After the interview you don't want to ask questions just for the sake of asking questions, it could actually make you look more obnoxious than interested. I would only write back to your POI if you have a legitimate question to ask. Did you have any concerns post-interview? Anything remain unclear? Do you know how many of the POI's students get funded? Do you know how they are funded? Did you get to talk to any of the POI's current students? If you really have everything answered that you need answered I think you a better off acknowledging the email and how well people answered your questions on interview day (but this is just my opinion!).