I'm no adcom but I can certainly tell you it would have behooved you to send a thank you letter within 48 hrs of your interview. By the fact that you don't know the name of your interviewer I am sure you didn't. In this regard I imagine you could call the admissions office and ask who interviewed you with the back drop that he really left a good impression on you and that you wanted to send him a thank you letter. Good luck.
I'd suggest you try the sheepishly honest approach -- "I was really nervous at my interview and didn't get the interviewer's name. I want to write him/her a brief 'thank you' note, and I'm hoping you can look up his/her name for me?" This is so common and so understandable -- so long as you sound apologetic and a little bit embarrassed, then they'll probably be happy to get you the information.