incident plane wave in femlab?

I'd like introduce an obliquely incident plane wave as source in my simulation. I tried using surface current and electric field boundary, but no satisfactory results. So could u be kind enough to give me some hints?

Actually it seems that this is a not so easy job, I asked this question in COMSOL Multiphysics emailing list without answer, making me disppointed ...

If anyone incidently is using this software and has a proper license no., I hope u can help me contact its technical support.

Thanks!